ML4821
100
80
60
40
20
0
0
INV
7
V
V
= 15V
CC
O
L
= 1.0V TO 5.0V
–30
60
R
= 100kΩ
GAIN
T = 25°C
A
+8V
–
+
90
EA OUT
6
PHASE
+8V
120
150
V
REF
S.S
9
6.2kΩ
–20
180
10
100
1.0k
10k
100k
1.0M
10M
f, FREQUENCY (Hz)
Figure 3. Error and Current Amplifier Configuration
Figure 4. Error Amplifier Open-loop Gain and Phase vs.
Frequency.
0.5
0
OPERATING BOUNDRY
SOURCE SATURATION
V
CC
(LOAD TO GROUND)
V
= 15V
CC
DESIGN
0.4
–1.0
–2.0
80µs PULSED LOAD
T
= 25°C
A
FOR
DESIGN
120 Hz RATE
NORMAL
FOR
OPERATIONS
BROWNOUT
T
= –55°C
A
0.3
THIS IS THE MINIMUM
OPERATING
K
VOLTAGE POINT
0.23
0.2
0.1
0
3.0
2.0
1.0
0
THIS GAIN CURVE TAKES
2
OUT THE 1/(VIN
)
T
= –55°C
A
DEPENDENCY OF THE
VOLTAGE CONTROL LOOP
T
= 25°C
A
SINK SATURATION
(LOAD TO V
)
CC
GND
600
0
1
2
85VAC
3
4
5
6
7
0
200
400
800
220VAC
120VAC
I
O
, OUTPUT LOAD CURRENT (mA)
V
RMS
Figure 5. Output Saturation Voltage vs. Output Current.
Figure 6. K-factor. Gain Modulator gain with
respect to the voltage at V
.
RMS
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:
Figure 6 shows the gain adjustor (K) with respect to the
voltage at V . The curve has been separated in two
RMS
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 operating conditions as input
voltage decreases the gain increases compensating for the
drop in the loop gain.
I
= K ×I
× (V − 0.8)
GM
SINE EA
where:
I
is the current in the dropping resistor, V
EA
SINE
is the output of the error amplifier and K is a
constant determined by the V
input.
RMS
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.
The output current of the gain modulator is limited to:
2.5
I
=
GM(MAX)
R
T
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+.
7
Micro Linear
MICRO-LINEAR [ MICRO LINEAR CORPORATION ]
