FSDH321_04 FAIRCHILD [Fairchild Semiconductor], FSDH321_04 Datasheet - Page 12

• Manufacturer Part Number: FSDH321_04
• Description: Green Mode Fairchild Power Switch
• Manufacturer: FAIRCHILD [Fairchild Semiconductor]

FSDH321, FSDL321
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
0.35V/0.5V
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
0.5V
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
(350mV). 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 97KHz to 100KHz (from 48.5KHz to
51.5KHz ; FSDL321)in 4mS for the FSDH321. 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.
Figure 11. Frequency Modulation Waveform for FSDH321
waveform
0.5V
0.3V
Current
0.5V
0.35V
Feedback
Figure 10. Circuit for Burst Operation
Drain to
Drain to
Drain to
Drain to
voltage
voltage
Burst Operation
Source
Source
Switching OFF
Turn-on
Turn-on
Oscillator
Oscillator
Internal
Internal
Waveform
Waveform
97kHz
97kHz
Vds
Vds
103kHz
103kHz
100kHz
100kHz
103kHz
103kHz
Burst Operation
Switching OFF
Turn-off
Turn-off
6kHz
6kHz
Normal Operation