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LM5108: HB pin config

Minghao Yin
Prodigy 365 points
Part Number: LM5108

Hi Team,

Our customer has some question about LM5108.

How to design the current capability of the power supply VDD of the LM5108 driver chip? According to calculations, at startup, the instantaneous current of VDD is related to the resistance on the bootstrap capacitor link. Could you please help confirm whether there is a current limiting design inside the chip on the link from VDD to HB and the bootstrap capacitor? And also help to confirm the current capacity specification of the internal bootstrap diode?

And what is the purpose of connecting electrolytic capacitors in parallel across the bootstrap capacitor on the recommended circuit?

What's the meaning of the VHBHYS? Is there any relationship with VHBH?

  • 0
    TI__Genius 9865 points

    Hey Minghao,

    Thank you for your question regarding the LM5108.

    There is not a current limiting circuit within the chip for VDD to VHB. There is not a specification on the diode current capabilities so there is no guarantees made on this. We have performed some testing that has shown forward currents above 1A and the diode not failing. But again, we do not specify this parameter so if you need to determine that, you would need to do additional testing.

    The electrolytic capacitor in parallel with the bootstrap capacitor is used to assist in minimizing the low frequency ripple and droop on HB.

    For VHBHYS, this is a threshold put in place for when the device switches on, the current draw can cause the supply voltage to drop and fall below the UVLO threshold causing the device to turn off. If the voltage drop exceeds this falling edge hysteresis, the device can turn on and off before reaching steady state. More information on this can be found in the "Understanding Undervoltage Lockout in Power Devices" at this link.

    In this equation, VHBH appears to be VHBSYS from the datasheet.

    Let me know if you have any further questions.

    Thank you,

    William Moore