JP4848689B2 - Semiconductor integrated circuit - Google Patents

Description

  The present invention relates to a semiconductor integrated circuit having a function of directly driving an external light emitting diode.

  In electronic devices such as cellular phones, light emitting diodes (LEDs) are widely used as illumination for displaying operation buttons and light sources for backlights. In particular, since a mobile phone uses a battery as a power source, it is necessary to drive the LED at a constant current so that the brightness of the LED does not change due to a change in the voltage of the battery and to cope with a low power supply voltage.

  As a related technique, the following Patent Document 1 discloses a light-emitting diode driving circuit that achieves low voltage operation and low power consumption. In this light emitting diode drive circuit, the constant current circuit for generating the drive current of the light emitting diode and the current switch circuit for switching the drive current are constituted by MOS transistors, thereby making it necessary to connect the collector and the emitter required for the bipolar transistor. The power supply voltage is reduced by eliminating the voltage margin. However, the output stage of the light emitting diode driving circuit is configured such that the MOS transistor of the constant current circuit and the MOS transistor of the current switch circuit are connected in series, and the voltage applied to the light emitting diode is from the power supply voltage. Since the voltage between the drain and source of the two transistors is subtracted, the voltage applied to the light emitting diode may be insufficient in the case of a low power supply voltage.

Patent Document 2 below discloses a backlight light emitting diode drive circuit provided with a reference voltage control circuit that changes a reference voltage for generating a constant current in accordance with a change in battery voltage. Thereby, when the battery voltage is high, the light emitting diode emits light brightly, and when the battery voltage becomes low, the light emitting diode emits light darkly to prevent the battery from being consumed. The output stage of this light emitting diode driving circuit is composed of one MOS transistor, but a resistor is connected in series with the MOS transistor, and a voltage drop due to the resistance occurs. The applied voltage cannot be taken sufficiently.
Japanese Patent Laid-Open No. 2000-4202 (first page, FIG. 1) JP 2001-325824 A (first page, FIG. 1)

  In view of the above, an object of the present invention is to provide a semiconductor integrated circuit that has a function of driving an external light emitting diode at a constant current and can easily cope with a low power supply voltage.

  In order to solve the above problems, a semiconductor integrated circuit according to the present invention includes a constant voltage generation circuit that generates a predetermined voltage when a control signal is activated, and a predetermined voltage generated by the constant voltage generation circuit. A differential amplifier circuit that generates an output potential based on the difference between the feedback voltage and a first P channel that generates a first current between the source and drain when the output potential of the differential amplifier circuit is applied to the gate To generate a MOS transistor, at least one resistor for generating a feedback voltage by supplying a first current from the drain of the first P-channel MOS transistor, and a second current proportional to the first current A second P-channel MOS transistor and a third P-channel MOS transistor constituting the first current mirror circuit, and a light emitting diode having an anode connected to the power supply potential A first N channel MOS transistor and a second N channel MOS transistor constituting a second current mirror circuit for generating a drive current proportional to the second current in order to drive the cathode of the anode; A fourth P-channel MOS transistor having a drain connected to the drains of the two N-channel MOS transistors and cutting off the light-emitting diode by raising the cathode potential of the light-emitting diode when the control signal is deactivated; It comprises.

  Here, the at least one resistor may include a plurality of resistors whose resistance values are variable by changing the connection state of the wiring in the wiring layer so that the drive current of the light emitting diode can be adjusted. Further, the second and third P-channel MOS transistors constituting the first current mirror circuit may have an absolute value of the threshold voltage between the gate and the source that is smaller than other P-channel MOS transistors. Further, the second current mirror circuit may generate a driving current N times the second current (N> 1).

  According to the semiconductor integrated circuit of the present invention, the first P-channel MOS transistor connected to the output terminal of the differential amplifier circuit generates a constant current, and the first current mirror circuit including the P-channel MOS transistor, N Since the LED is driven via the second current mirror circuit by the channel MOS transistor, it becomes easy to cope with the low power supply voltage.

Hereinafter, the best mode for carrying out the present invention will be described in detail with reference to the drawings. The same constituent elements are denoted by the same reference numerals, and the description thereof is omitted.
FIG. 1 is a circuit diagram showing a configuration of an LED drive circuit built in a semiconductor integrated circuit according to an embodiment of the present invention.

As shown in FIG. 1, the LED drive circuit 100 has two inverters 11 and 12 connected in series that invert and output an input control signal, and a predetermined signal when the control signal is activated to a high level. a constant-voltage generating circuit 20 for generating a voltage (constant voltage), and N-channel MOS transistors QN10 to fix the output terminal of the constant voltage generating circuit 20 to the power supply potential V _SS when the control signal is deactivated, the differential P-channel MOS transistors QP21 to QP22 and N-channel MOS transistors QN21 to QN23 constituting the amplifier circuit, and a P-channel MOS for fixing the output terminal of the differential amplifier circuit to the power supply potential V _DD when the control signal is deactivated a transistor QP23, P-channel MOS output potential of the differential amplifier circuit generates the drain current I ₁ is applied to the gate A transistor QP30, (in FIG. 1, shows a plurality of resistors R11~R13 connected in series) at least one resistance drain current I ₁ of transistor QP30 generates a feedback voltage is supplied and has a ing.

Furthermore, LED drive circuit 100 includes a P-channel MOS transistors QP41 and QP42 that constitute the first current mirror circuit which operates based on the drain current _{I 1} of transistor QP30, the transistor when the control signal is deactivated QP41 and a P-channel MOS transistor QP43 to cut off QP42, N-channel MOS transistors QN51 and QN52 constituting the second current mirror circuit which operates based on the drain current _{I 2} of the transistor QP42, the control signal is deactivated and N-channel MOS transistors QN53 to cut off the transistor QN51 and QN52 when the, P channel MOS transistor the control signal to fix the output terminal of the LED driving circuit 100 when it is deactivated in the power supply potential _{V DD} And a QP51.

A light emitting diode (LED) 200 is connected between the power supply potential V _DD and the output terminal of the LED drive circuit 100. The transistor QN52 allows a drive current to flow through the LED 200 from the anode connected to the power supply potential V _DD toward the cathode connected to the output terminal of the LED drive circuit 100.

The differential amplifier circuit composed of the transistors QP21 to QP22 and QN21 to QN23 is based on the difference between the constant voltage generated by the constant voltage generation circuit 20 and output to the node A and the feedback voltage applied to the node B. To generate an output potential. The output potential of the differential amplifier circuit is applied to the gate of the transistor QP30, the transistor QP30 generates the drain current I ₁ between the source and drain.

A plurality of resistors R11 to R13 are connected to the drain of the transistor QP30. Since the resistance values of these resistors R11 to R13 are variable by changing the connection state of the wirings in the wiring layer of the semiconductor integrated circuit, the resistors thus connected are referred to as “resistance circuits”. . By drain current I ₁ of transistor QP30 flows to the resistor circuit, the feedback voltage is generated.

If the open-loop gain of the differential amplifier circuit is sufficiently large, the voltage applied to the node A and the voltage applied to the node B are substantially equal to each other according to the principle of negative feedback. Accordingly, the drain current I ₁ of transistor QP30 has a value determined by dividing the value of the constant voltage generated by the constant voltage generation circuit 20 by the resistance value of the resistor circuit, a constant current. By changing the resistance value of the resistor, by changing the value of the drain current I ₁ of transistor QP30, it is possible to adjust the driving current of the LED 200.

Drain current _{I 1} of transistor QP30 is also transmitted to the transistors QP41 constituting the first current mirror circuit. First current mirror circuit generates a drain current _{I 2} of the transistor QP42, which is proportional to the drain current _{I 1} of transistor QP41. Here, it is assumed that the drain current I ₂ is substantially equal to the drain current I ₁ .

Furthermore, the drain current _{I 2} of the transistor QP42 is also transmitted to the transistors QN51 constituting the second current mirror circuit. Second current mirror circuit generates a drain current _{I 3} of the transistor QN52, which is proportional to the drain current _{I 2} of transistor QN51. Here, the drain current _{I 3,} and is N times the drain current _{I 2} (N> 1). This can be realized by setting the size of the transistor QN52 to N times the size of the transistor QN51. Drain current _{I 3} of the transistor QN52 is, since the drive current of the LED 200, for example, by setting N to a range of 10 to 100, can generate a large driving current by a small internal current.

  Thus, in the present embodiment, the P-channel MOS transistor QP30 connected to the output terminal of the differential amplifier circuit generates a constant current, and the P-channel of the first current mirror circuit is generated based on the generated constant current. Since the MOS transistors QP41 and QP42 operate and the N-channel MOS transistors QN51 and QN52 of the second current mirror circuit operate based on the drain current of the transistor QP42 to drive the LED 200, it becomes easy to operate even at a low power supply voltage. ing.

  In addition, by lowering the absolute value of the threshold voltage between the gate and source of the P-channel MOS transistors QP41 and QP42 constituting the first current mirror circuit, it becomes easier to cope with a lower power supply voltage. . This can be realized, for example, by raising the impurity concentration of the N well in the P channel MOS transistor higher than usual.

  Further, since the P-channel MOS transistor QP51 and the N-channel MOS transistor QN52 in the output stage have the same configuration as a general output driver used in the input / output cell, it is possible to use a general output driver layout as it is. In addition, there is no problem with electrostatic breakdown. Note that the transistor QP51 is cut off when the control signal is inactivated, and therefore does not affect the operation of driving the LED. In general, an N channel MOS transistor has a higher driving capability than a P channel MOS transistor of the same size, so that the transistor QN52 can strongly drive an LED with the same area. As a result, the layout is compact.

On the other hand, when the control signal is deactivated to a low level, the constant voltage generation circuit 20 stops operating, the transistor QN10 is turned on, and the output terminal (node A) of the constant voltage generation circuit 20 is connected to the power supply potential. fixed to V _SS, the transistor QP23 is turned on to fix the output terminal of the differential amplifier circuit to the power supply potential _{V DD.} Further, the transistor QP43 is turned on to cut off the transistors QP41 and QP42 constituting the first current mirror circuit, and the transistor QN53 is turned on to turn on the transistors QN51 and QN51 constituting the second current mirror circuit. Cut off QN52. Further, the transistor QP51 fixes the output terminal of the LED drive circuit 100 to the power supply potential V _DD , thereby raising the cathode potential of the LED 200 and cutting off the LED 200.

The circuit diagram which shows the structure of the LED drive circuit in one Embodiment of this invention.

Explanation of symbols

  11, 12 Inverter, 20 Constant voltage generation circuit, 100 LED drive circuit, 200 Light emitting diode, QP21 to QP51 P channel MOS transistor, QN10 to QN53 N channel MOS transistor, R11, R12, ... Resistance

Claims (4)

A constant voltage generation circuit for generating a predetermined voltage when the control signal is activated;
A differential amplifier circuit that generates an output potential based on a difference between a predetermined voltage generated by the constant voltage generation circuit and a feedback voltage;
A first P-channel MOS transistor for generating a first current between a source and a drain by applying an output potential of the differential amplifier circuit to a gate;
At least one resistor for generating the feedback voltage by supplying the first current from the drain of the first P-channel MOS transistor;
A second P-channel MOS transistor and a third P-channel MOS transistor constituting a first current mirror circuit for generating a second current proportional to the first current;
A first N-channel MOS transistor constituting a second current mirror circuit for generating a drive current proportional to the second current in order to drive a cathode of a light emitting diode having an anode connected to a power supply potential And a second N-channel MOS transistor;
A fourth P having a drain connected to the drain of the second N-channel MOS transistor, wherein when the control signal is deactivated, the cathode potential of the light emitting diode is raised to cut off the light emitting diode; A channel MOS transistor;
A semiconductor integrated circuit comprising:   The at least one resistor includes a plurality of resistors whose resistance values are variable by changing a connection state of the wiring in the wiring layer so that the drive current of the light emitting diode can be adjusted. Semiconductor integrated circuit.   3. The second and third P-channel MOS transistors constituting the first current mirror circuit have an absolute value of a gate-source threshold voltage smaller than other P-channel MOS transistors. Semiconductor integrated circuit.   4. The semiconductor integrated circuit according to claim 1, wherein the second current mirror circuit generates a drive current N times as large as the second current (N> 1). 5.

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