High-Voltage LED Driver Provides Software-Controlled Intensity
Figure 1. This APD driver (U1) has been modified to provide high-voltage LED modules with software-adjustable intensity control.
The APD-bias IC (U1) has been reconfigured to allow its low-voltage DAC output to modulate the high-voltage current-sense feedback through a high-voltage-output transconductance stage consisting of Q2 and Q3. These two complementary transistors provide first-order temperature compensation sufficient for the application.
Equations from the MAX1932 data sheet are used to select components for the step-up DC-DC converter. The current-adjust transfer function is given by
where VCL is the current-limit threshold (2V), CODE is the digital code to the DAC in decimal format, and IOUT is the desired output current. For the circuit shown, these conditions correspond to a full scale of 39mA and a resolution of 150µA. The 3-wire serial interface that controls U1 allows the user to shut U1 down by writing code 00hex to the DAC. The circuit also provides an output-voltage limit. If an LED fails open, the R5/R7 divider limits the output voltage to 50V (in this case). Simultaneously, the CL pin goes high to indicate the open-fault condition.
A similar version of this article appeared in the September 30, 2004 issue of EDN magazine.