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TPS25910: Slew Rate Control Calculations

James Morrison30
Intellectual 560 points
Part Number: TPS25910
Other Parts Discussed in Thread: TPS2595, TPS2596,

Hello,

I'm a bit confused trying to understand equations 11 and 12.  It seems some terms aren't defined exactly, or at least I can't decode it with confidence.

In equation 11, how does the term I(C-LOAD) relate to I(CHARGE)?  We're calculating the time to charge the cap and I understand that this would use the net current into the capacitor (I.e. current in minus any current out to the load).  However, with I(CHARGE) = 11uA, any load current over that would mean that you'd never charge the cap.

Also, I'm not quite sure what the sentence "Select the device load to be less than 5W / VIN." is referring to.  I can't just select what current I want the load to be.

For my application, VIN=5V, I(load)=1A, C(load) = 200uF.  I'd like to ramp up the voltage over 50mS or so.  During startup, the load will have minimum draw as it will be in reset.  It's a simple micro-controller circuit with a couple of low-power sensors.

James.

  • 0
    TI__Guru 57737 points

    Hi James,

    I would recommend you to consider our latest parts, TPS2595 and TPS2596 for your application. 

    In equation 11, Icharge is the current charging the Cext cap (dvdt cap) on the Gate pin. This current is not charging the load capacitance. The total capacitance on the gate pin and the current flowing into it will result in a dv/dt. This dv/dt will also reflect on the output capacitance during start up.

    The sentence "Select the device load to be less than 5W / VIN." is referring to the load current during startup. Since the device has a power limit of 5W, it is not recommended to have a load current during startup greater than 5W/Vin as this will result in failed start up. This is because all the current allowed by the efuse will be flowing into the load rather than charging the capacitance.

    For your application specification, TPS25910 can be used easily without any issue.

  • 0 in reply to Praveen GD
    Intellectual 560 points

    Thanks Praveen,

    I would suggest that equation 11 be updated then because the terms used there are confusing at best.  I think I have the information I need now.

    James.