www.fairchildsemi.com

May 2005

FAN7311 Rev. 1.0.0

AN731

LCD Backlight

Inverter Drive

FAN7311

LCD Backlight Inverter Drive IC

Features

· High-Efficiency Single-Stage Power Conversion

· Wide Input Voltage Range : 5V to 25.5V

· Back Light Lamp Ballast and Soft Dimming

· Reduces Number of Required External Components

· Precision Voltage Reference Trimmed to 2%

· ZVS Full-Bridge Topology

· Soft Start

· PWM Control at Fixed Frequency

· Analog and Burst Dimming Function

· Programmable Striking Frequency

· Open Lamp Protection

· Open Lamp Regulation

· 20-Pin SSOP

Description

The FAN7311 provides all the control functions for a
series parallel resonant converter as well as a pulse width mod-
ulation (PWM) controller to develop a supply voltage. Typical
operating frequency range is between 30kHz and 250kHz,
depending on the CCFL and the transformer's characteristics.
The FAN7311 uses a new patent-pending phase-shift control.

20-SSOP

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FAN7311 Rev. 1.0.0

AN731

LCD Backlight

Inverter Drive

Internal Block Diagram

BDIM

S_S

EA_OUT

OSCILLATOR

Error Amp.

EA_IN

6uA

max. 2V

min. 0.5V

ADIM

OUTB

OUTA

OUTC

OUTD

VIN

2.5VREF

ENA

UVLO 5V

UVLO

REF

BCT

max. 2V

min. 0.5V

OLR

1.4V

Solr

SET

CLR

UVLO

OLP

PGND

Sburst

Voltage

Reference

Internal

Bias

VIN

1.4uA

UVLO

Output

Driver

Output

Control

Logic

AGND

Solr 105uA

Sburst 85uA

Output

Driver

2.5V

Va+

RT1

Striking

Logic

OLP

S_S

OLP

2.5V 1.5V

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FAN7311 Rev. 1.0.0

AN731

LCD Backlight

Inverter Drive

Pin Assignments

Pin Definitions

Name

Function/Description

Name

Function/Description

OLP

Open Lamp Protection

BCT

Burst Dimming Timing Capacitor

OLR

Open Lamp Regulation

Timing Resistor

ENA

Enable Input

Timing Capacitor

S_S

Soft Start

OUTD

NMOSFET Drive Output D

GND

Analog Ground

OUTC

PMOSFET Drive Output C

REF

2.5V Reference Voltage

PGND

Power Ground

ADIM

Analog Dimming Input

VIN

Supply Voltage

BDIM

Burst Dimming Input

OUTA

PMOSFET Drive Output A

EA_IN

Error Amplifier Input

OUTB

NMOSFET Drive Output B

EA_OUT

Error Amplifier Output

RT1

Striking Frequency Resistor

FAN7311

OLP

OLR

ENA

S_S

OUTC

REF

ADIM

BDIM

EA_IN EA_OUT

RT1

OUTB

OUTA

VIN

PGND

OUTD

BCT

GND

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FAN7311 Rev. 1.0.0

AN731

LCD Backlight

Inverter Drive

Absolute Maximum Ratings

For typical values Ta=25

C, Vcc=12V and for min/max values Ta is the operating ambient temperature range with

-25

C and 5V

Vcc

25.5V, unless otherwise specified.

Note:

1. Thermal resistance test board

Size: 76.2mm * 114.3mm * 1.6mm(1S0P)
JEDEC standard: JESD51-3, JESD51-7

2. Assume no ambient airflow

Characteristics

Symbol

Value

Unit

Supply Voltage

5 ~ 25.5

Operating Temperature Range

Topr

-25 ~ 85

Storage Temperature Range

Tstg

-65 ~ 150

Thermal Resistance Junction-Air (Note1,2)

112

C/W

Power Dissipation

1.1

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FAN7311 Rev. 1.0.0

AN731

LCD Backlight

Inverter Drive

Electrical Characteristics

For typical values Ta=25

C, Vcc=12V and for min/max values Ta is the operating ambient temperature range with

-25

C and 5V

Vcc

25.5V, unless otherwise specified.

Characteristics

Symbol

Test Condition

Min.

Typ.

Max.

Unit

REFERENCE SECTION ( Recommend X7R Capacitor )

Line Regulation

Vref

25.5V

2.5V Regulation Voltage

V25

2.45

2.5

2.55

OSCILLATOR SECTION(MAIN)

Oscillation Frequency

fosc

Ta = 25

C, Ct = 270pF

Rt = 18k

108

115

122

kHz

Ct = 270pF, Rt = 18k

106

115

124

CT High Voltage

Vcth

2.0

CT Low Voltage

Vctl

0.5

OSCILLATOR SECTION(BURST)

Oscillation Frequency

foscb

Ctb = 10nF, Rt=18k

195

225

255

BCT High Voltage

Vbcth

BCT Low Voltage

Vbctl

0.5

ERROR AMP SECTION

Open Loop Gain

Unit Gain Bandwidth

1.5

MHz

Feedback Output High Voltage

Veh

EA_IN = 0V

2.0

2.27

2.54

Output Sink Current

lsin

EA_OUT = 1.5V

-1

Output Source Current

lsur

EA_OUT = 1.5V

EA_IN Driving Current On OLR

Iolr

105

135

EA_IN Driving Current On Burst Dimming

Iburst

109

Feedback High Voltage On Burst Dimming

Vfbh

R(EA_IN) = 60k

Va+0.1

Va+0.4

Va+0.7

SOFT START SECTION

Soft Start Current

S_S=2V

Soft Start Clamping Voltage

Vssh

PROTECTION SECTION

Open Lamp Protection Voltage 0

Volp0

Start at open lamp

2.2

2.5

2.8

Open Lamp Protection Voltage 1

Volp1

Normal -> open lamp

1.3

1.5

1.7

Open Lamp Regulation Voltage

Volr

1.75

2.25

Open Lamp Protection Charging Current

Iolp

0.7

1.4

2.1

UNDER VOLTAGE LOCK OUT SECTION

Start Threshold Voltage

Vth

Start Up Current

Ist

= Vth-0.2

130

180

Operating Supply Current

Iop

= 12V

1.5

Stand-by Current

Isb

= 12V

200

370

ON/OFF SECTION

On State Input Voltage

Von

Off Stage Input Voltage

Voff

0.7

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FAN7311 Rev. 1.0.0

AN731

LCD Backlight

Inverter Drive

Electrical Characteristics

(Continued)

For typical values Ta=25

C, Vcc=12V and for min/max values Ta is the operating ambient temperature range with

-25

C and 5V

Vcc

25.5V, unless otherwise specified.

Characteristics

Symbol

Test Condition

Min.

Typ.

Max.

Unit

OUTPUT SECTION

PMOS Gate High Voltage

Vpdhv

= 12V

Vcc

PMOS Gate Low Voltage

Vphlv

= 12V

Vcc-10.5

Vcc-8.5

Vcc-6.5

NMOS Gate Drive Volgate

Vndhv

= 12V

6.5

8.5

10.5

NMOS Gate Drive Volgate

Vndhv

= 12V

PMOS Gate Voltage With UVLO Activated

Vpuv

= Vth-0.2

Vcc-0.3

NMOS Gate Voltage With UVLO Activated

Vnuv

= Vth-0.2

0.3

Rising Time

= 12V, Cload=2nF

200

500

Falling Time

= 12V, Cload=2nF

200

500

MAX./MIN OVERLAP

Min. Overlap between diagonal switches

fosc=100KHz

Max. Overlap betwwen diagonal switches

fosc=100KHz

100

DELAY TIME

PDR_A/NDR_B

Rt=18k

450

PDR_C/NDR_D

Rt=18k

450

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FAN7311 Rev. 1.0.0

AN731

LCD Backlight

Inverter Drive

Function Description

UVLO: The under voltage lockout circuit guarantees stable
operation of the IC's control circuit by stopping and starting it as
a function of the Vin value. The UVLO circuit turns on the con-
trol circuit when Vin exceeds 5V. When Vin is lower than 5V, the
IC's standby current is less than 200uA.

ENA:

Applying voltage higher than 2V to the ENA pin enables

the operation of the IC. Applying voltage lower than 0.7V to the
ENA pin will disable the operation of the inverter.

Soft start: The soft start function requires that the S_S pin is
connected through a capacitor to GND. A soft start circuit
ensures a gradual increase in the input and output power. The
capacitor connected to the S_S pin determines the rate at which
the duty ratio rises. It is charged by a 6uA current source.

Main oscillator: The timing capacitors (CTs) are charged by the
reference current source, which is formed by the timing resistor
(RT). The timing resistor's voltage is regulated at 1.25V. The
sawtooth waveform charges up to 2V. Once this voltage is
reached, the capacitors begin discharging down to 0.5V. Next,
the timing capacitors start charging again and a new switching
cycle begins. The main frequency can be programmed by
adjusting the Rt and CT values. The main frequency can be cal-
culated as shown below.

Burst oscillator & burst dimming: The timing capacitors
(BCTs) are charged by the reference current source, which is
formed by the timing resistor (RT). The timing resistor's voltage
is regulated at 1.25V. The sawtooth waveform charges up to 2V.
Once this voltage is reached, the capacitors begin discharging
down to 0.5V. Next the timing capacitors start charging again
and a new switching cycle begins. The burst dimming frequency
can be programmed by adjusting the Rt and BCT values. The
burst dimming frequency can be calculated as shown below.

To avoid visible flicker, the burst dimming frequency should be
greater than 120Hz.

What does the following sentence mean? Please rewrite.To
compare the input of BDIM pin with the 0.5~2V triangular wave
of burst oscillator makes the PWM pulse for burst dimming. The
PWM pulse controls EA_OUT's voltage by summing 85uA into
the EA_IN pin.

fop

32 RT CT

-------------------------------

fburst

3.75

64 RT BCT

-----------------------------------

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FAN7311 Rev. 1.0.0

AN731

LCD Backlight

Inverter Drive

Open lamp regulation & open lamp protection: It is neces-
sary to suspend power stage operation if an open lamp occurs,
because the power stage has high gain. When a voltage higher
than 2V is applied to the OLR pin, the part enters regulation
mode and controls the EA_OUT voltage. This limits the lamp
voltage by summing 105uA into the feedback node. At the same
time, the OLP capacitor, connected to the OLP pin, is charged
by the 1.4uA internal current source. Once it reaches 2.5V, the
IC enters shut down where all the output is high.

Output Drives: The four output drives are designed so that
switches A and B, C and D never turn on simultaneously. The
OUTA-OUTB pair is intended to drive one half-bridge in the
external power stage. The OUTC-OUTD pair will drive the other
half-bridge.

8.5V

VIN

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FAN7311 Rev. 1.0.0

AN731

LCD Backlight

Inverter Drive

Typical Application Circuits

OLR

OUTA

TX1

C10

15p

C19

2.2n

OLR

OUTD

OUTC

FDS8958A

R2
56k

FDS8958A

C1 1u

C14
20n

10u

R22

10k

D11

D1N4148

100k

R15

C6 1u

BAV99

DIM(0~3.3V)

ENA

C22

C4 4.7n

R5 27k

22k

C16

2.2n

C28
10n

CCFL

4.7n

R26

10k

OLP1

IC1 FAN7311

S_S

GND

REF

OLR

ENA

BDIM

OLP

VIN

OUTA

PGND

OUTB

RT1

OUTC

ADIM

EA_IN

EA_OUT

OUTD

BCT

0.1u

OLP

OUTB

KST2222

D1N4148

R6 120k

R16

1.5k

R9
8.2k

REF

C25

100u

R25

10k

C27
1u

C21

10n

20k

BAV99

OLP1

C5 220p

R14

100k

R24

10k

R1
330k

REF

VCC

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