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Part Number: TPS50601A-SP
Customer has modified an TPS50601A EVM to look at his desired configuration: L = 5.6 uH, Cout = 2x18uF + 8uF, all ceramic. Vin = 5V. Fsw = 200 kHz
With output set to 2.5V, load = 250 mA, output ripple captured on scope:
This seems reasonable.
When output voltage adjusted to 1.8V, (Iload = 250 mA), output ripple waveform looks like this:
Amplitude of ripple (pk-pk) seems reasonable, but waveshape is different from 2.5V case. For example, the rise/fall times don't seem to align with duty cycle (36%), and signal appears "noisier". Any explanations as to why the different response?
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In reply to Ramesh Khanna:
Thanks for the comments and vote of confidence on the design. We are trying to make a small sized solution, so the design has relatively high ripple current compared to the output load current to allow a smaller inductor (K>2).
I will pass along the recommendation on how to remeasure the ripple.
By the way, here are the compensation values (selected using Webench). Rcomp = 301 ohm, Ccomp = 330 nF, Ccomp2 = 620 pF, Ccomp3 = 750pF.
Also, speaking of compensation. The Re-Comp feature of Webench shows the power stage response has a complex double pole instead of the expected real poles for the typical current mode control device. This seems to be an artifact of the low values chosen for the Inductor and output caps, again to save space. The double pole makes it difficult to add much phase boost so phase margin is not as high as I like. Tests so far show the design to be stable and working as expected, except for understanding the ripple waveform at 1.2V (original post has an error. Output voltage at lower setting is 1.2V, not 1.8V).
In reply to Mark Guastaferro:
I don't have exact PNs for the caps. I'll try to get better information.
Here is how the EVM was modified for the testing:
In reply to Kyle Rakos:
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