Led Circuit

Led Circuit

Battery-powered products LED control problem

1 Overview

LED devices during training, the intensity of light emitted by the current flowing in the decision of diodes. Figure 1 is a voltage source circuit and current limiting bias resistor consists of a simple, flowing through the LED current by the following formula: = IDIODE (VCC-VF) / [+ RDS rlimit (ON)] in this way are more expensive than low,Hp laptop battery but the different diode VF (forward voltage) parameters of the same thing. Figure 2, Figure 3-25 ℃ LED forward voltage (typical value) and the current curve. Can be seen from the current indicators: For the GaAsP diode, VF can be up to 2.7 V (+40%) for InGaN diodes, VF can be on the program to 4.2 V (+20%).HP Pavilion dv2000 Battery(hp dv2000 battery), HP HSTNN-LB31 Battery, HP EV088AA Battery, If the system needs more LED, like cell phones back display 8 LEDs, the design of Figure 1 will require more than the current limit resistance, occupy a large circuit board area. toshiba PA3191U-3BRS toshiba PA3191U-4BRS

If Vcc is increased to more than 10 times VF VF could reduce the impact of changes, but consumes more, did not meet the demand for battery-powered products. Using a single system Li + + Li voltage range of the battery is 4.2 ~ 3V. If bias circuit LED simply by Li + and provide the current limiting resistor, the output will be significant changes in brightness. Reasonable alternatives should be based on the circuit bias current.

2 circuit bias current

Circuit bias current is actually a power source to supply through the LED. If the power source has sufficient dynamic range, this method will not offset the impact of changes in VF. Figure 4 shows the layout of the current program offset. The circuit of Figure 1, the current limiting resistor with an alternative power source. The intensity of light output and forward voltage independent, as long as there is sufficient voltage to provide a source of bias current and the LED can be. HP EV088AA Battery, Figure 4 in order to be able to control switching Q1.

MAX1916 LED driver chips, and other special offers, an advanced LED circuit bias current. In the small SOT23 package includes three groups MAX1916 current source, current flows through RSET mirror for 3 outputs, in Figure 5.HP Pavilion dv2000 Battery(hp dv2000 battery), HP HSTNN-LB31 Battery, HP EV088AA Battery, Several circuits have the same phase gate MOSFET – the source of electricity, so they channel the same current, the current size of the decision taken by the ISET current mirror. MAX1916 offset current maximum of ± 5%, the factor "image" that the 200A / A. In other words, when the ISET of 50μA, each output current 10 (1 ± 0.05) mA (max). toshiba PA3284U-1BRS toshiba PA3285U-1BAS

MAX1916 drain-source resistance in the temperature range for a maximum of 50Ω, a current of 2mA work GaAsP diode to ensure the minimum required for the normal operating voltage: VF 100 mV to 2.71V voltage entry may be the GaAsP LED voltage protection to 2.7V. To obtain a pressure drop and increased current output, the MAX1916 can output in parallel form factors of three "mirror" of the source current 600, shown in Figure 6, parallel drain-source resistance of 50 / 3 = 16.67Ω (max). This connection allows a single white LED at 20mA 3V supply when the current reaches more than respond to current products such as portable cellular requirements backlight of the phone. Used to set the end of the voltage source can be strong with a load capacity of the main power separately, for example, cell phones, VSET may be radio frequency (RF) circuits, low noise 2.8 V supply. If you directly from a single Li + battery, MAX1916 ideal for driving low forward voltage LED GaAsP, and increased tension in the future InGaN white LED driver is needed for other programs. Because when Li + batteries, with low battery, the LED input voltage can not meet the required bias voltage. toshiba PA3285U-1BRS toshiba PA3285U-2BAS

3, charge pump boost converter to drive white LEDs

The voltage from 3.5 to 4.2 V before (at 20 mA) conditions usually white LED boost converter can charge pump (like MAX682 MAX684 ~) and that together constitute the drive circuit MAX1916 LED Figure 7 said. MAX682 MAX684 ~ 2.7V input voltage to convert to 5.05V output, the output current, respectively, 250mA, 100mA, 50mA. Control closure, with MAX684 or MAX1916 pins allow control pin can turn off the LEDs. Figure 7 circuit, HP HSTNN-LB31 Battery, MAX684 In stop mode, supply current down to 22μA, RSET = 43kΩ time LED current is 22mA;

Figure 8 is using a charge pump LED current control circuit is a voltage regulation score is typically 1.235V, IPK = 1.235 / R SENSE, check the resistance of 24Ω can provide 50mA LED current. Charge pump, the output voltage is increased switching voltage LED, the LED started to turn. Typical forward voltage white LED is 3.5 (1 ± 0.1) V, coupled with feedback control voltage, MAX1759 provides a bias voltage of 4.735V output. The voltage ripple in the output circuit 40mV not lead to meaningful change in the production of LED products are generally aware of the human eye can not. In addition, the circuit of Figure 8 in a shutdown state, no DC between input channels and output. toshiba satellite a105 battery toshiba satellite a80 battery

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