Recent advances in operating efficiency have expanded the use of LEDs from one of mere indicators to becoming driving forces in electronic lighting. Increased reliability and ruggedness (versus other lighting technologies) gives the LED a bright future indeed. Vendors in recent years have introduced many ICs for driving LEDs, but the problem of driving serial chains of LEDs has received less attention. One approach to that problem adapts a bias-supply IC for APDs (avalanche photodiodes) to provide adjustable-current, software shutdown, and logic indication of open-circuit faults (Figure 1). This design reconfigures the APD-bias IC, IC₁, to allow its low-voltage DAC output to modulate the high-voltage, current-sense feedback via a high-voltage-output transconductance stage comprising Q₂ and Q₃. These two complementary transistors provide first-order temperature-compensation sufficient for the application.

Equations from the MAX1932 data sheet help you select components for the step-up dc/dc converter. The current-adjustment transfer function is:

where V_CL is the current-limit threshold (2V), CODE is the digital code to the DAC in decimal format, and I_OUT is the desired output current. For this circuit, these conditions correspond to a full-scale output of 39 mA and a resolution of 150 µA. The three-wire serial interface that controls IC₁ allows you to shut down IC₁ 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, in this case, to 50V. Simultaneously, the CL pin goes high to indicate the open-fault condition.

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