I've been using the National "Simple Switchers" with good results for
awhile, and would like to take a step beyond the usual methods of setting
the output voltage. I typically use the fixed voltage versions of the LM2676
with a resistor in series with R2 to increase the output voltage when the
predetermined +5 or +12 needs adjusted. But now I'd like to control the
output voltage using a microcontroller. I'm pretty sure I could get this
done by using the adjustable version which does not have R1 or R2, and
placing a digital potentiometer in place of R1 with an appropriate value of
fixed resistor for R2. However, I have a spare channel from an offboard 5
volt D/A which would be cheaper and easier from a BOM standpoint. I was
hoping to get it done with some spare R to R opamps which are also
available.

My analog skills have gotten a little lazy over the years and I'm having a
tough time thinking this one out. I guess the bottom line is that I need to
control the feedback current from the switcher output using a 0-5v DAC
output.

Any ideas? I would tend to think this has already been done somewhere by
someone...

Chris

"Christopher Ott" chrisott at ottelectronics dot com wrote in message
...
I've been using the National "Simple Switchers" with good results for
awhile, and would like to take a step beyond the usual methods of setting
the output voltage. I typically use the fixed voltage versions of the
LM2676 with a resistor in series with R2 to increase the output voltage
when the predetermined +5 or +12 needs adjusted. But now I'd like to
control the output voltage using a microcontroller. I'm pretty sure I
could get this done by using the adjustable version which does not have R1
or R2, and placing a digital potentiometer in place of R1 with an
appropriate value of fixed resistor for R2. However, I have a spare
channel from an offboard 5 volt D/A which would be cheaper and easier from
a BOM standpoint. I was hoping to get it done with some spare R to R
opamps which are also available.

My analog skills have gotten a little lazy over the years and I'm having a
tough time thinking this one out. I guess the bottom line is that I need
to control the feedback current from the switcher output using a 0-5v DAC
output.

Any ideas? I would tend to think this has already been done somewhere by
someone...

Chris


How about an LM2636.

Bob

"sycochkn" wrote in message
m...

"Christopher Ott" chrisott at ottelectronics dot com wrote in message
...
I've been using the National "Simple Switchers" with good results for
awhile, and would like to take a step beyond the usual methods of setting
the output voltage. I typically use the fixed voltage versions of the
LM2676 with a resistor in series with R2 to increase the output voltage
when the predetermined +5 or +12 needs adjusted. But now I'd like to
control the output voltage using a microcontroller. I'm pretty sure I
could get this done by using the adjustable version which does not have
R1 or R2, and placing a digital potentiometer in place of R1 with an
appropriate value of fixed resistor for R2. However, I have a spare
channel from an offboard 5 volt D/A which would be cheaper and easier
from a BOM standpoint. I was hoping to get it done with some spare R to R
opamps which are also available.

My analog skills have gotten a little lazy over the years and I'm having
a tough time thinking this one out. I guess the bottom line is that I
need to control the feedback current from the switcher output using a
0-5v DAC output.

Any ideas? I would tend to think this has already been done somewhere by
someone...

Chris


How about an LM2636.

Bob

Looks like the LM2636 only does 3.5 volts (I'm wanting a window of approx
12v to 36v). It's also just a controller. The simple switchers are nice in
that they have integrated FET's in addition to a controller. Much easier on
the BOM and board design end.

Chris

I've used LM317 types with dac outputs. Attached is a pdf of the circuit.
This was driving a 12V lamp and was able to run it from 4v to full. Not
perfect but was good enough for the purpose of providing a stable
illumination for a measurment application. We had a power op-amp previously
that was not protected. On occasion, the lamp would short out and take out
the driver. The LM317 is very well protected and never failed in this
configuration.
Oppie

"Christopher Ott" chrisott at ottelectronics dot com wrote in message
...
I've been using the National "Simple Switchers" with good results for
awhile, and would like to take a step beyond the usual methods of setting
the output voltage. I typically use the fixed voltage versions of the
LM2676
with a resistor in series with R2 to increase the output voltage when the
predetermined +5 or +12 needs adjusted. But now I'd like to control the
output voltage using a microcontroller. I'm pretty sure I could get this
done by using the adjustable version which does not have R1 or R2, and
placing a digital potentiometer in place of R1 with an appropriate value
of
fixed resistor for R2. However, I have a spare channel from an offboard 5
volt D/A which would be cheaper and easier from a BOM standpoint. I was
hoping to get it done with some spare R to R opamps which are also
available.

My analog skills have gotten a little lazy over the years and I'm having a
tough time thinking this one out. I guess the bottom line is that I need
to
control the feedback current from the switcher output using a 0-5v DAC
output.

Any ideas? I would tend to think this has already been done somewhere by
someone...

Chris