TPS22913CEVM-656: Reverse Current Protection and current through the body Diode

Hi Tony, 

Thanks for reaching out on E2E!

Can you describe how much larger the load capacitance will be, compared to the input capacitance? In general, there's two conditions to look out for when COUT is far greater than CIN:

1. As the load switch is enabled, you’ll have a sudden change in voltage (large dv/dt). Since I  = C_LOAD * dv/dt, if you have a large amount of output capacitance, then you’ll have a large inrush current event as the switch is enabled. As this large inrush current event occurs, depending on the input supply it could dip due to the sudden change in load current. By having a 10:1 ratio, the large input caps can provide an immediate source of current for an inrush event, preventing dip on the input supply.
2. The second benefit to a 10:1 ratio is if the input supply is suddenly removed or turned off while enable is on. If you have a larger ratio of capacitance on the output vs input, this could cause the voltage on VOUT to exceed the voltage on VIN during this event. This may result in current flowing from VOUT to VIN through the body diode, potentially damaging upstream circuitry or the load switch itself.

Do you see condition #2 potentially happening in your system? 

Thanks,

Arthur Huang

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Hi Arthur,
Thanks for answering my post. We'll have about 220uf on the output of the switch and was planning for 220uf at the input. Our power supply design will handle the inrush current. Item 1 condition is not a concern.
Condition 2: We are using the switch for sequencing the 3.3V and 1.2V for the C6747 DSP. We have the power up sequence covered. For the power down sequence we picked the TPS22913. Under normal shutdown the switch will be off before the our power supply shuts down. However if the condition occurs that the input to our power supply causes our 3.3V supply to shut down first, is the Reverse Current Protection in the switch enough to not damage the switch in some way? Thank you, Tony

Hi Arthur, Thanks for the quick reply. I thought the same thing however why does the data sheet state in 11.1.4 as shown below. Is this leftover from a data sheet that did not have Reverse current protection?
Thank you, Tony

11.1.4 Output Capacitor (Optional)
Due to the integrated body diode in the PMOS switch, a CIN greater than CL is highly recommended. 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.