Does the ADM1272 require a minimum output capacitor?

Dear Sir,

My customer is now testing their own ADM1272 board. Some ADM1272s are not designed with output capacitors. Testing is a bit problematic now. The test with output capacitance is normal. As shown in the first picture. The test without output capacitance is abnormal. As shown in the second picture. Does the ADM1272 require a minimum output capacitor? Thanks.

CH1, VOUT (means MOSFET Source pin)
CH2, VG (means MOSFET Gate pin)
CH3, short current
CH4, VIN (means MOSFET Drain pin)
M1, MOSFET VDS (means CH4 subtraction CH1)

CH1, VOUT (means MOSFET Source pin)
CH2, VG (means MOSFET Gate pin)
CH3, short current
CH4, PG

    •  Analog Employees 
    on Mar 11, 2020 4:13 PM 8 months ago

    Hi Andy,

        This is a good question. It looks to me like the output is actually an LC tank with resonance excited to ring by the turn off event. You can see that the ADM1272 has clamped the gate to the source of the FET, so the FET is off, but there is ringing at the output that is not driven by current through the FET itself. The frequency of the ringing indicates that it is not driven by a feedback circuit within the ADM1272, which can't move that fast. I suspect that there is substantial inductance in the cables at the output of the FET, and with very little capacitance or resistance the circuit is underdamped.

        The ADM1272 is intended for use with substantial capacitive loads, but there is no specified minimum capacitance. My advice is to make sure that the load network has a damped characteristic, or no resonances at all. You can test with less inductance at the output, or add a load resistor to damp the ringing.

    Thanks!
      Nathan