May 22, 1997

APD-bias circuit has adjustable output

Larry Suppan, Maxim Integrated Products, Sunnyvale, CA

The avalanche-photodiode detector (APD), like the PIN diode, serves as a receiver in optical communications systems. The APD is more sensitive, but you must properly bias it to produce the appropriate electron flow for a given flux of photons. An external D/A converter allows dynamic, computer-controlled adjustment of an APD-bias generator (Figure 1). To generate the bias voltage, V_OUT, a boost converter (IC₁, L₁, and Q₁) drives a diode-capacitor charge pump comprising D₃C₄, D₂C₃, and D₁C₅.

IC₁ regulates V_OUT with respect to a setpoint that the external control voltage, V_DAC, establishes. As V_DAC ranges from 2 to 0V, V_OUT varies from 28V to approximately 71V (Figure 2). At 70V with 0.5-mA output current, the maximum ripple is typically 0.5V (0.7%). The circuit produces less than 0.3% ripple at 1 mA, and its maximum output current is approximately 3 mA. The output capacitor, C₅, should be a low-ESR type. (DI #2038)

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