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# TPS62122: Power up using quantity 2 of the TPS62122 and having only 1 ma of current available

Part Number: TPS62122

I have a 6.6 volt input for Vin and only 1 ma of current available limited by a constant current diode. My supply has 2 outputs 2.7 and 1.8 using 2  of the TPS6212s.  I power up fine but if I raise the input power slowly or jiggle power on I can get the Vout on either or both supplies to "stall" at 1.3Vdc.  The supplies were drawing too much current, probably because it was trying to switchmode the current at too low of input voltage. I had the Enable pin tied to Vin.  I now took my enable pin to a voltage divider dividing Vin to about 2 volts when Vin is 6V and tied that to the enable on the TPS62122s (sharing the divider  to both parts.)  works perfect now.  I am respectful of the spec that enable needs 50uA of current and sized the resistors to accommodate the 50uA.  Do you see any downside of doing that?  I basically have the enable pin sitting at 2 volts in normal operation with my 6.6 volt input.  Any better ideas?

sch1.docx

• Richard,

As long as the EN voltage stays above the on threshold (1.1V) and within voltage limits for the EN pin (-0.3V to VIN+0.3 but less than 17V), the supplies should work. So, as long as your input voltage VIN range does not violate the EN pin voltage limits through the voltage divider, it should be fine. I did not see a 50uA current limitation mentioned in the datasheet. Also, please ensure the input current that the input supply VIN can deliver is sufficient for the required output power drawn by the loads at 2.7V and 1.8V outputs. Typically it will be  around (2.7/VIN*(max current drawn by 2.7V load) + 1.8/VIN*(max current drawn by 1.8V load)) amperes with additional margin required due to power losses during operation. The lower the VIN, the higher the input current requirement will be so you can find the max current limit based on your minimum input voltage you are designing for.

Regards,

Amod

• Thanks for the reply.  I am only drawing about 2 ma on the 2.7 and 1 ma on the 1.8V supply.  very very light load and very limited input current (plenty of voltage)

• Richard,

Thanks for the update. That should be fine - the device will run in a PFM mode of operation and regulate the output. Please let us know if you have any more questions.

Regards,

Amod