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Hi

I am working on a tracking system for vehicles (both cars and trucks). The system power supply is the vehicle's battery (which supplies 12V or 24, for cars and trucks respectively), which may have many fluctuations (spikes, negative voltages, noise, transient voltages, etc.). The tracking system needs 5V and 500mA constant (it may peak at 700mA), which means a regulator circuit from the vehicle battery (12-24V) to 5V (500mA~700mA) is necessary. Price is an important factor, it should be as cheap as possible.

I have designed the following circuit to deal with the voltage regulation:

What do you think?

Do you have a different suggestion? I am open to different circuits (specially more simple ones), which regulate the voltage to 5V and protects from the voltage irregularities in the vehicle's battery.

Thanks in advance. If you need to know anything else let me know.

Looks OK but the chip you've chosen has a low (50kHz) switching frequency so the inductor will be quite large.

As far as I can tell from the datasheet, it's a voltage mode (VM) switcher and has no feedforward compensation (FFC). Without FFC, VM converters are not generally as good as current mode (CM) converters in line regulation.

If you use ceramic caps, use X7R dielectric not Y5V. Y5V has awful temperature coefficient and capacitance drops off steeply with voltage. X5R might be OK.

Thanks for your feedback and advice on the caps.

In your opinion, which chip (circuit) should I use?

A couple of other things:

Don't forget that the vehicle supply can drop way below the nominal voltage during startup, particularly in very cold or hot conditions.

Make sure you design the inductor to cope with the temperature range of the application. Both core and copper losses increase with temperature which heats up the core even more.

You may want to rethink the whole design and use a chip and proven designs, specified and tested to survive in automotive electrical environments.

You see when the starter motor and solenoid kick out, you have several Henries carrying 300 amps discharging through a spark gap. The voltage transients can be frightful, if the car battery or cables or connections have some ESR. The window motors and door locks pass fewer amps, but through thinner wires, so those can make some dandy transients also. More than enough to fry your diode, Zener, and IC.

Also any switching regulator is going, without extensive and expensive bypassing and shielding, going to put out warbly harmonics across the AM band. Are you assuming everybody is going to be listening to FM, iPod, or satellite radio?

Also unless you use a $1.50 toroid instead of a 19-cent bar inductor, the regulator is going to be spewing out all sorts of magnetic spikes into nearby wiring.

Sorry to be such a stick-in-the-mud, but autos and regulators are really hard to prove out in simulation. No simulator that I know of adequately models all the incoming and outgoing EMI in a car.

Thanks everyone for your help. I have read and looked into everyone's advice and I will get back to each of you tomorrow (I'm sorry, today was really busy).

After your ideas I have decided to go for a regulator designed for automotive use, and I found the MIC2941AWU (http://www.farnell.com/datasheets/29446.pdf) which is said to be "ideally suited for automotive applications for a variety of reasons", such as a wide range of inputs (4V - 60V), deals with reverse battery, and load dump conditions (positive transients up to 60V), etc.

As an automotive environment may have positive transient voltages higher than 100V, I was thinking about adding a TVS diode to the suggested application design in MIC2941AWU's datasheet and that would do it.

What do you think about this strategy?

Do a little mental math. An inductor carrying 200 amps at 12 volts when open circuited, will push how many amps into 120 volts? About 20. Urp. What's the size and cost of a surge suppressor that can repeatedly take that kind of jolt? The answer might astound you. Just like you shouldn't take a manicure scissors to a gunfight, don't take a 1N34 to a car's electrical system.