ML4877

7

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

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

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

T1

©

Q3

Q4

Lm

Lm

T1

1:N

Figure 2. Kelvin Sense Connections

SOURCE OF

U2-A

T1-CNTR-PRI

DRAIN-Q3

L GATE2

DRAIN-Q4

L GATE1

CLOCK

Figure 3. Operating Waveforms

of the Lamp Driver Section