Sony RM-PJM610, Service Manual

Page: 75 / 177

3-22
VPL-PX20/PX30
3-12-7. GB, GC, and GD Boards
1. Structure
The GB, GC, and GD boards are each composed of the DC-DC converter for step-down voltage as
shown below.
GB board : 5.9V
±
0.3V
→
3.3V
±
0.3V
GC board : 9.0V
±
2.0V
→
5.0V
±
0.3V
GD board : 17.5V
±
0.4V
→
10.6V
±
0.3V
2. Operations of the DC-DC Converter
As the structure of each DC-DC converter is more or less the same, the basic circuit operations are
described here.
Fig. 6 shows the DC-DC converter control circuit structure. This converter adopts the step-down
chopper method in which the input voltage is directly switched by two MOS-FETs.
In moderate loads, the DC-DC converter sets into the idling mode and skips most of the internal
oscillator pulses to decrease the switching frequency and reduce MOS-FET switching losses.
In heavy loads, the converter controls the MOS-FET pulse width used for switching, and turns ON
the High side MOS-FET only for the time determined by the duty ratio.
When the MOS-FET at the High side goes OFF, the Low side MOS-FET turns ON after 60 ns. The
Low side MOS-FET maintains the ON state until the current flowing to the L inductor zero-crosses.
The aim of these operations is to attain high efficiency in both moderate and heavy loads.
+5V LIHEAR
REGULATOR
SHDH
V+
OUT
PW
LOGIC
TO
CSL
4.5V
SECF B
REF
+5V AT 5mA
MAIN
OUTPUT L
C
GND
SHDN
SS
1V
4V
+2.505V
AT 100 A
ON/OFF
SYNC
ADJ FB
FB
CSL
CSH
P GND
DL
LX
DH
BST
VL
5V FB
3.3V FB
BATTERY VOLTAGE
High side MOS-FET
Low side MOS-FET
+2.505V
REF
LPF
60kHz
PWN
CONPARATOR
Fig. 6 DC-DC Converter Control Circuit