ML4827 Fairchild, ML4827 Datasheet - Page 11

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

PRODUCT SPECIFICATION
To put some numbers into the discussion, with a given
V
1.
2. For δ = 55%: V
3. For δ = 60%: V
4. For δ = 64% (Data Sheet Lower Limit Value): V
5. For δ = 70%: V
6. For δ = 74% (Data Sheet Upper Limit Value): V
It is economically desirable to design for the lowest mean-
ingful voltage on the output MOSFET. It is simultaneously
necessary to design the circuit to operate at the lowest guar-
anteed value for δ, to ensure that the magnetics will deliver
full output power with any individual ML4827. In actual
operation, the choice of δ
REV. 1.0.1 6/27/01
BUSS(MAX)
For δ = 50%: V
(1–δ)]} x 400V = 0.50/0.50 x 400V = 400V
0.64/0.36 x 400V = 711V
0.74/0.26 x 400V = 1138V
of 400V:
RESET
RESET
RESET
RESET
MIN
= {[(1/f
= 0.55/0.45 x 400V = 489V
= 0.60/0.40 x 400V = 600V
= 0.70/0.30 x 400V = 933V
Figure 3. ML4827-PWM Duty Cycle Clamp for Voltage-Made Operation
= 60% will allow some toler-
ERROR
PWM
AMP
PWM
C RAMP2
R RAMP2
) x δ]/[(1/f
PWM
R1
RESET
RESET
) x
R FB1
R FB2
R2
=
=
V FB
RAMP2
V REF
V DC
δ MAX =
PFC V BUSS
ance for the timing capacitors and resistors. A tolerance on
(R
to achieve the desired result. This should be combined with
an external duty cycle clamp. This protects the PWM cir-
cuitry against the condition in which the output has been
shorted, and the error amplifier output (V
wise be driven to its upper rail. One method which works
well when the PWM is used in voltage mode is to limit the
maximum input to the PWM feedback voltage (V
voltage available to this pin is derived from the ML4827’s
7.5V V
RAMP2 capacitor. This will be true whether the RAMP2
capacitor is charged from V
done in voltage-mode applications, from the output of the
PFC Stage (the “feedforward” configuration). Figure 3
shows such a duty cycle clamp.
If the ML4827-2’s PWM is to be used in a current-mode
design, the PWM stage will require slope compensation.
This can be done by any of the standard industry techniques.
Note that the ramp to use for this slope compensation is the
voltage on RAMP1.
RAMP2
R 2 V REF
R 1 + R 2
REF
x C
, it will be in close ratio to the charging time of the
RAMP2
V RAMP2 (PEAK)
) of ±2% is the simplest “brute force” way
REF
, or, as is more commonly
DC
) would other-
DC
). If the
ML4827
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