FSDL0365 Fairchild Semiconductor, FSDL0365 Datasheet - Page 12

• Manufacturer Part Number: FSDL0365
• Description: Green Mode Fairchild Power Switch (FPSTM)
• Manufacturer: Fairchild Semiconductor

FSDL0165RN
6. Burst operation :
standby mode, the FPS
As the load decreases, the feedback voltage decreases. As shown in
figure 10, the device automatically enters burst mode when the
feedback voltage drops below V
tinues but the current limit is set to a fixed limit internally to mini-
mize flux density in the transformer. The fixed current limit is
larger than that defined by Vfb = V
driven down further.
voltage drops below V
stops and the output voltages start to drop at a rate dependent
on the standby current load. This causes the feedback volt-
age to rise. Once it passes
The feedback
mode operation alternately enables and disables switching of
the power Sense FET thereby reducing switching loss in
Standby mode.
12
FB
3
I
delay
0.3/0.5V
S W IT C H
Vcc
0.5V
voltage then falls and the process repeats. Burst
O F F
Figure 8. Soft Start Function
Figure 9. Circuit for Burst operation
Vcc
+
-
Switching continues until
BURL
In order to minimize power
I
FB
TM
5 V
(300mV). At
2.5R
V
enters burst mode
I_ o v e r
BURH
BURH
R
Normal
BURH
(500mV) switching resumes.
(500mV). Switching still con-
and therefore, Vfb is
this point switching
R s e n s e
Burst
D R A I N
D R A I N
D R A I N
D R A I N
G N D
G N D
G N D
G N D
Vcc
operation.
the feedback
I
dissipation in
B_PEAK
MOSFET
MOSFET
MOSFET
MOSFET
Current
Current
Current
Current
PWM
7. Frequency Modulation : EMI reduction can be accom-
plished by modulating the switching frequency of a switched
power supply. Frequency modulation can reduce EMI by
spreading the energy over a wider frequency range than the
band width measured by the EMI test equipment. The
amount of EMI reduction is directly related to the depth of
the reference frequency. As can be seen in Figure 11, the fre-
quency changes from 65KHz to 69KHz in 4mS for the
FSDM0265RN. Frequency modulation allows the use of a
cost effective inductor instead of an AC input mode choke to
satisfy the requirements of world wide EMI limits.
waveform
0.5V
0.3V
Current
Feedback
Figure 11. Frequency Modulation Waveform
D rain to
S ource
D rain to
current
S ourc e
voltage
Figure 10. Circuit for Burst Operation
Burst Operation
T urn-on
Switching OFF
O scillator
Internal
W aveform
65kH z
V ds
69kH z
67kH z
69kH z
Burst Operation
Switching OFF
T urn-off
point
4k H z
Normal Operation