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Series/Parallel LDO or Zener + LDO?

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dknguyen

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So I have a 10-36V source, and require a 5V and 15V output. A switching regulator conditions the source to 15V. It's big and expensive and because of the current levels involved (unregulated, these currents don't pass through the regulators), stacked boards are involved and it just makes board organization restrictive, and the project already requires small size, so a second switching regulator is not possible.

I have to step down the 15V from the switching regulator down to 5V. Let's say 500mA maximum (probably 200mA average. I can either use a 10V+5V LDO in series, two 5V LDOs in parallel, or use use a zener to chop off 5V and let the LDO chop off the remaining 5V. Using just one cuts too close to the thermal limit.

Any ideas which is best? I think the zener is cheapest but I can't find a large enough zener that can handle 500mA with dropping out. I think the series LDOs might have lower noise than the parallel ones, but I don't really know. But the problem with using series/parallel LDOs is that the 15V input cuts pretty close (less than 1V) or if you look at another spec, it's dead on the maximum input rating.

The LDO I am using is from Ti, the Reg103 fixed voltage version.

EDIT: JUst noticed no 10V version of the regulator exist so if I use two series regulators, I'll have to find another first stage LDO which may have a higher max input voltage.
 
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audioguru

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An LDO regulator is Low DropOut. A 6V input for a 5V output.
You have a 15V input so you don't need a LDO regulator. An ordinary regulator that needs a minimum input voltage of 7.5V will be fine.

A zener diode wastes a lot of power by making a lot of heat. The huge resistor that feeds the zener diode and also feeds the load wastes more power by making more heat.
 

Sceadwian

Banned
Isn't that all linear regulators do to regulate the voltage audioguru? Generate heat.. Zeners don't regulate very well with load changes
 

Oznog

Active Member
Linear regulators (and zeners, for that matter) CANNOT be put in parallel. It won't work. One will usually end up taking all the current and overheat before the other even switches on.

Zeners aren't great. They don't usually come in great heat-dissipating packages like the TO220 you'd find a regulator in. A zener capable of dissipating several watts is a pretty special part.

A 10V reg can be made with an adjustable reg. Also you can use a configuration with an external transistor which can handle a lot of heat. I recommend doing that.
 

audioguru

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Most Helpful Member
A linear regulator heats up when the load current is high. It is cold when the load current is low.

A zener diode heats up when the load current is low.
The resistor that feeds the zener diode and feeds the load heats up when the load current is low, medium and high. A complete waste of power that makes lots of heat.

Linear regulators do not change their voltage when the input voltage changes, when the load current changes or when their temperature changes.
Some zener diodes regulate very poorly when anything changes.
Low voltage and high voltage zener diodes regulate very poorly.
 

Hero999

Banned
Sorry I don't think I fully understand the question.

Whats wrong with a switching regulator?

A SEPIC is perfect if you want a positive output that's higher or lower than the input voltage and a buck boost is good is you're not bothered about the output being negitive.

There again you probably already know all of the above.
 
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