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Low-Voltage Current Sink Controls High-Voltage LED String
Most portable products with displays that use white light emitting diode (WLED) backlights also need auxiliary LED lighting. Two ICs are generally needed: an inductive boost to obtain maximum efficiency (>80%) for the backlight LEDs; and a charge pump to allow independent control of each auxiliary LED. In addition, each IC requires a programmable current sink for brightness control or color blending, so the cost and complexity can increase quickly. This design tip shows how a single programmable LED driver can be combined with a low-cost boost converter to achieve a flexible, high-efficiency, easy-to-program solution. Figure 1 shows an implementation using the ADP1612 (see Figure 2) boost converter and the ADP8860 (see Figure 3) parallel LED driver.
1. ADP1612 boost converter and ADP8860 LED driver implement programmable
In this application, FB of the ADP1612 boost converter is connected to D2, one of the current sinks on the ADP8860 LED driver. The 5-V Zener diode protects that current sink in case of a fault or rapid shutdown. The OVP Zener diode protects the output capacitor, COUT, and the ADP1612 in case of an open-circuit fault in one of the backlight LEDs.
When current sink D2 is off, the voltage on FB is pulled to VIN, shutting down the ADP1612. When D2 turns on, the voltage on FB is pulled low, and the boost starts switching. The ADP1612 regulates the output voltage to provide 1.2 V on FB and thus on D2. This is enough to allow accurate current regulation. As the current through D2's current sink changes, the ADP1612 automatically scales the output voltage to deliver exactly enough voltage to power the LEDs and the current sink. The ADP8860 has independent control of each sink, so the same programming used for the auxiliary LEDs can also be applied to the backlight LEDs.
DC-to-DC Switching Converters Operate at 650 kHz/1300 kHz
Figure 2. ADP1612/ADP1613 block diagram.
Smart LED Driver Includes Charge Pump, I2C
Figure 3. ADP8860 functional block diagram.
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