LM5141-Q1: delta Vbst

Dennis McGlumphy

Part Number: LM5141-Q1
Other Parts Discussed in Thread: CSD87381P,

Dear Mr. Hegarty

In the section 7.3.17 High- and Low-side Drivers, I'm not sure what the delta Vbst parameter is. Where did the 100mV to 300mV come from? Is this the MOSFET switch parameter or the HO/HOL output parameter? Again, I'll be using the CSD87381P dual MOSFET.

over 3 years ago

0 Timothy Hegarty over 3 years ago

TI__Mastermind 49317 points

Hi Dennis,

This is the change in voltage on the boot cap for a given MOSFET gate charge supplied during MOSFET turn-on and a given boot cap value (derated for voltage as it's a ceramic).

Regards,

Tim

0 Dennis McGlumphy over 3 years ago in reply to Timothy Hegarty

Expert 1390 points

Tim

I'm sorry I'm so dense but I'm not getting it. The Cbst capacitor is between the HB pin and the SW pin. Are you saying the voltage at the SW pin will change 100mV to 300mV or are you saying this 100mV to 300mV voltage change comes from the initial voltage at the gate and the final voltage at the gate once the gate capacitance has charged? The definition given is ΔVBST is the voltage variation allowed on the high-side MOSFET driver after turn-on. This says to me that the HO and HOL output tied to the gate of the high side MOSFET will very. Or is this the variation allowed on the gate of the MOSFET specified by the MOSFET specifications?

Insofar as the slew-rate control is concerned, I see in the specifications that the switch over time for the high and low side MOSFETs is 20ns. In selecting the values for the slew-rate resistors, is it to say that the resistor/gate capacitance time constant needs to be less than 20ns?

Sorry for all the trouble.

0 Timothy Hegarty over 3 years ago in reply to Dennis McGlumphy

TI__Mastermind 49317 points

Hi Dennis,

delta VBst is the change in voltage of the boot capacitor (from BST to SW). This cap is floating on Sw and acts as the supply for the high-side gate driver.

You can use the LM5141-Q1 quickstart calculator to assist with component selection. A 100nF boot cap is ideal. No need for low-side gate resistors, just put a few Ohms on HO to control the high-side FET urn-on transition.

Regards,

Tim

0 Dennis McGlumphy over 3 years ago in reply to Timothy Hegarty

Expert 1390 points

Tim

Are you saying that BST is the HB pin?

0 Timothy Hegarty over 3 years ago in reply to Dennis McGlumphy

TI__Mastermind 49317 points

Yes, the HB pin (most of our parts label this as BST or BOOT, sorry for the confusion).

0 Dennis McGlumphy over 3 years ago in reply to Timothy Hegarty

Expert 1390 points

Thank you.

0 Timothy Hegarty over 3 years ago in reply to Dennis McGlumphy

TI__Mastermind 49317 points

Thanks for the enquiry, Dennis.

Regards,

Tim

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LM5141-Q1: delta Vbst