[FAQ] TPS61220: what happen if VIN is higher than VOUT

the TPS61220 can operate with Vin greater than Vout. But there will be higher ripple on the output due to the behavior of the output rectifier and output mosfet.

The TPS61220 will always regulate the output to at least what it is set to. The only question is how much higher than the setpoint can it go. WIth Vout = 3.3V, load of 10 mA, and various Vins, there were 4 modes of operation identified:

1. Vin = 3V. Normal boost operation. Vout equal to the setpoint with expected low output voltage ripple.

2. Vin = 3.6V. Boost and pass through mode. The IC turns on when Vout drops below the setpoint. It boosts for a very short time and then turns off. The body diode still conducts with a forward voltage of around 700 mV. So, it and the output cap supply the load current until the next switching cycle.

3. Vin = 3.9V. Boost mode and pass through mode. This is the same operating principle as mode 2, but the ripple is higher since Vin is higher. Thus, when the IC outputs a boost pulse it will transfer more energy to the output resulting in higher output voltage ripple and a longer time between bursts.

4. Vin = 4.2V. Pass through mode. Vin is high enough that Vout can be supplied purely through the body diode. Since Vout is above the setpoint, the IC will not switch.

The above operating modes are true all the time for this IC but the operating points (Vin, Vout, Iload) at which the different modes are entered will vary based on the operating conditions.

the TLV61220, TPS61291, TPS61322x also have similar behavior at Vin higher than VOUT