ml4841 Fairchild Semiconductor, ml4841 Datasheet - Page 11

• Manufacturer Part Number: ml4841
• Description: Ml4841 Variable Feedforward Pfc/pwm Controller Combo
• Manufacturer: Fairchild Semiconductor

PRODUCT SPECIFICATION
The ML4841 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-
lation is determined during the ON time of the switch. Figure
4 shows a typical trailing edge control scheme.
REV. 1.0.3 6/13/01
+
DC
VIN
OSC
REF
L1
I1
U4
+
–
EA
RAMP
CLK
U3
Figure 4. Typical Trailing Edge Control Scheme
SW2
SW1
+
–
U1
I2
C1
I4
I3
R
D
DFF
CLK
U2
RL
Q
Q
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
output voltage, the switch will be turned ON. The effective
duty-cycle of the leading edge modulation is determined
during the OFF time of the switch. Figure 5 shows a leading
edge control scheme.
One of the advantages of this control teccnique 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 synchro-
nized switching, the ripple voltage of the first stage is
reduced. Calculation and evaluation have shown that the
120Hz component of the PFC’s output ripple voltage can be
reduced by as much as 30% using this method.
RAMP
VSW1
VEAO
TIME
TIME
ML4841
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