SX1308 boost converter, strange behaviour on feedback pin? - Page 1
Forum section: electronics / projects

author: TERRA Operative
author_rank: Super Contributor
timestamp: August 03, 2023, 01:03:13 am
content: Hi all.I've built up an LED backlight driver for my Tek THS720 scopes to replace the standard CCFL and driver.I used the SX1308 boost converter as per this post here:https://www.eevblog.com/forum/testgear/tektronix-ths7xxx-scope-ccfl-controller-drop-in-replacement-for-led-conversion/Now, with my design, I am getting some weird stuff happening....When I adjust the trimpot, it varies the output voltage from the expected 12.7V at full scale down to 9.8V at roughly half scale, but won't drop the voltage any further regardless of where the trimpot is set between half and minimum adjustment..The weird thing is, when I try to probe the pin to check voltages/noise/etc with my multimeter or scope, the circuit starts behaving normally.Upon further inspection, I found that simply touching anywhere around the feedback pin/resistor divider even with a meter probe that is not plugged in to anything causes the circuit to work properly...I suspect the additional capacitance has something to do with it maybe?..Is there any boost converter guru's who can take a look at my design linked below and see what I'm missing with my design?

author: uer166
author_rank: Super Contributor
timestamp: August 03, 2023, 01:10:15 am
content: Layout wise, your switch node is very large and well coupled to the sensitive feedback node. It's possible that it's enough to make the control loop unstable until you add some 10s of pF of capacitance to filter that out.To fix this particular kind of problem, keep the feedback away from switch node, maybe pour a ground between it and the FB trace, and place the resistive divider as close to the FB pin as possible.

author: TERRA Operative
author_rank: Super Contributor
timestamp: August 03, 2023, 01:16:46 am
content: Yeah, I just thought of that now as I was looking at it actually, I'll add a ground pour between the switch node and voltage divider.

author: ArdWar
author_rank: Frequent Contributor
timestamp: August 03, 2023, 09:00:34 am
content: You're also sacrificing your GND connection by making SW node way too wide. Actually, Isn't your GND basically ends up as a very long and thin trace there? It's usually the other way around for switchers. GND-CINloop as short as possible, GND trace as wide as possible and SW as small as possible.

author: mikerj
author_rank: Super Contributor
timestamp: August 03, 2023, 11:07:11 am
content: You may find a low value (10's of pF) feed forward capacitor in parallel with R3 will help with stability, and it would be easy to tack a cap on top of R3. Measuring the load step response of the regulator would be a good way of optimising the value, but you will need a working scope to do this!TI have a pretty good app note on the role of feed forward caps in switching regulator design:https://www.ti.com/lit/an/slva289b/slva289b.pdf?ts=1691025975803

author: TERRA Operative
author_rank: Super Contributor
timestamp: August 03, 2023, 11:13:13 am
content: Cool, I'm working on an improved layout, but will try the feed-forward cap idea on the PCB I have here.No problems with scopes, I have them coming out my ears all the way up to 2GHz.

author: TERRA Operative
author_rank: Super Contributor
timestamp: August 03, 2023, 03:00:18 pm
content: Ok, I've had a play with the layout, Got the feedback stuff scrunched up closer and isolated the switching node from the feedback with some ground pour and via stitching.I also added a feed-forward capacitor in the feedback loop that can be added if needed after testing.I'll have to borrow/buy/build a transient load tester to test stuff properly, but I've found a few designs around the place. TI have a design using 555 IC's that looks easy to build...https://www.ti.com/lit/an/snoa507/snoa507.pdf?ts=1691068745714

author: ArdWar
author_rank: Frequent Contributor
timestamp: August 03, 2023, 05:43:57 pm
content: Maybe rotate R4 90 degree CW parallel to pot R5 so it does not "choke" the ground plane and pick up "noisy" ground there. Put GND vias on the now vacated area.Put R6 to the right of pot R5 to further reduce clutter on GND plane (not strictly needed but good practices nonetheless).Rotate C1 90 degree CCW and put it below the polyfuse so it does not filter through what's basically an almost isolated ground island.IDK if there's ringing problem in your design, but I'd usually put a small C0G cap between Vin and GND very close to the switchers just between it and inductor.Also, that EN trace very near mounting hole makes me sweat a bit

author: TERRA Operative
author_rank: Super Contributor
timestamp: August 04, 2023, 02:00:23 pm
content: Ok, had another go at it, let me know what you think.The trace near the mounting hole is not so bad, it simply sits on a post with no screw etc, and there's some clearance between the trace and hole.I added the cap on the input, and also one on the output side too, just in case..I forgot to move R6, but can do that next anyway.

author: TERRA Operative
author_rank: Super Contributor
timestamp: August 09, 2023, 01:45:17 pm
content: I think I have a final-ish revision.I moved the main caps well away from the feedback loop and minimised the loop length. Plus a little optimising and tidying up of placement.