“31WOC” on a boost converter, but what datasheet goes with it?

Question

Long shot, I know...

I have this Boost convert 1-5V in out 5.1-5.2V and I hope to modify it for a different output voltage.

So I'm trying to identify and find the datasheet for the main chip.

I think the chip reads 31WOC, but I can't find a datasheet for that number. Anyone familiar what the type is of the chip?

There is no more datasheet than the following:

IN 1-5V
OUT 5.1-5.2 
Current: Rated 600mA (single lithium input), the maximum 700mA (single lithium input)

€ 1,50

Answer 1

The 31WOC is probably not the actual part number, rather some type of code described in the datasheet for different variants of the chip.

I think I would forget about what the actual part number is, and just locate the feedback pin and adjust the resistor divider as necessary (assuming it's an adjustable regulator). Most use an internal reference of ~1.2V. Look for the pin with this voltage on, attached to the centre of a resistor divider.
If it's a fixed regulator then it's a little more tricky, you will probably have to experiment with a few resistor values from the feedback pin to ground (start with 1-10k?). You will only be able to raise the output voltage this way.

EDIT - on the picture you have added, it looks like the divider is the two resistors at the top right marked "18C" (15k?) and "473" (47k). Measure these resistors, and also the voltage at the pin they are connected to (top right pin of IC). The output voltage formula should be 1.24V * ((Rtop/Rbottom) + 1) which in your case is 1.24V * ((47k / 15k) + 1) = 5.12V, which is around what you are seeing.
Adjust as desired, but be aware if you are increasing the output, that the chip may not be rated for voltages much above it's current level.

Answer 2

What Oli said looks very good.
Comment on voltage may not fully apply - see below - but is irrelevant if you are reducing voltage.

The SOT23? device labelled 141M is almost certainly a MOSFET which does the actual inductor drive and which is driven by the IC.
Lower pin as shown is source, upper is gate - connected to IC and middle pin in drain - ie std MOSFET pinout. It looks like they MAY have a high side current sense resistor - the R100 = 0.1 Ohm? just to right of MOSFET. This is unusual. It just may be in series with output to limit FET current but that would also be unusual.

The SS22 labelled device is almost certainly a Schottky diode acting as the main output rectifier. SS22 data sheet here - 2A 14V RMS. Usually they use an SS12 1A diode. They may have had a few zillion 2A devices to use up - 1A is usually ample for this sort of device.

The 31WoC is probably a batch code. Alibaba sellers often list these as if they were part numbers but I could not see it listed.

The diagram below is not for your specific IC but shows a typical arrangement which will be close to what you have. Yours seems to use all 6 pins which is, again, unusual, as most functions can be accomodated with 5 pins. Top circuit is with MOSFET (as in your example) and the bottom circuit is for internal main-FET versions.

Normal practice is to feed the IC Vdd from the output (so supply voltage is bootstrapped) if Vin is low and from Vin if Vin_min is say 3 Volts. Startup is usually down to 0.8V to 1.0V range but they tend not to pull the skin off a rice pudding at these voltages and may not start under very heavy static load.

As you are reducing Vout and not changing Vin the arrangement of Vdd that exists now should be appropriate. If you had been changing to a much higher Vout it may have been desirable to change where Vdd is driven from.

The high side resistor from Vout to feedback pin usually benefits from a small capacitor in parallel with it. This can be thought of either as coupling Vout changes to the feedback pin directly without division or as wreaking dark magic on the overall transfer function to assist stability. Either view works. It is not obvious from the diagram whether such a capacitor is present and if not it may not be needed but bear it in mind.

The 4R7 labelled inductor MAY be 4.7 uH but these ICs tend to be not terribly high frequency (100 kHz range typically) and a value of 47 uH would be more normal.