TPS709: Vdo vs VIN curve for TPS70933

Hi Koyo Asaishi,

Thank you for asking this question. There are no graphs for the TPS70933 in regard to this.

We can make some assumptions though based on the data in the datasheet. If you compare the 6.5 V against the 3.3 V we see that the 3.3 V has a larger VDO max at 1400 mV compared to the 1000 mV. Look at the image attached. 

This is when Vin = 4.3 V for the 3.3 and when Vin = 7.5 V for the 6.5 V part.

This being said, you cannot assume that the dropout would be the same for the 3.3 V compared the 6.5 V. I would make the assumption based on the data attached that the dropout would be higher for the 3.3 than the 6.5. 

Based on the data, I would assume that the MAX would be around 1600 mV and the TYP would be around 1100 mV with the conditions of Iout = 150 mA and Vin = 4.3.

Also, you mentioned with a Vin of 2.7-3.3. This is impossible or the LDO will not be regulating properly. Dropout Voltage is the minimum voltage between Vin and Vout for the LDO to regulating properly. If you have a 3.3 V LDO it will not regulate properly if your voltage is below this value or even equal to this value. The Vin >= Vdo + Vout in order for it to operate properly.

The Rds (on) is consistent depending on Vin. Rds(on) is the resistance of the FET between Vin and Vout and is controlled by the internal error Amplifier. The gate of the transistor changes Rds(on) making it either larger or smaller depending on if the gate voltage increases or decreases. This is how the device regulates. The beauty of the LDO is that no matter what Vin you have, the Vout stays consistent. If Rds were to change based on Vin then Vout would not stay consistent. 

Hope this helps,

Josh Nachassi