LM5122-Q1: Help needed to design 100V 6A output power rail

Hello Yamamoto-san,

Thanks for also asking this question in e2e.

The main problem with LM5122 is the rating of the SW pin, which is 105V abs max.
In boost topology, the SW pin will jump up and down between VOUT and GND, so 100V output would in theory be feasible, but as soon as there is an overshoot on the SW, the voltage will go above the abs max and customer is at risk of damaging the device.

Unfortunately, we have no synchronous devices which can support this output voltage of 100V without being close to the abs max voltage.
You do not have this limitation in a asynchronous controller like LM5155, but for 600W, you losses on the output diode will be very high.

An alternative solution would be still use an asynchronous design, but make it multi phase, so the losses on each output diode become small enough to be acceptable.

LM5155 does not support multi phase designs, but it should be possible to connect several controller to the same external clock.
A second option would be to use LM5122, but do not connect the high side switch and use a diode instead. This avid the problem with abs max ratings and customer can still take advantage of the integrated multi phase synchronization.

I found a reference design for a SEPIC topology with LM5122 multiphase. Here you can see how there is no high side FET and SW pin is shorted to GND.
https://www.ti.com/tool/PMP23320

Please let me know if there are additional questions on this.
Best regards,
Niklas

Hi Niklas-san,

Thank you so much for the clear instruction how to achieve my power requirement. I think PMP23320 is a good reference. I will communicate with my customer regarding specific requirements based on PMP23320. I guess PDS(Power Design Services) can also help us, so I will ask the help to them as well if my customer is really interested in PMP23320.

Best regards,

Shunsuke Yamamoto

Hi Yamamoto-san,

I agree the PDS team should be the able to give the best suitable proposal here.
Feel free to come back to me if there are any additional questions on the LM5122 device.

Best regards,
Niklas

Hi Niklas-san,

Sorry for getting back to the first questions.

1. What is the maximum phase when LM5122-Q1 is designed as multiphase? I guess 4 is the maximum phase.

2. What is the maximum output power per phase? I guess 100W is the maximum output power per phase.

Best regards,

Shunsuke Yamamoto

Hi Yamamoto-san,

The engineer is out of office today. Please expect a delayed reply.

Best Regards,
Feng