ML4876

7

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.

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

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.

Figure 2. Simplified Lamp Driver Circuit

Figure 3. Operating Waveforms of the

Lamp Driver Section

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.

T1

©

Q4

Q5

Lm

Lm

T1

1:N

SOURCE OF

U2-A

T1-CNTR-PRI

DRAIN-Q4

L GATE2

DRAIN-Q5

L GATE1

CLOCK