PRODUCT SPECIFICATION
ML4824
ML4824’s voltage error amplifier has a specially shaped
nonlinearity such that under steady-state operating condi-
tions the transconductance of the error amplifier is at a local
minimum. Rapid perturbations in line or load conditions will
cause the input to the voltage error amplifier (V
FB
) to devi-
ate from its 2.5V (nominal) value. If this happens, the
transconductance of the voltage error amplifier will increase
significantly, as shown in the Typical Performance Charac-
teristics. This raises the gain-bandwidth product of the volt-
age loop, resulting in a much more rapid voltage loop
response to such perturbations than would occur with a con-
ventional linear gain characteristic.
The current amplifier compensation is similar to that of the
voltage error amplifier with the exception of the choice of
crossover frequency. The crossover frequency of the current
amplifier should be at least 10 times that of the voltage
amplifier, to prevent interaction with the voltage loop. It
should also be limited to less than 1/6th that of the switching
frequency, e.g. 16.7kHz for a 100kHz switching frequency.
There is a modest degree of gain contouring applied to the
transfer characteristic of the current error amplifier, to
increase its speed of response to current-loop perturbations.
However, the boost inductor will usually be the dominant
factor in overall current loop response. Therefore, this con-
touring is significantly less marked than that of the voltage
error amplifier. This is illustrated in the Typical Performance
Characteristics.
For more information on compensating the current and
voltage control loops, see Application Notes 33 and 34.
Application Note 16 also contains valuable information for
the design of this class of PFC.
Oscillator (RAMP 1)
EXAMPLE:
For the application circuit shown in the data sheet, with the
oscillator running at:
1
f
OSC
=
100kHz
=
---------------
-
t
RAMP
t
RAMP
=
C
T
×
R
T
×
0.51
=
1
×
10
–
5
Solving for R
T
x C
T
yields 2 x 10
-4
. Selecting standard
components values, C
T
= 470pF, and R
T
= 41.2kΩ.
The deadtime of the oscillator adds to the Maximum PWM
Duty Cycle (it is an input to the Duty Cycle Limiter). With
zero oscillator deadtime, the Maximum PWM Duty Cycle is
typically 45%. In many applications, care should be taken
that C
T
not be made so large as to extend the Maximum
Duty Cycle beyond 50%. This can be accomplished by using
a stable 470pF capacitor for C
T
.
PWM SECTION
Pulse Width Modulator
The oscillator frequency is determined by the values of R
T
and C
T
, which determine the ramp and off-time of the
oscillator output clock:
1
-
f
OSC
= --------------------------------------------------
t
RAMP
+
t
DEADTIME
(2)
The deadtime of the oscillator is derived from the following
equation:
V
REF
–
1.25
-
t
RAMP
=
C
T
×
R
T
×
In
-------------------------------
V
REF
–
3.75
(3)
The PWM section of the ML4824 is straightforward, but
there are several points which should be noted. Foremost
among these is its inherent synchronization to the PFC
section of the device, from which it also derives its basic
timing (at the PFC frequency in the ML4824-1, and at twice
the PFC frequency in the ML4824-2). The PWM is capable
of current-mode or voltage mode operation. In current-mode
applications, the PWM ramp (RAMP 2) is usually derived
directly from a current sensing resistor or current trans-
former in the primary of the output stage, and is thereby
representative of the current flowing in the converter’s output
stage. DC I
LIMIT
, which provides cycle-by-cycle current
limiting, is typically connected to RAMP 2 in such applica-
tions. For voltage-mode operation or certain specialized
applications, RAMP 2 can be connected to a separate RC
timing network to generate a voltage ramp against which
V
DC
will be compared. Under these conditions, the use of
voltage feedforward from the PFC buss can assist in line
regulation accuracy and response. As in current mode
operation, the DC I
LIMIT
input is used for output stage
overcurrent protection.
No voltage error amplifier is included in the PWM stage of
the ML4824, as this function is generally performed on the
output side of the PWM’s isolation boundary. To facilitate
the design of optocoupler feedback circuitry, an offset has
been built into the PWM’s RAMP 2 input which allows V
DC
to command a zero percent duty cycle for input voltages
below 1.25V.
PWM Current Limit
The DC I
LIMIT
pin is a direct input to the cycle-by-cycle
at V
REF
= 7.5V:
t
RAMP
=
C
T
×
R
T
×
0.51
The deadtime of the oscillator may be determined using:
2.5V
-
t
DEADTIME
= -----------------
×
C
T
=
490
×
C
T
5.1mA
(4)
The deadtime is so small (t
RAMP
>> t
DEADTIME
) that the
operating frequency can typically be approximated by:
1
f
OSC
= ---------------
-
t
RAMP
REV. 1.0.6 11/7/03
(5)
current limiter for the PWM section. Should the input
voltage at this pin ever exceed 1V, the output of the PWM
will be disabled until the output flip-flop is reset by the clock
pulse at the start of the next PWM power cycle.
9
FAIRCHILD [ FAIRCHILD SEMICONDUCTOR ]
