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Original question:

TPS22810: CT capacitor

TPS22810: CT capacitor

Wim De Witte
Intellectual 300 points
Part Number: TPS22810
Other Parts Discussed in Thread: TPS22917

Hi,

Can you please advise if this device fits my use case.

Power supply is 5V, max 100mA

My device needs capacitor storage, in case of sudden power-off.

I calculated that the capacitance has to be 3000uF; (sufficient to write critical data to flash)

Therefore, I will use the TPS22810 to limit the inrush current towards the power supply which cannot handle large inrush currents.

I calculated and simulated with tina a Ct of 2uF.

When the device is disconnected from the supply, the output voltage will gradually drop to 3V within 250ms.

I added a series resistor of 5 Ohm to limit max current to 1A to preserve Imax

2 questions:

when sudden power off; Vin = 0V and Vout is max 5V; can this harm the device?

when powering on: can the device handle the power dissipation?

Regards, Wim

  • 0
    TI__Genius 15780 points

    Hi Wim,

    When using a large CT capacitor to turn on the device with a large capacitive load, the power dissipation across the device causes the device to heat up. The longer the turn on time, the longer the device is put in the high power range and the more it heats up.

    The good thing about the TPS22810 in your application is that the device has thermal shutdown, so even a long turn on time may not be a problem for the device since it can cycle thermal shutdown until the capacitor is fully charged. The problem is that the device does not have reverse current blocking, so under the case where VOUT = 5V and VIN=0V, current will flow across the body diode of the device, and this will not be regulated. If VIN is left floating, then it will just bring VIN up to about 4.3V and the device will not be damaged. But if there is a load on VIN, then it may pull current enough to damage the device.

    One potential solution would be to add an NMOS FET to the other side of the TPS22810 with the body diode facing the output. With the CT pin tied to the gate of this FET, the device will block reverse current when turned off. When turned on, the NMOS will also turn on slowly. However, the NMOS will not have any thermal shutdown protection, so you would have to make sure the FET is within its SOA.

    Another solution would be the TPS22917 device. It also has a CT pin for slow turn on, and it also features reverse current blocking (even under the case of VOUT=5V and VIN=0V). But the TPS22917 doesn't have thermal shutdown, so you would need to test the slow turn on with the device to make sure it doesn't overheat when turning on the capacitance in your system.

    Let me know if you have any additional questions.

    Thanks,

    Alek Kaknevicius

  • 0 in reply to Aleksandras_Kaknevicius
    Intellectual 300 points

    Hi Alek,

    thanks for the explanation.

    Changed the schematic to prevent a load at Vin; thus reverse current blocking is not an issue anymore. 

    I also calculated the thermal dissipation. I don't see a problem with (Vin-Vout) 5V/0.1A going to 0V/0.1A within 200msec.

    Regards,

    Wim