ML4802
FUNCTIONAL DESCRIPTION
The ML4802 consists of a combined average-current-
controlled, continuous boost Power Factor Corrector (PFC)
front end and a synchronized Pulse Width Modulator
(PWM) back end. It is distinguished from earlier combo
controllers by its unique Green Mode operation and
dramatically reduced start-up and operating currents. The
PWM section has been optimized for use in current mode
topologies. The PWM stage uses conventional trailing-
edge duty cycle modulation, while the PFC uses leading-
edge modulation. This patented leading/trailing edge
modulation technique results in a higher useable PFC error
amplifier bandwidth, and can significantly reduce the size
of the PFC DC bus capacitor.
The synchronization of the PWM with the PFC simplifies
the PWM compensation due to the reduced ripple on the
PFC output capacitor (the PWM input capacitor). The
PWM section of the ML4802 runs at twice the frequency
of the PFC, which allows the use of smaller PWM output
magnetics and filter capacitors while holding down the
losses in the PFC stage power components.
In addition to power factor correction, a number of
protection features have been built into the ML4802. These
include soft-start, PFC over-voltage protection, peak
current limiting, brown-out protection, duty cycle limit,
and under-voltage lockout.
GREEN MODE OPERATION
Green Mode automatically improves efficiency by up to
20% or more during low power operation. This feature is
particularly helpful in meeting the demands of Energy
Star™ programs. When the PWM’s output falls to
nominally 17% of its design maximum power, Green
Mode operation is initiated. The upper Green Mode
threshold corresponds roughly to 1/3 of rated full power
level. In Green Mode, the PWM operates in a pulse-
START
skipping mode. This significantly reduces the frequency
of operation, and therefore the dissipation in the PWM
output driver and switch. Since the pulse-skipping is
synchronous to the PWM’s master clock, the noise
spectrum of the PWM retains a strong relationship to its
spectrum during continuous-mode operation, which eases
input and output filter design. PWM pulse frequency
reductions in excess of 10:1 are common, with no
increase in peak-to-peak output ripple. During Green
Mode, the PFC also cycles on and off, running only as
often as necessary to maintain its feedback voltage (VFB)
between 2.25V and 2.5V (corresponding typical values of
VBUSS are 382V and 425V). The PFC uses a built-in soft-
start to minimize line current peaks and component stress
when turning on. See Figure 1 for a flow chart detailing
Green Mode and Normal Mode operation.
2.25V
–
+
S
R
Q
Q
PFC ON
VFB
2.5V
VDC
VTH1
VTH2
+
–
–
+
PWM OFF
Figure 2. Green Mode Section Block Diagram
Entering Green Mode
The Green Mode Controller is detailed in Figure 2. Key to
the ML4802’s operation in Green-Mode is the fact that the
PWM’s output power is related to the voltage on the VDC
(PWM Duty Cycle Control Voltage) pin by a known
transfer characteristic. Therefore, the output power POUT
drawn from an ML4802 supply can be inferred by
monitoring the value of VDC fed back to the ML4802
from the (external) reference/error amplifier combination.
When the output power taken from the PWM is reduced,
the voltage on VDC will decrease. When VDC falls below
VTH1 (1.30V typical), the part enters Green Mode
operation. Once this happens, the threshold to which VDC
is compared for further PWM operation is set to a higher
value VTH2 (1.58V typical). This causes the PWM to enter
a pulse-skipping mode while maintaining the desired
output voltage. Pulse-skipping occurs because VTH2 is a
higher voltage than VTH1, and because the PWM drive
(PWMOUT) is disabled until VDC
³
VTH2. Since the
primary current of the PWM output transformer is
determined by VTH2 in Green Mode, and VTH2 > VTH1,
each PWM output pulse will carry slightly more energy
during Green Mode operation than during all but the
highest duty cycle regimes of continuous-mode operation.
In Green Mode, the power in each PWM output pulse is:
PPULSE
µ
IPRIMARY(PWM) x VBUSS
NORMAL MODE
OPERATION
NO
VFB
≥
2.50V?
VDC
<
1.30V?
YES
NO
YES
TURN PFC OFF
VFB
<
2.25V?
YES
NO
VDC
≥
2.25V?
YES
ISSUE PWM
POWER PULSE
NO
TURN PWM ON
TURN PFC ON
Figure 1. ML4802 Operational Flow Chart
PPULSE
µ
(VGMT/RSENSE[PWM]) x 380V
6
Datasheet
August 2000
MICRO-LINEAR [ MICRO LINEAR CORPORATION ]
