Hi everyone,

I need to regulate a 12V supply to 2.5V @ 400mA. Using a traditional linear regulator will make it too hot. Using a buck switcher will make the duty cycle too low, which may cause instability.

What is a good way to do this? I am thinking of using 2 regulators to step down the voltage, but it will be costly.

Is there any switcher out there that perform well under high input voltage and low output voltage/current?

Thank you in advance!
James

 

If you use linear regulators, you will be dissipating 3.8 Watts in the regulators and 1W in the load. Not very efficient.

What makes you think that the duty cycle will be too low with a switcher?

You'll have more noise on the output, but even that can be further reduced by adding a pi filter.

 

With a buck regulator, you can make the duty cycle anything you want it to be. What is the problem?

 

If the duty cycle is higher (the inductor will be on longer), the voltage output will be higher. Is it not?

I tried a Ricoh switcher and found that the duty cycle is about 15% when I draw 300mA from the 2.5V rail (with 12V input).

The datasheet states that when the duty cycle is below 35%, it will alter from PWM mode to VFM mode (pulse skip mode?). I think pulse skip mode will introduce more noise into the system.
James

 

Depends entirely on which chip you choose.

 

Try using National Semiconductor's Webbench tool on their website.
In just a couple minutes, it came up with a design for a 94% efficient buck regulator using an LM3100 and 10 external components; six capacitors, three resistors and an inductor (I manually adjusted efficiency to maximum)