# what happen with the fix output voltage adm7171 ?

Hi all,

I used the the both model, the fix(3.3V and 5.0V) and adjustable output voltage with the same footprint, the enable signal in common. but, the adjustable one worked, the other didn't. the adjust one give the expected output while with the fix, the output is equal to the input voltage (6.0V). I tried both case, do not place the feedback resistor and do place the fb resistor. they are the same results, the output is equal to the input. so, anyone can help me, plz.?

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• Hi Quyen,

The formula for the output voltage in adjustable mode is: VOUT = VREF x (1 + R1/R2). When using the adjustable model, the VREF = 1.2V, therefore the formula becomes: VOUT = 1.2V x (1 + R1/R2).

The fixed model you used, the 5V model (ADM7171ACPZ-5-R7), has a VREF = 5V. When you use the 5V version in adjustable mode (with feedback resistors), the formula becomes (see below example): The output will be different when you use a different model on the same footprint. What likely happened is you exceeded the set output voltage when you replaced the adjustable model with a fixed model (since the VREF becomes larger).

When you use the same footprint and remove the installed resistors, is the SENSE pin float? If so, that may be the explanation why the VOUT ≅ VIN since it will not regulate. The same outcome happens when you short SENSE pin to GND. But if you short the SENSE pin to the VOUT pin, you get the designated fixed output (i.e. VOUT = 5V for ADM7171ACPZ-5-R7).

• Hi Quyen,

The formula for the output voltage in adjustable mode is: VOUT = VREF x (1 + R1/R2). When using the adjustable model, the VREF = 1.2V, therefore the formula becomes: VOUT = 1.2V x (1 + R1/R2).

The fixed model you used, the 5V model (ADM7171ACPZ-5-R7), has a VREF = 5V. When you use the 5V version in adjustable mode (with feedback resistors), the formula becomes (see below example): The output will be different when you use a different model on the same footprint. What likely happened is you exceeded the set output voltage when you replaced the adjustable model with a fixed model (since the VREF becomes larger).

When you use the same footprint and remove the installed resistors, is the SENSE pin float? If so, that may be the explanation why the VOUT ≅ VIN since it will not regulate. The same outcome happens when you short SENSE pin to GND. But if you short the SENSE pin to the VOUT pin, you get the designated fixed output (i.e. VOUT = 5V for ADM7171ACPZ-5-R7).

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