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Original question:
TPS552892: Efficiency does not meet specifications(5Vin/ 5vout)
Tool/software:
Hi there,
I'm trying to use the TPS552892 in my application with specifications Vout = 12V, Vin = 8-24V, and Iout = 2A. I have copied many of the values from the evaluation module TPS552892EVM-111, but I am still running into some issues with lower Vin at a higher output current.
I am using a programmable DC electronic load, which should be resistive. With Vin = 8V and currents above 750mA or so, the output turns into a triangular wave. With higher Vin, even still in boost mode, it is able to handle more load (I haven't characterized this relationship too carefully). I played a little with output capacitance and it helped some stability issues at even lower load currents, but did not help at 750mA.
I then tried using the programmable load with the evaluation module. The EVM does better than my board, but still seems to fail its own specs. With Vin > 6V it should be able to handle 3A. However, when I set Vin = 8V and Iout = 1.5A, the output produces a triangular wave similar to what I saw with my own board. I measured the following waveforms on the scope:
Ch1(yellow) = Vout, Ch2(blue) = SW1, Ch3(red) = SW2
Using Vin=8V, Iout=1.2A:
Using Vin=8V, Iout=1.5A:
(Note the timescale is very different in the two images). Changing Vin and Iout create different versions of these two waveforms. The patterns make sense in the working conditions, but I'm not sure what is happening when it's failing at the higher loads. At 1.5A it's still able to hit 12V for a little bit of time, but as Iout increases it stops hitting 12V and turns into a triangular wave. I thought it might be a pattern that's familiar to somebody who has more experience with switching regulators.
I'm hoping to use this as a clue to troubleshoot my own design, since it seems to be failing in the same way (just at a lower load than the EVM). But since the EVM should work to its rated spec, I'm not sure if there is something else wrong with my setup.
Thanks!
Hi Bryce,
Thanks for the response. Somewhat embarrassingly, that was indeed the problem with my EVM setup. However, it's not the problem with my own board which is also failing with a triangular Vout.
My own board doesn't have test points for SW1 and SW2 (the two sides of the inductor), but I tried to probe them anyways via the capacitors' pads. This takes some fidgeting and somewhere in the process, something seems to break. After "breaking", the regulator outputs a lower Vout (and draws some non-insignificant current) even with open load. I've broken two regulators on two boards this way and I'm not sure how or why. It's possible I shorted something while probing.
I'm having trouble getting consistent behavior out of my regulators/boards, and they keep breaking before I can look for patterns. I'll keep trying when more regulators arrive in the mail. In the meantime, any chance you recognize these waveform patterns?
Ch1 = Vout, Ch2 = SW1, Ch3 = SW2, Ch4 = Vin.
This is happening at low load this time (Iout = 250 mA) as this incarnation of the regulator/circuit seems unable to handle low loads.
Thanks!
Irene
Hi Bruce,
1. Yes, the EVM seems to operate as expected now. Thank you!
2. Here are the schematic and layout (4 layers). Power for the TPS552892 (Lamp_Pwr_Unreg) is coming through a high side switch (TPS27S100ARRKR) set with a current limit of 3A which has an additional 10uF on its output (feeding input to FB2, which itself is left over from an older design and we are thinking of removing it). Main power that goes through the high side switch to TPS552892 has about 100uF located nearby, so I had considered all of this to help with input capacitance, which might not be valid.
The layout is sort of difficult to see, but we did try to keep everything close and follow the layout guidelines. I did not separate analog and digital grounds for the regulator, though.
Re: 3 and 4, I installed a new regulator and observed the same behavior, including a similar waveform on SW1. I also realized that I was probing the wrong Vin last time since I probed system power; instead, I should probe what comes through the high side switch. This new Vin looks much noisier, perhaps additional capacitance would help. But would this cause the strange behavior on SW1?
Using Vin=8V, target load 500mA, I observed the following waveforms. Ch1=Vout, Ch2=SW1, Ch3=SW2, Ch4=Vin (pin 2 of J11 in the schematic). I also probed EN separately but it is voltage divided off Vin and looked stable in comparison.
The parts are manually soldered and it is possible there are some assembly errors, but at this point I have observed the behavior twice on two separate boards. Please let me know if you have any ideas... thanks!
Irene
