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Part Number: TPS54332
I have implemented what should be a reasonably close replica of the reference design for this chip.
Input voltage is nominally 12V and output 5V. The circuit works apparently OK until output current reaches close to 700 mA at which time the output voltage collapses. Removing the load will then restore operation.
The layout looks like this:
It is a 4 layer board with ground fill on all layers (gray in the image, unfilled for clarity). The colored outlines are other unfilled copper areas. Orange Vin UVLO and yellow FB tracks are on the bottom side.
The related schematic looks like this:
Output caps are 47uF 6.3V tantalums. The upper feedback resistor R22 is in fact 12k, not 10k as shown. Also, R21 is 100k instead of 560k.
I have tried the following:
-Moving the compensation nodes has no effect (C26 -> 470pF; R27 -> 22k) no visible effect. The circuit appears stable before and after and the current limit is unchanged.
-Increase the inductor 2.2uH -> 4.7uH; no effect on current limit.
-Increase the boot cap from 220nF to 470nF; no effect as makes sense (an earlier test circuit failed to work with the recommended 100nF cap but did work with a 220nF)
- Introduce a modest lowpass to the FB by paralleling R26 with a 470pF cap; no effect
- Increase input voltage from 20VDC to 24VDC; THE CIRCUIT NOW WORKS CLOSE TO 800mA but then crashes again!? WTF?
At all times the scope trace from the switching node was very clean. Output voltage has modest spiking and some ringing that attenuates quickly - could be partially from the scope ground lead.
At this time i can't come up with any other moving parts that could be tweaked. Is there a fundamental problem with the layout because the schematic is practically identical to the reference? Any advice is appreciated.
Will output restart again even you keep 700mA Iout?
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In reply to Vincent Zhang(BSR):
No, it will not restart. So far i have turned the electronic load off and back on and that works. I haven't explicitly tested the necessary load reduction to restore operation. But i can do that and report back.
At Uin=20V the output crashes at ~650mA and the load needs to be reduced to 250mA befor output voltage is restored.
At Uin = 24V the crash occurs at ~750mA but the load needs to go down to 195mA before recovery. Also i noticed a curious behavior where the output voltage started rising as the load decreased but stayed well below the setpoint 5V. Once Iout reached 195mA the output voltage snapped to 5V again.
In reply to Martti Paalanen:
The phenomenon looks strange for me. Please help check:
1. Whether supply source has current limit?
2. Where is the GND copper layer poured for thermal dissipation?
3. The VIN trace is too thin?
4. Please check whether the waveform of all nodes works normal
Thank you sir! Indeed, my power supply current limit was too low! I am unable to believe i have committed such a blunder but there it was. Maybe because I normally use that particular output for low current loads and never checked, but that is a pisspoor excuse.
Anyway, problem completely solved and now the circuit behaves just as expected. Just for the record, your points 2-4 are all in order. Thermal camera shows no abnormal heating and voltage drop in the input traces is negligible. All waveforms are now ok as well.
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