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Single device ±5V Regulator/DC-DC converter from ±12V rails

S

Shahid Sheikh

Jan 1, 1970
0
Hi all,

I am looking for a single ic regulator or dc-dc converter that can
give me +5 and -5V from +12 and -12V rails and upto 250 mA of current
from Vcc to Vee. Only external components I can afford to have in my
application are caps.

Thanks,

Shahid
 
W

Walter Harley

Jan 1, 1970
0
Shahid Sheikh said:
Hi all,

I am looking for a single ic regulator or dc-dc converter that can
give me +5 and -5V from +12 and -12V rails and upto 250 mA of current
from Vcc to Vee. Only external components I can afford to have in my
application are caps.

That seems like a very peculiar requirement. Why would you be able to have
caps and not, say, resistors? If it needs to be a single device, is there
some sort of limitation on its size? I can't understand why you could have,
say, a 1"x2"x0.5" modular DC-DC converter, but you couldn't have two
SO-sized LM7805's. Can you dissipate the necessary amount of heat for a
linear regulator? If not, then you're forced to have a switch-mode
regulator of some sort, which may help you decide.
 
S

Shahid Sheikh

Jan 1, 1970
0
Walter Harley said:
That seems like a very peculiar requirement. Why would you be able to have
caps and not, say, resistors? If it needs to be a single device, is there
some sort of limitation on its size? I can't understand why you could have,
say, a 1"x2"x0.5" modular DC-DC converter, but you couldn't have two
SO-sized LM7805's. Can you dissipate the necessary amount of heat for a
linear regulator? If not, then you're forced to have a switch-mode
regulator of some sort, which may help you decide.

Main objective is to keep the cost to a bare minimum by using as few
components as possible. Space is limited to about 0.5" x 0.5".

7805 and 7815 are a very good possibility if a single package
regulator device is not available. Have plenty of copper on the PCB to
dissipate the heat from linear regulators.

Thanks,

Shahid
 
W

Walter Harley

Jan 1, 1970
0
Shahid Sheikh said:
Main objective is to keep the cost to a bare minimum by using as few
components as possible. Space is limited to about 0.5" x 0.5".

7805 and 7815 are a very good possibility if a single package
regulator device is not available. Have plenty of copper on the PCB to
dissipate the heat from linear regulators.

I am not aware of any dual +/-5v regulator chips. If they exist, I suspect
they cost enough more than a jellybean 7805 and 7905 that any additional
assembly cost will be outweighed by the savings. Maybe someone else knows
of one.

Watch out for that "have plenty of copper". If you're really sourcing 250mA
and dropping 7V with each one, then each one is dropping about 2W; what's
the thermal resistance to air of the amount of copper you can afford? If
it's 30C/W, then your boards are going to be pretty toasty - at least hot
enough that you'd better use high-temp electrolytics, thereby adding more
cost.
 
S

Shahid Sheikh

Jan 1, 1970
0
Walter Harley said:
I am not aware of any dual +/-5v regulator chips. If they exist, I suspect
they cost enough more than a jellybean 7805 and 7905 that any additional
assembly cost will be outweighed by the savings. Maybe someone else knows
of one.

Watch out for that "have plenty of copper". If you're really sourcing 250mA
and dropping 7V with each one, then each one is dropping about 2W; what's
the thermal resistance to air of the amount of copper you can afford? If
it's 30C/W, then your boards are going to be pretty toasty - at least hot
enough that you'd better use high-temp electrolytics, thereby adding more
cost.

Thanks for the info. You're right. I wasn't able to find any dual
voltage regulator like the one I needed. So I'll just go with the 7805
and 7905. 250mA is the max current of the device. Where I am using it,
it doesn't draw more than 35mA even when its working the hardest. That
is about a 1/4W dissipated through the linear regulators. Should be
okay as far as heat dissipation goes.

Thanks,

Shahid
 
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