Re: Using a uC for DC-DC conversion?

From: Joerg (notthisjoergsch_at_removethispacbell.net)
Date: 10/13/04 

Date: Wed, 13 Oct 2004 21:20:46 GMT

Hi Dave,

>I used the Atmel Tiny-26 running on internal 8 MHz R/C osc, with internal
>PLL giving me 64 MHz to the timer, which resulted in 500kHz and 7 bits
>resolution, as the controller for a buck mode constant current source for a
>NIMH charger. The T26 also managed all the charging with a fairly
>complicated algorithm, and communicates to the host through a clocked serial
>interface telling it what's going on with the battery.
>
>
That's the king size edition. I never had anything that could run above
12MHz. Everything else was out of the $$$ range.

> The T26 has eight channels, 10 bit resolution.
>
>I corrected the current every mS based on the ADC reading of a shunt
>resistor.
>
>
This would be the leather seats that I usually can't use. Any uC with a
decent ADC on board I saw was over $2 in qties.

>The T26 also has a programmable gain amp for doing this.
>One bad thing, it's internal "precision" 2.5V ref ends up at about 2.7V
>I measured input voltage, battery voltage, battery current (high side)
>battery temperature, and ambient temperature.
>
>
Mostly I rely on the TLV431, LMV431 and the like for around 20 cents a pop.

>>Then there is the issue of making the code that runs the PWM safe and fast
>>enough. After all, one minor hangup in this area could result in a plume
>>of smoke.
>>
>>
>
>That's where I've had the least problems. (none!) ...
>
Probably you were the HW designer and also did the firmware. Then it's
under your control. It can become challenging when you are asked to join
the uC party as an additional tenant.

>... The batteries themselves gave me the most grief. NEVER buy cheap Chinese NIMH cells! I spec'd Sanyo
>or Panasonic, and that's what they ended up buying, but only after some interesting developments.
>
>
It's not just batteries, electrolytic capacitors can also put on quite a
pyrotechnic show. Wonder where fireworks were invented ;-)

>>Next, there is the trend to ever lower VCC levels which renders the task
>>of turning on a FET hard enough non-trivial.
>>
>>
>
>There, I had to use a driver chip, the old NPN to ground with a pullup just
>couldn't take the fast switching, but I needed to keep the L small, or I
>would have gone to 100kHz and had more resolution, and probably not needed
>the driver.
>
>
You can prevent saturation on the ol' bipolar transistors with a
Schottky or other pulldown when the collector goes below base. But the
beta still puts a damper on things if you want to do the whole switching
with that one transistor.

Regards, Joerg

http://www.analogconsultants.com

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