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Problem with building a step up with variable Vout for MAX1709

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federico.disante

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I have a problem building a step up with variable Vout.

I first build the circuit with a fixed Vout of 5V, and the IC works perfectly.

If I try to build a step up with two resistor as described in the data sheet (pages 9
and 10, fig. 4) the Vout is only about .1 - .2 V over the input voltage, and follow the inout voltage variation. If I change the function mode to output 5V the circuit again works correctly!

What's wrong? I connected the 3.3/5 to ground and the FB to the center point of the partitor resistor: I choose R4=22k and R3 varying from 50k and 70k (to get a Vout beetwen 4V and 5V).

There is anyone that used the 1709 with a Vout different fromt 3.3V or 5V?

Thank you in advance. Best regards,
Federico
 

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