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# STBB3J Feedback Voltage Resistors

Associate II

Hello, I'm new here. I'm doing a project with STBB3J components and I believe I didn't correctly understand how to calculate the resistors that control the output voltage. I followed the example of the same application circuit, which is this (page 4 technical sheet):

Datasheet: STBB3J.fm

And to configure the output voltage close to 3.3V. I used this equation (page. 18 datasheet):

Considering Vfb = 0.5V, R1 = 150K, and Vout = 3.3V, I have R2 to be approximately 26.8K. When I assemble this set of resistors, the output voltage is approximately 0.6V. As I am using the same application circuit, I believe I did not correctly understand the calculation of the resistors. If anyone could help me I would be grateful.

1 ACCEPTED SOLUTION

Accepted Solutions
ST Employee

Many thanks for the success story!

Regarding the capacitors, section 8.3 of the data sheet also mentions that ceramic caps with at least 10µF should be used. Although the data sheet also uses 4x10µF each, it should be noted that it was also created in 2013, when larger capacitance values were even rarer and/or more expensive. In the meantime, there is nothing to be said against a single ceramic cap on each input and output of e.g. 47µF if its ESR is small enough.

Regards
/Peter

7 REPLIES 7
ST Employee

Welcome @Maihac, to the community!

You have calculated the resistors correctly, but this alone is no guarantee of a functioning converter. How are EN and MODE/SYNC connected and what are the values of the passive components, i.e. coil and capacitors?

The layout also plays an important role and it would be useful to add it here for checking.

Regards
/Peter

Associate II

ok, below is the schematic and layout.

ST Employee

Schematic and layout look good.

Now the type of the 10µH coil L2 would be important. However, the value of the saturation current of L2 and the flowing load current are also sufficient for me to assess the suitability of L2.

Associate II

Ok. I was wondering if it would be possible to connect the MODE/SYNC pin directly to GND or if I would need a resistor.

As for the inductor, I'm using this model "MWSA0402S-100MT Sunlord | C408339 - LCSC Electronics", the typical saturation current is 2A. I can't say what type of coil it is, just that it is a power inductor.

Other details.... I had this board manufactured/welded in a specialized company... I bought 3 boards and they all have the same problem. I've already tested the possibility of shorts, measured the resistance of all the resistors to check, and everything is fine. The output voltage is locked at 0.6V. I also checked that it wasn't a lack of current at the power input, but everything is normal, to test and understand the problem I'm putting a 5V 3A source in the VBAT.
I also checked that there wasn't a short in the rest of my circuit, but between 3.3V and GND it is giving more than 80K of resistance. And for information, my circuit does not consume more than 0.5A.

One detail that I didn't understand is how the datasheet indicates that Vfb is constant at 0.5V and that it varies with the output voltage.

ST Employee

The coil should still be sufficient for the output current, but according to the data sheet your specified 2A is the typical value, for worst case scenarios I would use the maximum value of 1.6A - which should actually be sufficient with a load of 0.5A. For test purposes, however, it may be useful to use a coil with a larger isat, e.g. two MWSA0402S-4R7MT connected in series, which then offer 9.4µH and 2.4A.

You can also do the following to be on the safe side:

• double-check orientation of U52 (dot on package at marking of the board)
• startup according to the data sheet, Fig. 23, by briefly connecting pin E4 (EN, R43.1) to GND and releasing it again, while recording the signals at SW1 (D1...D3, L1.1) and SW1 (B1...B3, L1.2) with an oscilloscope, if possible also the current curve in L2.

Associate II

I found the problem.

The component I bought is the STBB3JCC, which has a feedback voltage of 0.1V and I was looking at the STBB3JC datasheet, which has a feedback voltage of 0.5V.

I will produce a new board with the resistors already corrected, and with the inductor recommended in the XFL4020-152 datasheet.

I would just like to know if there is any difference between using a 47uF capacitor instead of using 4 10uF capacitors in the input voltage and output voltage?

ST Employee

Many thanks for the success story!

Regarding the capacitors, section 8.3 of the data sheet also mentions that ceramic caps with at least 10µF should be used. Although the data sheet also uses 4x10µF each, it should be noted that it was also created in 2013, when larger capacitance values were even rarer and/or more expensive. In the meantime, there is nothing to be said against a single ceramic cap on each input and output of e.g. 47µF if its ESR is small enough.

Regards
/Peter