Re: schottky tempco

On Dec 12, 4:55 pm, gearhead <nos...@xxxxxxxxxxxx> wrote:

I need a diode with a low voltage drop and a known temperature
coefficient.
I'm going to put it in series with the adjust terminal of a lm317 in
constant-current
configuration, so the voltage with respect to ground at the output pin
of the 317 will be 1.2 volts plus the voltage drop ...
circuit. Since I need about 2000 ppm/C, or maybe a little more, a
"regular" silicon diode won't work. It would have 2.2 mV/C

Any diode will have about .003 * Vf tempco; Shottky diodes
have lower Vf, so lower tempco. If you need something
higher than a silicon diode, use two silicon diodes in series, OR
make a difference with some fixed voltage, as

(A+ B*T ) - C = result

so as to subtract from the non-temp-dependent A term; B can be
any fraction of the result that you want, just chose C correctly...

Schottky diodes aren't specified for their operating conditions under
modest currents, so you'll get nothing from their data sheets. Use
a transistor instead as a planar diode (connect B and C together)
for temperature sensing.

I think if you make the LM317 current source with a fixed resistor,
and
feed the output current into an R + diode series pair, you'll find
the tempco you want is achievable that way.
.

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