LM5171-Q1: Combining Gate drivers

Preerag M

Part Number: LM5171-Q1
Other Parts Discussed in Thread: LM5171

Tool/software:

We are designing a topology as shown in the figure. To have a bypass mode where the power stage itself is used as the bypass path, another high side driver output that supports 100% duty cycle is, tied together with the LM5171 gate driver outputs. It is made sure that only one of the IC is powered ON at a time by using a power multiplexer configuration to supply the VCC to the IC's. Is there any risks involved in this topology or am i good to go? (PS:It will be made sure that there is no time when both will be in ON condition by providing sufficient delay).

1 month ago

0 Alicia Hao 1 month ago

TI__Intellectual 940 points

Hi Preerag,

There will be some delay as expert is out of office today, please help expect feedback tomorrow. Thank you.

0 Feng Ji 1 month ago in reply to Alicia Hao

TI__Mastermind 22365 points

Hi Preerag,

This is not a typical application. I have a few concerns:

Please make sure the "High-side driver" can survive fast switching of SW and HO when LM5171 is operating.
When the "High-side driver" is driving HO high, LM5171 HB-SW will be powered from HO-SW. HO will switch low once HB-SW is powered.

From my view, you can use an external bypass FET as shown below to avoid the risk mentioned above.

Best Regards,

Feng

0 Preerag M 1 month ago in reply to Feng Ji

Prodigy 80 points

Okay, got it, Thank you Feng.

0 Preerag M 27 days ago in reply to Feng Ji

Prodigy 80 points

Hi Feng, can you give me a more elaborate explanation, why HO will be pulled down when HO-SW is driven by an external MOSFET. In the circuit, we are deriving power to the boot strap capacitor from VCC, which we will be turning OFF completely. So is this still an issue?

0 Feng Ji 26 days ago in reply to Preerag M

TI__Mastermind 22365 points

Hi Preerag,

there is a diode from HO to HB. When HO-SW is driven by an external MOSFET, HB-SW will also see the voltage.

Best Regards,

Feng

0 Preerag M 25 days ago in reply to Feng Ji

Prodigy 80 points

Okay, got it. Thank you for the explanation Feng.

What if we add a solid state switch (NMOS or PMOS) between the switch node and the bootstrap capacitor as shown in the architecture given below. The switch will be powered by Vcc itself and will be turned off when Vcc is disabled. Do you see any risk in this architecture?. Also, if possible can you please suggest which device will be best for the switch and its specifications required so that it won't affect the working of LM5171.

0 Feng Ji 25 days ago in reply to Preerag M

TI__Mastermind 22365 points

Hi Preerag,

You drawing is wrong, SW is in the middle of the two FETs.

I don't think disconnect boot strap cap will help as internal diode from HO to HB is still connected.

Maybe you can use a SPDT as below.

Best Regards,
Feng

0 Preerag M 24 days ago in reply to Feng Ji

Prodigy 80 points

Okay, thanks for pointing out the accidental mistake. Those two were supposed to be connected to the switching node. Does TI has any automotive qualified SPDT switch which will be suitable for our application. We will be operating at 100kHz and the maximum total gate charge of the NMOS is 275nC.

0 Feng Ji 24 days ago in reply to Preerag M

TI__Mastermind 22365 points

Hi Preerag,

I am not familiar with the product.

Best Regards,
Feng

0 Preerag M 24 days ago in reply to Feng Ji

Prodigy 80 points

Okay, Thank you Feng

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LM5171-Q1: Combining Gate drivers