JP5145704B2 - Power control circuit - Google Patents

Description

  The present invention relates to a power supply control circuit that controls the operation of a switching power supply, and more particularly to an IC (integrated circuit) power supply control circuit.

  An AC-DC switching power supply that converts an alternating current input from a commercial alternating current power source into a direct current and outputs the direct current is used in a wide range of applications, and various types of power supplies have been put into practical use. A control circuit for controlling the operation of such a switching power supply is in the form of an IC, which is a packaged product.

  FIG. 6 is a diagram showing a circuit configuration of a switching power supply controlled by such an IC power supply control circuit. Here, an example configured as a flyback converter is shown.

  In the circuit configuration shown in the figure, the alternating current from the alternating current power supply AP1 is full-wave rectified by the diode stack DS1, and the direct current smoothed by the capacitor C1 is supplied to the primary winding N1 of the output transformer T1. A power MOS transistor Q1, which is a switching element, is connected in series to the primary winding N1, and the power MOS transistor Q1 is turned on (ON) and turned off (OFF) by a drive signal from the control IC 1 constituting the power supply control circuit. As a result, a pulsating flow is generated in the secondary winding N2 of the output transformer T1. This pulsating current is rectified by the diode D1, smoothed by the capacitor C2, and supplied to a load (not shown).

  The output voltage to the load is divided and detected by the resistors R1 and R2, and the detected value is input to the FB terminal of the control IC 1 as a feedback signal via the photocoupler PC1. Further, when a current flows through the primary winding N1 of the output transformer T1, a voltage is also generated in the auxiliary winding N3. This voltage is rectified by the diode D2, smoothed by the capacitor C3, and Vcc which is the power supply terminal of the control IC1. Supplied to the terminal.

  In addition, a temperature detection element (thermistor) RT for detecting an overheating state as an abnormal state of the switching power supply is externally attached to the OTP terminal of the control IC 1. C4 is a capacitor, ZD1 is a shunt regulator, R3 is a limiting resistor for limiting the current from the high voltage system, R4 is a detection resistor for detecting the current of the power MOS transistor Q1, R5 is a filter resistor for reducing noise to the IS terminal, R6 Is a resistor for adjusting the gate drive current of the power MOS transistor Q1.

  Here, in the case of the switching power supply as described above, the control IC 1 that drives the external power MOS transistor Q1 as a power switching element generally uses a relatively inexpensive general-purpose IC having an 8-pin external terminal configuration. Is. In this case, in addition to the above three essential terminals of the Vcc terminal for power supply input, the GND terminal for grounding, and the OUT terminal for gate drive of the power MOS transistor Q1, the IS terminal for detecting the drain current of the power MOS transistor Q1. If two terminals for control of the feedback signal input FB terminal are set, the rest becomes three terminals.

  Among the remaining 3 terminals, the VH terminal has a high voltage switch function that supplies current (starting current and latch holding current) from the high voltage system to the Vcc terminal only when necessary, and reduces power consumption at light loads. And the non-connected NC terminal that is an adjacent pin of the VH terminal is excluded from the safety standard requirement, there is only one terminal that can be used for other functions.

  The remaining one terminal has the most important functions of what the switching power supply requires, such as soft start, frequency adjustment during brownout light load, latch timer, and overcurrent limit level on the primary side of the output transformer T1. It may be assigned to various functions such as adjustment. The role of this terminal changes depending on which function is emphasized, and incorporates as many functions as possible.

  In a switching power supply that needs to suppress the temperature around the switching element with high accuracy, the overheat protection function that uses a temperature detection element that has a negative temperature characteristic (resistance value decreases at high temperatures) as an internal function may be given top priority. (For example, refer to Patent Document 1). In this case, the one terminal is used as a drive terminal for the temperature detecting element, and a constant current source for always supplying a certain discharge current at this terminal is provided. Then, the terminal voltage is monitored, and when it falls to a preset threshold level, it is detected as an overheat state, and the detection signal is sent to the delay timer that activates the protection function. I do.

  FIG. 7 is a diagram showing the characteristics of the temperature sensing element having the negative temperature characteristics described above, and shows the relationship between temperature and resistance value. In the figure, the characteristics of three temperature detecting elements are shown, and the resistance value decreases as the temperature increases.

  FIG. 8 is a diagram showing a configuration of a main part of a conventional power supply control circuit that performs a protection operation by detecting an overheat state as described above. This power supply control circuit is configured in the control IC 1 and has a built-in activation element (not shown), and an OTP terminal is assigned for overheat protection.

  A temperature detecting element RT having a negative temperature characteristic is externally attached to the OTP terminal, and a discharge current of 70 μA is supplied from the +5 V power supply line through the constant current source IS101. The terminal voltage is compared with the threshold level of 1.0 V by the comparator CP101 composed of an operational amplifier, and is reduced to the threshold level, that is, the resistance value of the temperature detection element RT is 14.286 kΩ (1 V / 70 μA = 14.286 kΩ). When the voltage drops, the output of the comparator CP101 is inverted, and an overheat state is detected.

  The overheat detection signal is input to the delay circuit 101 with a delay time of 50 μs to prevent malfunction due to noise. If the output of the comparator CP101 remains inverted until the delay time elapses, the delay circuit 101 outputs a latch signal. The set signal is input to the flip-flop FF101. This flip-flop FF101 is for issuing a switching stop signal. When a set signal is input, a disable signal is output from the PWM (Pulse Width Modulation) signal generation circuit 102 to the output circuit 103 to which the PWM signal is input. Output. As a result, the PWM signal from the OUT terminal is stopped, and the switching is fixed to the stopped state. The latch flip-flop FF101 is reset when the control IC 1 is started or restarted.

In the power supply control circuit of Patent Document 1, a thermistor is added to protect the circuit from temperature rise, and a capacitor is connected in parallel with the thermistor, but the terminal to which this thermistor is connected is a single function. Yes, capacitors are not always necessary.
JP 2004-242427 A

  In the conventional power supply control circuit configured as described above, if the overheat detection protection function is built in, the temperature detection element externally attached to the external terminal changes the resistance value depending on the temperature, so that this acts as a constant current source. The voltage at the external terminal also varies depending on the temperature.

  For this reason, it is difficult to add something to the external terminal to operate other functions, and there is a problem that it cannot be used for anything other than a stop function for forcibly stopping switching by shorting the external terminal to GND.

  The present invention has been made in view of these points, and an object of the present invention is to provide a power supply control circuit capable of easily adding other functions even when an overheat detection protection function is incorporated.

In the present invention, in order to solve the above-described problem, in an integrated circuit power supply control circuit for controlling the switching operation of a switching power supply, a capacitor connected to a predetermined external terminal and a first current for supplying a current to the external terminal are provided. A constant current source, a second constant current source that discharges current from the external terminal, a voltage level of the external terminal that is higher than a first threshold level for detecting an abnormal state, and second, third When the voltage level of the external terminal rises to the third threshold level due to the current supply of the first constant current source and the first, second and third comparators respectively comparing with the threshold level of Switching from the current supply of the first constant current source to the current discharge of the second constant current source, the voltage level of the external terminal is changed to the second by the current discharge of the second constant current source. By switching from the current discharge of the second constant current source to the current supply of the first constant current source when the level drops to the threshold level, the second threshold level and the third threshold level are changed between the second threshold level and the third threshold level. comprising a switching means to provide an oscillator function voltage changes, and based on the change in the voltage level of said external terminals, a power supply control circuit according to claim Rukoto by modulating the switching frequency of the switching power supply is provided The

According to such a power supply control circuit, it has an oscillator function in which a voltage changes between a second threshold level and a third threshold level higher than the first threshold level for detecting an abnormal condition, and a capacitor is connected. and an external terminal based on a change of the voltage level of the applied modulation on the switching frequency of the switching power supply Runode, even when an internal thermal detection protection, be utilized oscillator functions easily add other functions it can.

The power supply control circuit of the present invention has an oscillator function in which a voltage changes between a second threshold level and a third threshold level higher than the first threshold level for detecting an abnormal state, and a capacitor is connected Runode by modulating the switching frequency of the switching power supply based on a change of the voltage level of the external terminal, even when an internal thermal detection protection, that may utilize the oscillator function easily add other functions There are advantages.

  The present invention realizes multiple functions by using one external terminal of the control IC of the switching power supply. When the overheat detection protection function is built in, the temperature at which the resistance value changes with temperature at the external terminal. Since the sensing element is connected, there are some difficulties in applications that require high-precision control, but other functions are operated by using a region higher than the threshold level for overheating detection. Specifically, a capacitor with a temperature detection element is connected to the external terminal, and charging and discharging of this capacitor are switched according to the second and third threshold levels, thereby enabling an oscillator with a long period of 5 ms or more that is difficult to incorporate an IC. Configure. The frequency jittering function that reduces the peak of the noise component by modulating the switching frequency of the switching power supply using this long-cycle voltage change, restarts when the switching power supply is started or overload is detected. A soft start function that gradually widens the width of the switching on-pulse is realized.

  However, because it is also used as an overheat detection protection function, the fluctuation due to temperature is large. In particular, it is difficult to operate as an oscillator near the temperature at which the overheat detection protection function works. Even if the fluctuation is about 5 to 3 times, it is necessary not to cause a big problem in using the switching power supply. For this reason, it is effective to incorporate frequency jittering and soft start functions.

  FIG. 1 is a diagram showing a configuration of a main part of the power supply control circuit according to the first embodiment of the present invention. This power supply control circuit is configured in the control IC 1 of the switching power supply shown in FIG. 6, and controls the switching operation. The VH terminal to which the starting current is supplied from the switching power supply, the unused unconnected NC next to it. Terminal, Vcc terminal to which power supply voltage Vcc is supplied, OUT terminal for driving a power MOS transistor, GND terminal for grounding, FB terminal to which a feedback signal of a switching power supply is input, and a detection signal of a current flowing through the power MOS transistor Q1 Is input, and an OTP terminal (predetermined external terminal) for detecting overheating has an 8-pin external terminal. Here, a part of the circuit is shown.

  A temperature detecting element RT and a capacitor Co are externally attached to the OTP terminal, a first constant current source IS1 that supplies a discharge current (70 μA) to the OTP terminal, and a second current that discharges a sink current (70 μA) from the OTP terminal. Constant current source IS2. A P-channel MOS transistor PM1 is connected as a switch element in series to the first constant current source IS1, and an N-channel MOS transistor NM1 is connected as a switch element in series to the second constant current source IS2.

  Further, the first threshold level (1.0 V) for detecting the abnormal state (overheated state) of the voltage level of the OTP terminal, and the second and third threshold levels (1.1 V, 1.5 V) that are higher in that order. Are respectively provided with a first comparator CP1 and second and third comparators CP2 and CP3. The overheat detection signal output from the first comparator CP1 is input to the delay circuit 11 having a delay time of 50 μs to prevent malfunction due to noise, and the output of the comparator CP1 remains inverted until the delay time elapses. Then, the set signal is input from the delay circuit 11 to the latching flip-flop FF1. The flip-flop FF1 is for issuing a switching stop signal. When a set signal is input, the flip-flop FF1 outputs a disable signal to the output circuit 13 to which the PWM signal is input from the PWM signal generation circuit 12. As a result, the PWM signal from the OUT terminal is stopped, and the switching is fixed to the stopped state. The latch flip-flop FF1 is reset when the control IC 1 is activated or restarted.

  Further, when the voltage level of the OTP terminal rises to the third threshold level due to the current supply of the first constant current source IS1, the current emission of the second constant current source IS2 from the current supply of the first constant current source IS1. When the voltage level of the OTP terminal drops to the second threshold level due to the current emission of the second constant current source IS2, the current of the first constant current source IS1 is changed from the current emission of the second constant current source IS2. A flip-flop FF2 is provided as switching means for providing an oscillator function in which the voltage changes between the second threshold level and the third threshold level by switching to supply.

  In the power supply control circuit configured as described above, switching of the switching power supply is controlled based on a change in the voltage level of the OTP terminal due to the action of the oscillator, but the capacitor Co that controls the oscillation timing is externally attached. A capacitor Co having a large capacitance (0.1 μF or more) can be used, and an oscillator with a slow period of 5 ms or more, which is difficult to incorporate in an IC, can be configured.

  In the normal temperature range, when the terminal voltage of the OTP terminal falls below 1.1V, the output of the comparator CP2 is inverted, the flip-flop FF2 is reset, the Q output becomes L (low) level, and the MOS transistor PM1 is turned on. The MOS transistor NM1 is turned off, and a discharge current of 70 μA is supplied from the first constant current source IS1. The capacitor Co connected to the OTP terminal is charged with the remaining current obtained by subtracting the current flowing to the GND side by the resistance of the temperature detection element RT from this current, and the voltage level of the OTP terminal rises.

  At this time, the resistance value of the temperature detection element RT is assumed to be a value that can be increased to 1.5 V or more with a current of 70 μA in the normal temperature range. That is, the range of the resistance value of 1.5V ÷ 70 μA = 21.428 kΩ or more is the normal temperature range here.

  When the voltage level of the OTP terminal rises to 1.5V, the output of the comparator CP3 is inverted, the flip-flop FF2 is set and the Q output becomes H (high) level, the MOS transistor PM1 is off, and the MOS transistor NM1 is The second constant current source IS2 is turned on, and a sink current of 70 μA is released. The capacitor Co is discharged by a current obtained by adding this current and the current flowing through the temperature detection element RT, and the voltage level of the OTP terminal decreases.

  By repeating the above operation, the OTP terminal operates as a low frequency oscillator. This OTP terminal signal (terminal voltage) can be used as a frequency jitter modulation signal, and the switching frequency can be modulated.

  Further, at a high temperature, the terminal voltage of the OTP terminal decreases to 1.1 V, and when the capacitor Co is switched from discharging to charging as described above, if the temperature detection element RT passes a current of 70 μA or more, the terminal voltage further increases. When the voltage level drops to 1V below the overheat detection threshold level, the comparator CP1 detects the overheat state. This detection signal is input to the delay circuit 11 as described above, and enters the overheat protection state after the delay time has elapsed.

  As described above, the oscillator function of changing the voltage between the second threshold level and the third threshold level higher than the first threshold level for detecting the abnormal state has the function of the OTP terminal to which the capacitor Co is connected. Since switching of the switching power supply is controlled based on a change in voltage level, even when an overheat detection protection function is incorporated, other functions can be easily added using the oscillator function.

  There is an intermediate state between the normal state and the overheat protection state in which the terminal voltage of the OTP terminal can be raised only to 1V to 1.5V with a charging current of 70 μA. There is a region that stops (frequency jitter does not work), and this is based on the assumption that the specifications of the switching power supply are acceptable. In order to reduce the intermediate region, the threshold level of the second and third comparators CP2 and CP3 for frequency jitter may be set as close as possible to the threshold level for overheat detection, and improvement is possible.

FIG. 2 is a circuit diagram showing a configuration example of an oscillator in the power supply control circuit according to the first embodiment.
When 2.5 V, for example, is input as the reference voltage Vref1, the operational amplifier OP1 controls the voltage applied to the timing resistor R11 to be the same as the reference voltage Vref1. The current flowing through the timing resistor R11 is copied by a current mirror circuit including the MOS transistors PM11 to PM13, and the current flowing through the MOS transistor PM13 becomes a constant current source for charging the timing capacitor Ct. Further, the current flowing through the MOS transistor PM12 is folded back by a current mirror circuit including the MOS transistors NM12 and NM13, and the current flowing through the MOS transistor NM13 becomes a current source for discharging the timing capacitor Ct.

  The comparators CP11 and CP12 monitor the voltage of the timing capacitor Ct, invert the flip-flop FF11 every time the threshold level becomes 3V or 1V, and alternately switch the MOS transistor PM14 and the MOS transistor NM11 which are charge / discharge changeover switches. Turn it on. Thereby, it operates as an oscillator.

FIG. 3 is a diagram illustrating a configuration example of the frequency jittering circuit in the power supply control circuit according to the first embodiment.
When modulating the frequency of the oscillator for switching the power MOS transistor Q1 of FIG. 6, if the reference voltage Vref1 is periodically changed, for example, by ± 2.5% with respect to 2.5 V, the timing of FIG. Similarly, the charging / discharging current for the capacitor Ct also changes by ± 2.5%, and the switching frequency can be changed by ± 2.5%.

  The terminal voltage (oscillated from 1.1 V to 1.5 V) of the OTP terminal is input to the base of the transistor Tr11 as the J_IN input of the circuit of FIG. 3, and a current buffer constituted by the transistor Tr11, the current source IS11, and the transistor Tr12. Impedance conversion is performed by the circuit B1 to ensure a sufficient drive current. At this time, the base potential of the transistor Tr11 and the emitter potential of the transistor Tr12 are substantially equal.

The J_OFF input is a signal that enables the jitter function when it is at the L level. At this time, the switch SW1 is turned on and the switch SW2 is turned off.
Resistors R21 to R23 constitute a reference voltage Vref1 generating circuit and divide a voltage of 5V. When the voltage on the switch SW2 side of the resistor R24 is 1.3V, the reference voltage is adjusted to 2.5V. Since the voltage on the switch SW1 side applied to the resistor R24 becomes the voltage of the OTP terminal, the voltage varies between 1.1V and 1.5V across 1.3V. At this time, the current flowing through the resistor R24 to the resistor R23 is added to the current flowing from the resistors R21 and R22 to the resistor R23, and the voltage drop at the resistor R23 is changed. By utilizing this change, the potential change of the reference voltage Vref1 can be adjusted to 2.5V ± 2.5%. Further, if the resistance value of the resistor R24 is decreased and the current is increased, the change in the reference voltage Vref1 can be increased accordingly.

  FIG. 4 is a diagram showing a configuration of a main part of the power supply control circuit according to the second embodiment of the present invention. The same reference numerals as those in FIG. 1 indicate the same components. In this power supply control circuit, when the switching power supply is started or restarted, the voltage level of the OTP terminal is once increased to the second or third threshold level with a discharge current larger than the supply current of the first constant current source IS1. I have to.

  Specifically, a pull-up circuit B2 comprising a flip-flop FF3 and a P-channel MOS transistor PM2 is added to the power supply control circuit of FIG. 1, and the function of latching the voltage at the OTP terminal is activated when the switching power supply is started or restarted. Try to increase rapidly to no voltage.

  By connecting the capacitor Co to the OTP terminal, it takes time until the terminal voltage rises at the time of start-up or restart after being stopped by the protection function, and 50 μs until the latch function for dealing with the abnormal state works. During this delay time, the voltage may not rise above the threshold level of 1.0 V and the latch may stop. In the second embodiment, a pull-up circuit B2 is provided, the gate of the MOS transistor PM2, which is a pull-up switch, is connected to the Q output side of the flip-flop FF3, the drain of the MOS transistor PM2 is connected to the OTP terminal, This prevents the latch from stopping.

  That is, the flip-flop FF3 is reset at the time of starting or restarting, and the MOS transistor PM2 is turned on. Thereby, the terminal voltage of the OTP terminal is reliably raised to 1.0 V or more within the delay time of 50 μs in the latch with respect to the resistance of the temperature detection element RT connected to the OTP terminal and the capacitance of the capacitor Co.

  In the circuit of FIG. 4, when the output of the comparator CP3 having a threshold level of 1.5V is inverted, the flip-flop FF3 is inverted in response to the output signal, and the MOS transistor PM2 is turned off to stop the pull-up current. It has become.

  If it is not desired to increase the pull-up current, the pull-up circuit B2 or the MOS transistor PM2 is not provided, and the latch flip-flop until the output of the comparator CP3 or the Q output of the flip-flop FF3 is inverted. A logic gate circuit that invalidates the input of the set signal to the FF 1 may be added.

  FIG. 5 is a diagram showing the configuration of the main part of the power supply control circuit according to the third embodiment of the present invention. The same reference numerals as those in FIG. 1 indicate the same components. This power supply control circuit has both the above-described frequency jittering function and soft start function.

  In other words, the third embodiment has a comparator that compares the voltage level of the OTP terminal with the fourth and fifth threshold levels that are higher in order than the first threshold level. A function of gradually decreasing the voltage after increasing to the threshold level, and switching based on the voltage change of the OTP terminal while the voltage level of the OTP terminal decreases from the fifth threshold level to the fourth threshold level. The on-switching width or on-duty of power supply switching is gradually increased, and the soft start function is also realized at the same time. Actually, the third threshold level and the fourth threshold level are combined and set to the same threshold level.

  The circuit shown in FIG. 5 includes a one-shot circuit 14 to which the output of the oscillator shown in FIG. 2 is input and a flip-flop FF4 instead of the PWM signal generation circuit 12 shown in FIG. OG1 and OG2 are OR gates, and AG1 is an AND gate. Further, a comparator CP4 that compares the terminal voltage of the OTP terminal with a threshold level of 2.0V and a comparator CP5 that compares the terminal voltage of the FB terminal with a threshold level of 0.8V are provided. The output of the comparator CP4 is a flip-flop. The signal is input to the pull-up circuit B3 including the FF5 and the P-channel MOS transistor PM3, and the output of the comparator CP5 is input to the one-shot circuit 14 as a clear signal. B4 is an amplifier circuit for amplifying the terminal voltage of the FB terminal, and is composed of a negative feedback amplifier OP2 to which a comparison voltage of 2.25 V is input and resistors R31 and R32. B5 is a PWM processing circuit composed of PWM comparators CP6 and CP7, and B6 is a voltage dividing circuit composed of resistors R33 and R34.

  The soft start function will be described. Like the pull-up circuit B2 in FIG. 4, the pull-up circuit B3 raises the terminal voltage of the OTP terminal to 2V (fifth threshold level) at the time of start-up or restart, and then from 2V to the capacitor The terminal voltage of the OTP terminal is changed to the PWM comparator CP6 of the PWM processing circuit B5 using the first (first time) period when the discharge current of Co is lowered to 1.5 V (fourth threshold level) which is the upper limit of jitter oscillation. To enter. The PWM comparator CP6 has the same function as the PWM comparator CP7 that processes the feedback signal from the FB terminal, and the comparison level is a detection signal of the drain current of the power MOS transistor Q1 input from the IS terminal. The PWM comparators CP6 and CP7 cause the drain current detection signal to be lower than the higher of the terminal voltage of the OTP terminal or the output voltage of the voltage dividing circuit B6 (which corresponds to the lower current as the current level). When this is detected and an increase in the drain current of the power MOS transistor Q1 is detected, a signal for turning off the power MOS transistor Q1 is output. This signal is input as a reset signal to the flip-flop FF4 through the OR gate OG2.

  Here, in the switching power supply of FIG. 6, the drain current detection of the power MOS transistor Q1 by the IS terminal of the control IC 1 is performed with a negative voltage with respect to the GND (ground) level of the control IC 1. That is, when the drain current of the power MOS transistor Q1 is zero, the input voltage to the IS terminal is also zero, but the input voltage to the IS terminal becomes a negative value as the drain current increases. (Absolute value increases). This signal is converted into a signal that decreases by a positive value by the voltage dividing circuit B6, and is input to the PWM comparators CP6 and CP7. In order to achieve a match with the characteristic that the current detection signal decreases as the drain current of the power MOS transistor Q1 increases, a signal input to the non-inverting input terminal of the PWM comparator CP6 in order to realize the soft start function is also used. It is assumed that 0V is lowered as an initial value.

  The present invention can also be applied to the case where the drain current detection is performed with a voltage on the plus side with respect to the GND level, but in this case, the drain current detection signal (voltage signal) decreases as the current increases. If it is. In a normal sensor, the detection signal increases as the current increases. However, the signal may be inverted by an inverting amplifier circuit using an operational amplifier.

  In the circuit of FIG. 5, PWM control is performed in a current mode in which the drain current of the power MOS transistor Q1 is detected on the minus side. Therefore, the terminal voltage of the FB terminal is 1V (minimum on pulse width of the power MOS transistor Q1) to 3V. The input range (maximum pulse width) is converted from an input range of 2 V (minimum pulse width) to 1.5 V (maximum pulse width) by the negative feedback amplifier OP2 of the amplifier circuit B4 and input to the PWM comparator CP7. The detection signal of the IS terminal is also converted from the input range of 0V to -1V, the reference voltage 4V inside the control IC 1 is converted to a positive potential by resistance voltage division by the voltage dividing circuit B6, and converted to the range of 2V to 1.5V. Are input to the PWM comparators CP6 and CP7.

  At the time of start-up and restart, since the voltage on the secondary side of the output transformer T1 of the switching power supply is still low, the terminal voltage of the FB terminal rises to 3 V or more in order to output more power. This terminal voltage is converted to 1.5 V or less by the negative feedback amplifier OP2, and the ON pulse is not stopped until the terminal voltage at the IS terminal becomes -1 V or less. At this time, if the pulse width is suddenly maximized, abnormal noise is generated or an overvoltage is generated due to an overshoot of the secondary voltage of the output transformer T1, so the pulse width is gradually increased. The function of gradually increasing the on-pulse width of the power MOS transistor Q1 at the time of starting or restarting is a soft start function.

  In addition, when starting up or restarting, a signal (terminal voltage) of the OTP terminal that gradually changes from 2 V to 1.5 V or less is input to the PWM comparator CP6 having the same function as the PWM comparator CP7, and the signal from the IS terminal When either of the two PWM comparators CP6 and CP7 outputs a signal for turning off the power MOS transistor Q1, the OUT terminal becomes L level and the power MOS transistor Q1 is turned off. Thereby, the pulse width can be gradually increased. After that, since the OTP terminal does not rise to 1.5V or higher, the PWM comparator CP6 does not operate.

  Here, the capacitor Co attached externally to the OTP terminal is discharged with a current that is the sum of the current of 70 μA of the second constant current source IS2 and the current flowing through the temperature detection element RT, so the soft start time varies depending on the temperature. Occur. That is, when the temperature detection element RT does not flow current at a low temperature, the capacitor Co is discharged at 70 μA. If the temperature is just before the stop, the current of about 140 μA is applied to the temperature detection element RT at the overheat detection threshold level of 14.286 kΩ. Are discharged at a total of 210 μA. For this reason, the variation of the soft start time of about three times (70 μA to 210 μA) at the maximum occurs, and it is assumed that this can be allowed by the specification of the switching power supply.

  Note that the variation in the soft start time can be improved by increasing the internal discharge current from the second constant current source IS2 to suppress the influence of the change in the current flowing through the temperature detection element RT. For example, if the internal discharge current is 140 μA, the maximum is about twice (140 μA to 280 μA). However, since the current increases, the time is shortened unless a large capacitor is connected as Co.

  Furthermore, if the J_OFF input is kept at the H level until the voltage at the OTP terminal shown in FIG. 3 falls below the fourth threshold level, the jitter function can be disabled while the soft start function is operating. Similarly, in the first and second embodiments, the J_OFF input is set to the H level until the voltage of the OTP terminal enters between the second threshold level and the third threshold level. Can disable the jitter function.

  Although the third embodiment has been described as having both the frequency jittering function and the soft start function, it may have only the soft start function in order to reduce the circuit scale. When only the soft start function is used, the second and third threshold levels are not necessary, and only the first, fourth and fifth threshold levels may be left. In this case, the fourth and fifth threshold levels in the third embodiment are the second and third threshold levels.

It is a figure which shows the structure of the principal part of the power supply control circuit of the 1st Embodiment of this invention. FIG. 3 is a circuit diagram illustrating a configuration example of an oscillator in the power supply control circuit according to the first embodiment. It is a figure which shows the structural example of the frequency jittering circuit in the power supply control circuit of 1st Embodiment. It is a figure which shows the structure of the principal part of the power supply control circuit of the 2nd Embodiment of this invention. It is a figure which shows the structure of the principal part of the power supply control circuit of the 3rd Embodiment of this invention. It is a figure which shows the circuit structure of a switching power supply. It is a figure which shows the characteristic of the temperature detection element with a negative temperature characteristic. It is a figure which shows the structure of the principal part of the conventional power supply control circuit.

Explanation of symbols

1 Control IC
DESCRIPTION OF SYMBOLS 11 Delay circuit 12 PWM signal generation circuit 13 Output circuit 14 1 shot circuit AP1 AC power supply Co capacitor Ct Timing capacitor CP1 1st comparator CP2 2nd comparator CP3 3rd comparator CP4, CP5, CP11, CP12 Comparator CP6, CP7 PWM Comparator D1 Diode DS1 Diode stack FF1 to FF5 Flip-flop IS1 First constant current source IS2 Second constant current source NM1, NM11 to NM13 N-channel MOS transistor OP1 Operational amplifier OP2 Negative feedback amplifier PM1 to PM3, PM11 to PM14 P Channel MOS transistor Q1 Power MOS transistor R11 Timing resistance RT Temperature sensing element T1 Output transformer

Claims (9)

In an integrated circuit power supply control circuit that controls the switching operation of a switching power supply,
A capacitor connected to a predetermined external terminal;
A first constant current source for supplying a current to the external terminal;
A second constant current source for discharging current from the external terminal;
First, second, and third comparators respectively comparing a voltage level of the external terminal with a first threshold level for detecting an abnormal state and second and third threshold levels that are sequentially higher than the first threshold level;
When the voltage level of the external terminal rises to the third threshold level due to the current supply of the first constant current source, the current discharge of the second constant current source from the current supply of the first constant current source And when the voltage level of the external terminal drops to the second threshold level due to the current emission of the second constant current source, the current emission of the second constant current source is changed to the first constant current source. Switching means for providing an oscillator function that changes a voltage between the second threshold level and the third threshold level by switching to the current supply of
On the basis of the change in the voltage level of the external terminals, the power supply control circuit according to claim Rukoto by modulating the switching frequency of the switching power supply. A comparator that compares the voltage level of the external terminal with fourth and fifth threshold levels that are higher in order than the first threshold level;
The switching means has a function of raising the voltage level of the external terminal to a fifth threshold level and then lowering the voltage level;
Based on the voltage change of the external terminal while the voltage level of the external terminal decreases from the fifth threshold level to the fourth threshold level, the ON width or ON duty of the switching power supply is gradually increased. power control circuit according to claim 1, wherein the go. 3. The power supply control circuit according to claim 2, wherein the third threshold level and the fourth threshold level are the same threshold level . When starting or restarting the switching power supply, the voltage level of the external terminal is temporarily increased to the second or third threshold level with a supply current larger than the supply current of the first constant current source. The power supply control circuit according to claim 1 . 2. The power supply control circuit according to claim 1 , wherein an on-width or an on-duty of a switching pulse is controlled when the switching power supply is started or restarted based on a change in voltage level of the external terminal . At the time of starting or restarting the switching power supply, the on-width of the switching pulse is only during the time when the voltage level of the external terminal drops from the third threshold level to the second threshold level for the first time. 2. The power supply control circuit according to claim 1, wherein the on-duty is controlled . 3. The power supply control circuit according to claim 2 , wherein when the control is performed to gradually increase the ON width or ON duty of the switching pulse, the switching frequency of the switching power supply is not modulated.
The switching frequency of the switching power supply is not modulated until the voltage level of the external terminal enters between the second threshold level and the third threshold level when the switching power supply is started. power control circuit according to claim 1 Symbol mounting to. The external terminal, an external terminal to which a starting current is supplied to the switching power supply, an unconnected external terminal that is not used next to the external terminal, an external terminal to which a power supply voltage is supplied, an external terminal for driving a switching element, and an external for grounding terminal, an external terminal a feedback signal of the switching power supply is input, and a power supply of claim 1 Symbol mounting detection signal of the current flowing through the switching element and having an external terminal of the 8-pin external terminal input Control circuit.

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