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LMG5200: Bootstrap Capacitor

Danilo A.
Mastermind 30410 points
Part Number: LMG5200


Hi Team,

We would like to ask your help regarding our customer's inquiry below.

The requirement of our below circuit is to operate in pulsed mode and in CW ( continuous on) mode.  

I completely understand the requirement for a “real load” to keep the boot-strap circuit charged, but even with a significant load we’re seeing the output oscillate (mostly ON, with rapid short drops to OFF) when held ON full time.

The smaller pulsed Radar load seem too “fit” within the ON periods, so it’s only the CW (or fully ON) condition where the output jumps around.

Increasing the boot-strap Cap (C226 here) from 0.1uF to 1.0uF improves the ON period.

Any thoughts on just how large this Cap must become to keep this circuit continually charged independent of ON time?  Is there a calculation/formula available?

Regards,

Danilo

  • 0
    TI__Expert 8960 points

    Hi Danilo,

    Bootstrap caps do not work at 100% duty cycle; larger caps will extend the on time but no amount of capacitance will allow 100% duty cycle. You might consider using an isolated power supply or charge pump to maintain the bootstrap cap charge.

    Thanks,

    Travis