TPS746: Output capacitance

Hi,

The maximum capacitor recommended is assuming the typical minimum ESR for ceramic capacitors. Since you have a pi filter at the output, the impedance introduced by the inductor L1-N4 makes the effect of C456_N4 and the other capacitors negligible.  One thing you might see with excessive capacitance at the output is a non-monotonic rise. The reason for this is that the LDO will startup into the fold-back current limit and then may enter current limit as it tries to bring up VOUT. If the capacitance is excessive, there is a chance that the LDO can enter into thermal shutdown, and depending on the load conditions it may cycle on and off as it tries to bring up the output voltage. 

Another concern is with so much output capacitance, if VIN is suddenly disabled or shorted to GND, care must be taken to ensure that reverse current through the parasitic body diode does not damage the device. This is normally mitigated with an external diode to bypass the LDO. 

I hope this helps to answer your question. 

Hi,

Reducing COUT could help to avoid any startup issues. It would be best if you could test this on an EVM to see if you can accept the startup profile.

With respect to protecting against the reverse current, it really depends if you think that VIN can be suddenly shorted to GND  If this can happen, it is our standard recommendation to add an external parallel diode to shunt current from VOUT to VIN around the LDO. 

Another approach might be to just use a much higher PSRR and lower noise LDO like the TPS7A91. This is a fast transient response low noise LDO which can help minimize the need for a lot of output capacitance. 

Does this help to answer your question?