Micro Linear Corporation ML4877ER, ML4877CR Datasheet

JULY 2000

NEW!

30W CCFL Inverter Design

ML4877*

LCD Desktop Backlight Lamp Driver

GENERAL DESCRIPTION

The ML4877 is an ideal solution for driving multiple cold
cathode fluorescent tubes (CCFL) used in liquid crystal
display (LCD) backlight applications. It provides dimming
ballast control for the LCD display.

By utilizing differential drive the ML4877 can deliver the
same light output with significantly less input power
compared to existing single ended drive schemes.
Improvements as high as 30% can be realized when using
low power lamps and advanced LCD screen housings. This
increased light output is achieved because the differential
drive configuration is much less sensitive, and therefore
less power is wasted in the capacitive parasitics that exist
in the backlight housing. An additional benefit of this
configuration is an even distribution of light.

The IC includes an adjustable lamp out detect circuit that
latches the IC off when a lamp fault is detected. Also, the
unique architecture of the ML4877 allows the
development of a backlight system that will inherently
meet the UL requirements for safety.

The ML4877 is optimized for large LCD applications
applications where high efficiency is critical to maximize
battery life. The high efficiency is achieved by a resonant
scheme with zero voltage switching.

FEATURES

■ Ideal for 30W inverter designs, 1 to 8 lamp design

■ PWM dimming capability

■ Backlight lamp driver with differential drive

■ Up to 30% lower power for same light output

■ Low standby current (<10µA)

■ Improved efficiency (»95%)

■ Allows all N-channel MOSFET drive

■ Adjustable lamp out detect with latch

■ Resonant threshold detection and synchronous

rectification

■ Positive input for dimming control

* THIS PART IS END OF LIFE AS OF JULY 1, 2000

BLOCK DIAGRAM

AZR

2

REGULATOR

5

PGND18GND

LINEAR

MASTER

BIAS

&

UVLO

17

ON/OFF

HVDD

VREF

12

15

VDD

13

VDD

Q

S

QR

B ON

19

DR3 DR1 DR1

ONE

SHOT

SS

+

–

VDD

CLK

S

Q

RQ

B OFF

RESONANT

THRESHOLD

DETECTOR

OSCILLATOR

4

R

T

B SYNC OUT

20

11

L RTD

10

L GATE 1

DR2

Q

NEG

EDGE

DELAY

9

T

T

Q

DR2

–

+

SS

3

SS CAPC

14

L GATE 2

16

6

+

–

–

+

0.5V
8

7

1

L ILIM

LEA OUT

LEA–

LEA+

1

ML4877

PIN CONFIGURATION

PIN DESCRIPTION

LEA+

AZR

SS CAP

R

VREF

L ILIM

LEA–

LEA OUT

C

L RTD

ML4877

20-Pin SSOP (R20)

1
2
3

T

T

4
5
6
7
8
9

10

TOP VIEW

20
19
18
17
16
15
14
13
12
11

B OFF
B ON
GND
PGND
L GATE 2
ON/OFF
L GATE 1
VDD
HVDD
B SYNC OUT

PIN NAME FUNCTION

1 LEA+ Positive input for lamp error amp

2 AZR Connection to gate of external FET for

high voltage regulator. Internally a
zener diode to ground.

3 SS CAP Connection of optional external soft

start capacitor

4R

T

Oscillator timing resistor

5 VREF Voltage reference output

6 L ILIM Input to current limit amplifier

7 LEA- Negative input for lamp error

amplifier

8 LEA OUT Output of lamp error amplifier.

External compensation capacitor
connects between this pin and LEA.

9C

T

Oscillator timing capacitor

10 L RTD Input to resonant threshold detector

PIN NAME FUNCTION

11 B SYNC Output of MOSFET driver to gate of

OUT synchronous FET catch diode.

12 HVDD Battery power input to linear regulator

13 VDD Output of linear regulator. Positive

power for IC.

14 L GATE1 Output of MOSFET driver. Connection

to gate of one side of inverter FET
drive pair.

15 ON/OFF Logic input for chip

16 L GATE2 Output of MOSFET driver. Connection

to gate of one side of inverter FET
drive pair.

17 PGND Power ground

18 GND Signal ground

19 B ON Connection to primary side of gate

pulse transformer

20 B OFF Output of MOSFET driver. Connection

to gate of FET that disables the input
power.

2

ABSOLUTE MAXIMUM RATINGS

ML4877

Absolute maximum ratings are those values beyond which
the device could be permanently damaged. Absolute
maximum ratings are stress ratings only and functional
device operation is not implied.

Voltage on HVIDD .................................................... 20V

Current into L RTD ............................................... ±10mA

Junction Temperature.............................................. 150ºC

Storage Temperature Range .......................-65ºC to 150ºC

Lead Temperature (Soldering 10 sec.) ..................... 260ºC

Supply Current (ICC) ............................................... 75mA

Thermal Resistance (qJA).................................... 100ºC/W

Output Current, Source or Sink ............................. 250mA

Voltage on Pins LEA+, AZR, SS CAP, RT, VREF,

OPERATING CONDITIONS

L ILIM, LEA-, LEA OUT, CT, B SYNC OUT, VDD,
L GATE 1, ON/OFF, L GATE 2, PGND, GND,

B ON, B OFF ............................... –0.3V to VDD +0.3V

Temperature Range

ML4877C ................................................... 0ºC to 70ºC

ML4877E...............................................–20ºC to 70ºC

ELECTRICAL CHARACTERISTICS

Unless otherwise specified, VDD = 5V ±5%, TA= Operating Temperature Range, CT = 47pF, RT = 82kW (Note 1)

SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS

CURRENT REGULATOR

ERROR AMPLIFIER

Open Loop Gain 60 70 dB

Output High I

Output Low I

Bandwidth (-3dB) 1 MHz

= 5µA 2.8 3.0 V

LOAD

= 25µA 0.4 0.7 V

LOAD

Common Mode Voltage Range 0 1.0 V

Input Bias Current 50 100 nA

Input Offset Voltage -5 0 5 mv

Soft Start Charge Current V

Soft Start Threshold (LEA OUT) V

CURRENT LIMIT COMPARATOR

Current Threshold 450 500 550 mV

Input Bias Current V

Propagation Delay (Note 2) 150 250 ns

OUTPUT DRIVERS

Output High - B SYNC OUT, B OFF VDD 5V, I

Output Low - B SYNC OUT, B OFF I

Rise & Fall time - B SYNC OUT, B OFF C

Output High - B ON VDD 5V, I

Output Low - B ON I

Fall Time - B ON C

ONE SHOT

= 1V 550 750 950 nA

SSCAP

= 1V 2 2.5 V

SSCAP

= 0.1V 50 100 nA

ILIM

= 12mA 4.625 4.8 V

LOAD

12mA 0.2 0.375 V

LOAD

=100pF 20 50 ns

LOAD

= 12mA 4.625 4.8 V

LOAD

50mA 0.2 0.375 V

LOAD

= 2400pF (Note 2) 45 80 ns

LOAD

Pulse Width 100 150 200 ns

3

ML4877

ELECTRICAL CHARACTERISTICS (Continued)

SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS

DELAY TIMER

Delay Time 20 35 55 ns

HIGH VOLTAGE INVERTER

Oscillator

Nominal Frequency 68 80 92 kHz

Discharge Current VCT = 2V 500 700 900 µA

Peak Voltage 2.3 2.5 2.7 V

Valley Voltage 0.8 1 1.2 V

Output Drivers

Output High - L GATE 1, 2 VDD = 5V, I

Output Low - L GATE 1, 2 I

Rise & Fall Time - L GATE 1, 2 C

Resonant Threshold Detector

Threshold 0.45 0.8 1.15 V

Hysteresis 0.15 03 0.45 mV

Lamp Out Detect

Threshold -2 VDD 2 %

Latch Inhibit Threshold (SSCAP) LRTD > VDD + 0.1V 2.5 V

Under Voltage Detector

Start Up Threshold 3.8 4.1 4.4 V

Hysteresis 150 300 450 mV

Logic Interface (ON/OFF)

V

IH

V

IL

Input Bias Current ON/OFF = 3V 10 25 µA

Linear Regulator

12mA 4.625 4.8 V

LOAD

= 50mA 0.2 0.375 V

LOAD

=1000pF 20 50 ns

LOAD

2.5 V

0.5 V

Aux Zener Reference Voltage (AZR) I

Regulator Voltage (VDD) HVDD = 12V 4.75 5.0 5.35 V

Regulator Source Current External to device 10 mA

Drop Out Voltage I

Drop Out Voltage l

HVDD Input Voltage Range 5 18 V

= 10µA 12.3 13.5 14.7 V

AZR

= 1mA 30 90 mA

HVDD

= 5mA 125 275 mA

HVDD

4