ML4835CS datasheet(10/17 Pages) MICRO-LINEAR

10 / 17 page

ML4835

(C1) in the frequency compensation network. The

compensation network shown in Figure 5 will introduce a

zero and a pole at:

1 2

(2)

Figure 4 shows the output configuration for the

operational transconductance amplifiers.

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

input is given by VOS = io/gm. For an io of 1µA and a gm

of 0.05 µW the input referred offset will be 20mV.

Capacitor C1 as shown in Figure 5 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 ML4835.

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, as illustrated in Figure 7.

END OF LAMP LIFE

At the end of a lamp’s life when the emissive material is

depleted, the arc current is rectified and high voltage

occurs across the lamp near the depleted cathode. The

ballast acts as a constant current source so power is

dissipated near the depleted cathode which can lead to

arcing and bulb cracking. Compact fluorescent lamps are

more prone to cracking or shattering because their small

diameter can’t dissipate as much heat as the larger linear

lamps. Compact fluorescents also present more of a

safety hazard since they are usually used in downlighting

systems without reflector covers.

EOL and the ML4835

The ML4835 uses a circuit that creates a DC voltage

representative of the power supplied to the lamps through

the inverter. This voltage is used by the ML4835 to latch

off the ballast when it exceeds an internal threshold. An

external resistor can be used as the “EOL latch resistor” to

set the power level trip point, as shown in by R9 in Figure

12. See Micro Linear ML4835 User Guide and

applications notes for more details. Figure 4 illustrates a

simplified model of ML4835 EOL functionality.

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

discharging time tDIS of the oscillator capacitor CT.

OSCILLATOR

The VCO frequency ranges are controlled by the output

of the LFB amplifier (RSET). As lamp current decreases,

LFB OUT falls in voltage, causing the CT charging current

to increase, thereby causing the oscillator frequency to

increase. Since the ballast output network attenuates high

frequencies, the power to the lamp will be decreased. The

oscillator frequency is determined by the following

equations:

OSC

CHG

DIS

(3)

and

tR C In

VII

CHG

REF

CHG

REF

CHG

+´

(4)

The oscillator’s minimum frequency is set when ICHG = 0

where:

MIN

051

(5)

The oscillator's start frequency can be expressed by:

START

´´

051

(5a)

Both equations assume that tCHG >> tDIS.

When LFB OUT is high, ICHG = 0 and the minimum

frequency occurs. The charging current varies according

to two control inputs to the oscillator:

1. The output of the preheat timer

2. The voltage at LFB OUT (lamp feedback amplifier

output)

In preheat condition, charging current is fixed at

CHG PREHEAT

SET

()

(6)

In running mode, charging current decreases as the

voltage rises from 0V to VOH at the LAMP FB amplifier.

The charging current behavior can be expressed as:

LEAO

CHG

SET

25%

(7)

The highest frequency is attained when ICHG is highest,

which is attained when voltage at LFB OUT is at 0V:

CHG

SET

()

(8)

FUNCTIONAL DESCRIPTION (Continued)

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