Page 374 - Complete Wireless Design

Support Circuit Design

Support Circuit Design 373

increasing or decreasing the pulse’s duty cycle into this LC filter, the average
DC output voltage will increase or decrease. D protects Q from the inductive
1 1
kickback of the stored charge within L , which would normally produce a very
1
high spike of voltage into the emitter during switching. The voltage divider of
R and R program the desired output voltage of the switching regulator by
1 2
using the voltage dropped across R as a comparison to the zener’s reference
2
voltage, which then turns on or off the comparator, and thus controls the VCO,
all depending on the voltage level across R . Most of the above circuit is found
2
in IC form, with built-in current-limiting and thermal protection.
A widely used integrated version, with typical support components, is dis-
played in Fig. 8.49. This is a common regulator arrangement in many produc-
tion switching-mode power supplies. As above, R and R program the desired
1 2
output voltage, with the voltage across R being fed back by the feedback line
2
to the internal comparator circuits. The low-pass filter is composed of L and
1
C , with D shunting the inductive kickback of a discharging L to ground
OUT 1 1
(when the IC switches off), instead of across the IC. C is used to give the reg-
1
ulator stability at higher current draws. Further filtering at the regulator’s
output may be required if undesirable ripple amplitudes are still present.
8.7.3 Regulator design
Three-terminal regulators are the easiest type of regulator to design and
implement, and are also the most prevalent. The only parts needed for a

Figure 8.48 Switching regulator operation.

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