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LM5113-Q1: Charge pump circuitry appears to be non-functional

Jose Lasso
Prodigy 10 points
Part Number: LM5113-Q1
Other Parts Discussed in Thread: LM5113

The driver is being used as an independent high and low-side MOSFET driver.

The high side charges 1300uF of capacitive load, and its use is for inrush current limit. That means that HS is connected to the positive terminal of the capacitor and the source terminal of the MOSFET transistor with a shunt resistor measuring the inrush current.

The HI input at the LM5113 driver is connected to the output of a voltage comparator that sees the Vrshunt.

When 0V are @ HS terminal the misfit transistor switches properly, however as the voltage increases the Vgs decrease until the MOSFET goes off. With the comparator enabled the the voltage at HS with the circuit active goes only up to 1.46V as the Vgs threshold of the transistor is 1.6V tap 2.5V max, and HB is measured @ 4.5V, independent of the voltage @ HS.

We have: Changed the 0.1uF bootstrap capacitor to 0.22uF, decreased the 100Ohm bootstrap resistor to 0, also, we removed that resistor so only the internal bootstrap diode would work, the only relatable change is when we shortcircuit R bootstrap, the Voltage @ HS goes up to 1.6V, from 1.4V with the original circuit. 

DCI024D95 LM5113 case CS2292932.pdf

  • 0
    TI__Expert 4655 points

    Hi Jose,

    Our expert will get back to you shortly on this issue. Please give us couple more days.

    Thanks

    Sasi

  • 0 in reply to Sasikala Thangam
    TI__Prodigy 560 points

    Hi Jose,

    Thanks for reaching out to TI.

    Could you point to the charge pump circuitry the title and last page of the document you provided refer to? The schematic on page one doesn't seem to have a charge pump to charge the bootstrap capacitor. There's also a FAQ going over the charge pump configuration here.
    Additionally, R49's resistance is quite large, which would path the current more to the diode. Could you try reducing this value to something much lower like 1 ohm?

    Thanks,

    Jeremiah

  • 0 in reply to Jeremiah Vo
    Prodigy 10 points

    Well, I just assumed that the driver has a charge pump circuitry internally, it may not but the bootstrap rail or the voltage @ HB terminal is what I mean. As per an external circuitry to charge the bootstrap capacitor, the driver is not meant to need one, please see the schematic diagram for the evaluation board at:

    https://www.ti.com/lit/ug/snva484b/snva484b.pdf?ts=1713218733887

    and for R49, I mention in the document:

    "Removed R49 (left that open so only the internal bootstrap diode

    was used) VBUS dropped to 1.28-1.44V

    Shorted R49, VBUS went up to 1.48V roughly."

    Please note that you can find applications with no R49 at all, nor external bootstrap diode (as the driver has an internal one) like this circuit:

    https://epc-co.com/epc/Portals/0/epc/documents/guides/EPC9201_qsg.pdf

  • 0 in reply to Jose Lasso
    TI__Prodigy 560 points

    Hi Jose,

    Firstly, we'd recommend keeping R49 shorted in this application.

    Secondly, I didn't notice the full low-side circuit or FET for LOH/LOL in your schematic on page one. The low-side circuit in a half-bridge configuration charges the bootstrap capacitor when switching. Without this low-side switching, the system is being operated at 100% duty cycle, which is preventing the charging of the bootstrap capacitor. In your current configuration, you'd want to consider designing some extra circuitry on the high-side. We have an FAQ linked HERE which goes over design considerations.

    Thanks,

    Jeremiah