ML4821
INV
7
+8V
A
VOL
, OPEN-LOOP VOLTAGE GAIN (dB)
100
80
GAIN
60
40
PHASE
20
0
–20
10
100
1.0k
10k
100k
f, FREQUENCY (Hz)
1.0M
120
150
180
10M
0
V
CC
= 15V
V
O
= 1.0V TO 5.0V
–30
R
L
= 100kΩ
T
A
= 25°C
60
90
PHASE (DEGREES)
–
+8V
V
REF
+
EA OUT
6
S.S
9
6.2kΩ
Figure 3. Error and Current Amplifier Configuration
Figure 4. Error Amplifier Open-loop Gain and Phase vs.
Frequency.
0.5
OPERATING BOUNDRY
0
V
SAT
, OUTPUT SATURATION VOLTAGE (V)
V
CC
–1.0
–2.0
T
A
= 25°C
SOURCE SATURATION
(LOAD TO GROUND)
V
CC
= 15V
80µs PULSED LOAD
120 Hz RATE
0.4
DESIGN
FOR
BROWNOUT
DESIGN
FOR
NORMAL
OPERATIONS
THIS IS THE MINIMUM
OPERATING
VOLTAGE POINT
T
A
= –55°C
K
0.3
3.0
T
A
= –55°C
2.0
1.0
0
SINK SATURATION
(LOAD TO V
CC
)
GND
0
200
400
600
800
T
A
= 25°C
0.2
0.23
0.1
THIS GAIN CURVE TAKES
OUT THE 1/(V
IN
)
2
DEPENDENCY OF THE
VOLTAGE CONTROL LOOP
0
0
1
2
85VAC
3
120VAC
4
V
RMS
5
220VAC
6
7
I
O
, OUTPUT LOAD CURRENT (mA)
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:
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 determined by the V
RMS
input.
The output current of the gain modulator is limited to:
I
GM(MAX)
=
2.5
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+.
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 operating 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.
Micro Linear
7
MICRO-LINEAR [ MICRO LINEAR CORPORATION ]
