[SOLVED] Can't track down SMD component

So I have this small board which has a 3-pin (SOT-23 package) on it clearly marked "MS6R3"


Should be easy to figure out what it is.... BUT after consulting many many sources on the web, I found the following :

marking of 6R could be FMMV2101 varicap diode, OR an MMBFJ112 JFET purposed as an 'analog switch'
marking of R3 could be a 2SC2351Q RF BJT
marking of MS** could be MCP112T-240E/TT which is a voltage brownout detector

None of these make sense in the circuit at hand. Full pcb view --


As you can see this connects to an antenna on the left - which is an active antenna using a BFP740 in a simple LNA config.
Also the circuit layout seems to imply that this component should be a 3-pin LDO, outputting not more than 4V, since the BFP740 optimal operation is stated to be 4V.

However I am unable to find out exactly what it is !! I have found several 3-pin sot23 LDO's, but either their marking is different, and even my expected pinout is different.

Please suggest ?

thanks !

Markings are chosen by manufacturers individually and thus not unique.

The component is clearly in the DC path, I agree about LDO or similar function. Apparently you don't have found the right device yet.

Agreed on what FvM said.
6R3 sounds like a 6.3ohm resistor value but does not make sense in respect to "antenna" port. Can you measure with multimeter?

FvM said:

Markings are chosen by manufacturers individually and thus not unique.

The component is clearly in the DC path, I agree about LDO or similar function. Apparently you don't have found the right device yet.

Click to expand...

Nope not found it yet. If not LDO, then which "similar" function would you suggest ?

Can you measure DC resistance with multimeter?

ge said:

Can you measure DC resistance with multimeter?

Click to expand...

Yes of course I can. But why would I do that for a SOT23 package ?

The Electrical properties may tell you what it is. You may need to remove the component from the board to get an accurate measurement. As example, the 51 ohm resistor would also show up in your measurement.

Resistor makes no sense in this situation. Voltage regulator or similar function is most likely, a simple thing like reversal protection diode is also possible.

Why don't you measure the electrical characteristics using a power supply and a load?

FvM said:

Resistor makes no sense in this situation. Voltage regulator or similar function is most likely, a simple thing like reversal protection diode is also possible.

Why don't you measure the electrical characteristics using a power supply and a load?

Click to expand...

I don't think it's a reverse protect diode, considering the very standard looking filtering components around it. And also the fact that it's supplying a fairly low voltage BJT in the later LNA part.

I agree that a quick check with power applied should throw light on the LDO theory. Will do it tomorrow. Thanks !

It seems confirmed that it's a 3.3V LDO, based on connecting up variable power & measuring output. Output was rock steady at 3.3V

With this info, I re-looked at several LDO's datasheet specifically for the 3.3 versions, and found two which matched. Both from Texas Instr - LM3480 & TLV760xx
The package, pin-out, and of course output voltage match !! In addition both are capable of handling inputs upto 30V !! A useful feature for equipment mounted in a car/ truck.

There are of course several other mfgs who make this, but all that I have looked at have a different pinout. (see pic below)

However what did NOT match are the package markings !! So even though I have my answer, I am still curious about the markings. Is it just a case of 'chinese' knock-off ? If so, why put different markings ? Or is there a 3rd IC with these features AND same marking ?

The two Texas IC's seem to have very similar features (quick look), but they ARE different part numbers, and DO have different markings.

Can someone dig up a third IC which has the correct marking ?

I repeat here again -

Marking : MS6R3 (see pic in 1st post)
Package : sot23
function : 3 pin fixed 3.3v LDO
pinout : as per pic below