JP2014093842A - Led drive circuit for flash and electronic apparatus provided with the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an LED drive circuit for flash having excellent transition responsiveness from low current driving to flash current driving and having high efficiency.SOLUTION: The LED drive circuit for flash includes: a booster circuit whose input voltage is voltage of a battery; an electric double layer capacitor that is charged by the booster circuit and drives an LED element with an output current; a step-down circuit whose input power supply is the electric double layer capacitor and whose output is a cathode side of the LED element; and a power switch element for feeding a large current to the LED element.

Description

  The present invention relates to a flash LED drive circuit.

  In a portable device such as a smartphone, a booster circuit for charging an electric double layer capacitor with a battery as a power source in order to drive a flashlight LED (light emitting diode) with a large current as the performance of the camera increases. A driver circuit that drives the LED by passing a flash current from the electric double layer capacitor to the LED is used. An electric double layer capacitor is a large-capacity capacitor using a physical phenomenon called an electric double layer, and a capacitor having a capacity of about 0.3 V or more with a withstand voltage of about 5.5 V, even if it is large enough to be mounted on a portable device. is there. A battery includes a large internal resistance, and it may be difficult to pass a large current, and a large capacity capacitor is used.

  As inventions related to flash LED drive circuits, there are inventions disclosed in, for example, Patent Document 1 and Patent Document 2. In particular, in the invention disclosed in Patent Document 1, an electric double layer capacitor is charged from a battery to a sufficient voltage by a booster circuit, and then the LED is flashed by a constant current circuit.

JP 2010-4692 A JP 2005-93566 A

  Here, FIG. 5 shows a configuration of a conventional flash LED drive circuit in which the flash LED drive circuit of Patent Document 1 is slightly modified. A conventional flash LED drive circuit 110 shown in FIG. 5 includes a booster circuit 102, an electric double layer capacitor 103, a constant current circuit 105, and a flash current drive circuit 106.

  A booster circuit 102 that uses the voltage of the battery 101 as an input voltage includes a coil L101, an NMOS 1021, a PMOS 1022, and a boost control circuit 1023 that controls on / off of the NMOS 1021 and the PMOS 1022.

  One end of the electric double layer capacitor 103 is connected to the output terminal of the booster circuit 102 and also connected to the anode of the LED 104.

  A constant current circuit 105 and a flash current driving circuit 106 are connected to the cathode of the LED 104. The flash current drive circuit 106 includes a power MOS 1062 whose drain is connected to the cathode of the LED 104, whose source is connected to one end of the current limiting resistor R101, and whose gate is connected to the flash LED current control circuit 1061. The flash LED current control circuit 1061 controls on / off of the power MOS 1062.

  Next, the operation of the conventional flash LED drive circuit 110 shown in FIG. 5 when shifting from low current driving to flash current driving will be described separately when efficiency is emphasized and responsiveness is emphasized.

  First, FIG. 6 shows an example of each waveform of the voltage VCAP of the electric double layer capacitor 103, the cathode voltage VLED of the LED 104, and the LED current ILED flowing through the LED 104 when importance is placed on the efficiency.

  At the time of low current driving in which a low current is supplied to the LED 104 by the constant current circuit 105, the power MOS 1062 is turned off by the flash LED current control circuit 1061, and the VLED is set low so that a high voltage is not applied to the constant current circuit 105. The loss of the circuit 105 is reduced to improve efficiency. Thereby, VCAP which is a voltage higher than VLED by the forward voltage VF of LED 104 corresponding to a low current is also set low.

  Therefore, if it is attempted to shift from the low current drive to the flash current drive (the power MOS 1062 is turned on), the electric double layer for increasing VCAP to a high voltage corresponding to the forward voltage VF of the LED 104 through which the flash current can flow. A charging period for the capacitor 103 is required. For example, if the electric double layer capacitor 103 has a capacity of 1 F, a charging current of 0.5 A, and the required forward voltage VF of the LED 104 is different by 1 V between low current driving and flash current driving, the charging time is 2 seconds. Therefore, it takes a considerable time to shift from low current driving to flash current driving.

  On the other hand, FIG. 7 shows an example of each waveform of the voltage VCAP of the electric double layer capacitor 103, the cathode voltage VLED of the LED 104, and the LED current ILED that flows through the LED 104 when emphasizing responsiveness.

  In this case, as shown in FIG. 7, VCAP is set to a voltage high enough to allow the flash current to flow during the low current drive so that the low current drive can be immediately shifted to the flash current drive. However, since the VLED is also set to a high voltage at the time of low current driving, the loss of the constant current circuit 105 is increased and the efficiency is lowered.

  In view of the above problems, an object of the present invention is to provide a flash LED drive circuit that can improve the response of the transition from low current drive to flash current drive and achieve high efficiency.

In order to achieve the above object, the present invention provides:
A booster circuit using the battery voltage as an input voltage;
An electric double layer capacitor that is charged by the booster circuit and drives the LED element by an output current;
A step-down circuit having the electric double layer capacitor as an input power source and the cathode side of the LED element as an output;
A power switch element for flowing a large current through the LED element;
The flash LED drive circuit is provided.

  In the above configuration, the booster circuit may include a coil, a main switching element, a rectifying element, and a control circuit that controls on / off of the main switching element and the rectifying element.

  In any of the above-described configurations, the step-down circuit may include a coil, a main switching element, a rectifying element, and a control circuit that controls on / off of the main switching element and the rectifying element.

  Further, in the above configuration, a configuration may further include a changeover switch for using the coil, the main switching element, and the rectifying element of the booster circuit for the step-down circuit.

  According to another aspect of the present invention, there is provided an electronic apparatus including the flash LED driving circuit having any one of the above configurations and an LED element driven by the flash LED driving circuit.

  According to the present invention, the response from the low current driving to the flash current driving can be improved and the efficiency can be improved.

1 is a configuration diagram of a flash LED drive circuit according to a first embodiment of the present invention. FIG. It is a figure which shows an example of the voltage and current waveform which concern on 1st Embodiment of this invention. It is a block diagram of the LED drive circuit for flash | flash which concerns on 2nd Embodiment of this invention. It is a figure which shows an example of the voltage and current waveform which concern on 2nd Embodiment of this invention. It is a block diagram of the LED drive circuit for flash | flash which concerns on a prior art example. It is a figure which shows an example of the voltage and current waveform concerning a prior art example when importance is attached to efficiency. It is a figure which shows an example of the voltage and current waveform concerning a prior art example when emphasizing responsiveness.

(First embodiment)
An embodiment of the present invention will be described below with reference to the drawings. FIG. 1 shows the configuration of the flash LED drive circuit according to the first embodiment of the present invention. The flash LED drive circuit 121 shown in FIG. 1 includes a booster circuit 2, an electric double layer capacitor 3, a step-down circuit 4, and a flash current drive circuit 6. The difference between the configuration of the flash LED drive circuit 121 and the conventional configuration shown in FIG. 5 described above is that the constant current circuit 105 is not provided, the electric double layer capacitor 3 is used as an input power source, and the cathode side of the LED 5 (LED element). Is provided.

  The battery 1, the flash LED drive circuit 121, and the LED 5 illustrated in FIG. 1 are provided in an electronic device such as a smartphone.

  The step-up circuit 2 has a function of charging the electric double layer capacitor 3 with a constant current using the voltage of the battery 1 composed of a lithium ion battery or the like as an input voltage.

  The NMOS 21 (main switching element) and the PMOS 22 (rectifier element) in the booster circuit 2 are on / off controlled by the booster control circuit 23, and operate so that one of them is on and the other is off during a necessary period of the switching operation. It operates so that both are off during periods other than. The PMOS 22 may be replaced with a diode or a Schottky diode.

  The step-up control circuit 23 has a circuit for detecting the current of the coil L1 or the PMOS 22 or the NMOS 21 in order to control the on / off period of the NMOS 21 and the PMOS 22, and restricts the current IL1 of the coil L1, thereby supplying the electric double layer capacitor 3 Control charging current.

  The boost control circuit 23 controls VCAP using a detection circuit that detects the output voltage VCAP of the boost circuit 2. For example, when VCAP becomes a certain set voltage or less, a constant current is supplied to the electric double layer capacitor 3 by the boosting operation to start charging, and when VCAP becomes a certain set voltage or more, the current output is stopped. May be. Further, feedback control for always controlling VCAP to a certain target voltage value may be employed.

  The step-down circuit 4 uses the electric double layer capacitor 3 as an input power source when the LED 5 is driven at a low current, and sets the cathode voltage VLED of the LED 5 so as to secure a forward voltage VF necessary to flow a certain target current to the LED 5. It has a function to control.

  The step-down circuit 4 includes a PMOS 41 and an NMOS 42 connected in series between a connection point between one end of the electric double layer capacitor 3 and the anode of the LED 5 and the ground potential, a step-down control circuit 43, and a connection point between the PMOS 41 and the NMOS 42. One end is connected to the connection point between the coil L2 having one end connected, the capacitor C1 having one end connected to the other end of the coil L2, and the connection point between the coil L2 and the capacitor C1, and the connection point between the cathode of the LED 5 and the drain of the power MOS 62. And a switch SW1 to which the other end is connected.

  The PMOS 41 (main switching element) and the NMOS 42 (rectifier element) in the step-down circuit 4 are on / off controlled by the step-down control circuit 43, and operate so that one is on when the low current drive is on and the other is off. Both work to turn off. The NMOS 42 may be replaced with a diode or a Schottky diode.

  The switch SW1 is turned on when the step-down circuit 4 is operated, and is turned off otherwise.

  The step-down control circuit 43 has a circuit for detecting the current of the coil L2 or PMOS 41 or NMOS 42 in order to control the on / off period of the PMOS 41 and NMOS 42, and determines the current of the LED 5 by controlling the current IL2 flowing through the coil L2. Can do. For example, the control method may be current control that feeds back the current IL2, and the current of the LED 5 can be controlled by operating as a current source without performing voltage feedback.

  The flash current drive circuit 6 has a power MOS 62 (power switch element), a current limiting resistor R1, and a flash LED current control circuit 61. By turning on the power MOS 62 during the flash current drive period, a flash current ( Large current). When adjusting the flash current, the duty ratio may be controlled by adjusting the ON period, and PWM (pulse width modulation) may be used. Note that the switch SW1 is turned off during the flash current driving period.

  FIG. 2 shows an example of voltage and current waveforms when switching from low current driving to flash current driving in the configuration according to the present embodiment. In FIG. 2, the LED current includes ripples in the low current driving period. This causes the step-down circuit 4 to operate in the low current driving period, and the current flowing in the coil L2 becomes a triangular wave, and the capacitor C1 has a large capacity. This is because the triangular wave affects the LED current to some extent because it is not.

  Since the voltage VCAP of the electric double layer capacitor 3 holds the voltage necessary for driving the flash current (the voltage corresponding to the forward voltage VF of the LED 5 through which the flash current can flow), switching from the low current to the flash current is immediate. Can be done. Further, compared to the conventional configuration shown in FIG. 5 in which the voltage required for the flash current drive is held in VCAP, the power consumed by the constant current circuit 105 during low current drive is more efficient. In other words, the efficiency is replaced with 80% or more), so that the low current driving can be performed with the high efficiency.

(Second Embodiment)
Next, FIG. 3 shows the configuration of the flash LED drive circuit according to the second embodiment of the present invention. 3 includes a switching circuit 8, an electric double layer capacitor 9, a flash current driving circuit 11, a constant current circuit 12, a switching control circuit 13, switches SW2 and SW3 (switching switches). ) And a capacitor C2.

  Note that the battery 7, the flash LED drive circuit 122, and the LED 10 illustrated in FIG. 3 are provided in an electronic device such as a smartphone.

  The switching circuit 8 includes a coil L3, an NMOS 81, a PMOS 82, and a switching control circuit 83, and can perform both step-up and step-down operations.

  The NMOS 81 and the PMOS 82 in the switching circuit 8 are ON / OFF controlled by the switching control circuit 83, and operate so that one of them is ON when the switching operation is necessary, and the other is OFF during the other periods. To work.

  The flash current drive circuit 11 includes a flash LED current control circuit 111, a power MOS 112, and a current limiting resistor R2. The power MOS 112 is turned off during low current driving and turned on during flash current driving.

  FIG. 4 shows an example of voltage and current waveforms when the flash LED drive circuit 122 according to the present embodiment is driven at a low current, and the operation during the low current drive will be described below.

  When the switching control circuit 13 detects that the voltage VCAP of the electric double layer capacitor 9 has become lower than a certain set voltage Vth1, the switch SW2 is turned on, the switch SW3 is turned off, and the switching control circuit 83 starts the boosting operation. Command. As a result, the switching circuit 8 starts a boosting operation using the voltage of the battery 7 as an input voltage (in this case, the NMOS 81 functions as a main switching element and the PMOS 82 functions as a rectifying element), and charging of the electric double layer capacitor 9 starts. VCAP rises. At this time, the switching control circuit 13 turns on the constant current circuit 12 and uses the constant current circuit 12 as an auxiliary to drive the LED 10 at a low current.

  Further, when the switching control circuit 13 detects that VCAP has become higher than a certain set voltage Vth2, the switch SW2 is turned off and the switch SW3 is turned on, and the switching control circuit 83 is instructed to start a step-down operation. At this time, the switching control circuit 13 turns off the constant current circuit 12.

  As a result, the switching circuit 8 and the capacitor C2 operate as a step-down circuit that uses the electric double layer capacitor 9 as an input power supply and outputs the cathode side of the LED 10 (in this case, the PMOS 82 functions as a main switching element and the NMOS 81 functions as a rectifying element). The current ILED flowing through the LED 10 is controlled to a certain set current, and the LED 10 is driven at a low current. At this time, VCAP falls. In FIG. 4, ILED includes ripples because the current IL3 flowing through the coil L3 during the step-down operation is a triangular wave, and the capacitor C2 is not large in capacity, so that the ILED is affected to some extent by the triangular wave.

  In the low current driving period by the control as described above, the VCAP is controlled so as to hold the voltage necessary for the flash current driving. Therefore, when the power MOS 112 is turned on, a flash current can be immediately supplied to the LED 10. In the present embodiment, there is a period during which the switching circuit 8 is stepped down without using the constant current circuit 12 during low current driving, so that power loss can be reduced and high efficiency can be achieved.

  Furthermore, according to the present embodiment, the coils can be combined into one as compared with the configuration of FIG. 1, and the cost can be reduced.

  The embodiment of the present invention has been described above, but the embodiment can be variously modified within the scope of the gist of the present invention. In addition, the flash LED drive circuit according to the embodiment of the present invention is particularly suitable for use as an electronic device such as a mobile phone, a smartphone, or a digital camera as a small and highly efficient circuit.

1 Battery 2 Booster Circuit 21 NMOS
22 PMOS
23 Step-up control circuit L1 Coil 3 Electric double layer capacitor 4 Step-down circuit 41 PMOS
42 NMOS
43 Step-down control circuit L2 Coil C1 Capacitor SW1 Switch 5 LED
6 Flash current drive circuit 61 Flash LED current control circuit 62 Power MOS
R1 Current limiting resistor 7 Battery 8 Switching circuit 81 NMOS
82 PMOS
83 Switching control circuit L3 Coil 9 Electric double layer capacitor 10 LED
11 Flash Current Drive Circuit 111 Flash LED Current Control Circuit 112 Power MOS
R2 current limiting resistor 12 constant current circuit 13 switching control circuit SW2, SW3 switch C2 capacitor 121, 122 LED drive circuit for flash

Claims (5)

A booster circuit using the battery voltage as an input voltage;
An electric double layer capacitor that is charged by the booster circuit and drives the LED element by an output current;
A step-down circuit having the electric double layer capacitor as an input power source and the cathode side of the LED element as an output;
A power switch element for flowing a large current through the LED element;
A flash LED driving circuit comprising:   2. The flash LED drive according to claim 1, wherein the booster circuit includes a coil, a main switching element, a rectifying element, and a control circuit that controls on / off of the main switching element and the rectifying element. circuit.   3. The step-down circuit includes a coil, a main switching element, a rectifying element, and a control circuit that controls on / off of the main switching element and the rectifying element. LED drive circuit for flash.   4. The flash LED drive circuit according to claim 3, further comprising a changeover switch for using the coil, the main switching element, and the rectifying element of the step-up circuit for the step-down circuit.   5. An electronic device comprising: the flash LED drive circuit according to claim 1; and an LED element driven by the flash LED drive circuit.

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