FS8S0765RCB
external sync signal. The internal sync comparator detects
the sync signal and determines the SenseFET turn-on time.
The SenseFET is turned on at the negative edge of the sync
comparator output. The reference voltage of the sync com-
parator is an inverted saw tooth with a base frequency of
20kHz and a varying range between 5.8V and 7.2V, as
shown in the figure 8. The inverted saw tooth reference gets
rid of the excessive switching noise at the first synchronized
turn-on. The external sync signal is recommended to have an
amplitude higher than 4.2V.
Vref
sumption in standby mode, it is recommended to set the
value of Vcc during normal operation as high as possible
(about 29V).
V
5V
3V
Vsync
V
Vfb
SYNC COMP.
Css
5
7.2V
1V
V
External
Sync
Rss
D3
5.8V
4
Cfb
2.5R
OSC
PWM COMP.
D
S
Vds
2uA
0.9mA
R
GATE
DRIVER
V
Vcc
Voffset
FS8S0765RCB
Vcc
15V
12V
11V
9V
Figure 8. The circuit for the synchronization with external
sync
Section 4
Section I
Section 2
Section 3
Section 5
Section 6
time
6. Burst mode operation :
In order to minimize the power
dissipation at standby mode, FS8S0765RCB has a burst
mode operation. In burst mode, the FPS reduces the effective
switching frequency and output voltage. The FPS enters into
burst mode when the voltage of the soft start pin is higher
than 3V, no sync signal is applied and the feedback voltage is
lower than 1V. During the burst mode operation, Vcc is hys-
teresis controlled between 11V and 12V. Once the FPS
enters into burst mode, it stops switching operation until Vcc
drops to 11V. When Vcc reaches 11V, the FPS starts switch-
ing with switching frequency of 40kHz and peak MOSFET
current of 0.6 A until Vcc reaches 12V. When Vcc reaches
12V, the switching operation is terminated again until Vcc
reduces to 11V. Figure 9 shows operating waveforms. The
soft start during the initial start-up is shown in the section 1.
During this period, there is no external sync signal and the
switching frequency is 20kHz. The section 2 represents the
normal mode operation. The switching frequency is synchro-
nized with the external sync signal. In the section 3, the
external sync signal is removed. However, the load still
exists and thus the feedback voltage (Vfb) is higher than 1V.
In this period, the FPS does the normal switching operation
with switching frequency of 20kHz. The section 4 and 5
show the burst mode operation. At the end of the section 3,
the load is eliminated and the feedback voltage (Vfb) drops
below 1V forcing the FPS to stop switching operation. Dur-
ing the section 4, Vcc goes down to 11V. During section 5,
Vcc is hysteresis controlled between 11V and 12V. When the
external sync signal is applied on the pin 5, the FPS resumes
its normal operation. In order to minimize the power con-
Figure 9. The operation of the FS8S0765RCB at the nor-
mal mode and the off mode
13
FAIRCHILD [ FAIRCHILD SEMICONDUCTOR ]
