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AMC1200-Q1: AMC1200

robin gangopadhya18
Mastermind 6326 points
Part Number: AMC1200-Q1
Other Parts Discussed in Thread: AMC1200, UCC28711

Tool/software:

Hello:

Pl take a look at the attached sch : it has  AMC1200 amp in a very std way of isolated current sensing.

In the application, voltage across the current sensing resistor R2 is not allowed to exceed 300mv differential  and inrush at turn-on voltage across R2 is clamped to safe levels for AM1200.

The source of the current through R2 is a DC-DC converter using UCC28711- but this is not relevant to the issue at hand.

Here is the issue we find .

With converter return current causing R2 voltage to be 140mV

Yet at pin 2, we notice only 90mV. Leakage &  input offset etc do not seem to be causing this huge drop.

Second issue is that differential output of AMC1200 is expected to be 8* 90 mV

But it is not. In fact  output might be oscillating.

We have not yet taken clean waveforms. 

While that effort is ongoing, we need to address this discrepancy.

-Why 90mV at pin 2?

-Why there is no gain at the outputs?

What needs to change for these numbers to be consisAMC1200_SCH_TI.pdftent with the Spec sheet expectations.

Any help will be highly appreciated

-r

  • 0
    TI__Guru 56532 points

    Hi, 

    Maybe try removing the diodes on the inputs, perhaps they are causing leakage. 

    How is the VDD1 supply being generated for the device? 

  • 0 in reply to Alexander Smith
    Mastermind 6326 points

    Hello Alexander:

    Thnx for the review.

    Indeed, I have removed all shunting devices from pin3 excepting 1 resistor..

    It turns out that we cannot put more than a few tens of Ohms  from pins 2 & 3.( as in the eva kit sch)

    I had 49.9 Ohms of size. Replacing the 6.96k fixed the input levels for sure..

    With 49.9, we do see full 140 mV at pin 2.

    Almost 0 at pin 3.

    Bear in mind input is DC due to the converter input current . 

    I just dbl checked the diff to single opamp in the sch: that is still working .

    But I busted the AMC1200 somehow....waiting to get AMC13xx series tomorrow.. A better choice.

    To recap: we are measuring dc current on the return path by R2( 1 Ohm)...so the amp should have constant gain in DC as well.

    The VDD1 side 5V is derived from an adj LDO- it should have been TPS24AXX FAMILY but for the proto, I had to use some other part. This is LDO  powered from one of the outputs of the DC DC converter using UCC28711. Return line for that is GNDEXT.

    We are expecting to see 140mV*8=1.12V diff output. That would be 2.5-.6=1.9v on Voutn, 2.5+.6=3.1v on Voutp.

    Instead, we see 1.24v & 3.84V..

    While troubleshooting, I blew the amp.

    Waiting to replace it tomorrow.

    Will share the results asap.

    r