Step-up switching regulator

[ericmar]

step-up switching regulator

Hi,

May I know if it is harmful for the input voltage supplied to a step-up switching regulator to be higher than the step-up voltage at the output?

In fact, I found a sentence in the datasheet of a step-up regulator (MAX668) stating that "regulation cannot be maintained if V_IN exceeds V_OUT". What does this actually mean?

If it is and there is no way to lower down the input voltage, then should I use a zener diode to clamp down the input voltage? Would this waste the power on the supplying power?

Please advise. Thanks.

Regards,
Eric

 

[VVV]

A step-up regulator will always attempt to produce an output voltage that is HIGHER than the input. That is why it is called "step-up".
However, if you set the output voltage to a certain value and somehow the input voltage increases above that value, then the regulator cannot maintain regulation and the output voltage will closely follow the input voltage (usually minus one diode drop).

For example, if you have a 6 to 12V step-up, it will produce 12V until the input reaches about 12.5V. After that point, if the input increases further, the regulator cannot do anything to lower the output voltage (we established that it is a step-up).

Generally the input should not exceed the output for a step-up. That is why you use a step-up, because you need the output to be higher than the input.
Using a Zener is not a good choice, because it will wast a lot of power.

If the input can exceed the output, you should use a buck-boost regulator, which will maintain regulation for input voltages both below and above the set output. Search for example the Linear Technology website for buck-boost regulators.

 

[ericmar]

Hi VVV,

Thanks for your reply. I know buck-boost regulator or SEPIC configuration can take in input lower or higher than the output voltage but I just cant get it work somehow.

I tried design SEPIC configuration twice with 2 different switching regulator, they just dont work so I kinda give up and try the step-up and step-down method.

Thanks.

Regards,
Eric