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AM3358: Voltage clamp

Derek
Mastermind 9701 points
Part Number: AM3358
Other Parts Discussed in Thread: TLVH431, REF3318

Hello, looking through AM335x voltage rail clamp between 1.8V and 3.3V rail documented in figure 7 of slvu731b, the default design uses 5 parts including the shunt voltage reference TLVH431, the PNP 2N2907A etc. The goal (as far as I understand it) is to short the rails if the voltage between them gets well above the 1.5V nominally.

Couldn't this design be done with just two parts - a series voltage reference and an NFET? Use a ~1.6V or 1.8V series reference like REF3318, and connect the output to the gate of an NFET (like DMG1012, Vgs=1.0V) between the two rails. When the voltage between the rails exceeds Vreference +Vdropout (1.802 for the REF3318) then the NFET will turn on, clamping the rails.

I'm working on a design that already has a ton of unique parts so we're trying to avoid any more. I'm already using that FET all over the place so the only additional part would be the ref.

Would this work?

  • 0
    TI__Guru**** 393215 points

    Hi,

    Could you post the proposed schematic? It's not very clear how the REF3318 is connected.

  • 0 in reply to Biser Gatchev-XID
    Mastermind 9701 points

    I did more calculations and I don't think the REF3318 would work because it would not turn on until the difference in the rails is 1.802V, and we want it to turn on when the difference in the rails is 1.55V. If we had a fixed 1.55 reference then that would work perfectly and we could omit the two resistors.

    However, with the clamp circuit shown in figure 7 of slvu731b, can I replace the PNP transistor (MMBT2907) with a PFET? (DMP2035U-7, Vgate is 1V). That would eliminate a unique part. Based on the specs of the PFET it appears that it will work. Both parts are SOT23, with compatible pinouts (base of PNP is same pin as the gate of FET package, source of PNP is same pin as emitter of PNP, and collector is same pin as drain)

  • 0 in reply to Derek
    TI__Guru**** 393215 points

    Derek9531 said:
    can I replace the PNP transistor with a PFET?

    I don't see any issue here.