ML4835
FUNCTIONAL DESCRIPTION (Continued)
The ML4835 implements a triple frequency operation
scheme: programmable three-frequency sequence for pre-
heat, ignition, and dimming, that extends lamp life,
simplifies lamp network design, and starts lamps at any
dimming level without flashing. This addresses the need
for a high-Q network for starting sequence and low-Q
network for operation, minimizing parasitic losses and
improving overall power efficiency. The values for the
pre-heat, start, operation, and restart can be programmed
or selected (Figure 2).
POWER FACTOR SECTION
The ML4835 power factor section is a peak or average
current sensing boost mode PFC control circuit in which
only voltage loop compensation is needed. It is simpler
than a conventional average current control method. It
consists of a voltage error amplifier, a current sense
amplifier (no compensation is needed), an integrator, a
comparator, and a logic control block. In the boost
topology, power factor correction is achieved by sensing
the output voltage and the current flowing through the
current sense resistor. Duty cycle control is achieved by
comparing the integrated voltage signal of the error
amplifier and the voltage across R
control timing is shown in Figure 3.
. The duty cycle
SENSE
PREHEAT
f
1
SET TIME VALUES
HIGH Q
FOR PREHEAT,
START AND OPERATION,
AND RESTART
ML4835
LOW Q
f
f
3
2
START
OPERATION
Figure 2. Three Frequency Design Model
L
V
OUT
SW2
LAMP
RA
RB
LAMP
NETWORK
EMI
FILTER
INVERTER
SW1
R
SENSE
LAMP
3
4
18
1
PFC OUT
PIFB
PIFBO
PVFB/OVP
–A
+
–
R
S
Q
PIFBO
CLK
OSC
PEAO
–
RAMP
V
TO I
VREF1
+
CLK
C
RAMP
PEAO
PFC OUT
13
2
C
RAMP
R1
C1
C2
Figure 3. ML4835 PFC Controller Section
7
MICRO-LINEAR [ MICRO LINEAR CORPORATION ]
