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TPS22990: TPS22990 circuit design related questions.

Domenic Diolaiti
Prodigy 40 points
Part Number: TPS22990
Other Parts Discussed in Thread: TPS22920,

I am using the TPS22920 load switch part in a design of mine. That switch is difficult to solder, and seems prone to damage due to reverse current from Vout > Vin on shutdwon. So I decided to  replace it with the TPS22990 load switch, and I have some questions:

1) I want to keep the rise time as close as possible between the two designs. I find myself using values of between 33nF-47nF for the CT part. So I assume that a long rise time could affect the thermal aspect of the  TPS22990, since the internal FET has an SOA that needs to be respected.

How would I go about determining if this is an issue? 

2) Regarding reverse current due to body diode, I see that the TPS22990 does not have an absolute maximum rating which states that that Vout <= Vin.

So does that mean that it is less susceptible to reverse current through the body diode? (I only need to verify the current in a power shutdown condition.)

How much current can the body diode take before damage?

Do I need to add extra bleeders in order to mitigate this issue?

Can I use the PG pin to bleed, and if so what is the maximum current that it can sink?

  • 0
    TI__Genius 13650 points

    Hi Domenic,

    Welcome to E2E!

    Before going into the questions I would like to mention that both of these devices do not have a reverse current blocking feature. Without reverse current blocking both of devices can be damaged by the effects of VOUT bring greater than VIN. For this reason we recommend using an input capacitor that is greater than the output capacitor. A 10 to 1 ratio is typically recommended in the datasheet.

    Regarding the questions on the TPS22990, when using a large CT capacitor to turn on the device onto a large capacitive load, the power dissipation will cause the device to heat up. The longer the rise time, the longer the device will be exposed to this power dissipation. As this device was not tested with higher CT capacitance than the ones listed in the datasheet, the only indications would be size of the output capacitance and the load current. Additionally the device would have to be tested to observe the behavior under your loading conditions.

    The TPS22990 will also be susceptible to damage by reverse current through the device for which the 10:1 ratio on the input to output capacitor is recommended.

    Adding extra bleeders could be an option to mitigate the device, but the 10:1 ratio from input to output capacitor should be enough to eliminate this problem during power down. The PG pin was not tested to sink current from the output, so it would not be recommended.

    What is the output capacitance of the device?

    What is the input capacitance?

    Best regards,

    Andy Robles

  • 0 in reply to Andy Robles
    Prodigy 40 points

    Hi Andy,

    Thanks for the answers.

    So the question of how much capacitance does not have a clear cut answer. We have a massive plane with tons of capacitors spread all around. We also have some local capacitance at the input of the TPS22920 device. So overall  on the PCB Cin > 10x Cout, but locally there are about equal. I have not performed an impedance simulation to see the effects of the ratio. 

    However it is my understanding that the body diode could handle some current for some time, no? 

    Here is what we see, Vout vs Vin

  • 0 in reply to Domenic Diolaiti
    Prodigy 40 points

    Hi,

    So i think i understand now, given that there is a slew rate the inrush current will be constant during startup. Even if I have 30-60nF of CT value, and output capacitance of 100uF-200uF on a 3.3V rail, the inrsuh current would only be in the hundreds on mA. I would assume that since the device can handle 10A @125oC Junction, even if my ambient temperature is 85oC, a few hundred mA at startup would not  be an issue.

    Thanks

  • 0 in reply to Domenic Diolaiti
    TI__Genius 13650 points

    Hi Domenic,

    For this device, the integrity of the body diode was not tested. We would not be able to guarantee that the device won't get damaged when VOUT>VIN.

    A device with reverse current blocking would be needed in order to guarantee the device won't get damaged. What are some key care-about characteristics for the load switch so I may suggest a device with reverse current blocking?

    You may also visit the product page on this link, and select reverse current blocking on the function tab to view the devices with reverse current blocking in our portfolio.

    Let me know if you have any questions.

    Andy Robles