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LM5117-Q1: Using bootstrap to create additional rail

Thomas Stegmeijer
Prodigy 95 points
Part Number: LM5117-Q1
Other Parts Discussed in Thread: LM5117, TPS274160

We are developing a supply based around the LM5117, in constant current mode

Input voltage range 24V-28V. Output current 0-9A and maximum output voltage of 21V. 

The output needs to be switched via high side switches to loads. (only one active at a time)

Prefferably with n-channel fets,for lower rsdson and we can use the same fets as used for the switcher. 

for this I need a supply above vout, output current can be very low. 

Can I use the bootstrap from the LM5117 for this? By adding a diode from the HB pin to a small capacitor, and thus create a voltage rail of: Vin + VCC - (2 x diode drop)

  • 0
    TI__Guru 71935 points

    Hello

    It is not a good idea to use the boot-strap supply for your high-side switch drive.

    The only supply for the buck FETs comes from the boot capacitor, so any load will 

    tend to drag this supply down and you will loose gate drive.  Further, this supply is used 

    for other internal circuits.  Any unintentional disturbance on this supply can upset the 

    functioning of the device.

    It would be better to put a charge pump on the switch node and use FPWM mode (connect DEMB pin to VCC) to get your high side FET drive.

    Alternatively, TI has many high side switches with built in charge pumps to fill your application need; such as the TPS274160 and others.

    Thanks

  • 0 in reply to Frank De Stasi
    TI__Guru 71935 points

    Hello

    I will close this post due to inactivity.

    If you need more help, you can re-open.

    Thanks