ML4821IS MICRO-LINEAR [Micro Linear Corporation], ML4821IS Datasheet - Page 6

• Manufacturer Part Number: ML4821IS
• Description: Power Factor Controller
• Manufacturer: MICRO-LINEAR [Micro Linear Corporation]

ML4821
FUNCTIONAL DESCRIPTION
OSCILLATOR
The ML4821 oscillator charges the external capacitor
connected to C
capacitor voltage reaches the upper threshold, the
comparator changes state and the capacitor discharges to
the lower threshold through Q1.
The oscillator period can be described by the following
relationship:
The ML4821 oscillator includes a SYNC input for
synchronizing to an external frequency source. A positive
pulse on this pin of 2V (typ) resets the oscillators
comparator and initiates a discharge cycle for C
and C
oscillator frequency should be selected to produce a
lower frequency than the external frequency source.
6
RAMP VALLEY
RAMP PEAK
where:
and:
T
T
T
CLOCK
OSC
RAMP
DISCHARGE
T
component values which set the ML4821
= T
Figure 1. Oscillator Block Diagram.
= C(Ramp Valley to Peak)
C
C
R
RAMP
T
T
T
T
= C(Ramp Valley to Pk)
with a current equal to 2.5/R
+ T
DISCHARGE
C
R
8.4mA
t
T
T
SYNC
D
I
R T
Q1
2k
1k
(I
I
R T
2
RT
+
–
Q2
(8.4mA – I
/2)
T
. When the
T
V
. The R
REF
Micro Linear
RT
/2)
T
VOLTAGE AND CURRENT ERROR AMPLIFIERS
The ML4821 voltage error amplifier is a high open loop
gain, wide bandwidth amplifier with a class A output. The
soft start circuit controls the input to this amplifier for
closed loop soft start operation.
The current error amplifier (IA) is similar to the voltage
error amplifier but is designed for very low offsets to allow
the selection of a low value resistor for R
OUTPUT DRIVER STAGE
The ML4821 Output Driver is a 1A peak output high
speed totem pole circuit designed to quickly drive
capacitive loads, such as power MOSFET gates. The driver
circuit’s output voltage is internally limited to 17V.
GAIN MODULATOR
The ML4821 gain modulator responds linearly to current
injected into the I
fashion to voltage on the V
on the V
limit, or ”brown-out protection”, upon the overall PFC
circuit (Figures 6 and 7). The rectified line input sine wave
is converted to a current for the I
resistor. In this way, most ground noise produces an
insignificant effect on the reference to the PWM
comparator. This gives the ML4821 a high degree of
immunity to the disturbances common in high-power
switching circuits.
Figure 2. Oscillator Timing Resistance vs. Frequency.
1000
100
10
RMS
0
pin, the gain modulator enforces a power
10
SINE
pin, and in an inverse-square
1nF
20
RMS
R
T
pin. At very low voltages
(k )
150pF
SINE
30
680pF
input via a dropping
SENSE
330pF
40
.
470pF
50