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TPS62750: switching current limit during load

Nate Enos
Genius 9430 points
Part Number: TPS62750

Hi team,

Is there any concern about switching the current limit with the H/L pin while the part is loaded? We're running into some issues when we transition from 100mA to the 500mA limit, which happens while the part is switching. When this happens the part stops switching, but stays on so the output voltage rises up to 4V (output should be 3.3V nominal)

Here are a view scope shots to show the behavior


 

Ignore the labels on the scope shots, the channels are labeled as follows:

 

Image CURRENT 1.PNG:

Trace 1 is the output voltage.

Trace 2 is the H/L control pin 3.

Trace 3 is the USB 5V input current to the regulator.

You can see that approximately 620msec after turn power was applied trace two transitions low to high (0>>5V) which should transition from the low to high current limit.  At this point you can also see that the voltage on the output steps up, very puzzling.

 

Image CURRENT 2.PNG:

This image shows the same trace 1 and 3 as in CURRENT 1.PNG above but trace 2 is now the switching side of the inductor.  You can see that at the 620msec transition period of the low to high on pin 3 the regulator stops switching.

 

Image LOW TO HIGH TRANSTITION 3.PNG:

This image shows the same traces as in CURRENT 2.PNG but at a higher resolution.  We did this to show that we do not think the device is current limiting during this transition period.  You can see where the switching waveform stops which is when we control the low to high transition on pin 3.

Thanks!

Nate

  • 0
    TI__Guru*** 147340 points
    Can you post your schematic? What else does the signal driving H/L go to in their system? What is connected on the output?

    In the first image, you can also see that the input current goes down a lot after H/L goes high. This does not make sense, since Vout only went up a little. The output power is thus lower overall.
  • 0 in reply to Chris Glaser
    TI__Guru*** 147340 points
    Their schematic shows several errors: R70 blocks the input cap from being next to the Vin pins where it should be. This is the most important connection for any buck converter. This device is optimized and designed to work with very large output caps (>1000uF). The RC circuit on EN will violate the EN pin's abs max rating when Vin is removed. EN will be held high while Vin will drop. A schottky diode from EN to Vin should fix this.

    I suggest retesting their setup on the EVM to determine if the observed behavior is related to the device's operation or their usage of it.