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TPS22810: Maximum Output Capacitance / Slew Rate Limit

Jessica Riley66
Prodigy 110 points
Part Number: TPS22810

Hello -  I would like to use the TPS22810 to switch on power to a large capacitor bank (3.3mF) while limiting the inrush current to ~500mA.  My input voltage is 12V and I think this would require a CT capacitor value of around 0.33µF, giving a turn-on time of about 67ms to reach 80% of the input voltage.  I believe the power during turn on would be ~12V*500mA/2 = 2.8W.  

Are these calculations correct?  Can the TPS22810 handle that slow a turn-on?  Is there an SOA curve available for this part?

  • 0
    TI__Mastermind 32650 points

    Hi Jessica,

    Your calculations for the timing are correct. There is no maximum CT value but large CT values may cause concern from a thermal perspective depending on the loading conditions.

    You might need to increase your allowed inrush current to turn on the load as your load cap is very large to turn on faster.

    Edit: Initial statement about power dissipation was incorrect. Removing to avoid inconsistency

  • 0 in reply to Shreyas Dmello
    Prodigy 110 points

    Thank you for the quick reply.

    Suppose I increase the inrush current limit to ~1.5A, with CT = 0.1µF.  The slew rate is then 0.4662 V/ms and it will charge the output to 80% * 12V in ~21ms.  The power during this turn-on would be ~12V*1.5A/2 = 9.2W.  Faster, but with higher power loss - can the TPS22810 handle that?

    Can you share an SOA curve for this part?  That would be very useful to determine what transient turn-on power would be within an acceptable range.

  • 0 in reply to Jessica Riley66
    Prodigy 110 points

    I've used TINA Spice to simulate this circuit, where I've added a 10 Ohm series resistor between the output of the load switch and the bank of bulk capacitors to limit the current during turn-on.  Are there any issues with this two-stage design?  I believe it should allow the load switch to turn on quickly then more slowly charge up the bulk capacitor bank and allow a high-current transient load to discharge the bulk capacitors without exceeding the current limit of the load switch.  

    My design requirements are:

    Input voltage = 12V

    Direct output maximum load current = 1A

    Bulk capacitor continuous load current = 1mA

    Bulk capacitor transient load current = 6.2A for 2.6ms

    Minimum output voltage on bulk capacitor under transient load = 6V

    slvmcp1.tsc

  • 0 in reply to Jessica Riley66
    TI__Mastermind 32650 points

    Hi Jessica,

    Apologies for the slower reply. Looking at the application further, I believe the device will have a better chance of dissipating a smaller power over a longer period of time than a higher peak power over a short period of time so as to charge the cap. This is because it is more likely that a faster turnon would result in a power transient that would trigger thermal shutdown. I also believed that your application needed to turn on quicker to work correctly, which I do not think is the case.

    As stated in my previous post, there is no max Ct cap so I think the first test is to try your initial design case with a 500mA limit over a longer turn on. This will avoid the extra resistance in your two stage design. 

    Unfortunately, we do not have an SOA curve for this device but would highly recommend running your use case on an EVM first.

  • 0 in reply to Shreyas Dmello
    Prodigy 110 points

    Thank you for the suggestion - I will try my circuit on the EVM and see if there are any issues.

    And you are correct - fast turn-on time is not critical for my application.  But I think the solution I have simulated achieves both: fast turn-on but lower in-rush current.  Are you able to review the simulation attached?

    0827.slvmcp1.tsc

  • 0 in reply to Jessica Riley66
    TI__Mastermind 32650 points

    Hi Jessica,

    This is an interesting setup. You are right, looking at the simulation (I switched out the current source with a 1k resistor), you will minimize the inrush curent and turn on quickly, but all the power is "offloaded" to the 10Ohm resistor. While you could use a power resistor to build this application, we usually do not recommend this because the resistor will decrease the DC efficiency of the application.

    If this is not a concern for you, you could try this out with the EVM as well.