Texas Instruments 99/4A - Page 93

PG HER SUPPLY BOARD
G ENERAL D E E CRIPTION
The US power supply consists of an external wall transfcr^er and a
regulator board inside the Home Computer. Connection between the
console and wall transformer is made by a 4 pin connector. The re 
gulator board supplies the 99/4A three regulated voltages: + 12 „ -i-s
and — 5 volt. The + 12 and -5 volt supplies uses a normal three pin
Fixed regulator end the +5 volt supply uses a switching regulator
c i r c u 11 «
SWITCHING REGULATOR
The switching regulator used is a ua 72-3 C. A schematic diagram is
given in Figure 49, p!29. In essence the ua 723 is a normal series
regulator, but it is connected in such a wav that it behaves like
a switching regulator. From the block diagram it can be seen that
the regulator consists of a compensated voltage reference. and
error amplifier and a current limiting circuit.
HARDWARE DESCRIPTION +12 VOLT SUPPLY
The + 12 volt regulator circuit shares the rectifier circuit with
the + 5 volt regulator. This circuit uses an input Filter consis 
ting oF LI, L2, Cl, C2, C3 and C4. The Filter is used to reduce RF
interference on the mains. DI, D2, D3 and D4 compose the brides
rectifier. C9 and C17 are used to reduce the ripple. The DC re-
voltage is fed to the three pin fixed regulator UI. which regul 
ates the input voltage down to + 12 volt. CIO is used to reduce
voltage swing due to fast changing 1 oad conditions.
-5 VOLT SUPPLY
The -5 volt supply uses a separate winding on the tranFormer. L3
together with C5 and C6 compose an RF Filter. The AC voltage is
rectified by D5, C8 reduces the ripple on the output of the
rectifier. U2 is a 5 volt Fixed regulator. C7 on the output
reduces voltage swing due to Fast changing load conditions.
+5 VOLT SUPPLY
REFERENCE VOLTAGE
The reference voltage for the regulator is derived from the +12
volt. RI, D7 and zener diode DC compose a 7 volt reference. D7
is used to compensate for temperature variations. C12 is used to
reduce noise. The 7 volt reference is divided by Rll, R15 and R16
through R19 v RIS through R19 are supplied with jumper wires which
can be cut in such a way that the voltage on the non-inverting
input of comparator U4 is 5 volts. Since this comparator has an
open collector output, R12 is used as a pull-up. R14 connected be 
tween the output and the inverting input (pin 2) sets the voltage
gain to 1. R13 provides a small positive feedback. Since m this
configuration the comparator behaves as a unity gain amp 1 ifier ,
the voltage on the inverting input (pm 2) is equal to that of the
divider. This voltage — is used as a reference for the switching
regulator U3. CIS is used to ensure a slow starting up of the -^-5
volt regulator; thus the starting condition that the -5 volt is
applied First to the circuitry inside the 99/4A is met. CIS else
determines the frequency st which the -5 volt regulator switches.
34