Welch Allyn Atlas Service Manual page 79

controller performs the following functions:
1. Monitors the status of the front panel ON-OFF key. If the unit is off and the ON-OFF key is pressed,
the controller will drive U10-6 high, which will enable power to the remainder of the instrument.
2. At power up, the microcontroller will drive the beeper for about 1 second.
3. At power up, the microcontroller will reset Shift Register U2. This will cause the following:
The front panel LED's are blanked.
The NIBP pump drive is placed in the off state.
4. When the unit is powered on the microcontroller will communicate with the system CPU. When the
front panel ON-OFF key is pressed, the CPU will store away present operating conditions, then issues a
command to the microcontroller to shut instrument power off. U10-6 is driven low which will remove
power from the remainder of the instrument.
1.2.2 18V boost converter and FET ON/OFF switch
The PWM controller, U301, is configured as an 18V boost-converter. An N-Channel MOSFET transistor,
Q303, is used to switch power to the 12V regulator. As Q303 is configured as a high side switch, it is
necessary to develop a gate voltage of proper magnitude to turn Q303 on. Then, to enable power to the
12V regulator, the following takes place:
1. The microcontroller drives U10-6 high, turning transistor Q302 on.
2. Transistor Q301 is switched on, supplying power to the PWM controller, U301.
3. The boost regulator develops 18VDC, which will switch transistor Q303 On.
HS301
Heatsink
for D301
F303
4Amp
6078
1
AC+12V
2
3 4
Battery
D301 C301
+
MBR2535CTL 330uF
R303
51.1K
Vbackup
R22
10K
(pg 1)
PIC-Data
On/Off-Key
(Front Panel Key)
On/Off
(pg 3)
U2-Reset
(To U2)
Appendix A
U307
8
VIN VOUT
u307-6 7
FDBK ERR
C331 3
+
SHTDN
4.7uF
6
TAP
2
SENS
4
D
GND
LP2951CD-3.3
D
+Vfuse
R305
1
Q301
MPSW51A
1 3
R301
10K C302
+
Q301-b 1uF
R302
1.96K
Q302-c
R304
100K 3
Q302-B 1 Q302
2 MMBT3904L
C339
+
1uF
C42
.047uF
D
U10
2 13
ClkIn/Osc1/RB5 RB0
3 12
Clkout/Osc2/RB4 RB1
4 11
Vpp/MCLR/RB3 RB2 PIC-Clk
5 10
TC5/TOCK1 RC0
6 9
PIC-audio
RC4 RC1
7 8
RC3 RC2
D620361-MX10000
1
VBackup
5
C332
+
4.7uF
D
On/Off Circuit
18V Boost - Mosfet Gate Voltage
U301-7
U301
MC34063AD
8 U301-sw
IDC
7
IPK
1
ISWC
U301-pwr 6
Vcc
2
ISWE
U301-ct 3 5 U301-comp
TCAP COMP
C303
470pf
10/14/99
On/Off Switch
4
2 3
V
Q303
MTP30N06VL
L301
1mH
D302
BAV99L
1 2
+18V
R306
61.9K
+ C304
R307 100uF
4.64K