LM5175: operation mode transition of Buck, Buck-Boost and Boost

Vijay-san,

Thak you for your answer.
I checked your URL, however I couldn't see Figure. If you have material, please share.

And I understood as the following,

LM5175 operation mode transit from buck to buck-boost when LDRV1 on time reduces and it reduces below min offt ime.
LM5175 operation mode transit from buck-boost to boost when LDRV2 on time increases and it increases more than min on time.

Is my understood correct?
However LM5175 doesn't have LDRV1 min off time and LDRV2 min on time.
Please let me know the parameters.

Best Regards,
Kohei Sasaki

Vijay-san,

Thank you for your answer.
It is described min on timer is 250ns, however according to my test with EVM, it seems LM5175 does buck-boost operation though Vin is 30V.

EVM Vout is 12V and Fsw is 300kHz. I think LDRV1 on time is over 250ns at 30Vin, 12Vout.

Why does boost FET operate?

Best Regards,
Kohei Sasaki

Hi Sasaki-san,

can you share the conditions and SW1, SW2 waveforms that you observe.

Also which EVM are you using?

Regards,

Vijay-san,

I used LM5175EVM. (User's Guide: SNVU440)

I attache the waveform of SW1 and SW2. Ch1 is SW1, Ch2 is SW2.
According to the waveform, there is on period of both Q2 and Q3. It seems that is isn't boost operation.

Vin=30V

VIN 30V.TIF

Vin=15V

VIN 15V.TIF

Vin=10V 

VIN 10V.TIF

Best Regards,
Kohei Sasaki

Vijay-san,

I attache the another board waveform.

This is not same condition as EVM.   Vout is 6V and Fsw is 400kHz setting.

When Vin is 10V, it is buck operation only.

When Vin is 9.9V and 6.8V, it becomes buck-boost operation. And Fsw is not stable. Is this expected operation?

When Vin is 5V, it is buck-boost operation, however Fsw is stable.

Could you explain about above behavior transition?

LM5175 waveform.pdf

Best Regards,
Kohei Sasaki

Hi Kohei-san,

Can you share the schematic?