PRODUCT SPECIFICATION ML4821
REV. 1.0.2 6/19/01 7
Figure 3. Error and Current Amplifier Configuration
Figure 4. Error Amplifier Open-loop Gain and
Phase vs. Frequency.
6
7
9
–
+
S.S
INV
+8V
V
REF
+8V
6.2kΩ
EA OUT
V
CC
= 15V
V
O
= 1.0V TO 5.0V
R
L
= 100kΩ
T
A
= 25°C
GAIN
PHASE
100
80
60
40
20
0
–20
10 100 1.0k 10k 100k 1.0M 10M
0
–30
60
90
120
150
180
f, FREQUENCY (Hz)
A
VOL
, OPEN-LOOP VOLTAGE GAIN (dB)
PHASE (DEGREES)
Figure 5. Output Saturation Voltage vs. Output Current. Figure 6. K-factor. Gain Modulator gain with respect to
the voltage at V
RMS
.
SOURCE
SATURATION
V
CC
T
A
= 25°C
T
A
= –55°C
T
A
= –55°C
T
A
= 25°C
V
CC
= 15V
80µs PULSED
LOAD
SINK
SATURATION
GND
0
.0
.0
.0
.0
.0
0
0 200 400 600 800
I
O
, OUTPUT LOAD CURRENT (mA)
0.5
0.4
0.3
0.2
0.1
0
1357460
85VAC
0.23
DESIGN
FOR
NORMAL
OPERATIONS
THIS IS THE MINIMUM
OPERATING
VOLTAGE POINT
THIS GAIN CURVE TAKES
OUT THE 1/(V
IN
)
2
DEPENDENCY OF THE
VOLTAGE CONTROL LOOP
120VAC
OPERATING BOUNDRY
220VAC
K
2
V
RMS
DESIGN
FOR
BROWNOUT
The output of the gain modulator is a current which appears
on IA+ to form the reference for the current error amplifier
and is given as:
I
GM
= K × I
SINE
× (V
EA
– 0.8)
where:
I
SINE
is the current in the dropping resistor, V
EA
is the
output of the error amplifier and K is a constant deter-
mined by the V
RMS
input.
The output current of the gain modulator is limited to:
This sets the system current limit.The multiplier output
current is converted into the reference voltage for the current
(IA) amplifier through a resistor to ground on IA+.
Figure 6 shows the gain adjustor (K) with respect to the
voltage at V
RMS
. The curve has been separated in two parts.
The right hand part is for operation under normal conditions
in the voltage range from minimum line voltage to maximum
line voltage (90VAC to 260VAC). 85VAC on the curve has
been chosen to account for tolerances. Under normal operat-
ing conditions as input voltage decreases the gain increases
compensating for the drop in the loop gain.
Under brownout conditions (below 85VAC) the gain
decreases to limit the amount of current that is drawn from
the line thus preventing an overload condition. This is a very
useful feature since in many cases the load for a PFC is a
constant power load. The input current has to go high to
compensate for a drop in the input voltage.
I
GM MAX()
2.5
R
T
--------=
