Texas Instruments UCC2977EVM User manual

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July 2002 PMP Systems Power
User’s Guide
SLLU049

IMPORTANT NOTICE
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any product or service without notice. Customers should obtain the latest relevant information before placing
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TI warrants performance of its hardware products to the specifications applicable at the time of sale in
accordance with TI’s standard warranty. Testing and other quality control techniques are used to the extent TI
deems necessary to support this warranty. Except where mandated by government requirements, testing of all
parameters of each product is not necessarily performed.
TI assumes no liability for applications assistance or customer product design. Customers are responsible for
their products and applications using TI components. To minimize the risks associated with customer products
and applications, customers should provide adequate design and operating safeguards.
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Use of such information may require a license from a third party under the patents or other intellectual property
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Reproduction of information in TI data books or data sheets is permissible only if reproduction is without
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Mailing Address:
Texas Instruments
Post Office Box 655303
Dallas, Texas 75265
Copyright 2002, Texas Instruments Incorporated

EVM IMPORTANT NOTICE
Texas Instruments (TI) provides the enclosed product(s) under the following conditions:
Thisevaluationkit being soldbyTI is intendedforuse for ENGINEERING DEVELOPMENT OR EVALUATION
PURPOSES ONLY and is not considered by TI to be fit for commercial use. As such, the goods being provided
may not be complete in terms of required design-, marketing-, and/or manufacturing-related protective
considerations, including product safety measures typically found in the end product incorporating the goods.
As a prototype, this product does not fall within the scope of the European Union directive on electromagnetic
compatibility and therefore may not meet the technical requirements of the directive.
ShouldthisevaluationkitnotmeetthespecificationsindicatedintheEVMUser’sGuide,thekitmaybe returned
within 30 days from the date of delivery for a full refund. THE FOREGOING WARRANTY IS THE EXCLUSIVE
WARRANTY MADE BY SELLER TO BUYER AND IS IN LIEU OF ALL OTHER WARRANTIES, EXPRESSED,
IMPLIED,ORSTATUTORY,INCLUDINGANYWARRANTY OF MERCHANTABILITY ORFITNESSFORANY
PARTICULAR PURPOSE.
The user assumes all responsibility and liability for proper and safe handling of the goods. Further, the user
indemnifies TI from all claims arising from the handling or use of the goods. Please be aware that the products
receivedmaynotberegulatorycompliant oragency certified (FCC,UL, CE,etc.). Dueto theopen construction
oftheproduct,itistheuser’sresponsibilitytotakeanyandallappropriateprecautionswithregardtoelectrostatic
discharge.
EXCEPT TO THE EXTENT OF THE INDEMNITY SET FORTH ABOVE, NEITHER PARTY SHALL BE LIABLE
TO THE OTHER FOR ANY INDIRECT, SPECIAL, INCIDENTAL, OR CONSEQUENTIAL DAMAGES.
TI currently deals with a variety of customers for products, and therefore our arrangement with the user is not
exclusive.
TI assumes no liability for applications assistance, customer product design, software performance, or
infringement of patents or services described herein.
Please read the EVM User’s Guide and, specifically, the EVM Warnings and Restrictions notice in the EVM
User’sGuidepriortohandlingtheproduct.Thisnoticecontainsimportantsafetyinformationabouttemperatures
and voltages. For further safety concerns, please contact the TI application engineer.
Persons handling the product must have electronics training and observe good laboratory practice standards.
No license is granted under any patent right or other intellectual property right of TI covering or relating to any
machine, process, or combination in which such TI products or services might be or are used.
Mailing Address:
Texas Instruments
Post Office Box 655303
Dallas, Texas 75265
Copyright 2002, Texas Instruments Incorporated

EVM WARNINGS AND RESTRICTIONS
It is important to operate this EVM within the input and output voltage ranges specified in the
user’s guide.
Exceeding the specified input range may cause unexpected operation and/or irreversible
damage to the EVM. If there are questions concerning the input range, please contact a TI
field representative prior to connecting the input power.
Applyingloadsoutsideofthespecifiedoutputrangemayresultinunintendedoperationand/or
possible permanent damage to the EVM. Please consult the EVM User’s Guide prior to
connecting any load to the EVM output. If there is uncertainty as to the load specification,
please contact a TI field representative.
Duringnormaloperation,somecircuitcomponentsmayhavecasetemperaturesgreaterthan
60°C.TheEVM isdesignedto operateproperlywith certaincomponentsabove 60°C aslong
astheinputandoutput rangesare maintained.Thesecomponentsincludebut arenot limited
tolinearregulators,switchingtransistors,passtransistors,andcurrentsenseresistors.These
typesof devicescanbeidentifiedusingtheEVM schematiclocated intheEVMUser’sGuide.
When placing measurement probes near these devices during operation, please be aware
that these devices may be very warm to the touch.
Mailing Address:
Texas Instruments
Post Office Box 655303
Dallas, Texas 75265
Copyright 2002, Texas Instruments Incorporated

Related Documentation From Texas Instruments
iii
Preface
Read This First
About This Manual
This users guide describes the characteristics, operation, and use of the
UCC2977 CCFL backlight converter evaluation module (EVM). The users
guide includes a schematic diagram, bill of materials and test results.
How to Use This Manual
This document contains the following chapters:
-Chapter 1—Hardware
-Chapter 2—Design Procedure
-Chapter 3—Test Results
Related Documentation From Texas Instruments
-Texas Instruments Users Manual, UCC3976/7 Users Manual, TI
Literature Number SLUU108
-Texas Instruments Data Sheet, UCC2977, TI Literature Number
SLUS499A
FCC Warning
Thisequipmentisintendedforuseinalaboratorytestenvironmentonly.Itgen-
erates,uses,andcanradiateradiofrequencyenergy,andhasnotbeentested
for compliance with the limits of computing devices pursuant to subpart J of
part 15 of FCC rules, which are designed to provide reasonable protection
against radio frequency interference. Operation of this equipment in other en-
vironmentsmaycauseinterferencewithradiocommunications,inwhichcase
the user at his own expense will be required to take whatever measures may
be required to correct this interference.

iv

Contents
v
Contents
1 Hardware 1-1. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1.1 Introduction 1-2. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1.2 Operating Specifications 1-2. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1.3 Schematic 1-3. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1.4 Bill of Materials 1-4. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1.5 Board Layout 1-5. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2 Design Procedure 2-1. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.1 Push-Pull Inductor Selection 2-2. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.2 Frequency Range Setting 2-2. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.3 Analog Dimming of the Lamp 2-2. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.4 Open-Lamp Voltage Programming 2-3. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.5 Shutdown 2-3. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.6 No Lock 2-3. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.7 Burst Dimming 2-4. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.8 Clamp Circuit (Optional) 2-4. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3 Test Results 3-1. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.1 Test Setup 3-2. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.1.1 Lighting CCFL 3-2. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.1.2 Analog Dimming Test 3-3. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.1.3 Burst Dimming Test 3-4. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.2 Test Results 3-5. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Contents
vi
Figures
1–1 UCC2977EVM Schematic 1-3. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1–2 Top Silk Screen With Top Copper Layer 1-5. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1–3 Top Layer Copper 1-5. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1–4 Bottom Layer Copper 1-5. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3–1 Test Setup 3-2. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3–2 Test Setup for Analog Dimming 3-3. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3–3 Test Setup for Burst Dimming 3-4. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3–4 Efficiency With a Low Profile Sumida Inductor 3-5. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3–5 Efficiency With a High Profile Toko Inductor 3-5. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3–6 Transformer Input/Output Voltages at VI= 3 V 3-6. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3–7 Transformer Input/Output Voltages at VI= 6 V 3-6. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3–8 Burst Dimming With 50% Duty Cycle 3-6. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3–9 Burst Dimming With 80% Duty Cycle 3-7. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3–10 Open Lamp Protection 3-7. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Tables
1–1 Operating Specifications 1-2. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1–2 Bill of Materials Required for the UCC2977 (SLUP178) 1-4. . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1-1
Hardware
The UCC2977 evaluation module (SLUP178) provides a reference design for
evaluating the performance of a high efficiency CCFL backlight converter
usingthe UCC2977 (push-pull backlightcontroller).Thedevice contains all of
the circuitry necessary to control a backlight converter. This chapter contains
the schematic, board layout, and the bill of materials. The evaluation module
(EVM) performance specifications are also given.
Topic Page
1.1 Introduction 1-2. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1.2 Operating Specifications 1-2. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1.3 Schematic 1-3. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1.4 Bill of Materials 1-4. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1.5 Board Layout 1-5. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Chapter 1

Introduction
1-2
1.1 Introduction
TheUCC2977EVM(SLUP178)isa1.8-Wdc/acinvertermoduleusedtodrive
a cold cathode fluorescent lamp (CCFL) with a piezoelectric transformer
(PZT). This EVM consists of a push-pull circuit using the UCC2977 controller.
Theprincipleofoperationfortheinverterisexplainedintheapplicationsection
of the UCC2977 data sheet.
1.2 Operating Specifications
This section summarizes the performance specifications of the SLUP178
converter. Table 1–1 lists the operating specifications for the SLUP178.
Table 1–1.Operating Specifications
Input voltage (VIN)3~6 Vdc
Output voltage (Lamp_HV, Lamp LV) 300 Vac, rms
Output current (lamp current) 4.7 mA, rms
Operating frequency (kHz) 100 kHz
Burst dimming (BD )†Square wave, 120 Hz, 0 to 3 V
Analog dimming (AD) 0~3 Vdc
CCFL lamp JKL BF3100–20B (100 mm, 270 VRMS)
†Use a function generator.

Schematic
1-3
Hardware
1.3 Schematic
Figure 1–1.UCC2977EVM Schematic

Bill of Materials
1-4
1.4 Bill of Materials
Table 1–2.Bill of Materials Required for the UCC2977 (SLUP178)
Count Ref Des Description Size MFR Part Number
1 C1 Capacitor,ceramic,2200pF,50V,
X7R, 20% 603 Murata GRM188R71H222KA01
1 C2 Capacitor, ceramic, 220 pF, 50 V,
X7R, 20% 603 Murata GRM188R7H221KD01
2C3, C7 Capacitor, ceramic, 0.1 µF, 25 V,
X7R, 20% 805 Murata GRM21BR71E104KA01
1 C4 Capacitor, ceramic, 0.022 µF,
25 V, X7R, 20% 603 Murata GRM188R71E223KA01
2C5, C6 Capacitor, ceramic, 4.7 µF, 10 V,
X5R, 20% 1206 Murata GRM31CR61A475KA01
2D1, D2 Diode, switching, 1.0 mA, 75 V SOD123 Motorola MMSD914T1
1 D3 Diode, dual ultra fast, series,
200 mA, 70 V SOT23 Fairchild BAV99
1 JP1 Header, 3 pin, 100 mil spacing,
(36-pin strip) 0.100 ×3ISullins PTC36SAAN
2L1, L2 Inductor, SMT, 22 µH, 1.2 A,
128 mΩ6.7 ×6.7 ×3 mm Sumida CDRH6D28-220NC
(or Toko inductor, 22 µH, 115 mΩ7.6 ×7.6 ×5.1 mm Toko 646CY-220M
1 Q1 Transistor, NPN, high perfor-
mance, 500 mA SOT23 ZeTek FMMT491A
1 Q2 XSTR, MOSFET, N-channel,
30 V, 4.6 A, Rds 35 mΩTSSOP8 IR IRF7752
1 R1 Resistor, chip, 22 kΩ, 1/16 W, 1% 603 Std Std
1 R10 Resistor, chip, 50 kΩ, 1/16 W, 1% 603 Std Std
2R12, R13 Resistor, chip, 10 Ω, 1/16 W, 1% 603 Std Std
1 R15 Resistor, chip, 909 Ω, 1/16 W, 1% 603 Std Std
1 R2 Resistor,chip,150kΩ,1/16W,1% 603 Std Std
1 R3 Resistor,chip,182kΩ,1/16W,1% 603 Std Std
1 R4 Resistor, chip,8.2kΩ,1/16W,1% 603 Std Std
2R5, R6 Resistor,chip,2.0 kΩ, 1/16 W,1% 603 Std Std
8R7, R8. R9.
R11, R14,
R16, R17,
R18
Resistor, chip, 1.0 MΩ, 1/10 W,
1% 805 Std Std
1 T1 Transformer, custom,
Piezoelectric, SMT 0.760 ×0.244 Panasonic EFTU11R8MX50
2TP1, GND Test point, black, 1 mm 0.038 Farnell 240-333
9 TP2–TP5,
VIN, BD, AD,
LAMP_HV,
LAMP_LV
Test point, red, 1 mm 0.038 Farnell 240-345
1 U1 IC, CCFL push-pull driver TSSOP-08 TI UCC2977PW
1—PCB, 3.55I×0.905I×0.62IAny SLUP178

Board Layout
1-5
Hardware
1.5 Board Layout
Figure 1–2.Top Silk Screen With Top Copper Layer
Figure 1–3.Top Layer Copper
Figure 1–4.Bottom Layer Copper

1-6

2-1
Design Procedure
Design Procedure
This chapter describes how to design the UCC2977EVM.
Topic Page
2.1 Push-Pull Inductor Selection 2-2. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.2 Frequency Range Setting 2-2. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.3 Analog Dimming of the Lamp 2-2. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.4 Open-Lamp Voltage Programming 2-3. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.5 Shutdown 2-3. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.6 No Lock 2-3. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.7 Burst Dimming 2-4. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.8 Clamp Circuit (Optional) 2-4. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Chapter 2

Push-Pull Inductor Selection
2-2
2.1 Push-Pull Inductor Selection
The push-pull topology requires a different approach for calculating the
externalinductorvalues.ReferringtoFigure1–1,Q2AandQ2Baredrivenout
of phase at 50% duty cycle. When Q2A is on, current is ramped up in L1.
During the next switch cycle Q2B is turned on, Q2A is turned off, and the
energy stored in L1 is transferred through the piezoelectric transformer. L1
resonates with the piezoelectric primary capacitance forming a half sinusoid
at Q2A’s drain. In order to achieve zero-voltage switching, the drain voltage
must return to ground before the next switching cycle. This dictates that the
LC resonant frequency must be greater than the switching frequency.
Lt1
4 p2 ƒ2 Cp
(1)
In the EVM with a 100 kHz switching frequency and 61.6 nF piezoelectric
transformer, L1andL2must be lessthan41µH for zero-voltage switching,so
22-µH inductors are used.
A suitable capacitor must be chosen so that the peak-to-peak output ripple is
within the limits allowable for the application.
2.2 Frequency Range Setting
The VCO frequency range is programmed with external components R1, R3
andC2.The frequency is determined by thevoltagedecayfrom1.7Vto0.6 V
at the OSC pin. When the voltage reaches 0.6 V, an internal current source
charges OSC back to 1.7 V. The decay time is determined by the value of C2
and the discharge currents generated in R1 and R3. An accurate NPO
capacitor is recommended for C2 and 1% resistors are recommended for R1
and R3. The output frequency range can be calculated by equation 2:
ƪR1 )R3
R1 R3 C2ƫ(2)
2 ln
ȧ
ȱ
Ȳ
1.7 V (R1 )R3) *VCOMP R1
ǒ0.6 V *VCOMPǓ R1 )0.6 V R3ȧ
ȳ
ȴ
ƒǒVCOMPǓ+
WithC2setat220pF,R1at22kΩandR3at182kΩ,thefrequencyis100kHz.
2.3 Analog Dimming of the Lamp
For analog dimming, enable the open-lamp detection by connecting pin 1 to
pin2ofjumperJP1andgroundBD.Lampintensityiscontrolledwiththesignal
AD.
The RMS lamp current becomes:
ILAMP +ƪ1.5 V (R2 )R10) *VAD R10ƫp
2
Ǹ R2 R15
(3)

Open-Lamp Voltage Programming
2-3
Design Procedure
Zero volts on VAD commands full current while 3 V commands minimum
current. For the initially configured EVM, maximum current is 4.8 mA. R15 is
selected to be 909 Ωsetting VAD to 0 V and lamp current to 4.8 mA. With R2
setto 150 K, R10iscalculatedto be 50 K.Thecontrol-voltagetolamp-current
equation for the EVM is:
ILAMP(mA) +4.89 *1.22 VAD (4)
2.4 Open-Lamp Voltage Programming
It is necessary to suspend the power-stage operation if an open lamp occurs
because the piezoelectric transformer has a high gain. The open-lamp
detection circuit is composed of voltage divider R7, R8, R9, R11, R14, R16,
R17, R18, R4, R6, D2, and C1. A 1.5 V comparator at the OPEN pin shuts
down the inverter if an open lamp is triggered. The RMS secondary voltage at
which an open lamp shutdown occurs can be calculated by equation (5).
VOPEN +
ǒ1.5 V )VdiodeǓ ǒS
9
i+7Ri)R11 )R14 )S
18
j+16Rj)R4 )R6Ǔ
2
Ǹ (R4 )R6) RMS
(5)
With R7~R9, R11, R14, R16~R18 at 1 Meg each, R6 at 2 k and R4 at 8.2 k,
theRMSvalueofopenlampthresholdvoltageatthesecondaryis1220VRMS.
The value of capacitor C1 should be large enough to filter the sinusoid
waveform into dc. For this board, 2200 pF was chosen. If C5 is too large, it
takes extra time for OPEN/SD to climb to 1.5 V when the output voltage hits
the open-lamp threshold, resulting in a higher output voltage than the set
value.
Theopen-lamplevelshouldbesethighenoughtoavoidtrippingduringnormal
operation. Open-lamp detection is disabled for the burst-dimming mode by
connecting pin 3 to pin 2 of JP1. Connect pin 1 to pin 2 of JP1 to enable open
lamp protection.
2.5 Shutdown
The OPEN/SD pin is used for both open-lamp detection and commanded
shutdown. When a voltage higher than 2.5 V is applied to OPEN/SD through
D1,thepartenterstheshutdownorsleepmodewheretheoscillatorisinactive
andbothoutputsarehigh.Inthis mode, the partdrawslittlecurrentattheVDD
pin and the OPEN/SD pin.
2.6 No Lock
If the part fails to achieve regulation before reaching minimum frequency
(comp >2.2 V), it causes an internal retry counter to increment and then
attempt another start up. If the application does not operate normally after 7
attempts, the controller enters an error-induced shutdown state removing
power to the load.

Burst Dimming
2-4
2.7 Burst Dimming
Burst dimming can be implemented at the OPEN/SD pin at the cost of
open-lamp detection.
Connect pin 2 to pin 3 of JP1 to disable the open-lamp detection circuit during
burst dimming mode.
Sincethe feedback loop doesnotneed to operatewithminimumlamp current
(as with analog dimming), the feedback capacitor, C4, can be reduced to 22
nF to improve the response time when the lamp restrikes.
To implement burst dimming on the EVM, AD should be grounded to set
maximum lamp brightness. A low-frequency 0 V –4 V square wave applied
to BD will modulate the lamp current between zero and full intensity at the
desired frequency. A low-frequency repetition rate greater than 120 Hz is
recommended to avoid visible flicker. Applying 5 V at BD forces the lamp
current to zero, where 0 V at BD will force maximum lamp current. The duty
cycleofthesquarewavedeterminesthelampbrightnessasapercentofrated
lamp current.
2.8 Clamp Circuit (Optional)
There is also a clamp circuit to limit the maximum secondary voltage.
This circuit is typically not needed since the CCFL strikes or the open-lamp
circuit triggers before destructive levels are reached.
If R5 and Q1 are added to the board, the peak clamp voltage for the EVM is
given by equation (6):
Sincethe feedback loop doesnotneed to operatewithminimumlamp current
(as with analog dimming), the feedback capacitor, C4, can be reduced to 22
nF to improve the response time when the lamp restrikes.
VCLAMP +ǒ1.5 V )VbeǓ (R7)R8)R9)R11)R14)R16)R17)R18)R6)R4)
R4 PEAK (6)
The peak clamped voltage for the evaluation board is 2200V.
The evaluation board components can be modified depending on the
application requirements. A supply between 3 Vdc and 6 Vdc is required for
thisboardtooperatea270~450Vcold-cathodefluorescentlamp(CCFL)from
1 mA to 4.8 mA. The input voltage range changes with different voltage lamp.

3-1
Test Results
Test Results
ThischapterdescribeshowtoproperlyconnectandsetuptheUCC2977EVM.
It also presents the test results, which cover efficiency, burst dimming, and
open-lamp protection.
Topic Page
3.1 Test Setup 3-2. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.2 Test Results 3-5. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Chapter 3

Test Setup
3-2
3.1 Test Setup
3.1.1 Lighting CCFL
A power supply with a power capability of 6 V/0.5 A is required for this test.
Figure 3–1 shows the input/output connections to the SLUP178. Connect a
270-V lamp (BF3100–20B). Connect pin 1 to pin 2 of jumper JP1.
Figure 3–1.Test Setup
Power Supply
CCFL
Lamp