ML4800
PRODUCT SPECIFICATION
The output of the gain modulator is a current signal, in the
form of a full wave rectified sinusoid at twice the line fre-
quency. This current is applied to the virtual-ground (nega-
tive) input of the current error amplifier. In this way the gain
modulator forms the reference for the current error loop, and
ultimately controls the instantaneous current draw of the
PFC from the power line. The general form for the output of
the gain modulator is:
I
AC
×
VEAO
I
GAINMOD
=
−−−−−−−
2
−− ×
1V
−−−−−−− −−
−
V
RMS
(1)
cycle until the voltage on I
SENSE
is adequately negative to
cancel this increased current. Similarly, if the gain modula-
tor’s output decreases, the output duty cycle will decrease, to
achieve a less negative voltage on the I
SENSE
pin.
Cycle-By-Cycle Current Limiter
The I
SENSE
pin, as well as being a part of the current feed-
back loop, is a direct input to the cycle-by-cycle current
limiter for the PFC section. Should the input voltage at this
pin ever be more negative than -1V, the output of the PFC
will be disabled until the protection flip-flop is reset by the
clock pulse at the start of the next PFC power cycle.
TriFault Detect
TM
More exactly, the output current of the gain modulator is
given by:
I
GAINMOD
= K
× (
VEAO – 0.625V
) ×
I
AC
where K is in units of V
-1
.
Note that the output current of the gain modulator is limited
to 500µA.
Current Error Amplier
To improve power supply reliability, reduce system compo-
nent count, and simplify compliance to UL 1950 safety
standards, the ML4800 includes TriFault Detect. This feature
monitors VFB (Pin 15) for certain PFC fault conditions.
In the case of a feedback path failure, the output of the PFC
could go out of safe operating limits. With such a failure,
VFB will go outside of its normal operating area. Should
VFB go too low, too high, or open, TriFault Detect senses the
error and terminates the PFC output drive.
TriFault detect is an entirely internal circuit. It requires no
external components to serve its protective function.
Overvoltage Protection
The current error amplifier’s output controls the PFC duty
cycle to keep the average current through the boost inductor
a linear function of the line voltage. At the inverting input to
the current error amplifier, the output current of the gain
modulator is summed with a current which results from a
negative voltage being impressed upon the I
SENSE
pin. The
negative voltage on I
SENSE
represents the sum of all currents
flowing in the PFC circuit, and is typically derived from a
current sense resistor in series with the negative terminal of
the input bridge rectifier. In higher power applications, two
current transformers are sometimes used, one to monitor the
ID of the boost MOSFET(s) and one to monitor the I
F
of the
boost diode. As stated above, the inverting input of the cur-
rent error amplifier is a virtual ground. Given this fact, and
the arrangement of the duty cycle modulator polarities inter-
nal to the PFC, an increase in positive current from the gain
modulator will cause the output stage to increase its duty
The OVP comparator serves to protect the power circuit
from being subjected to excessive voltages if the load should
suddenly change. A resistor divider from the high voltage
DC output of the PFC is fed to V
FB
. When the voltage on
V
FB
exceeds 2.75V, the PFC output driver is shut down.
The PWM section will continue to operate. The OVP
comparator has 250mV of hysteresis, and the PFC will not
restart until the voltage at V
FB
drops below 2.50V. The V
FB
should be set at a level where the active and passive external
power components and the ML4800 are within their safe
operating voltages, but not so low as to interfere with the
boost voltage regulation loop.
16
VEAO
VFB
15
2.5V
IAC
2
VRMS
4
ISENSE
3
RAMP 1
7
GAIN
MODULATOR
1.6kΩ
VEA
–
+
1
IEAO
0.5V
1.6kΩ
IEA
+
+
–
–
+
–
TRI-FAULT
2.75V
OVP
+
–
S
–1V
+
–
Q
R
S
Q
PFC OUT
Q
12
PFC ILIMIT
R
OSCILLATOR
Q
Figure 1. PFC Section Block Diagram
8
REV. 1.0.5 9/25/01
FAIRCHILD [ FAIRCHILD SEMICONDUCTOR ]
