Needed: 12-24V to 5V regulator @ 60-100mA

[Piotr Biernat]

Hi!

I need a power supply for a circuit that is supposed to work in a car/bus.
Requirements are as above - from 12V or 24V regulate 5V @ 60-100mA. (I
already know what to do with load dump, reverse polarity etc., so this
topic is not relevant now).

I'm just wondering if it is still ok to use linear regulator for such a
thing - quick calculation gives (24V - 5V) * 100mA = 1.9W of power to
dissipate. Quite a lot. 5V/24V = 20% efficiency.

On the other hand, most easy-to-get switching regulators (e.g. National's
SimpleSwitcher) seem optimized for higher currents, so efficiency is not so
good at lower current. I've found some chips from Linear, like LT3470 or
LT1394, but they seem hard to get here (unless obtained directly from
Linear's page).

Also I'm just wondering about the switching regulator reliability for such
an application. Maybe it's just my lack of experience with switching
supplies, but I "trust" LDO linear regs a bit more

Any suggestions welcome

BR,
Przemyslaw


--

 

[linnix]

Hi!

I need a power supply for a circuit that is supposed to work in a car/bus.
Requirements are as above - from 12V or 24V regulate 5V @ 60-100mA. (I
already know what to do with load dump, reverse polarity etc., so this
topic is not relevant now).

I'm just wondering if it is still ok to use linear regulator for such a
thing - quick calculation gives (24V - 5V) * 100mA = 1.9W of power to
dissipate. Quite a lot. 5V/24V = 20% efficiency.

On the other hand, most easy-to-get switching regulators (e.g. National's
SimpleSwitcher) seem optimized for higher currents, so efficiency is not so
good at lower current. I've found some chips from Linear, like LT3470 or
LT1394, but they seem hard to get here (unless obtained directly from
Linear's page).

Linear for car perhaps, but not bus. Even a less efficient switcher is
better than linear. My guess is more than 50% for 24V @ 100mA.

Also I'm just wondering about the switching regulator reliability for such
an application. Maybe it's just my lack of experience with switching
supplies, but I "trust" LDO linear regs a bit more

It depends on the designer/manufacturer.

 

[Piotr Biernat]

At those current levels, a switcher using an MC34063 is suitable. I use
them a
lot at that sort of current level. In fact, most of the el cheapo mobile
phone
in-car chargers use it. DIP-8 chip, one diode, four resistors, a cap and
an
inductor. Pretty simple, schematic in the data sheet.

Looks nice, dirt cheap. MC33063 is probably better for my needs, as it is
supposed to operate from -40 celsius. Gonna give it a try probably, thanx !

BR,
Przemyslaw

 

[budgie]

Hi!

I need a power supply for a circuit that is supposed to work in a car/bus.
Requirements are as above - from 12V or 24V regulate 5V @ 60-100mA. (I
already know what to do with load dump, reverse polarity etc., so this
topic is not relevant now).

I'm just wondering if it is still ok to use linear regulator for such a
thing - quick calculation gives (24V - 5V) * 100mA = 1.9W of power to
dissipate. Quite a lot. 5V/24V = 20% efficiency.

On the other hand, most easy-to-get switching regulators (e.g. National's
SimpleSwitcher) seem optimized for higher currents, so efficiency is not so
good at lower current. I've found some chips from Linear, like LT3470 or
LT1394, but they seem hard to get here (unless obtained directly from
Linear's page).

Also I'm just wondering about the switching regulator reliability for such
an application. Maybe it's just my lack of experience with switching
supplies, but I "trust" LDO linear regs a bit more

At those current levels, a switcher using an MC34063 is suitable. I use them a
lot at that sort of current level. In fact, most of the el cheapo mobile phone
in-car chargers use it. DIP-8 chip, one diode, four resistors, a cap and an
inductor. Pretty simple, schematic in the data sheet.

 

[jasen]

Hi!

I need a power supply for a circuit that is supposed to work in a car/bus.
Requirements are as above - from 12V or 24V regulate 5V @ 60-100mA. (I
already know what to do with load dump, reverse polarity etc., so this
topic is not relevant now).

car's aren't nice 12V supplies, your finished device should be able to
withstand various fault conditions

eg: a load dump where the battery goes flat and then opem-circuit,
and everything takes a hit from an altenator with the field current wound
all the way up, upwards of 80V can be developed, this'll destroy lm317 and
7805 regulators.

I'm just wondering if it is still ok to use linear regulator for such a
thing - quick calculation gives (24V - 5V) * 100mA = 1.9W of power to
dissipate. Quite a lot. 5V/24V = 20% efficiency.

auto electrics are often horribly inneficient 20% is no big deal,

that bus will be closer to 27V typical by the way.

On the other hand, most easy-to-get switching regulators (e.g. National's
SimpleSwitcher) seem optimized for higher currents, so efficiency is not so
good at lower current. I've found some chips from Linear, like LT3470 or
LT1394, but they seem hard to get here (unless obtained directly from
Linear's page).

Also I'm just wondering about the switching regulator reliability for such
an application. Maybe it's just my lack of experience with switching
supplies, but I "trust" LDO linear regs a bit more

Any suggestions welcome

the commercial units I've seen used switchers of some type

if you only need one it's easiest to just buy a "cigar-lighter to usb" adaptor.