ML4876
FUNCTIONAL DESCRIPTION
The ML4876 consists of a PWM regulator, a lamp driver/
inverter, a linear regulator, a flyback 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. In
addition it generates the DC voltage for the contrast
requirements of LCD screens. A typical application circuit
is shown in Figure 1. Please refer to Application Note 32
for detailed application information beyond what is
presented here.
The MOSFETs (Q4 and Q5) 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
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
150ns is applied to the primary of the 2:1 pulse
transformer T2, rectified by diode D2, 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 U2-B. The pulse width of the signal on
the gate of U2-B (B OFF) varies according to the
amplitude of the feedback signal on LEA–, which is
proportional to the AC current flowing in the lamp. This
feedback signal is developed by monitoring the current
through resistor R6 in the common source connection of
the inverter MOSFETs (Q4 and Q5). The lamp current, and
therefore brightness, is adjusted by varying the voltage
applied to R7 at the brightness adjust control point.
Increasing this voltage decreases the brightness.
Note:
Please read the Power Sequencing section below
prior to using the ML4876.
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
R
to set the resonant frequency of the inverter.
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.
C
T
I
C
©
T1
Lm Lm
C
OUT
T1
1:N
CLOCK
L GATE1
DRAIN-Q5
C
R
LAMP
L GATE2
Q4
Q5
DRAIN-Q4
T1-CNTR-PRI
SOURCE OF
U2-A
Figure 2. Simplified Lamp Driver Circuit
Figure 3. Operating Waveforms of the
Lamp Driver Section
7
MICRO-LINEAR [ MICRO LINEAR CORPORATION ]
