LMZ14202: Power management - INTERNAL forum

Steve Widener

Part Number: LMZ14202
Other Parts Discussed in Thread: LMP8646, LMZ14201

We have a situation where we are seeing approximately 0.9V on the output of the LMZ14202 when the EN pin is low (close to 0V) and the input supply is ~ 24V.

The load is a passive LED device so no leakage would come back through that. The impedance of the FB network is ~ 4kohms.

There is a current limit amplifier used to set-up a current source into the LED when the LMZ part is enabled - that device is powered from a separate source but the leakage currents are low (<23uA) and we measure 0.2V on the FB while the output is 0.9V.

Here is what the circuit basically looks like except the DC/DC is the LMZ part and the "super cap" is an LED.

Will the LMZ14202 have a leakage current to the output that would generate 0.9V output? What would be the limit of the leakage current?

over 2 years ago

0 Richard Fu22 over 2 years ago

TI__Mastermind 31315 points

Hi Steve,

The device should not have leakage current that high. The feedback input bias current is 5nA, and the max leakage current for the VIN pin appears to be 1mA if it is not switching.

Is it possible to isolate the converter from the current amplifier and check performance of the DC/DC converter when enabled/disabled that way?

Additionally, if possible, can you provide a schematic and layout?

Regards,

Richard

0 Steve Widener over 2 years ago in reply to Richard Fu22

TI__Genius 11010 points

Richard,

I'll advise the customer to try isolating the amplifier if possible.

Unfortunately, we cannot see the schematic and layout for this one. They've indicated the s/d is very similar to the LMP8646 example from above.

Have we tested the EVM of the LMZ14202 to verify the output voltage with 24Vin & the enable pin low?

Thanks!

Steve

0 Richard Fu22 over 2 years ago in reply to Steve Widener

TI__Mastermind 31315 points

Hi Steve,

Unfortunately we haven't tested this. Let me order an EVM and test it to see. It'll take 3-5 days for shipping so anticipate some delay for results.

Regards,

Richard

0 Steve Widener over 2 years ago in reply to Richard Fu22

TI__Genius 11010 points

Hi Richard - any progress on this?

I measured a LMZ14201 EVM and saw ~ 0.5V on the output with the EN tied low and 24V input.

0 Richard Fu22 over 2 years ago in reply to Steve Widener

TI__Mastermind 31315 points

Hi Steve,

Sorry for leaving this hanging. I saw two things.

1) When the setup on the EVM is held as following:

and the following test sequence is applied: VIN = 24V, 8V Enable ON, OUT = 3.3V; 8V Enable OFF, OUT = 0V.

2) When I tied EN to 0V using this configuration, I didn't see the increase in output voltage.

It might be a difference in setup. Can you share how you were able to replicate the customer's?

Regards,

Richard

0 Steve Widener over 2 years ago in reply to Richard Fu22

TI__Genius 11010 points

Hey Richard,

For my test, I used the std EVM "as-is"

The difference I see is that I tied EN to 0V initially and then brought up the 24V supply - slowly ramping.

Steve

0 Richard Fu22 over 2 years ago in reply to Steve Widener

TI__Mastermind 31315 points

Hi Steve,

What was your slew rate that you apply? Let me try that tomorrow and I'll come back with results.

Regards,

Richard

0 Steve Widener over 2 years ago in reply to Richard Fu22

TI__Genius 11010 points

Sure - it was about a 1 second ramp for my testing

0 Richard Fu22 over 2 years ago in reply to Steve Widener

TI__Mastermind 31315 points

Hi Steve,

Please see the attached waveform. I did an even slower slew rate (24V over 2 seconds) and could not replicate. I still had the top UVLO resistor DNP'd, but I do not think there would be any difference.

I also added Youhao, who is supporting E2E for this month. Can the customer replicate this issue across a known good board?

Regards,

Richard

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