Welch Allyn Atlas Service Manual page 82

F301
4Amp
Bat-fuse
Battery
F302
4Amp
F302-2
AC+12V
L306
C340 Bead
.047uF
R318
10K
U303-fb
R319
19.6K U303
UC3843A
3
VFB
U303-cp 1
COMP
14
VREF
C315 R320
.047uF 7.5K 7
RT/CT ISENSE
C316
2200pF
U303-ct
ACON
Ref-Batcharge
1.5 Linear Regulators – 5VDC and 3.3VDC
1.5.1 5V regulator
A series pass 5V regulator is built using transistor Q312 and op amp U305A. The reference for the
regulator is the 5V reference from the PWM control IC U302. The output of the 5V regulator is turned off
when the unit is turned off. When the 18V-boost circuit is shut down, power is removed to U302, and then
the U302 reference is driven to 0V. Setting the reference to 0V will cause op amp U305 to turn the series
pass transistor Q312 off.
When operating from AC, transistor Q310 is turned on, which will turn transistor Q311 off. Power to the
5V regulator is then provided through Diode D310 from the output of the battery charger circuit. Note that
this voltage tap is before the current sense resistor, then load current on +5V does not affect the battery
charger current limit circuit. When AC is removed, the AC-On signal goes low, and Q310 turns off. The
gate of Q311 is then pulled up to 11.4V (12V – Vdiode). Transistor Q311 then turns on, and the 5V
regulator is powered from the battery.
1.5.2 3.3V Regulator
3.3VDC is derived using a three terminal regulator. The output of the 5V regulator is used to power the
3.3V regulator. The 3.3V regulator does not have an independent shutdown, but powers down as the 5V
regulator shuts down.
Appendix A
Battery
Charger
Q308
MTP23P06V
AC+12V-fuse 3 2 Q308-d
4
C337 C318
+
.047uF 330uF
Q308-g D306
MBRS340T3
R326
Q307 10
MMBT3904L
3 2 Q307-e
D305
C314 BAV99L
.047uF
R325 3
1.96K
Q306-c
R323 1.96K 3
10 1 Q306
OUT
2 MMBT3904L
5 U303-5
R324
Current Limit
10K
(1.5 Amp)
R321 D308 BAV99L
27.4K
2 1
R322
10K
Batt-fb
R340
10K
U304-7
Float Voltage
L304
47uH R327
0.1
C319-1
C319 C338
+
1000uF .047uF
R328
1K
U305-6
R330
33.2K
U305-5
C320 R332
.01uF 19.6K
U304-2
Ref-Batcharge
2 R333
1 19.6K
3 U304-3
U304A
MC34072 C322
.047uF
AC+12V-fuse
R339 C321 R334
19.6K .01uF 19.6K
R339-2
Ref-Batcharge
R335
4.32K
6 U304-6
7
5 U304-5
U304B R336
MC34072 7.5K
R337
8.87K
AC+12V-fuse
Bat-Adj
R338
2
1K
10/14/99
To Linear Regulators
D307
Input to 5V and
MBRS340T3
3.3V linear
regulators.
D307-A
R329
1K
R331
33.2K
6
7
5
U305B
MC34072
U305-pwr
+12V
R341
10K
R342
2 19.6K
Q309 1
MMBT3906L 3
Q309-3
RT301
10K
R344
10K 3
Q310-b 1 Q310
ACON
2 MMBT3904L
R345
10K
R343
10K
R343-1
D309
BAV70L