Most portable products that use white LED (W LED ) backlit displays also require auxiliary LED illumination. Two ICs are typically required: an inductive boost converter that maximizes the efficiency of the backlight LED ("80%"); a charge pump that allows independent control of each auxiliary LED. In addition, each IC requires a programmable current sink for brightness control or color mixing, which leads to a rapid increase in cost and complexity. This design tip shows how to combine a single programmable LED driver with a low cost boost converter for a flexible, efficient and easy to program solution. Figure 1 shows an implementation using a boost converter ADP1612 (see Figure 2) and a parallel LED driver ADP8860 (see Figure 3).
Figure 1. The boost converter ADP1612 and the LED driver ADP8860 enable programmable drive of the backlight and auxiliary LEDs.
In this application, the FB of the boost converter ADP1612 is connected to a current sink D2 on the LED driver ADP8860. The 5 V Zener diode protects the current sink from damage or rapid shutdown. If an open LED fails, the OVP Zener diode protects the output capacitors COUT and ADP1612.
When current sink D2 is turned off, the voltage on FB is pulled up to VIN, turning off the ADP1612. When D2 is turned on, the voltage on FB is pulled low and the boost starts switching. The ADP1612 regulates the output voltage and provides 1.2 V on FB and D2. This is enough to achieve precise current regulation. As the current through the D2 current sink changes, the ADP1612 automatically scales the output voltage to accurately deliver enough voltage to power the LEDs and current sinks. The ADP8860 is capable of controlling each absorber independently, so programming for auxiliary LEDs is equally applicable to backlight LEDs.
The operating frequency of the step-up DC-DC switching converter is 650 kHz / 1300 kHz
The boost converters ADP1612 and ADP1613 can supply more than 150 mA at up to 20 V when powered from a single 1.8 V to 5.5 V supply or a single 2.5 V to 5.5 V supply. By integrating a 1.4 A/2.0 A, 0.13 Ω power switch with a current mode pulse width modulation regulator, its output changes by less than 1% with input voltage, load current, and temperature. The operating frequency is pin selectable and optimized for high efficiency or minimum external component size: 90% efficiency at 650kHz; at 1.3 MHz, the circuit can be implemented with minimal space, making it ideal for portable devices and LCDs A space-constrained environment.
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