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PMP21274: Purpose of Q5 and R9

Andreas Nestler
Intellectual 370 points
Part Number: PMP21274
Other Parts Discussed in Thread: LM5122,

Tool/software:

Hello,

due to some issues with LM5122 I have investigated some reference designs. In PMP21274 there is an assembly option for Q5 and R9. What is the purpose of this option?

Thanks,

Best Reagrds Andreas

  • 0
    TI__Mastermind 41800 points

    Hi Andreas, I've seen designers try to add offsets on the current sense to try and flatten the current limit with input voltage. It looks like this what this designer was trying to do and added placeholder in case they ever needed it. It looks like for his final design it was not needed. 

  • 0 in reply to Darwin Fernandez
    Intellectual 370 points

    Hello Darwin,

    thanks for your reply. I'm not sure if your explanation fits the actual design, since the base of the additional transistor is connected to VCC , which is stabilized voltage from LM5122 internal LDO. At least I have not seen in the design that another circuitry feeds voltage into VCC pin. So, the voltage at VCC pin is fixed. For sure, the added components will decrease the set point of LM5122 current limiter at the input. This was the reason, why I asked since I do not understand why the limit shall be decreased if VCC is ramped up.

    One reason of such circuitry could be to (fine) tune the limit threshold w/o changing the shunt resistor value. Tuning is only possible to a lower current limit set point.

    Please correct me if I missed something.

    I'm looking forward hearing from you.

    Best regards,

    Andreas

  • +1 in reply to Andreas Nestler
    TI__Mastermind 41800 points

    Hi Andreas, I also have not seen it designed this way in the past. Usually you want an offset as a function of input voltage. Unfortunately, the engineer is no longer at TI so we won't be able to get the details; however, it is reasonable that they were looking to fine tune the current limit threshold and they found a sense resistance that fit their specs so ended up not needing the circuit in the end. Thanks!