ml4877 Microsemi Corporation, ml4877 Datasheet - Page 7

• Manufacturer Part Number: ml4877
• Description: Desktop Backlight Lamp Driver
• Manufacturer: Microsemi Corporation

FUNCTIONAL DESCRIPTION
The ML4877 consists of a PWM regulator, a lamp driver/
inverter, a linear regulator and control circuits. This IC, in
conjunction with external components, converts a DC
battery voltage into the high voltage and high frequency
AC signal required to start and drive miniature cold
cathode fluorescent lamps. Typical application circuits
are shown in Figure 1 and Figure 5. Note: Please read the
Power Sequencing section below prior to using the
ML4877.
LAMP DRIVER
The lamp driver, sometimes referred to as a lamp inverter,
is comprised of a PWM regulator and a Royer type
inverter circuit to drive the lamp. The PWM regulator, in a
buck configuration, controls the magnitude of the lamp
current to provide the dimming capability. Figure 2 shows
a simplified circuit to more easily illustrate the operation
of the circuit.
Due to the presence of the buck inductor, L1, the circuit
shown in Figure 2 is essentially a current fed parallel
loaded resonant circuit. Lm is the primary inductance of
the output transformer, T1, which tunes with the resonant
capacitor C
The oscillator frequency is always set lower than the natural
resonant frequency to ensure synchronization. The current
source IC models the current through the buck inductor L1.
The MOSFETs, (Q3 and Q4) are alternately turned on with
a constant 50% duty cycle signal (L GATE1, L GATE2) at
one-half the frequency of the oscillator. In this way each
transistor pulses, or excites, the resonant tank on each
half cycle. The combination of these two signals appear
across the primary winding of the output transformer as a
I
C
Figure 2. Kelvin Sense Connections
R
Q3
to set the resonant frequency of the inverter.
Lm
C
R
Lm
T1
Q4
1:N
T1
C
OUT
sinusoidal waveform. This voltage is multiplied by the
step-up turns ratio of the output transformer and impressed
across the lamp.
The output transitions are controlled by feedback through
the L RTD pin by sensing the voltage at the center tap of
the output transformer. Each time this signal reaches the
minimum resonant threshold detection point an internal
clock pulse is generated to keep the system synchronized.
Figure 3 shows some of these representative waveforms at
the important nodes of the circuit.
The PWM regulator is comprised of a MOSFET (U2-A),
inductor L1, and the gate control and drive circuitry as
shown in Figure 1. A signal with a constant pulse width of
I 50ns is applied to the primary of the 2:1 pulse
transformer T2, rectified by diode D1, and used to charge
the gate capacitance of U2-A, thereby turning it on. The
turn off is controlled by discharging this capacitance
through MOSFET Q2. The pulse width of the signal on the
gate of Q2 (B OFF) varies according to the difference of
the amplitude of the feedback signal on LEA+, and LEA–.
The signal on LEA– is proportional to the AC current
flowing in the lamp, while the signal on LEA+ is a
function of the brightness control setting. The AC lamp
current feedback signal is developed by monitoring the
current through resistor R6 in the common source
connection of the inverter MOSFETs, Q3 and Q4. The
lamp current, and therefore brightness, is adjusted by
varying the voltage applied to R4, at the brightness adjust
control point. Increasing this voltage increases the brightness.
OSCILLATOR
The frequency of the oscillator in the ML4877 is set by
selecting the values Of C
T1-CNTR-PRI
SOURCE OF
DRAIN-Q4
DRAIN-Q3
L GATE1
L GATE2
CLOCK
Figure 3. Operating Waveforms
U2-A
of the Lamp Driver Section
C
T
T
and R
T
. Figure 4 shows the
ML4877
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