Issues with the OPA548 current limiting

Posted on Feb 7, 2014

In order to eliminate noise, I`ve used a 100uF capacitor and a 100nF bypass capacitor across the supply and two 10nF capacitors in parallel with the resistors Rda and Rdb (those are 10K each), As for the current limiting, I`ve used a 22K resistor paralleled with a 10nF capacitor, from pin 3 (Ilim) to V-. I`ve tied pin 7 (E/S) to V+. I`ve used two

Issues with the OPA548 current limiting
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free-wheeling diodes across the output and, of course, a zobel consisting of one 4, 7R resistor and a 100nF capacitor. The problem is that the OPA548 limits the current to around 700-800mA when sinking (output shorted to V+), and that current pretty much depends on the supply voltage. However, when sourcing (output shorted to V-), it behaves as expected, since the current is limited to 2A (Rlim = 22k). Why does the current limit doesn`t work properly when sinking Is it a defective design A damaged chip Or is it that the current limiting is only applied when the OPA548 is sourcing I`ve tried everything! I`ve changed the resistor values, taken out the zobel and the diodes, untied the E/S pin from V+, and the behaviour is still the same! What is wrong Have you looked at the "GND" output with a scope to see if there is a high frequency (>100KHz) oscillation This could be a small 100MHz squeak of a few hundred mV, or a several Vpp oscillation. Note that "floating" your scope by breaking the ground pin is cheating. You are still adding extra capacitance to the "GND" point through the AC line and may actually "snub" the oscillation. Ideally the scope should be "grounded" to one of the rails and the GND node probed. Use a battery powered, truly "floating" scope if possible. You can also use a AC voltmeter between one of the rails and "Ground" to check for any AC voltages. Any AC voltage more than a few mV across the rails may mean...

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