Re: Need explantion of part of schematic
- From: "Ban" <bansuri@xxxxxx>
- Date: Fri, 08 Apr 2005 03:15:32 GMT
Lessie wrote:
> The schematic concerned is at the end of the microchip
> document on microchip serial programming
> http://ww1.microchip.com/downloads/en/AppNotes/91013b.pdf
>
> The circuit generates VPP for the programming phase. D1 is
> the reference of 12.7V and Q2 forms a negative feedback
> with the opamp. This part I do understand. However, Q2 is
> strange, what does it do? It seems that it will be in a
> cut out state because base will be higher than emitter by
> 0.6V due to Q2 ?
Q1 is there to discharge the 12.7V line capacitance when Vpp_in goes low. In
programming operation it is isolated as you have rightly understood. But
when Vpp_out stays high, when the next instruction is send, it will be
misinterpreted as a programming bit, so the line has to be forced low within
one clock cycle and Q1 guarantees that.
--
ciao Ban
Bordighera, Italy
.
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