LM25117: LM25117 schematic

Hi Fred,

If the Vout is >9V, the VCC can be biased by Vout to save the power loss of internal VCC LDO to reduce the device thermal.

Ho Andy,

How about the diode function between VCC and HB pin , is it necessary?

And what's the spec need for this diode?

Hi 

is there a limit or spec for this bootstrap diode since  I heard large junction cap may lead to large reverse recovery

would you evaluate below is suitable or not?

Hi Fred,

Do you mean the D2 Bootstrap diode?

Voltage rating: >1.2*(Vin_max+VCC)

Current rating: average>50mA--100mA, peak 1A~2A

SK32 is too big for D2.

B R

Andy

Hi Andy,

I mean D2.

1. If Vin=24V, (24+7.6)x1.2=40V , does this mean SK32 can't afford 40V? 

and why is equation related to Vin, I though it connect only to VCC

2. by  too big ,do you mean average current rating of diode?

Hi Fred,

D2 20V rating is not enough as I already explained in the thread:

This is essential to block the high voltage when high side switch turn on, SW will equal to VIN, HB pin voltage is VIN+VCC

1A rating is enough for D2, the package size should be as smaller as possible to reduce the loop area.

B R

Andy

Hi Andy,

1. 1.2 in the equation is for margin, right?

2. is loop area related to junction cap? are they same 

1. Yes,  it require 1.2X or higher margin

2. junction cap is not critical for bootstrap diode

I see,

 what will loop area affect if it's too large?

it may generate higher EMI noise when VCC charge up Cboot....