LM5116: HB resistor

Kuramochi Tadahiko

Part Number: LM5116

Hi,

Following is mentioned on the datasheet.

"Use of an HB resistor for this function should be carefully evaluated so as not cause potentially harmful negative voltage to the high-side driver, and is generally limited to 2.2-Ω maximum.".

What does "harmful negative voltage" mean?

How behavior is supposed by large HB resistor?

Best Regards,

Kuramochi

over 6 years ago

0 Timothy Hegarty over 6 years ago

TI__Guru 73752 points

Hi Kuramochi,

Adding a resistor in series with the boot cap slows down the turn-on rate of the high-side FET. This helps to reduce the SW voltage overshoot/ringing, albeit with increased turn-on loss. This is equivalent to adding a series gate resistor but without the penalty of higher turn-off loss.

However, as the boot cap is the supply to the high-side gate driver, the boot rail will get perturbed when the gate current spikes during turn-on. Thus, the boot resistor should not be too large. Unless the resistance is very high and the FET has a large gate charge, a negative voltage should not occur. My recommendation is to keep it under 10 Ohms.

Regards,

Tim

0 Kuramochi Tadahiko over 6 years ago in reply to Timothy Hegarty

Guru 28715 points

TIm-san,

Thank you for your quick reply.

Where does "a negative voltage" appear on schematic when the resistance is very high and the FET has a large gate charge?

Best Regards,

Kuramochi

0 Timothy Hegarty over 6 years ago in reply to Kuramochi Tadahiko

TI__Guru 73752 points

Kuramochi-san,

If the resistance value is high, then a large gate current causes a large voltage drop. Thus, the supply to the HB pin may swing negative, depending on the profile of the gate current waveform. I think this is an unlikely scenario for most applications.

Regards,

Tim

0 Kuramochi Tadahiko over 6 years ago in reply to Timothy Hegarty

Guru 28715 points

Tim-san,

Thank you for your clarify.

I understood that negative voltage appears during HB pin-SW pin voltage.

Best Regards,

Kuramochi

0 Kuramochi Tadahiko over 6 years ago in reply to Kuramochi Tadahiko

Guru 28715 points

Tim-san,

I have an additional question.

I think that the current flows out from HB pin when the supply to the HB pin swing negative.

Can this happen?

Best Regards,

Kuramochi

0 Timothy Hegarty over 6 years ago in reply to Kuramochi Tadahiko

TI__Guru 73752 points

Kuramochi-san,

Typically, current flows from the boot cap into the HB pin and then out of the HO pin to supply current to the gate of the FET. The boot cap then gets refreshed by the boot diode from VCC when SW goes low.

Regards,

Tim

0 Kuramochi Tadahiko over 6 years ago in reply to Timothy Hegarty

Guru 28715 points

TIm-san,

Thank you for your reply.

Can the supply to the HB pin swing negative?

Or is there possibility that this mention is typo?

Best Regards,

Kuramoochi

0 Timothy Hegarty over 6 years ago in reply to Kuramochi Tadahiko

TI__Guru 73752 points

Kuramochi-san,

In my opinion, it wouldn't swing negative unless some parasitic inductance is in series with the boot cap connection. However, if the boot cap is located close to the HB and SW pins, there shouldn't be any issue.

Regards,

Tim

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LM5116: HB resistor