LED flash drive circuit design

5 Jun

On the market to increase each year, are hundreds of phone, the basic function of these cell phones are all the same, that is communication. The design is to increase mobile phone around mobile phone additional function, increase selling point and mobile phone RunDian inti the main way. The main difference between different mobile phone lives in peripheral functions, such as appearance, color screen brightness, multimedia function.

Now the market led strip lighting controller are integrated in the control circuit and boost switch tube, but inductor and used in the free-wheeling scuttle diodes or external, this increases the complexity of the circuit, cost and PCB area. In addition, because flash flooding power circuit, LED, display screen, mobile phone in the general mobile phone, and the phone of the circuit get close, so effectively prevent driving circuit the inductance of the EMI interference is also a very important question.

Charge pump adopts the capacitance for energy storage components, the charge pump don’t need external inductance, so that there is no problem of electromagnetic interference. In addition, the solution of the area of the PCB is small, but the relatively low efficiency. Because flash working time is very short, duration general for 100 ~ 300 ms, so for the battery life of the efficiency of the effect is not too big.

LED flash drive circuit design

Sioux company based on the charge pump working mode flash

Sioux flash drive series of products to the peripheral circuit is very simple, just need three capacitance two resistance, which Rsense and RSET to set the flash mode and ChangLiang mode LED current.

1. The SP6685 ChangLiang Mode:

SP6685 for constant flow type drive chip, in ChangLiang mode of 50 feet FB tube voltage Mv (typical values), this current on the LED to:

ILED = 50 mV/Rsense

Need to point out is, because FB voltage of 50 mA, so even if through the carrying 200 mA current, Rsense consumption of power for:

Rsense = 50 mV200mA = 0.01 W

This shows, with only 0603 package SMD resistor can meet the requirements. But in the constant pressure output work mode driver a circuit, the current limit resistance must select bigger encapsulation. The LED the forward voltage is commonly 3 V 4 V when the output voltage of 5 V, add in the current limit that the voltage across to 1 V 2 V, assuming that flash current 200 mA, the SFCL power consumption for:

PR = UI = (1 V ∽ 2 V) 0.2 A = 0.2 W ∽ 0.4 W

Then must use four 1206 package SMD resistor. Using a 0603 compared SP6685 resistance. The PCB area increases greatly.

With the constant pressure of the output charge pump, compared with SP6685 efficiency advantages.

The charge pump efficiency η depends on the input voltage VinLED positive voltage Vf and boost multiples K (1 X, 1.5 X, 2 X), the equations for:

?? = Vf / (Vink)

Because mobile phone batteries work range 3.6 V 4.2 V, when the output voltage of 5 V, charge pump must work in the booster mode, namely K must for or 2. And in fact, when the input voltage is higher than the forward voltage Vf certain LED amplitude, the charge pump can work at 1 X mode, then the power will be considerably more than the 1.5 X and 2 X model of efficiency.

2. SP6685 flash mode:

SP6685 flash mode voltage from RSET FB decision, computation formula is as follows:

VFB = (1.26 VRSET) 11.2 k Ω

Among them, the 1.26 V is chip internal reference voltage, current limit in the internal resistance. So, the LED on the current for:

ILED = VFB/Rsense

Because the current is not LED Tube by RSET, so almost no power consumption, can choose 0603 or 0402 chip resistors. In the flash drive circuit scheme, need only two chip resistors and three 0805 chip resistors, the PCB area is 5.43mm, with the minimum volume.

Comprehensive mentioned, the flash drive Sipex company series products have many advantages, including: the current control mode, can accurately control LED current on the and brightness; Peripheral devices at least, the minimum needed for packaging; Switch the highest frequency, can choose smaller net value filter capacitance and charge pump capacitor;

Don’t need inductance, won’t produce EMI interference problems; Can work at 1 X mode and low voltage feedback to 50 mV, so SP7685/6685/6686 of the efficiency in the use of charge pump mode for the chip supreme; SP7685 built-in Timeout function, in flash mode after 2.5 seconds automatic shutdown, thus protecting LED, flexible led strip lighting to avoid overheating; Provide the maximum driving current ($1.2 A), and 700 mA, 400 mA different levels of drive chip for choice.

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