ML4831

7

Figure 3 shows the output configuration for the

operational transconductance amplifiers.

CURRENT

MIRROR

IN

OUT

CURRENT

MIRROR

IN

OUT

IQ +

2

IQ –

2

Figure 3. Output Configuration

A DC path to ground or VCC at the output of the

transconductance amplifiers will introduce an offset error.

The magnitude of the offset voltage that will appear at the

of 0.08

µmhos the input referred offset will be 12.5mV.

Capacitor C1 as shown in Figure 2 is used to block the

DC current to minimize the adverse effect of offsets.

Slew rate enhancement is incorporated into all of the

operational transconductance amplifiers in the ML4831.

This improves the recovery of the circuit in response to

power up and transient conditions. The response to large

signals will be somewhat non-linear as the

transconductance amplifiers change from their low to high

transconductance mode. This is illustrated in Figure 4.

Linear Slope Region

0

Figure 4. Transconductance Amplifier Characteristics

BALLAST OUTPUT SECTION

The IC controls output power to the lamps via frequency

modulation with non-overlapping conduction. This means

that both ballast output drivers will be low during the

OSCILLATOR

The 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.

17

+

–

1.25/3.75

8

C(T)

CONTROL

R(T)/C(T)

R(T)

5 mA

CLOCK

C(T)

Figure 5. Oscillator Block Diagram and Timing

The oscillator frequency is determined by the following

equations:

F

tt

OSC

CHG

DIS

=

+

1

(3)

and

tR C In

VI

R

V

VI

R

V

CHG

T T

REF

CH

T

TL

REF

CH

T

TH

=

+−

+−









(4)