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.