5V buck converter like ACT4060A ?

[MathWizard]

I'm working on a treadmill control PCB, and found a buck regulator that is supposed to have a max rating of 6V on it's enable pin, but this 1 has a 10k resistor to 18V, and is showing about 7.4V on that pin. And I don't see any sign that it's switching. The output did slowly go up to 2.5V, but I'm pretty sure that was from somewhere else (haven't checked the output caps's ESR yet). And besides, the Vsense divider is set up for 5V, and so is the inductor.

Looking around, it seems it's an old or rare chip, and besides ebay/amazon, there's not much showing up for sale.

But it's 8-pin, common larger SMD size DIP, Vin 24Vmax, current mode, 2A, upto 5V output I think. IDK how common the pinout is, or how inter-changable such things really are, but the spec's must be pretty similar to lots of other chips. I can alter the circuit too. Besides the big caps, the whole circuit is on a little 3 pin standoff board.

Anyone know a similar chip ? This one used a 22uH inductor, and had 1mF caps on the input and output. Very basic circuit around it.

[DavidAlfa]

Not showing for sale? 5 for $2...
www.aliexpress.com/item/1005003113059097.html

[MathWizard]

Well I went ahead and ordered some ACT4060A off Ebay, the shipping was too much from China from the real stores I found.

With the chip back in, the circuit is making 5V ok, but I see the enable pin, is at 15V. It's pulled high, to the 18V rail from a 10k resistor. The datasheet, at least the one I found, says the pin is only rated for 6V max.

Right now across the pullup there's a 2.85V drop w/285uA current. So maybe I pop in a 62k resistor, and see if it drops to ~5V with closer to 200uA, or a 39k if the current stays about the same.



A 39k only dropped it to 12.2V, so IDK, try bigger again? Maybe it's no big deal, but 12V is still 2x what the datasheet says.

[fzabkar]

What about adding a 3K9 pulldown resistor between EN and Ground?

18V x 3.9K / (10K + 3.9K) = 5.05V

[MathWizard]

The datasheet says the pin threshold is ~1V, so I can work out what that would be. They mention a internal current of 2uA at that pin, when it's un-connected, and floating at 4.5V. So unless the the pin is doing under-voltage detection too, maybe I should just see if it really floats at 4.5V, and leave it like that.

[fzabkar]

I think the point of the ENable pin is to prevent the SMPS from switching on until its Vin has exceeded a stable threshold.

[MathWizard]

Yeah I added a voltage divider and a 10nF cap, so now it's at 5.2V.

IDK what to expect on the output ripple, the circuit is using about 100mA running a small LCD screen and some MCU type chips.

Does this look reasonable ? 36mVrms sounds ok I guess, but the peak to peak sure is high at 1V. That's running of 18V from my bench supply. I kept the original 1000uF input/output caps.

[DavidAlfa]

Set the BW to 20MHz and try again, might be noise picked by the probe.
36mV is very reasonable for a simple buck converter.