AMC1200: AMC1200 incorrect schematic

HIMANSHU SHARMA

Part Number: AMC1200
Other Parts Discussed in Thread: REF1933,

Hi all,

I want to measure fan speed using AMC1200BDWVR and OPA320AIDBVR. I am generating supply for ICs from TLV76050 and REF1933. However during testing, even with AMC input open, TLV76050 was failed. Looking over the schematic I noticed that decoupling capacitor for AMC (C23, C24) we used is 100nF, 0603 package, instead of 100nF,1206 package (as suggested in datasheet). Can it be the cause of failure or anything else is creating problem? Attached are the related schematic images.

Thanks,

over 3 years ago

0 Tom Hendrick over 3 years ago

TI__Guru**** 168605 points

Hi HS,

Welcome to our e2e forum! There should not be any issue with the use of an 0603 cap versus a 1206 size. There could be an issue with the inputs to the AMC1200 though. The analog input range to the AMC1200 is only +/-250mV. I don't know what you have off to the left in your schematic, but the +24V ISO Fan implies that connects to your fan power. Three wire fans with TACHO outputs usually have an open drain circuit, so can you tell us what voltage levels you are driving the inputs too?

0 HIMANSHU SHARMA over 3 years ago in reply to Tom Hendrick

Prodigy 80 points

Hi Tom,

Attached is the input circuitry for AMC1200. The maximum voltage input for AMC at full fan speed is 0.232V. Although I would like to mention that failure we obtained is at no signal to PWM_Hst port.

Regards,

0 HIMANSHU SHARMA over 3 years ago in reply to HIMANSHU SHARMA

Prodigy 80 points

Hi Tom,

I would like to add we were just testing output of TLV76050. So even the fan wasn't connected to HST Fan connector.

Regards,

0 Tom Hendrick over 3 years ago in reply to HIMANSHU SHARMA

TI__Guru**** 168605 points

Hi again HS,

Thank you for the extra detail! Unless I'm missing something, that +24V_ISO_fan rail is directly connected to pin 2 of the AMC1200 right? So if pin 2 relative to pin 4 of the AMC1200 is @ 24V, you are exceeding the max input voltage of the device.

0 HIMANSHU SHARMA over 3 years ago in reply to Tom Hendrick

Prodigy 80 points

Hi Tom,

Sorry for late reply. Yeah your observation is correct. Pin 2 input is at 24V wrt pin 4. But does it matter, as I was expecting that it is pin1-to-pin4 and pin2-to-pin voltages which will matter. In this case what is tha maximum voltage that we apply between pin2 and pin 4.

Thanks,

0 Tom Hendrick over 3 years ago in reply to HIMANSHU SHARMA

TI__Guru**** 168605 points

Hi HS,

You have to keep voltage to any pin lower than the respective VDD, so the 24V for the fan is a bit too high. The voltage between pins 2 and 3 is what you are going to measure, so that needs to stay within the operating input voltage range for proper measurements (+/-250mV in this case). Since you are essentially doing a high side voltage measurement, you could use a completely isolated 5V source and then reference it to the J1 side of R34. The VIN- is normally tied to GND1, so then you would tie the 24V_ISO_Fan side of R34 to VIN+. Another option would be to move R34 to the +24V_ISO_GND side of J1 and make this a low side measurement.

0 HIMANSHU SHARMA over 3 years ago in reply to Tom Hendrick

Prodigy 80 points

Hi Tom,

Now I understood the problem. I am sure suggestions given by you will solve the issue. Thanks fir your support.

Thanks & regards,

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AMC1200: AMC1200 incorrect schematic