EEVblog Electronics Community Forum
Electronics => Projects, Designs, and Technical Stuff => Topic started by: angust_uk on June 11, 2020, 01:23:24 pm
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Hi all
I have an odd issue with a design using a TI TPS63000 buck/boost controller; it is set up to give an output voltage of 3.6V.
When applying power (4.1V limited to 100mA in this case) with the enable pin low all is well, but when I enable the device with a 3.3V 'enable' signal the bench supply goes into CC mode as this part is trying to pull >100mA.
I'm a bit stumped as I've used the fixed voltage variant of this part in the past with no problems, and in this example have isolated all circuitry that is powered by this device.
The waveform is pretty ugly; the VBAT net is in yellow (i.e. the device's input, set to 4.1V and limited to 100mA from my bench supply), and the output is in blue. Note that for this test I tied the device's enable pin to the VINA and PS/SYNC pins as per the datasheet's example.
Have I missed something obvious?!
(By the way, the schematic shows the TPS63001, but the correct part (TPS63000) is indeed fitted)
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Is it struggling to charge C51 and come out of soft start?
What happens if you charge C51 and then turn it on?
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Is it struggling to charge C51 and come out of soft start?
What happens if you charge C51 and then turn it on?
I think you might be on to something! I just charged up C51 to around 3.5V then enabled power and it does indeed to start up correctly (although I am only seeing 3.3V on the 3.6V rail which is odd EDIT: no it's not. I'm an idiot and can't calculate my feedback resistor values correctly). Thanks!
As there is no simple way of controlling the soft-start, how would you get around this issue? Ideally I'm trying to find at least a temporary solution so I don't have to respin the board!
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FenTiger, I owe you a beer. Thanks! I swapped out the (probably overly large) output cap with 10uF just for testing, and I now get a nice stable supply. Phew!
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I'd have suggested turning up your 100mA input current limit a bit, and then perhaps reducing it once the supply is up, if this is just for initial bringup testing. I'm guessing you'll have more than 100mA available in production.
The data sheet says the soft start begins at 400mA output current, so perhaps 500mA input limit?