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TPSM82866A: Output Discharge Resistor Rating

Steve Widener
Genius 11010 points
Part Number: TPSM82866A
Other Parts Discussed in Thread: TPS62867, , TPSM82810

Is the output discharge resistor capable of discharging very large output capacitances without concern of damage?

For instance, we have a case where 1000uF will be connected on the output at 1.8V and an expected die temperature of 110C.

I see that the discharge resistance is typically ~ 3.5 ohms but more like 4.5 ohms when the die is hot (assuming 5V input)

For infinite capacitance (essentially a constant 1.8V on the output with the device forced off), I'd expect to see (1.8V^2)/4.5ohms = 720mW dissipation in the discharge resistorand a temperature rise of 25.4C/W * 0.72W = 18.3C. I think the 1000uF discharge would be ok ( ~ 4.5msec RC) but it's not clear due to the various thermal time constants. 

Will the device be ok for the 1000uF, 1.8V scenario described? Is there an energy rating on the discharge resistor?

Best Regards!

Steve

  • 0
    TI__Guru**** 150950 points

    Hi Steve,

    Yes, from the D/S abs max ratings, there should be no issue with sinking any amount of current through the VOS pin.  I will double check with design though.

    There is a thermal limit from the dissipation in that VOS resistor, as you noted.  But this power dissipation should be less than normal operation at full power, so it should not usually be an issue.  Because the IC is off when that resistor is on, the 2 power losses do not occur at the same time.

    I am more concerned with the 1000uF of Cout.  Is this all ceramic, or is some of it polymer or tantalum with ESR?

    Thanks,

    Chris

  • 0 in reply to Chris Glaser
    TI__Genius 11010 points

    Thanks Chris -

    There is still some concern about the reliability of the internal discharge "resistor" as there isn't a specific pulse rating called out anywhere. There have been instances of failure in similar situations and this a critical application - we'll need something analytically sound to close this one out. Would it be possible to place a series resistor (with surge/ pulse ratings) in series with the VOS pin? How high in resistance could we go before it would cause a problem?

    For the output capacitance, is the concern around stability? I see there is a datasheet note that values > 150uF can degrade the loop stability. Do we have a tool to help predict loop gain BW & phase margin?

  • 0 in reply to Steve Widener
    TI__Guru**** 150950 points

    Hi Steve,

    Since there is not a written limit in the D/S, there is not a limit.  I am double-checking if something was overlooked here.  Since the customer needs to know what the limit is, let's take this conversation to email.

    You should not insert a resistor in series with VOS, as this would also affect the control loop's feedback.

    Yes, that much output cap could be unstable.  A transient model for the TPSM82866A is in work, and you can use the one for the TPS62867 for now.

    Would a 6A version of the TPSM82810 be a better fit?  That one can support more output cap and has a higher output discharge resistor.

    Chris