Hi team,
My customer meet some issue on power off, SCH is below:
And below is the power off wave of TPS56221:
Input voltage is 12V, output is 3.3V, load current is 6A, when input power drop to 6.3V, TPS56221 input voltage has a big drop.
But when customer use 6V supply TPS56221, set output to 3.3V@6A load, TPS56221 works good.
We don`t know why this strange drop happened when input drop to 6.3V, do you have some comments?
Thanks very much.
Regards, Sunny
Sunny,
Assume 100% efficiency on TPS56221 DC DC Converter
IIN=Vout*Iout/VIN
IIN=~1.65A when VIN=12V
The UVLO of the TPS56221 is 4.3V.
The BP regulates to 6.5V, once the input voltage drops below 6.5V the BP linear regulator is in dropout and the gate drive voltage decreases and increases the on resistance of the power stage fets. The on resistance increasing, lowers the dc dc efficiency further increasing the IIN current.
I assume you are turning off the 12V source off and the voltage drifts lower. As the voltage drifts lower, the IIN current increases and as the input voltage goes lower than 6.5V the converter efficiency decreases because of the reduced gate drive.
Either Rpcb or the ESR of the Bulk cap is large and causing the voltage to drop.
Once the device stops switching the input current is ~0A and the input voltage increases to above the start voltage of the TPS56221 the device start up again and the input current increases causing the voltage to drop again
Regards,
David
Sunny,
Assume 100% efficiency on TPS56221 DC DC Converter
IIN=Vout*Iout/VIN
IIN=~1.65A when VIN=12V
The UVLO of the TPS56221 is 4.3V.
The BP regulates to 6.5V, once the input voltage drops below 6.5V the BP linear regulator is in dropout and the gate drive voltage decreases and increases the on resistance of the power stage fets. The on resistance increasing, lowers the dc dc efficiency further increasing the IIN current.
I assume you are turning off the 12V source off and the voltage drifts lower. As the voltage drifts lower, the IIN current increases and as the input voltage goes lower than 6.5V the converter efficiency decreases because of the reduced gate drive.
Either Rpcb or the ESR of the Bulk cap is large and causing the voltage to drop.
Once the device stops switching the input current is ~0A and the input voltage increases to above the start voltage of the TPS56221 the device start up again and the input current increases causing the voltage to drop again
Regards,
David
