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> I am looking for a cost sensitive (that is, cheap in very high volume
> manufacturing) DC/DC converter to create a low current, not regulated
> bias voltage in a circuit that already contains a PIC.
>
> The circuit should output 11 to 17 VDC, non reugulated and load current
> of no more than 3 to 5ma. The PIC is running on 4 to 5VDC but an higher
> voltage of 5 to 6 volt may also be available in the circuit.
>
> The circuit can use PIC outputs if needed (e.g. a continious square wave
> from a PWM) but should not require continious software operation since
> the PIC is already running a tight timing software.

Your subject line is your answer.
A 2 stage capacitor pump will do this.

Typical cct attached.
Drawn unusually as I was being lazy. Rearrange to get standard cct.

PWM is a square wave from a pin able to source/sink the desired current.
Caps 0.1 uF monolithic ceramics to start (cheap)
Higher freq - more power.
1N4148 diodes (cheap)
Higher the Vdd the better - if you have >5v then use it.
V gain per stage about Vpic_vdd - 1.2v.
eg at Vdd PIC = 5v it will add 2 x (5-1.2) = 7.6v.
If Vdd on diagram is 5v you get 12.6v out O/C and less under load. More
swing and more Vindc gives more Vout.

If you find the PIC doesn't provide enough drive current you can buffer
output with 2 x emitter followers - NO resistors needed and a cost of cents.

Schottky diode (eg BAT85) add about 0.6v per stage but cost far more.
_______________

If you want to you can do this with 1 x transistor, 1 x tiny inductor, 1 x
resistor, 1 x diode, 1 x output reservoir cap, 1 x output clamp zener (as
output will ring veeery high with inadequate load). Energy level
controllable with PWM mark space and frequency.  Arguable as to which is
cheaper.

The very enthused will replace the transistor and resistor with a PIC open
collector output (RA4). Beware exceeding max allowable voltage for this pin.
One diode, one inductor, 1 output cap. Cheap enough :-) ? Inductor here (and
above) may go to a supply above PIC Vdd (eg 6 volts).




        Russell McMahon


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