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TPS7A4700EVM Noisy Output

Nima Alamatsaz
Prodigy 20 points
Other Parts Discussed in Thread: TPS7A47

I tried out the TPS7A4700EVM regulator evaluation module for a couple of days and had two main issues with it which I was hoping to understand.

First, the regulated output of the board seemed quite noisy with an amplitude of up to 150mv over a 5v output. The noise was mainly consisted of both positive and negative spikes that were spaced couple of milliseconds apart. The input of the board was a reasonably low ripple 24v from a regular laptop adapter.
Second, the chip and the entire copper pours get quite hot while supplying 200mA at 5v. The heat can burn a finger by just touching the chip for a second. Is this normal at only 1 watt?
I'd appreciate any help!
  • 0
    TI__Guru 55940 points
    Hi Nima,

    It looks like you are dissipating 3.8W across the LDO. (24V - 5V) x 0.2A = 3.8W.

    Theta-Ja for TPS7A47 is 32.5C/W. As a result, the junction temperature of the LDO will be 124C higher than the ambient temperature.

    Very Respectfully,
    Ryan
  • 0 in reply to Ryan Eslinger
    Prodigy 20 points

    Hi Ryan,

    I did in fact misunderstand how LDOs actually function. Thank you so much for clearing that up for me.

    Nevertheless, my first issue is still up for grab. I thought maybe the noise is the result of a damage caused by overheating the chip, but isn't thermal shutdown supposed to protect it under high temperatures?

  • 0 in reply to Nima Alamatsaz
    TI__Guru 55940 points

    Hi Nima,

    Thermal shutdown is designed to protect against thermal overload conditions; however, this is not meant to be used in place of proper heat sinking.  Continuously running into thermal shutdown will degrade the device reliability.

    The Electrical Characteristic specifications are applicable only to the recommended operating conditions.  If your junction temperature is above 125C, then the device may be outside of the specified limits.  

    Can you share a DC scope shot of Vin, Vout, and Iout?

    Very Respectfully,

    Ryan