What could cause MAX17552 to switch the LX pin only at 8% duty cycle when the FB pin reads 100mV?

Question

I have small auxiliary power supply around a MAX17552 DCDC IC.

Pins:

- IN connected to 25V

- EN/UVLO connected to 25V

- RT/SYNC pulled to ground by 68kohm (reads 350mV)

- SS open

- MODE open

- 330pF capacitor on RESET

- VOUT grounded

- 100uH inductor connected to LX, the output capacitor is 10uF. (LX on-time appears to be 0.5uns, the off-time is 6.5 us)

- GND grounded

- FB gets the output voltage divided by 220K and 33K (reads 100mV)

And here comes the problem:

Even though the FB pin reads 100mV, the LX pin is only switching at 8% so the output votlage is only around 800 mV, instead of the expected 6.1V. What can cause this? Is there maybe a current limit on the LX pin?

jc2048 · Answer

LX on-time appears to be 0.5uns, the off-time is 6.5 us That would imply an output of 1.8V. Are you sure it isn't also doing some pulse skipping? From the datasheet, it looks as if your problem may relate to having selected the mode as PFM (MODE pin open) with a low value inductor. All their typical application examples with coils less than 150uH are with PWM operation. The only ones using the PFM mode (the ones marked 'high efficiency') have inductors higher than that. Depending what your board layout is like, you may be able to test that by grounding the MODE pin (before you apply power - it says not to change mode on the fly) to set PWM operation and see if it springs into life and gives you the 6.1V. I think you're right about the reason. It sounds (from a quick read, so don't rely on this) as if in PFM mode there's a current limit and with a small inductor it hits it too early and never gets the output up to the operating voltage. The trade-off that they are giving you here with the choice of modes is that if you want the higher efficiency, you need a larger inductor and consequentially the footprint is larger.

tomattila · Answer

Finally I decided to completely take apart the board. Sure enough there is nothing wrong with the MAX17552 circuit. There is a 1EDN7512G gate driver on the board and the official eagle library from Infineon is wrong. The pins are all mixed up so the 1EDN7512G shorts the output of the MAX17552 buck. As soon as I removed it, everything started to work. The morale of the story is that you should never trust a footprint library you download. Thanks for all the help.

genebren · Answer

Greetings,

Without seeing the schematic (yes, your description is fairly clear), or knowing the test conditions (load or no load), it is difficult to tell what is going wrong. As best I can tell, it seems that you have things connected correctly (it almost matches the example in the specification). What load are you testing at? The LX pin is limited to 0.21A. Did you verify the DC resistance and saturation current ratings of your inductor?

Good luck,

Gene

tomattila · Answer

It doesn't seem to pulse skip at all. I wish I could ground the MODE pin but it is too small for hand soldering. I didn't expect any trouble when I designed the circuit. I replaced the inductor with 220uH. Sadly the behavior hasn't changed which is funny because now the circuit is basically the same as Maxim's reference design. It must be a small stupid mistake but I can't find it. This is what the design looks like: https://ibb.co/ich62o https://ibb.co/ghjx8T https://ibb.co/ciJPoT

jc2048 · Answer

Has the voltage at the FB pin increased or is it still down at 100mV? The first of those links doesn't work for me (presumably it's the schematic). It looks to me like you need at least 180-190uH to be under the current limit (with Vin of 25V) by the time the minimum period is up, so 220uH should just about clear it (though given a fairly wide tolerance on the coil and particularly if the input voltage could go higher than 25V you might want to review that). Have you looked at their dev kit for the part. They use a 220uH part there. https://datasheets.maximintegrated.com/en/ds/MAX17552ATBEVKIT.pdf It might be worthwhile your looking at their board layout.

What could cause MAX17552 to switch the LX pin only at 8% duty cycle when the FB pin reads 100mV?

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