Boost Converter soft start attempt - Page 1
Forum section: electronics / projects

author: rx8pilot
author_rank: Super Contributor
timestamp: November 30, 2014, 12:15:49 am
content: First off, I am not an expert in power supply design. I have been successful with w few designs, but this one has a strange characteristic that I do not understand. It is an LM3488 based boost converter that takes 10-17v to 24 volts @ 3A. It is rather simple schematically (most of which comes from TI WebBench) and I spent quite a bit of time getting the PCB layout dialed in. The performance is "as-advertised" from TI and all seems well.....except.....The bus that powers this converter and a number of other circuits is brought up slowly as part of an inrush control scheme charging a large capacitor bank. For some reason, this circuit collapses at about 2.8v and seems to be a short - keeping the caps from charging and the bus stays low. If I just connect the circuit to a bus that is capable of taking the inrush - it starts just fine and idles at only 20ma or so. I don't have a test setup to measure what the inrush is yet.I cannot figure out why I cannot simply ramp the voltage up without it seemingly shorting out - while applying full voltage starts it up just fine. Any ideas?

author: johansen
author_rank: Super Contributor
timestamp: November 30, 2014, 12:27:28 am
content: The boost converter is probably stuck on, and waiting for the current in the inductor to reach some maximum value for volts across the current sense resistor to show up, which they won't, because you only have three volts.. which means the fet is operating in the linear region.. you may need to increase the current sense resistor such that the maximum current is below what your soft start circuitry is capable of supplying, or setting up some kind of creative soft start circuitry.. or, finding a boost controller that has a maximum duty cycle below 100%, which will allow for the output capacitor to charge up.. this particular boost controller stays on until the current sense comparator is tripped.

author: tom66
author_rank: Super Contributor
timestamp: November 30, 2014, 12:30:53 am
content: You might be able to disable the start-up of the regulator by pulling the COMP pin low via an NPN transistor or similar. Then you could start it up after the rail has reached the right voltage.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 30, 2014, 01:00:27 am
content: It's made for 3.0V+ supply, so you better use it on a system where 3.0V is available with amperes to spare.UVLO 2.85V. Don't forget to use logic level transistors with it...Why not use the classic UC3843? If the current sense resistor is a bit steep (1V peak..), you can always amplify it (PITA), add slope compensation (added slope reduces the current sense range), or use a current transformer (surprisingly practical for larger converters).Moral of the story: don't forget to do a slow supply voltage sweep too. I got bit by that once, fortunately my circuit was easy to modify.Tim

author: rx8pilot
author_rank: Super Contributor
timestamp: November 30, 2014, 03:22:29 am
content: Thank you for the responses. It seems strange that the controller would not have a UVLO set at a voltage that it can function to avoid any special tricks for a slow rising supply. The explanations make sense.I already have an MCU connection to the sync input that pulls high to disable the controller. This works just fine once the circuit is alive, but at 2.8v, the MCU is not yet awake and able to disable the boost controller. Maybe there is a solution to keep that pin high during startup although it would require a new PCB. I would love to avoid a new controller if I can - a layout do-over would be a bummer. I tweaked the noise and ripple of this one down to a very respectable level.Is it possible that merely changing the sense resistor, it may be able to deal with a slow rise on the supply?

author: johansen
author_rank: Super Contributor
timestamp: November 30, 2014, 03:51:56 am
content: Quote from: rx8pilot on November 30, 2014, 03:22:29 amyour supply isn't slowly rising, its clamped at 2.8 volts by the boost converter.if you insist on keeping the pcb you already have then you need to rework the inrush current management to provide the 5+ amps needed to trip the current sense comp.also, you need to verify that you won't damage the fet, because with only 3 volts to turn on the fet, you may find the rds on is rather high.

author: djQUAN
author_rank: Frequent Contributor
timestamp: November 30, 2014, 04:00:12 am
content: Have you tried adding a largish resistor (10K-100K) between mosfet gate to ground?It could be possible that the mosfet gate is floating when the chip is below operating voltage and a residual voltage on the gate causes it to slightly conduct?

author: rx8pilot
author_rank: Super Contributor
timestamp: November 30, 2014, 04:22:49 am
content: It seems that a PCB revision is the only right call to do it correctly. The inrush control circuit will never be able to tolerate the 5+ amps needed to get this converter started. On top of that, a few milliseconds after it starts, the MCU comes alive and shuts it back down until the user activates it.If I can get the MCU powered ahead of the LM3488, I would have the ability to pull the SHUTDOWN pin high as the rail voltage slowly rises to a usable level without being clamped.If I go all-in and choose a different controller, which one might be a better choice to deal with this scenario? I clearly did not anticipate this and did not have it on my mind when choosing the controller. I was looking for a reasonable combination of efficiency and low part count. If the device simply had a dedicated pin for shutdown/enable that was not tied to the switching frequency, this would be an easier solve. Any ideas on an alternate part that may help?

author: rx8pilot
author_rank: Super Contributor
timestamp: November 30, 2014, 04:36:11 am
content: Quote from: djQUAN on November 30, 2014, 04:00:12 amI will look at the data sheet for any clues on that idea. Not sure how the controller behaves below the 3v limit. The circuit clamps just above the gate threshold voltage (2v) and just below the operational limit of the controller (3v).

author: rx8pilot
author_rank: Super Contributor
timestamp: November 30, 2014, 08:02:09 am
content: I think this is solved....more study of the data sheet and some head scratching turned into an easy solve adding only two components.Pin 7 on the LM3488 is a multi-purpose pin that controls the switching frequency as well as shutdown. Putting DC >1.4v for more than 30us will stop the converter. I added a resister/zener fed from Vin that ensures the pin is above 1.4 volts when the ramps to more than 2.85v (the under-voltage lock out). This was the voltage was clamped while the converter was at 100% duty cycle - Thank you johansen and for pointing that out.I tested the modification with a slowly ramping voltage on power up and all is good. The output capacitors are gently charged to the input voltage (minus the diode drop) to reduce the inrush when the controller starts. When it reaches 11v, the MCU will allow the user to enable it. The best news is that the existing PCB layout is super easy to rev, nothing major.Thank you all for the information. I am very thankful for this resource.