ML4831

6

AVERAGE CURRENT AND OUTPUT VOLTAGE

REGULATION

The PWM regulator in the PFC Control section will act to

offset the positive voltage caused by the multiplier output

by producing an offsetting negative voltage on the current

sense resistor at Pin 4. A cycle-by-cycle current limit is

included to protect the MOSFET from high speed current

transients. When the voltage at Pin 4 goes negative by

more than 1V, the PWM cycle is terminated.

For more information on compensating the average

current and boost voltage error amplifier loops, see

ML4821 data sheet.

OVERVOLTAGE PROTECTION AND INHIBIT

The OVP pin serves to protect the power circuit from

being subjected to excessive voltages if the load should

change suddenly (lamp removal). A divider from the high

voltage DC bus sets the OVP trip level. When the voltage

on Pin 18 exceeds 2.75V, the PFC transistors are inhibited.

The ballast section will continue to operate. The OVP

threshold should be set to a level where the power

components are safe to operate, but not so low

as to interfere with the boost voltage regulation loop.

Figure 1. ML4831 Block Diagram

TRANSCONDUCTANCE AMPLIFIERS

The PFC voltage feedback, PFC current sense, and the

loop current amplifiers are all implemented as operational

transconductance amplifiers. They are designed to have

low small signal forward transconductance such that a

large value of load resistor (R1) and a low value ceramic

capacitor (<1

µF) can be used for AC coupling (C1) in the

frequency compensation network. The compensation

network shown in Figure 2 will introduce a zero and a

pole at:

f

RC

f

RC

ZP

==

1

2

1

2

11

1 2

ππ

(2)

–

+

18

2.5V

R1

C1

C2

Figure 2. Compensation Network

7

R(SET)

R(X)/C(X)

VCC

GND

IA OUT

IA +

10

16

17

11

2

4

OUT B

13

+

–

+

–

+

–

–

+

2.5V

–1V

PREHEAT

TIMER

OSC

UNDER-VOLTAGE

AND THERMAL

SHUTDOWN

–

+

S

R

Q

T

Q

Q

P GND

12

OUT A

14

PFC OUT

15

R(T)/C(T)

8

INTERRUPT

9

LFB OUT

6

LAMP F.B.

5

7K

PWM (PFC)

7K

I(SINE)

EA OUT

EA –/OVP

3

1

18

–

+

2.5V

–

+

2.75V

OVP

GAIN

MODULATORS