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Original question:

TPS73801: Overshoot

TPS73801: Capacitance Change

Kevin Chapman
Prodigy 20 points
Part Number: TPS73801

Tool/software:

Good Morning, 

I had a similar question regarding a capacitance change to a circuit using the TPS73801MDCQPSEP. A schematic snippet is shown below for reference:

The 0.001uF capacitor causes a large spike to occur on power-up. With no means to control the power-up ramp rate, I'd like to understand how changing this to a 0.1uF capacitor would change the behavior? Lets assume a 16V load with 100mA max draw. Is there any risk of damage to either the circuit or the load under these conditions? Let me know if you need any more information.

Respectfully,

-Kevin 

  • 0
    TI__Mastermind 37065 points

    Hi Kevin,

    The Cff capacitor across the top setpoint resistor will actually act as a soft start.  Fundamentally, what happens during turn on can be described in these steps:

    The internal voltage reference turns on and the output voltage tracks the reference voltage turn on ramp.

    If there is a Cff across the top setpoint resistor, then presents a time constant to the voltage that can appear across that top setpoint resistor.  So the FB pin will still ramp in accordance to the internal error amplifier but the output voltage will ramp slower since Vout = Vfb + V(top setpoint resistor).

    The overshoot you see on the turn on is usually only present in older LDO's (and this one is almost 15 years old).  This usually happens because the internal reference is turning on faster than the LDO feedback can react.  So by the time the internal feedback tries to slow down the pass FET, it is too late and you see some overshoot on Vout.  By adding a larger Cff you eliminate this overshoot.

    We have plenty of LDO's in our portfolio and we can suggest a modern LDO with better turn on performance, if you need us to.  Just give us Vin, Vout, Iout and any other specific needs in your system (noise, PSRR, thermal performance, size, cost, etc etc).

    Thanks,

    Stephen