ML4830

7

Mode

Pin 8

Definition

VCO

GND

Frequency Modulation

VCO-O

OPEN

Overlapping VCO F.M.

PWM

VREF

Pulse Width Modulation

Table 1. ML4830 Operating Modes

OSCILLATOR

In Table 1 above, the two VCO frequency ranges are

controlled by the output of the LFB amplifier (Pin 6). As

lamp current decreases, Pin 6 rises in voltage, causing the

C(T) charging current to decrease, thereby causing the

oscillator frequency to decrease. Since the ballast output

network attenuates high frequencies, the power to the

lamp will be increased.

set per (1) below.

18

+

–

1.25/3.75

9

C(T)

R(T)

CONTROL

R(T)/C(T)

5 mA

CLOCK

C(T)

Figure 2. Oscillator Block Diagram and Timing

Also, in both VCO modes, the when LFB OUT is high,

charging current varies according to two control inputs to

the oscillator:

1. The output of the preheat timer

2. The voltage at Pin 6 (lamp current output)

In preheat condition, charging current is fixed at

I

R SET

CHG PREHEAT

()

.

()

= 25

(1)

I

R SET

CHG( )

()

0

5

=

(2)

The oscillator frequency is determined by the following

equations:

F

tt

OSC

CHG

DIS

=

+

1

(3)

and

tR C In

IR

IR

CHG

T T

CHG

T

CHG

T

=

+

+









625

375

.

.

(4)

where:

F

RC

OSC

TT

≅

×

1

051

.

(5)

Highest lamp power, and lowest output frequency are

In this condition, the minimum operating frequency of the

ballast is set per (5) above.

For the IC to be used effectively in dimming ballasts with

value can be used, to yield a smaller frequency excursion

tC

DIS VCO

T

490

(6)