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Part Number: TPS61093
I have a TPS61093EVM evaluation board from TI, as well as a custom made PCB featuring this part in an embedded application. Both the EVM as well as the custom PCB are suffering from fairly extreme excursions in the transient response and I cannot figure out what is happening. The fact this occurs on both the EVM and the custom PCB suggests this is not a layout issue on the custom PCB.
First, the evaluation board TPS61093EVM. Input voltage is 2.2V. Output voltage is set for 6.5V ( R1 = 121k, R2 = 10.2k ) by unsoldering/soldering appropriate value R's. Load is pulsed between 0mA and 70mA, 250Hz, 50% duty cycle. The load is an electronic load, verified through thorough testing to be accurately loading this output at 70mA. Everything else on the TPS61093EVM is untouched and stock as in the datasheet & user guide. Here's the output waveform, measured using a low-inductance barrel ground lead, directly across the output capacitor, on a 1GHz Agilent scope:
As you can see, I'm seeing negative excursions of almost THREE VOLTS when the 70mA load is pulsed on, and then almost 600mV of overshoot and slow recovery when it is turned off. This looks almost nothing like the waveform in the TPS61093EVM datasheet.
Adding a Cff of 10nF seems to quell the problem somewhat, but WHY is such a large value for Cff required here?
Now, the TPS61093 on the custom PCB is laid out in more or less identical fashion as in the datasheet suggestion and the EVM. Component values are a bit different, however. In detail: Cin=4.7uF 0603. Cout=2.2uF 0603. Inductor is 4.7uH LQH2HPZ4R7MJRL with 1.2A Isat rating ( this is 2x the maximum L current in this design, btw. I think L saturation is not an issue here) SS pin is connected to 220k/220nF in parallel. Cp cap is 220nF. Vo cap is 220nF. The green waveform is the applied load current of 70mA. Yellow waveform is the Co output waveform, without using a Cff capacitor:
Now, adding Cff = 10nF to the custom PCB helps matters somewhat, but still seems out of spec with regards to the transient response:
In the yellow waveform above, it seems Vout doesn't even recover in time to reach the target voltage ... the long slow RC ramp of ~4ms seems quite out of spec, as well as all the other transient behavior of this design.
Can anyone at TI or external provide any insight to what is happening here?
Why the large excursions negative/positive on both the EVM and the custom PCB? Am I not allowed to pulse between zero load and nominal load with the TPS61093? For the record I have tried all sorts of inductor values, all sorts of output capacitor values, and a wide range of values for Cff; the values stated above in the custom PCB give the "best" response according to my empirical observations, but that is still very short of expected behavior.
Thank you for reading!
Best RegardsJasper LiBoost Converter Solution
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In reply to Jasper Li:
I have been researching this issue some more and I have found that many boost converters are subject to a minimum load; that is to say, below some minimum load, the output is not guaranteed to be in regulation. This may be because the device enters DCM or pulse-skipping mode, or because the minimum switch on time creates a lower current limit below which the control loop cannot function ... but I am not sure if that is what I see occurring in this device.
I am indeed pulsing the load current 70mA to 0mA, and might need a minimum load resistor.
Does TPS61093 have a minimum required load to operate properly? I cannot really find this stated in the datasheet directly, but the datasheet does state:
In my applicaiton, Vin = 2.2V, Vout=6.5V
So iout_skip = 4 * (65e-9)^2 * 1.2e6 / 2 * ( 6.5 + 0.8 - 2.2 ) * 4.7uH = 512 uA
If I keep the load current > 512uA would this help the response I am seeing? I am going to try and run this experiment this week and check it out.
In reply to Adam Pasternak:
Here is the input voltage waveform and the output voltage waveform before the Cout modification ( from the EVM ):
Now see below: I changed Cout to be 2x 4.7uF X7R 0805 25V capacitors in parallel. The waveform shows practically no improvement:
I doubt that Cout is the problem here. Using WEBENCH as well as equations from the datasheet, to satisfy my requirements for Vin = 2.2V Vout = 6.5V Iout = 100mA I need L = 10uH Cout = 1uF Cin = 4.7uF I used a smaller inductor of 4.7uH and a larger Cout of 2.2uF but this is within the tolerances in the datasheet. As an aside, I also tried using L = 10uH but that did not change the response at all. It seems to me something deeper is happening here causing the very long dropout and recovery times shown in the scope waveforms.
Unfortunately, the behavior after adding Cff = 10nF is still not acceptable. There is a large excursion of 400 mVp-p between voltage sag, recovery, and overshoot. It is not this absolute value in sag/overshoot that is the problem, but the recovery time. The recovery time appears to be very different from the TPS61093 datasheet & TPS61093-EVM . Even 2ms after the load is applied, it appears the output voltage does not recover/stabilize at its setpoint of 6.5V
I have tried adding a dummy load of 6.5k for 1mA load at the OFF time period in the PWM, but this does not help the transient response at all. I was concerned about the device entering DCM or PSM and its recovery from that, but it appears not to be the case.
I would try to find an alternate part from TI or another vendor, but the TPS61093 seems unique in that it has all the following features I require: (a) At least 1.8V min Vin (b) At least 6.5V Vout (c) <= 3mm x 3mm area (d) Synchronous rectifier (e) Can support 0.1A output current (f) Input/Output isolation switch So it seems that at least for now, I am stuck with the TPS61093
Jasper: thank you for all your help so far. Much appreciated! Look forward to hearing about your findings.
Ran some more experiments today in the lab. I think I have come to a solution.
For reference, the best transient response I was able to get with Cff = 10nF & Cout = 10uF was:
Changing Cff = 100nF resulted in:
Going even higher to Cff = 220nF did not improve anything:
Now, I needed to go back to my original Cout = 2.2uF. I was worried this would cause instability or again long transient response, however, going from Cout = 10uF --> 2.2uF did not affect the transient response that much, in fact, it was almost identical to the Cout = 10uF case:
For future reference or anybody else who has this problem, the design parameters for the above response are:
R1 = 121k R2 = 10.2k Cout = 2.2uF Cin = 4.7uF Cff = 220nF Css = 220nF Rss = 220k L = 4.7uH Cp = 220nF
Thank you Jasper for verifying these findings and helping me get the proper response. Even though this was solved, I am still very curious as to why in fact such a very, very large value for Cff is required. Usually Cff are in the range of pF to a few nF ...
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