EDN Access--05.22.97 APD-bias circuit has adjustable output

-May 22, 1997

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, VOUT, a boost converter (IC1, L1, and Q1) drives a diode-capacitor charge pump comprising D3C4, D2C3, and D1C5.

IC1 regulates VOUT with respect to a setpoint that the external control voltage, VDAC, establishes. As VDAC ranges from 2 to 0V, VOUT 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)

Figure 1


This avalanche-photodiode-detector bias generator produces a regulated, computer-controlled output of 45 to 70V.
Figure 2


The dc-output level for the circuit in Figure 1 is a linear function of the control voltage, VDAC.c

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