ML4804CS Micro Linear, ML4804CS Datasheet - Page 9

• Manufacturer Part Number: ML4804CS
• Description: Power Factor Correction and PWM Controller Combo
• Manufacturer: Micro Linear

Overvoltage Protection
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
voltage on V
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
The V
passive external power components and the ML4804 are
within their safe operating voltages, but not so low as to
interfere with the boost voltage regulation loop.
V
The V
TriFault
to prevent a damaged and broken connection or
component from causing an unsafe fault condition.
V
winding on the PFC boost inductor, or by some other
means whereby V
proportionality is exact, then a nominal V
V
the PFC down when V
The PFC will then remain in the shutdown state until V
declines to 13.0V, at which time the PFC will restart. If
the PFC V
the protection cycle will repeat. Note that the PWM stage
of the ML4804 remains operational even when the PFC
goes into V
For a real-world example, assume that the bootstrap
supply is derived from a conventional boost inductor
winding and rectified using Shottky diodes. Then it follows
FUNCTIONAL DESCRIPTION
OUTPUT
CC
CC
CC
Figure 2. Compensation Network Connections for the
PFC
OVP
OVP assumes that V
= 15.0V will cause the V
CC
FB
TM
15
OVP feature of the ML4804 works along with the
2
4
3
should be set at a level where the active and
CC
I AC
V FB
V RMS
I SENSE
2.5V
Detect as a redundant PFC buss voltage limiter,
Voltage and Current Error Amplifiers
CC
FB
again encounters an over voltage condition,
OVP shutdown.
exceeds 2.75V, the PFC output driver is
CC
–
+
VEA
VEAO
is proportional to V
MODULATOR
BUSS
CC
GAIN
16
is generated from a bootstrap
= [(16.4/15.0) x 385V] = 444V.
CC
OVP comparator to shut
FB
+
–
drops below 2.50V.
IEA
BUSS
FB
IEAO
BUSS
. When the
V REF
. If the
(Continued)
1
of 385V at
+
–
CC
that the voltage from the bootstrap winding must equal
15.8V during regular circuit operation, and will increase
to 17.2V at the point of V
output voltage from the PFC will have increased from a
noninal V
419VDC. When V
off, thereby protecting the output (BUSS) capacitor and
the semiconductors in both the PFC and PWM stages.
To assure reasonable headroom in which to operate this
device, V
threshold is always at least 2V above that of the UVLO.
To assure reliable operation of the ML4804, V
operated from a bootstrap winding on the PFC’s inductor,
or from an external power supply whose output is
regulated to 15.0V (nominal). In the case of a regulated
power supply powering the ML4804, the V
will be rendered non-operational.
Error Amplifier Compensation
The PWM loading of the PFC can be modeled as a
negative resistor; an increase in input voltage to the PWM
causes a decrease in the input current. This response
dictates the proper compensation of the PFC's two
transconductance error amplifiers. Figure 2 shows the
types of compensation networks most commonly used for
the voltage and current error amplifiers, along with their
respective return points. The current loop compensation is
returned to V
the PFC: as the reference voltage comes up from zero
volts, it creates a differentiated voltage on IEAO which
prevents the PFC from immediately demanding a full duty
cycle on its boost converter.
There are two major concerns when compensating the
Figure 3. External Component Connections to V
CC
BUSS
OVP tracks with UVLO. The V
REF
of 385VDC to (17.2/15.8) x 385V =
ML4804
to produce a soft-start characteristic on
V BIAS
GND
V CC
BUSS
R BIAS
reaches 419V, the PFC will shut
CC
OVP shutdown. Then the
CERAMIC
0.22µF
ZENER
15V
CC
CC
ML4804
OVP
OVP function
CC
must be
CC
9