N6774A - Agilent / Hewlett-Packard

Testing LCD Backlight Inverters

using the Agilent N6700

Modular Power System (MPS)

Application Note

Introduction

This application note describes

how to use the Agilent N6700

Modular Power System (MPS)

to test the backlight inverter

of a Liquid Crystal Display

(LCD). Testing the inverters

used to power and control

the backlights of LCDs (cold-

cathode-fluorescent-lamp,

or CCFL) requires multiple

sequenced output voltages of

different power ratings and

precise timing with up to mil-

lisecond resolution or better.

The N6700 MPS provides the

flexibility and features needed

in order to make the required

tests and reduce costs by elimi-

nating the need for multiple

instruments and providing

simple programming.

Description

During the design validation

process, it is essential for an

LCD manufacturer to test the

CCFL inverter to its specifica-

tions. They do this in order to

ensure that the inverter chosen

will perform as expected and

does not shorten the life of the

CCFL backlight, which is one

of the key components of an

LCD screen. Thousands of

on/off tests are performed over

a range of temperatures from

low to high in order to ensure

the proper operation of the

inverter and guarantee the

life of the backlight.

Problem

An inverter usually takes

multiple input signals, such as

VIN, which is the main power

input, analog dimming, digital

dimming, and on/off. To ensure

proper operation of the inverter

and ultimately the backlight,

these signals are precisely

sequenced to within a resolution

of one millisecond. In order to

properly test the inverter to

these input signal specifications,

you need to be able to precisely

create, control, and reproduce

these very different signals.

Trying to piece together the test

and measurement equipment

necessary to perform this task

can prove time consuming and

costly. This equipment can con-

sist of an oscilloscope with a

current probe or current shunt,

a DMM, a function generator,

and a power supply. However,

even after piecing together a

costly system, such as the one

above, the required functionality

may still be missing.

An alternative to piecing together

test equipment might be to use

the actual power/control board

used in the finished product,

however, this would also prove

very time consuming and

costly. The actual board does

not provide the capability to

simply and easily reprogram

for multiple tests nor does it

provide measurement or moni-

toring capabilities, which are

essential to confirming proper

operation. In order to measure

and monitor the tests being

performed, it would be neces-

sary to introduce test and

measurement equipment in

between the board and the

inverter being tested creating a

very complicated and potentially

unreliable system.

Example: 26 inch LCD TV

Most leading manufacturers of

LCD TVs have strict specifica-

tions regarding the backlight

inverters they use in their TVs.

Figure 1 above is a graph of four

typical signals and the neces-

sary timing between the signals

in order for the inverter to

function properly. Table 1 lists

common specifications for

these signals.

Signal Description Specifications

Minimum Typical Maximum Other

VIN Input Voltage 21.6 V 24 V 26.4 V

Max Power ~160 W

Input Current – 3.0 A 6 A

Analog Dimming

High – 3.3 V –

10 mV Resolution

Control (ADIM) Low – 0 V –

Digital Dimming

PWM 100 Hz – 300 Hz 0% to 100%

Control (DDIM) Frequency Duty Cycle

PWM Dimming 33% – 100% VIN = 24 V

Range VADIM = 3.3 V

On/Off On/High 2.3 V – 5.0 V

10 mV Resolution

Off/Low 0 V – 0.9 V

Table 1. Typical Inverter Signal Specification

Figure 1. Typical Inverter Input Timing Graph

VIN

ADIM Control Range: 0 ~ 3.3 V

ADIM

DDIM

On/Off

1.0 s (Min)

0.5 s (Min)

0.02 s (Min)

PWM

0.1 s (Min)

In order to properly reproduce

the four signals, you need a

system that can precisely create

as well as sequence them. You

also need a system that can

produce over 100 W of power

since most backlights require

this much power or more. The

power input signal V

IN, the

first signal in the graph, can

require up to ~160 W of power

(maximum voltage of 26.4 V

multiplied by a maximum

current of 6 A). Larger TVs

will require even more power

since they will have more

CCFL backlights.

Solution: The Agilent N6700 MPS

The N6700 MPS has the features

and covers the necessary power

ranges in order to properly

reproduce these signals without

the need for any extra equip-

ment. It is a compact, 1 U

mainframe and module based

system, where one mainframe

can hold up to four power

modules. There are three

mainframes (400 W, 600 W,

and 1200 W) as well as three

module performance tiers,

from basic to precision, which

can be mixed and matched

in any mainframe to suit the

needs of your test system.

Table 2 shows the requirements

for each signal along with the

recommended modules for this

test system. Each module meets

or exceeds the requirements.

VIN

VIN is supplied by an N6774A

module. The Agilent N6774A is

a 35 V/8.5 A Basic DC Power

Module that is capable of

supplying 300 W. It more than

adequately covers the power

for the backlight of the example

26 inch LCD TV, while also

providing ample margin.

However, if testing larger LCD

TVs, which have more CCFL

backlights that require more

power, you can easily parallel

it with other N6774A modules

for greater output current.

Other modules in the N6770

family could also be used if

your voltage requirements

are different (up to 100 V).

Slew rate control is used on

this output to control the speed

of turn-on in order to match

the specification illustrated

in Figure 1. Slew rate control

provides the ability to program

in Volts per second (V/s) the

speed at which the voltage will

slew from one programmed

value to the next. This feature is

used to either speed up or slow

down the transition from voltage

to voltage when necessary.

Although slew rate control

is only required on V

IN, all of

the modules available for the

N6700 MPS have this capability.

Signal Requirements Summary Recommended Required Module Features

N6700 MPS Module

VIN 20 V – 28 V N6774A 300 W Basic Module

6 A Maximum 35 V, 8.5 A Output Sequencing (1 ms Resolution)

Maximum Power ~160 W 300 W Slew Rate Control to 2400 V/s

Slew 0 to 24 V in 0.02 s

Analog 0 V – 5 V N6751A High Performance 100 W Module

Dimming 2 A Maximum 50 V, 5 A Output Sequencing (1 ms Resolution)

Control Maximum Power ~10 W 50 W

(ADIM) 10 mV Resolution

Digital 2 V – 5 V N6762A Precision 100 W Module

Dimming 1 A Maximum 50 V, 3 A Low (5.5 V) Range

Control Maximum Power ~5 W 100 W Output Sequencing (1 ms Resolution)

(DDIM) Pulse Width Modulated Signal LIST Mode for PWM Signal

100 Hz to 300 Hz Slew Rate of 3000 V/s or better

0% to 100% Duty Cycle for extreme case PWM signal

On/Off 0 V – 5 V N6751A High Performance 50 W Module

2 A Maximum 50 V, 5 A Output Sequencing (1 ms Resolution)

Maximum Power ~10 W 50 W

10 mV Resolution

Table 2. Requirements Summary and Recommended Modules

On/Off and Analog Dimming

The On/Off and Analog

Dimming

signals are supplied

by the Agilent N6750

High-

Performance,

Autoranging DC

Power Modules, which are

50 W or 100 W. Both signals are

provided by the N6751A

This

module provides the necessary

features for the signals. It can

also be coupled with Option 054,

High-Speed Test Extensions,

which adds LIST mode and

digitization. If more precise

programming and measurement

accuracy is needed, the Agilent

N6760 family could be used

for these signals.

Digital Dimming, PWM

The pulse-width modulated

digital dimming signal is sup-

plied by an N6762A Module.

The Agilent N6760 Precision

DC Power Module family pro-

vides a higher level of accuracy

and includes Option 054 as

standard equipment. These

modules also incorporate dual

voltage ranges (0 – 5.5 V or 0 –

50 V). The lower range pro-

vides more accurate program-

ming and measurement than

the higher range.

The N6762A also incorporates

LIST mode. LIST mode pro-

vides the

ability for up to 512

programmed

voltage and/or

current points. This ability

is best described as a “power

arbitrary waveform generator”

that can create almost any

waveform. Using the speed

(0 to 5 V in 160 µs) and accuracy

of the N6762A with LIST mode

will adequately reproduce the

PWM signal necessary for

digital dimming.

Output Delay (Sequencing)

Output delay is used on each

output channel of the system

in order to precisely time the

outputs to within the 1 ms

required resolution. Delays

can be programmed for each

output and when the output-

on command is sent the

programmed delay must pass

before the output will actually

turn on. This is done to

microsecond accuracy, but

is limited to one mainframe.

An alternate way of sequencing

outputs both within a main-

frame and between mainframes

is by using LIST mode and

LIST triggers, but this is beyond

the scope of this document.

Summary

Precise, repeatable reproduction

and sequencing of input signals

is essential in order to properly

test LCD backlight inverters.

Many different problems can

arise when trying to accomplish

this task using conventional

test and measurement equip-

ment, which can become

time consuming and costly.

The Agilent N6700 MPS solves

these problems in one simple

system. The N6700 MPS can

easily synchronize outputs

using the output delay feature.

The breadth of modules avail-

able for the N6700 MPS pro-

vide the varied levels of capa-

bilities needed to produce the

high-power VIN signal with an

N6774A and the complicated

low-power PWM digital dimming

signal with the N6762A.

The system is also easily

expandable when the need to

test larger LCDs arises. Larger

LCDs require either more or

larger backlights, which will

require more power. Modules

can be easily paralleled using

the unique virtual channel

feature in the N6700 MPS

to achieve the higher power

requirements. However, adding

modules to the system also

means adding additional main-

frames. The N6700 mainframes

have the ability to precisely

trigger between one another

by programming one of the

eight digital I/O pins to an

external trigger in/out. The

benefit of this system is that

no matter what the inverter

specification is, whether very

strict or not, low or high power,

analog or digital dimming, the

N6700 MPS has the flexibility to

accommodate each individual

situation.

Other Relevant Application

Notes/References

5989-1676EN

Application Note 1504

Simplify Multiple Bias Voltage

Sequencing and Ramping for

PC Motherboard Test

Agilent N6700 MPS Web Page

www.agilent.com/find/n6700

www.agilent.com

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Product specifications and descriptions in

this document subject to change without

notice.

Printed in the USA December 9, 2005

5989-4019EN

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