urgent , series or parallel voltage regulators

[Electro nS]

hi

i want to use LINEAR regulators (with D2pak footprint) to drop down 22-28v to 3.3v . at 250mA to 300mA MAX. SWITCHing regulators are not an option due to size , external components and price.

1- should i use LDOs ? are they better in this particular application than standard Linear regulators.

which is better using :

2- Series : 15v-5v-3.3v combo ? or 15v-6v-3.3v ...

or

3- parallel : 3x3.3v combo ?

plz advice more or less reg should be used , should diodes be added if parallel choice is made so that they donot supply current to each other .... etc ?

 

[barry]

Huh?

1)Why do you need to go through three stages to get to your desired 3.3 volts?
2) Why do you need to put anything in parallel?
3) LDOs only advantage is that they have a LOW DROPOUT. If you are going from 22 to 3, that's obviously not an issue.

 

[Electro nS]

Huh?

1)Why do you need to go through three stages to get to your desired 3.3 volts?
2) Why do you need to put anything in parallel?
3) LDOs only advantage is that they have a LOW DROPOUT. If you are going from 22 to 3, that's obviously not an issue.

thank for being "VERY SERIOUS"
about LDO , i thought they dissipate less heat because voltage drop across them is less , right ?, or is that negligable ??

and about combo: i want to play the regulators in series or parallel to reduce the power dissipation .
example P=(vin-vo)*I = (28-3.3)*0.3=7.41W which sounds high for d2pak heatsinked by 1/2inch sq area on PCB . if i can divide that on 3 regulators i think it would be bearable , right ??

does this make sense to u now my friend ??!!

 

[FvM]

Obviously the problem is power dissipation, worst case about 7.5 W.

Are you sure that you can easily hande 2.5 W per D2PAK, or do you consider SMD heatsinks?

 

[barry]

If you put 22volts into a regulator, and get 3.3 volts out, the voltage across the regulator is 18.7 volts, regardless of whether it's low dropout or not.

You need to understand the meaning of DROPOUT. It means the MINIMUM voltage between the input and output in order for it to work properly. That's it.

Do you think that 3 dpak regulators, and their required heatsinking/copper, will take up less space than ONE switching regulator? I'm not so sure.

You could also use a power resistor to drop some of the voltage, rather than MUCH more costly regulators.

 

[Electro nS]

Obviously the problem is power dissipation, worst case about 7.5 W.

Are you sure that you can easily hande 2.5 W per D2PAK, or do you consider SMD heatsinks?

how much the d2pak can handle safely and easily ? , without external heatsink ? just can u approximate.

and is series or parallel configuartion any different or both with make each device dissipate 2.5w?

 

[mtwieg]

hi

i want to use LINEAR regulators (with D2pak footprint) to drop down 22-28v to 3.3v . at 250mA to 300mA MAX. SWITCHing regulators are not an option due to size , external components and price.

I think you'll find that a SMPS will actually be smaller and maybe cheaper than a DPAK with appropriate heatsink for this application.

As an example, I found this SMPS in about a minute. It satisfied your specs, costs <$3 in quantities of 500, and takes up about 1/2" x 1/2".

 

[Audioguru]

how much the d2pak can handle safely and easily ? , without external heatsink ? just can u approximate.

and is series or parallel configuartion any different or both with make each device dissipate 2.5w?

Why not use a cheap resistor to drop most of the voltage then use a voltage regulator that will not get hot to regulate the remaining voltage?

 

[mtwieg]

A resistor doesn't work well if the circuit needs to be very compact, since its dissipation will just heat up nearby components. It only works well when the resistor is located remotely.

 

[Electro nS]

Why not use a cheap resistor to drop most of the voltage then use a voltage regulator that will not get hot to regulate the remaining voltage?

lets say i want to reduce 10-12v by a resistor , first problem is at different current consumption , different resistors should be used. second problem is 10v*300mA=3W , so i should use a 5w resistor which is huge !!

 

[crutschow]

See post #7. I also think a switching regulator makes more sense for those current and voltage requirements.

 

[Borber]

The size of above mentioned SMPS is only 12.5x13.5x3.5mm and it is many many times smaller than heatsink for 7.5W.

 

[Electro nS]

ok i am considering a "switching" drop in replacement for 78xx , i think it will solve the problem . maybe the suggested one or similar.

does these regulators work fine in circuits if the load is micro controller running at 40-70mhz , or should i worry about noise and uC reset or something ?? is it right that switching regulators work fine in digital circuits , but cause problems only in analog ??
The source is a battery and there are a couple of solenoids and relays on the battery bus . should i use big capacitors near regulator ? any advice ??