LM5176: LM5176QPWPTQ1 can not normally work in Boost mode

Hi Hins,

thank you for using the E2E forum.

Did you test and see this on the EVM or on your own board?

Best regards,

 Stefan

There is no EVM board, just test it on my own board. Please help to check the schematic diagram. Thank you. 

7624.LM5176 6-38V BUCK-BOOST 12V.pdf

Hi Hins,

the schematic do not show an issue.

Can you check in the boost case if the Gate signal for the Boost Low side MOSFET is switching LO2.

Can you share a scope plot with LO2, HO2 and SW2 when in Boost mode.

Best regards,

 Stefan

Hi Stefan,

In boost mode, if Vin is set to 8V, the waveforms of BOOT1, SW1, LDRV1, HDRV1 and BOOT2, SW2, LDRV2 and HDRV2 are shown below. 

If you set the Vin to 19V, the waveform is as follows. 

Regards

Hins Weng

Hi Hins,

thanks for plots, Can you add one more which is zoomed in to 4 or 5 switching cycles after startup finished.- so the right side of the plot top right.

Best regards,

 Stefan

Hi Stefan,

Please try to review below waveform (Vin=8V in boost mode), thanks.

Regards

Hins Weng

Hi Hins,

if i read your plots right you have a time scale of 5ms/div.

A typical switching frequency of the LM5176 is 300kHz (not sure what you are using).

This will give 3.33 us per switching period, so to show 4 or 5 periods a scaling of 2us/div would fit in that case.

The resolution you have used will show aliasing affects and cannot give the required details.

Best regards,

 Stefan

Hi Stefan,

All parameters are based on Calculation Tool, i.e. RT=33.2K. 

The RT resistor is related to the nominal switching frequency (Fsw) by RT=[(1/Fsw)-190 ns] / 116pF, so the Switching Frequency should be 250 kHz. 

4101.LM5176 Buck-Boost Quickstart Tool.xlsm

The waveform of SW1 and SW2 (Vin=11V)

Regards

Hins Weng

Hi Hins,

maybe this helps:

Best regards,

 Stefan

Hi Stefan,

Vin=11V, 4us/div

Moreover, please find the SW1/SW2 waveform of different Vin (4us/div)

Vin=6V

Vin=8V

Vin=11V

Vin=12V

Vin=13V

Vin=14V

Regards

Hins Weng

Hi Hins,

as you can see from the scope plots SW2 does not go down to 0V. This means something with the Boost low side MOSFET is not OK. Which is PQ9 and PQ10 in your schematic.

You should check this components, the Gate signals and the soldering for them.

How many boards you have already tested - does all show the same behavior?

Best regards,

 Stefan

Hi Stefan,

Yes. All sample boards showed the same behavior. 

However, I'm very sorry about the previous 4us/div scope plots. I mistakenly brought SW2 to BOOT2.

SW2 can go down to 0V.

Right now, I have corrected it and re-uploaded the scope plots, please help to review below screenshot.

Vin=8V

Vin=12V

Regards

Hins Weng

Hi Hins,

I think 2nd plot you have inserted is wrong and not with 8V input - it is identical to the 12V.

What was the load connected to the output when capturing this data.

The SW2 goes up to ~13V so this looks OK.

How does the output voltage look like - is this stable? can you please capture this as well?

Best regards,

 Stefan

Hi Stefan,

The plot should not have been inserted incorrectly. It shows an 8V input capture, where you can observe the changing state of SW2 while SW1 remains fixed.

1. The output is not connected to any load during the data capture process. 

2. Set the constant voltage/constant current power supply (Current=8A, Vin=6V~38V). The correctness of the output voltage can be confirmed if it measures 12V.

When the input voltage is 8V, please review the screenshot. Does it seem that there is nothing wrong? 

Regards

Hins Weng

HI Hins,

to the scope plots let me try again with following numbering

Vin = 8V : plot 1 and plot 2 show different timing in the waveform for SW2 -  this can not be the case.

If this plots are done with no load, VOUT needs to be identical to SW2 and should also keep the level, so if SW2 goes up to 12V, VOUT also needs to be 12v.

Even if the High side Boost MOSFET would not switch, then through the Body diode.

Please check again this measurements and the high side Boost MOSFET. 

Please also plot VOUT together with SW2.

Best regards,

 Stefan

Hi Stefan,

Yes, you are right.

Retest Vin=8V, please check it.

Regards

Hins Weng