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TPS22970: TPS22970 vs TPS 22920

David Tate
Prodigy 60 points
Part Number: TPS22970
Other Parts Discussed in Thread: TPS22920, TPS22997, TPS22990

I was comparing the TPS22920 with the TPS22970.

section 10.1.2 of the  TPS22920 suggests a CIN of 10X CL to limit Vin dropping during turn on and some bit about a body diode flowing current from VOUT back to VIN causing VIN to raise when shutting down. I did not find this verbiage in the TPS22970 datasheet. What is the recommended CIN to CL ratio for the TPS22970?

for reference TPS22920 datasheet excerpt:

10.1.2 Output Capacitor 
A CIN greater than CL is highly recommended due to the integral body diode in the NMOS switch. A CL greater
than CIN can cause VOUT to exceed VIN when the system supply is removed. This could result in current flow
through the body diode from VOUT to VIN. A CIN to CL ratio of 10 to 1 is recommended for minimizing VIN dip
caused by inrush currents during startup.

  • 0
    TI__Mastermind 29240 points

    Hi David,

    In general, the amount of input and output capacitance needed is based on the output requirements (for example if there is a switching converter on the output) and the input source strength. If the input source is strong (has good voltage stability during load steps) OR the startup slew rate is not fast, then usually not much input capacitance is needed compared to the output capacitance. But if you notice the input/output drooping when the load switch is starting, then more input capacitance is needed.

    Similarly to the above paragraph, if the load on the output is at least somewhat strong, then the output will fall quickly so there is very little risk of current flowing to VIN from the output capacitance. Quite a few of our devices take additional measures to prevent this including Quick Output Discharge (also called active discharge, it connects a resistor inside the device from VOUT to GND when the device is disabled) and blocking reverse current when the device is disabled - the newer devices in the TPS22920/TPS22970 family have the Quick Output Discharge feature.

    So, this 10:1 CIN:COUT recommendation and the excerpt surrounding it can be said for any device, but it is being phased out of data sheets as these values are mostly application-dependent.

    The devices you mentioned have newer generations to them, their family is as follows: TPS22920 -> TPS22970 -> TPS22990 -> TPS22997/8. I recommend using either TPS22997 or TPS22998, as they are the newest devices in the family and have the best specifications. Please let me know if you have additional questions.

    Thanks,

    Patrick