TPS74801: TPS74801 Output too high

Scott Grodevant

Part Number: TPS74801

Hi All,

I have designed a PCB using four TPS74801 regulators all running from a 5V input supply. They produce four different output voltages: 1.2, 1.8, 2.5 and 3.3 using the recommended R1/R2 values and output filters shown in the data sheet. Three of the regulator outputs are correct, e.g. within a few percent of their expected values. The 1.8V output however is substantially too high sitting around 2.7V I have measured two boards. One board is 2.68 the other at 2.72 volts. I have removed R1 and R2 from one of these regulator circuits and measured them to be the correct values: 3.57K and 2.87K. The boards are properly cleaned, so flux leakage isn't the issue.

The 1.8V regulator powers a Cypress CX3 CPU, a FSA2466 multiplexer a S25FS128SA serial flash memory and an accelerometer MMA8652FC. I don't see any reverse leakage paths that might be back-feeding the regulator.

Any thoughts most appreciated.

Thanks,

Scott

over 4 years ago

0 JCHK over 4 years ago

TI__Mastermind 43610 points

Hi Scott,

If the TPS748 on the 1.8V rail is removed, it the voltage still present?

When you measure Vout, is it with respect to the LDO GND or the PCB GND? The reason I am asking is maybe there is a grounding issue on your board.

The only other thing I can think of without seeing the schematic is maybe there is some parallel resistance on the FB pin that is impacting the divider ratio. Can you measure FB to GND, and FB to VOUT on the PCB with the TPS748, R1, and R2 removed?

Hopefully, this gives you some clues as to where to look.

Regards,

0 Scott Grodevant over 4 years ago in reply to JCHK

Intellectual 925 points

Hi John,

Thanks for the reply. We have tracked the issue back to another IC that's back-feeding the regulator.

Best,

Scott

0 JCHK over 4 years ago in reply to Scott Grodevant

TI__Mastermind 43610 points

Hi Scott,

Glad to hear it!

Regards,

John

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TPS74801: TPS74801 Output too high