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Chapter 7 Tutorial
Remote Programming
150
If no load resistor is attached to the power supply output terminal, then the
output voltage will rise linearly at a rate of C
O
/I
L
when programmed upward,
and T
R
= C
O
(E
2
-E
1
)/I
L
, the shortest possible up-programming time.
Figure 7-8. Speed of Response -Programming Down
Figure 7-8 shows that when the power supply is programmed down, the
regulator senses that the output voltage is higher than desired and turns off
the series transistors entirely. Since the control circuit can in no way cause
the series regulator transistors to conduct backwards, the output capacitor
can only be discharged through the load resistor and internal current source
(I
S
).
The output voltage decays linearly with slope of I
S
/C
O
with no load and
stops falling when it reaches the new output voltage which has been
demanded. If full load is connected, the output voltage will fall exponentially
faster.
Since up-programming speed is aided by the conduction of the series
regulating transistor, while down programming normally has no active
element aiding in the discharge of the output capacitor, laboratory power
supplies normally program upward more rapidly than downward.