# DC to DC conversion Help

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#### picstudent

##### New Member
Hello

I need to measure a DC voltage that varies from +55V to +75V approximately.
I can do it using a PIC Micro. But I need to derive the +5V DC from the voltage I need to measure.

Which is the best method to derive +5v from this source. Linear supply using 78xx reg will generate heat at this input output difference?

I need to run a PIC micro and three seven segments digits from this supply. Maximum 500ma or something altogether.

Thanks

Roy

#### MikeMl

##### Well-Known Member
Can the Pic Vss be connected to the minus side of the +55 to+75V supply?

The Pic A/D converter can strapped to be ratiometric w.r.t. it's own Vdd, or it can be fed from an external reference. You have two options: regulate the PIC's Vdd to sufficient accuracy, or dont worry about the exact PIC Vdd, and buy a voltage reference chip.

As to the efficiency question, use a "buck" switcher. Look at National's or Linear Technology's chip solutions, or buy a ready made one. Any decent switcher will have an efficiency of at least 85%.

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#### picstudent

##### New Member
Can the Pic Vss be connected to the minus side of the +55 to+75V supply?
Yes, Ground can be (is) common. The input to my system will be a ~ + 60Vdc and Ground. For reference Voltage I can use 2.5V or 2.048V ref chips.(2.5V is Very cheep and available locally) But my main requirement is the information to derive Vdd for PIC using a DC to DC converter. +5 Volt @ 500mA from +60vdc and Ground.

As to the efficiency question, use a "buck" switcher. Look at National's or Linear Technology's chip solutions, or buy a ready made one. Any decent switcher will have an efficiency of at least 85%.
To be frank, slight lower percentage of efficiency is not a problem in my case. But I need to avoid this heating up 78XX. I am interested in learning and implimenting a swicher based power supply. I think that is what we call by dc to dc converter. right?.(Correct me if I am wrong)

looking for app notes or example designs.

Thanks a lot

regards
Roy

#### picstudent

##### New Member
LM5085 seems to be little bit costly. Any other low cost solutions?

Thanks

#### picstudent

##### New Member
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