ML4827 Fairchild, ML4827 Datasheet - Page 13

• Manufacturer Part Number: ML4827
• Description: Fault-Protected PFC and PWM Controller Combo
• Manufacturer: Fairchild

PRODUCT SPECIFICATION
The ML4827 should be locally bypassed with a 10nF and a
1µF ceramic capacitor. In most applications, an electrolytic
capacitor of between 100µF and 330µF is also required
across the part, both for filtering and as part of the start-up
bootstrap circuitry.
Leading/Trailing Modulation
Conventional Pulse Width Modulation (PWM) techniques
employ trailing edge modulation in which the switch will
turn on right after the trailing edge of the system clock. The
error amplifier output voltage is then compared with the
modulating ramp. When the modulating ramp reaches the
level of the error amplifier output voltage, the switch will be
turned OFF. When the switch is ON, the inductor current will
ramp up. The effective duty cycle of the trailing edge modu-
REV. 1.0.1 6/27/01
+
DC
+
DC
VIN
VIN
REF
OSC
OSC
REF
L1
I1
I1
U4
L1
U4
+
–
+
–
EA
RAMP
CLK
EA
RAMP
CLK
U3
U3
Figure 6. Typical Leading Edge Control Scheme
Figure 5. Typical Trailing Edge Control Scheme
VEAO
SW2
SW2
SW1
SW1
+
–
+
–
CMP
U1
U1
I2
I2
I4
I4
I3
C1
C1
I3
D
R
R
D
DFF
DFF
CLK
CLK
U2
U2
RL
RL
Q
Q
Q
Q
lation is determined during the ON time of the switch. Figure
5 shows a typical trailing edge control scheme.
In the case of leading edge modulation, the switch is turned
OFF right at the leading edge of the system clock. When the
modulating ramp reaches the level of the error amplifier out-
put voltage, the switch will be turned ON. The effective
duty-cycle of the leading edge modulation is determined dur-
ing the OFF time of the switch. Figure 6 shows a leading
edge control scheme.
One of the advantages of this control technique is that it
requires only one system clock. Switch 1 (SW1) turns off
and switch 2 (SW2) turns on at the same instant to minimize
the momentary “no-load” period, thus lowering ripple volt-
age generated by the switching action. With such
RAMP
VSW1
RAMP
VSW1
VEAO
VEAO
TIME
TIME
TIME
TIME
ML4827
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