LDQ40PUR Adjustable Output Setting

[JoshuaT]

I am evaluating the LDQ40PUR (adjustable version) and there is some ambiguity in the datasheet for setting the output voltage.

The datasheet states:

The resistor divider should be selected as follows: VOUT = (1+R1/R2)*VADJ, considering IADJ negligible.

 

What is VADJ? Should this be VREF (1.2V)?

 

I assumed VADJ = VREF = 1.2V, and calculated my output with the following values:

R1 = 340kOhm

R2 = 102kOhm

VOUT = (1+340k/102k)*1.2V = 5V

 

However, when I built the circuit with these values, I am not getting this value at the VOUT pin.

 

When VIN is 3.6V or higher, I get VOUT = VIN (e.g. VIN=7V, VOUT=7V)

 

Here is the schematic for my circuit:

Schematic for LDQ40PUR test circuit

 

[Peter BENSCH]

Welcome ​@JoshuaT, to the community!

Your schematic is correct; with 340k+102k, your output voltage should be 5.2V when the input is 5.7V. In the worst-case scenario at 250mA, there could be a drop-out of up to 1V, meaning 6.2V would be required.

Are you sure you haven’t accidentally soldered the device on rotated by 180°?
What voltage do you measure at pin 5 when there is 7V at the input?

Regards
/Peter

[JoshuaT]

Hi,

There are some mistakes in my math in my post, I am actually targeting 5.2V as you deduced.

 

With 7V at pin 1, I see 3.87V on pin 5. 

I have it soldered as follows:

Is this the correct orientation?

[TDK]

If you’re getting the same output voltage as input voltage, there is a short somewhere and/or it’s being backpowered. At a minimum, you should see a different between them of the dropout voltage. Perhaps you could measure them more precisely.

Is there any load on the output? Consider setting a load of at least 10 mA or so.

[Peter BENSCH]

​@JoshuaT Something seems to be wrong with your setup and/or measurements: even if the 7V reach the output unchanged (as ​@TDK says, the cause must be found), then the divided voltage should be present at pin 5, i.e. 102*7V/(102+340) ~ 1.62V.

 

[JoshuaT]

Hi Peter,

 

The part must have been either damaged or not soldered properly. I guess I am not able to evaluate it in the DFN package.

 

Do you know if there is any availability for this part in the SOT23 package in quantities less than 3000?

[Peter BENSCH]

Well, in theory, the DFN6 (2x2) should be possible to solder by hand, but it requires an exceptionally steady hand, as well as plenty of experience in handling such packages with a 0.65 mm pitch.

The automotive variant, the LDQ40MRY in an SOT23-5L package, is indeed available in the eStore in quantities of 500; however, this is not a technical issue that should be discussed within the community, but rather a temporary availability issue that must always be clarified with the relevant distributor.

Good luck!

Regards
/Peter