LM5176-Q1: LM5176 Vout higher and then drop to normal output when start

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Hello Michael,

Did you observe any other differences between the functional boards and the ones that show this behavior? If possible, please check the startup on a working unit and a failing unit, e.g. the pin voltages at startup.

Can you share the schematic?

Did anything change on the external components?

Best regards,
Brigitte

Hi Brigitte,

We test the gate driving signal: LM5176 Vout setting volt is 15.5V. When LM5176 start up, there are 4 cases:

Case 1: Input volt is 13.5V, it first works as buck-boost, and output voltage reaches 16.4V, then transfer to boost. During the transfer duration, there is volt drop. And then, Vout rise to 15.5V and becomes stable.

Case2: input volt is till 13.5V, LM5176 always works in buck-boost mode, there is no volt rise at the beginning(not higher than 15.5V), and also no obvious volt drop during HDRV1&LDRV1 PMW duty cycle and frequency changing.

Case 3: Increase input volt to 14.5V, LM5176 will always work in buck-boost mode, there is no volt rise at the beginning(not higher than 15.5V), and also no obvious volt drop.

Case 4: Decrease input volt to 12.5V, similar to Case 1. It first works as buck-boost, output voltage reaches 15.8V, then transfer to boost. During the transfer duration, there is volt drop. And then, Vout rise to 15.5V and becomes stable.

Schematic attached, please explain volt rise(higher than setting volt 15.5V) in case 1, and obvious volt drop in case 1&4.

Thanks！

Michael.

Hi Michael,

unfortunately the resolution of the schematic is very low - so I can not identify details or read the values.

Can you provide a better one.

Best regards,

 Stefan

Hi Stefan,

     I try to upload schematic pdf file, but when I click the uploading area, it doesn't work. I use Chrome explorer, and already cleared cookies, do you have any suggestion?

I attached picture format schematic, please check!

Thanks!

Michael.

HI Michael,

best option to attach file here is to use drag and drop.

I have seen that you have some current sensing on the FB - can you try with removing/disabling that e.g. remove R10131.

Best regards,

 Stefan

3036.LM5176.pdf

Hi Stefan,

     Thanks for your advice, and uploading seems succeed.

     R10131 is voltage divider together with R10132 for ADC input. Please check schematic and confirm if it 's a intrinsic feature that there maybe voltage rising and drop when transfer from buck-boost to boost at start up, and depending on input voltage level.

Thanks!

Michael.

HI Michael,

It is not expected to see this output voltage behavior, therefore it would be good to understand what is triggering this. 

A first step would be to avoid other sources - like the ADC connection to the LM5176.

Best regards,

 Stefan

Hi Stefan,

     Thanks for advice. In theory, ADC input is high resistance input for MCU, there shall be no impact for Vout. We will remove ADC input resistor R10131 and check when getting back to office several days later.

Thanks!

Michael.

Hi Stefan,

     We removed ADC input resistor R10131, there is no change. According to tested waves, this rise higher and drop phenomenon are related with working modes transfer, as mentioned above,  when input voltage increase to 14.5V, LM5176 will always work in buck-boost mode, there is no volt rise at the beginning(not higher than 15.5V), and also no obvious volt drop during HDRV1&LDRV1 PMW duty cycle and frequency changing.

     Please help explain.

4

Thanks!

Michael.

we add additional 10mA load at Vout(1.5k resistor), this phenomenon is same.

Hi Michael,

Thanks for giving an update on the current results.
We'll get back to you later today with a reply.

Thank you and best regards,
Bryan

Hi Michael,

do you see this behavior only with no load?

What is the behavior if you have higher load during startup?

Is your source able to sink current in case the output voltage is higher then the expected?

Can you try with increased capacitance on the input - if the supply is not able to sink?

Best regards,

 Stefan