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Original question:

TPS50601-SP: Tracking start-up

TPS50601-SP: Power Good with tracking

Mark Guastaferro
Genius 11645 points
Part Number: TPS50601-SP


Tool/software:

Follow-on question to my previous post on two TPS50601 devices in a tracking start-up configuration. As noted in the previous post, the customer design is set up with device #1 output = 3.3V and the second tracks this for a 1.8V output with a voltage divider between Vout1 and the SS/Track input of device #2. Voltage divider is 5K/10K. 

This design basically works except the Power Good output of the second device does not come up when the output reaches 1.8V.  This appears to be because the SS pin voltage only reached about 1.12V when the ramp is complete. There's a comment in the data sheet under the Power Good description (8.3.13) that indicates power good will be off if the SS/TR pin is below 1.4V. 

Any ideas how we might be able to work around this? I think the only way is to change the divider to allow the SS pin voltage to reach 1.4V, but this will change the desired Ss tracking the customer is trying to meet. 

  • 0
    TI__Expert 6201 points

    Hi Mark,

    I agree with you that the way to get a valid Power Good will be to change the resistor divider so that the voltage on SS_TR is above the 1.4V requirement for Power Good to be valid.

    I did some back-of-the-napkin math and ran a couple simulations, based on what I'm seeing, a resistor divider with a multiplier of approximately 0.44 (instead of 0.33) would produce an accurate tracking ramp and have a final voltage above 1.4V. See my simulation below:

     

    Final SS_TR voltage in my sim was 1.457V using a 10k RTOP and 8k RBOTTOM resistor divider set. The Red trace is the simulated device output, and the blue trace is a ramped PSPICE voltage source being fed into the top of the resistor divider.

    How did the customer end up with their 10k + 5k resistor divider? I see in your previous post there is a PSPICE simulation that seems to show matching ramps, are the ramps matching well for them in real testing?

    Thanks,

    Andy

  • 0 in reply to Andy Fondaw
    TI__Genius 11645 points

    Hi Andy,

    Thanks for the suggestion and simulation.  I passed it along to the customer and this works.  Problem solved.   We're not sure why we chose 10K + 5k for the resistors when this was first tried some months ago, must have made an error somewhere and the tracking may have been close enough.