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, 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%). 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. sony vgp-bps8 battery sony vgp-bpl8 battery

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 sony vgp-bps9 batterypowered 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. 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. 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). sony vgp-bps12 battery

MAX1916 drain-source resistance in the temperature range for a maximum of 50O, 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 = 16.67O (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 + sony vgp-bpl9 battery, with low battery, the LED input voltage can not meet the required bias voltage. sony vgp-bps8 battery sony vgp-bps12 battery

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, MAX684 In stop mode, supply current down to 22µA, RSET = 43kO 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 24O 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. sony vgp-bps8 battery

4, inductor white LED controller

MAX1848 will boost converter with current control circuit integrated in the 6-pin SOT23 package, using the drive current sensing LED 3 groups, each including three LEDs in series connected LEDs, shown in Figure 9. Input voltage range of 2.6 ~ 5.5. MAX1848 architecture using feedback to adjust the voltage current through the diode, the small sense resistor (5O) help save energy and maintain high conversion efficiency. Analog controller is used to control the brightness all the LEDs. Typical Applications: L1 = 33µH, CCOMP = 150nF, COURT = 1.0µF, Rsense = 5O. LED current is determined by the control voltage: IOUT = VCTRL/13.33RSENSE.

LED brightness can be adjusted through the spit CTRL DAC or potentiometer divider circuit controlled voltage adjustment range is 250 mV ~ 5.5 V, the control pin allows the ground stop. When the load is 800mW power conversion efficiency of 88% of the circuit.

5 Conclusion

MAX1916 3-way adjustable structure internal configuration of the power source can control multiple LEDs, the direct use of the Li + single to drive red, green or yellow GaAsP LED, with the charge pump boost converter, MAX1916 can be used for driving white InGaN LEDs. For higher power requirements of the application, be based on inductor MAX1848 requires a minimum of external components, the output power of 800mW when the conversion efficiency of 88%.