# Avalanche photodiode current limiting/optimum APD bias point indication

I am trying to design an avalanche photodiode driver using the MAX1932 APD bias supply IC. Everything looks pretty understandable except selecting the APD current limit.

I looked through all APD datasheets and I cannot find any information that would help determine the current limit. For example:

MTAPD-07-011

All datasheets have dark current and forward current mentioned. Please correct me, but as I understand forward current is proportional to the incoming light intensity and doesn't have anything with bias current limiting.

Also, it is mentioned that it is possible to find the optimum APD bias point by measuring the current, but I cannot find any information on how it is related. Is there some fixed current value that can help correct bias voltage on temperature changes?

I have tried to read various semiconductor physics papers but without luck.

Where I could find more information about APD current limiting and how it is related to the optimum bias point?

Everything looks pretty understandable except selecting the APD current limit

The avalanche photo-diode current-limit is a protection feature. It is unrelated to the photo-current that flows (cathode to anode) when the APD is illuminated. The first linked device has a maximum current rating of 0.25 mA so, the MAX1932 should have an avalanche current limit set at no more than 0.2 mA (in order to protect the APD should it "avalanche" on over-voltage).

This won't normally happen because the reverse bias voltage shouldn't be set too close to the avalanche point. That point will be about 180 volts as per this graph in the first APD's data sheet: -

I've drawn a purple line at about 180 volts; it's my estimation of where the maximum reverse bias voltage is and, if a particular APD happens to start avalanching at a lower voltage, the current limit of less than 0.2 mA will protect it as per this picture from the first APD: -

Please correct me, but as I understand forward current is proportional to the incoming light intensity and doesn't have anything with bias current limiting.

No, photocurrent flows from cathode to anode as does the dark-current avalanche current should the device start to avalanche.

For the avalanche current limit, you start from the MAX1932 datasheet:

The current set resistor is the one connected to CS+ and CS-. Current to the photodiode has to go through that resistor.

You calculate the resistor value from the current limit you want to set, and the 2V drop mentioned in the current limit description:

Your AD1500 has an absolute maximum current rating of 0.25 mA. You'll want to stay well below that.

As a (probably not very good) example, you could choose to limit the current to 0.1 mA. A voltage drop of 2V at 0.1mA means a series resistance of 20k ohm. When the current exceeds 0.1mA, the voltage drop will be greater than 2V, and the MAX1932 will shut off the current to the photodiode.

The current limit is a safety circuit to prevent damage to the photodiode.

The optimum bias voltage will probably depend on how much sensitivity and speed your application requires.

Since the speed will be related to the device capacitance, and the capacitance varies with the bias voltage, it looks to me like you want to use a bias voltage above 45V for the AD1500:

Lower capacitance should translate to faster speed.