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TPS7A16: No output voltage vs input voltage rapid dropped

user4370495
Expert 1155 points
Part Number: TPS7A16

Dear E2E :

 I use TPS7A16-Q1  as a power for MCU.  and the system load is a motor. When i  turn on/off the load, i found the  LDO  Output Voltage turned off, 
and can't recover when the input voltage  turned normally. 


so i check  the input voltage when turn on/off the load, and  found the input voltage dropped .
Vavg=20V  Vdrop=7V. T<20US..  Waveform and Schematic as follows.

2. so i add a resistor 47 ohm between DI and C1.  the  Output voltage turn to be normal. the input voltage waveform as follow:
Vdrop=3V.

Question:

1. The reason why the output voltage abnormal, Cause the input voltage drop below the output voltage or not ?

2. Why the output voltage can't recover when the input voltage turn into normal voltage?

3.Can you give me some suggestion  for improve my schematic, except add a resistor ? 

thanks a lot.

  • 0
    TI__Guru 55940 points

    Hi ,

    The load transient on the output of the LDO is also causing a load transient on the upstream supply for the LDO.  The ringing you show on the input of the LDO is far exceeding the maximum slew rate on EN.

    By placing a resistor after the diode, you are creating an RC delay between the input recovering and the LDO attempting to startup again.  A better way to do this would be to enable the LDO after the input has recovered either by using a voltage supervisor (ti.com/svs) or an RC delay on enable.

    Very Respectfully,

    Ryan