Eight-String White LED Driver with Dynamic Output Voltage...

1 FP6742A-1.1-SEP -2009

FP6742A09A

fitipower integrated technology lnc.

Eight-String White LED Driver with DynamicOutput Voltage Control for LCD Panel Backlight

Description The FP6742A is a high efficiency driver for white LEDs. It is mainly designed for large LCD panels that employ an array of LEDs as backlight source. A step-up current mode PWM DC/DC converter with built-in power MOSFET drives up to eight parallel strings of multiple series-connected LEDs.

The FP6742A features dynamic output voltage control function, which automatically chooses the lowest active LED sense voltage to regulate the feedback voltage of step-up converter. Through the function, the FP6742A is able to dynamically adjust output voltage of step-up converter to optimize the system power efficiency.

The internal step-up converter provides soft-start feature which is determined by external compensation network. The switching frequency is programmable by an external resistor to provide users flexibility in application. In shutdown mode, current consumption can be reduced to only 1uA. It converts the input voltage ranged from 7V to 25V into an output voltage up to 40V.

The FP6742A also provides eight channel constant current sinks with maximum ±3% current matching. The LED current can be adjusted by an external resistor, which provides users flexibility to control the light intensity of LEDs. In addition, users can precisely adjust LED brightness from 0% to 100% via PWM pin with pulse width modulation.

The FP6742A has multiple features to protect the converter from fault conditions. To protect the step-up converter when the load is open, the adjustable over-voltage protection (OVP) function is provided. The converter features cycle-by-cycle current limit to ensure the system reliability and provide consistent operation. FP6742A also features thermal protection function which could against any combination of overload or ambient temperature that could exceed junction temperature.

The FP6742A is available in low-profile, space-saving thermal enhanced 20-pin TSSOP exposed pad and TQFN 4mm x 4mm packages.

Features Operating Voltage from 7V to 25V Output Voltage from Input Voltage to 40V Dynamic Output Voltage Control to Optimize

System Efficiency 90% Efficiency for Boost Converter Integrated Power MOSFET for Boost

Converter Programmable Switching Frequency Soft Start Function External Compensation Network Eight Constant Current Output Sink Channels LED Current Set by an External Resistor. Output Current Matching : ±3% (max.)

between Channels LED Brightness Dimming by External PWM

Signal. Adjustable Over-Voltage Protection Over-Temperature Protection RoHS Compliant

Applications Medium Size LCD Panel LED Backlight

Drivers Notebook, Subnotebook, and Tablet Computer

Displays Industrial Displays

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Pin Assignments TP Package (TSSOP-20 <Exposed Pad>)

WQ Package (TQFN-20, 4mm x 4mm)

Figure 1. Pin Assignment of FP6742A

Ordering Information

Typical Application Circuit

FP6742A

VIN

L1

CREF1µF

ISEN5

CGND

ISEN3

ISEN

2

SSTC

MP

ISET

ENA

VREF

VIN

AGND

RT

ON/OFF

CIN

10µF

RT 50K O

VP

RC1.2K

CC10n

ISEN4

ISEN6

10µH

PWM

PG

ND

SW

ISEN

8

ISEN

7

PWMDIMMING

COUT10µF

COUT110µF

ROV11M

ROV2

75K

RSET75K

D1

CP1n

ISEN

1

Figure 2. Typical Application Circuit of FP6742A

G: Green

TR: Tape / Reel

Package Type TP: TSSOP-20 (Exposed Pad) WQ: TQFN-20 (4X4)

FP6742A

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Functional Pin Description Pin Name Pin Function

ENA Enable pin. Pull this pin low to turn off IC

NC No connection

VREF Voltage reference output

VIN IC power supply pin

RT Switching frequency setting pin

OVP Over-voltage and over-drive protection threshold setting pin

ISET Pin used to connect an external resistor for setting LED current of each channel

SSTCMP Soft start and compensation pin

ISEN1 LED current sense for string 1








SW Switching pin

PWM PWM signal input pin for dimming control (If floating it will pull high to VREF)

CGND Constant current sinks power ground

PGND Boost converter power ground

AGND Analog ground.

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Block Diagram

Figure 3. Block Diagram of FP6742A

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Absolute Maximum Ratings VIN to GND----------------------------------------------------------------------------------------------------- 28V

SW to GND----------------------------------------------------------------------------------------------------- -0.3V to +45V

ISEN1~ISEN6 to GND--------------------------------------------------------------------------------------- -0.3V to +45V

ENA,VREF,RT,OVP,ISET,SSTCMP,PWM to GND---------------------------------------------------- -0.3V to +6V

Continuous Power Dissipation (TA=+25°C) TSSOP-20 (Exposed Pad)------------------------- 2.5W

Continuous Power Dissipation (TA=+25°C) TQFN-20 4mmx4mm-------------------------------- 2.0W

Package Thermal Resistance,TSSOP-20 (Exposed Pad) (θJA)---------------------------------- 40°C/W

Package Thermal Resistance,TQFN-20 4mmx4mm (θJA) ---------------------------------------- 50°C/W

Junction Temperature--------------------------------------------------------------------------------------- +150°C

Storage Temperature Range------------------------------------------------------------------------------- -65°C to +150°C

Lead Temperature (Soldering, 10sec.) ------------------------------------------------------------------ 260°C

ESD Susceptibility (HBM Mode)-------------------------------------------------------------------------- 1.5KV

ESD Susceptibility (MM Mode)---------------------------------------------------------------------------- 200V

Note1：Stresses beyond those listed under “Absolute Maximum Ratings" may cause permanent damage to the device.

Recommended Operating Conditions Supply Voltage, VIN------------------------------------------------------------------------------------------ 7V to 25V

Boost Converter Output Voltage ------------------------------------------------------------------------ ≦40V

Operation Temperature Range---------------------------------------------------------------------------- -40°C to +85°C

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Electrical Characteristics (VIN=12V, VENA=5V, RT=50kΩ, RSET=75kΩ, TA=25°C, unless otherwise specified)

Parameter Symbol Test Conditions Min Typ Max Units

INPUT

Operation Voltage Range VIN 7 25 V

VREF Under-Voltage Lockout VUVLO VIN rising, typical hysteresis is 100mV 3.3 3.9 4.5 V

Operating Current IIN Switching 6 9.5 mA

Shutdown Current ISD VENA=0V 1 5 µA

REFERENCE

VREF Voltage VREF 4.8 5 5.2 V

OSCILLATOR

Operating Frequency FOSC 0.8 1.0 1.2 MHz

Maximum Duty Cycle TDUTY 85 90 95 %

N-CHANNEL SWITCH

Current Limit (Note4) ILIM 2.5 A

On-Resistance (Note4) RON ISW= 200mA 0.25 Ω

Switch Leakage Current ISWOFF VSW=40V 5 20 µA

ENABLE CONTROL

ENA Input High Level VENH 1.8 V

ENA Input Low Level VENL 0.9 V

ENA Input Current IEN 0.01 1 µA

CONSTANT CURRENT SINKS

Sustaining Voltage at ISEN Pins VISEN(max 40 V

ISEN Pins Leakage Current ISENOFF VISEN=40V 1 20 µA

Average LED Current (Note3) ILED(avg) RSET=75kΩ 18 20 22 mA

LED Current Balance Rate (Note2) dILED ±1 ±3 %

PWM DIMMING CONTROL

Dimming High Level VDMH 1.3 V

Dimming Low Level VDML 0.5 V

Dimming Frequency Range (Note4) FPWM 0.1 30 kHz

CONTROL & PROTECTION

ISET Voltage VISET 490 500 510 mV

Soft-Start Charge Current ISST 1.9 2.5 3.1 µA

Over-Voltage Protection Threshold VOVP 2.6 2.75 2.9 V

TSD 160 °C Thermal Shutdown Temperature (Note4) Δ TSD Hysteresis 25 °C

Note 2： dILED = ± %100avgI2

)minImaxI( ××− ;

Note3： ILED (mA) = )k(R

1500

SET Ω

Note4：The specification is guaranteed by design, not production tested.

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Typical Performance Curves

-40 -20 0 20 40 60 80 100 120800

850

900

950

1000

1050

1100

1150

1200

VIN=12V

Osc

illat

or F

requ

ency

(KH

z)

Temperature(0C) -40 -20 0 20 40 60 80 100 120

0

1

2

3

4

5

VIN=12V

Qui

esce

nt C

urre

nt(m

A)

Temperature(0C)

Figure 4. Oscillator frequency vs. Temperature Figure 5. No Switching Current vs. Temperature

-40 -20 0 20 40 60 80 100 1200.46

0.47

0.48

0.49

0.50

0.51

0.52

0.53

0.54

VIN=12V

ISET

Vol

tage

(V)

Temperature(0C) -40 -20 0 20 40 60 80 100 120

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

VIN=12V

Shut

dow

n C

urre

nt(u

A)

Temperature(0C)

Figure 6. ISET voltage vs. Temperature Figure 7. Shutdown current vs. Temperature

6 8 10 12 14 16 18 20 22 240.0

0.5

1.0

1.5

2.0

2.5

3.0

No

Switc

hing

Cur

rent

(mA

)

Input Voltage(V) 8 10 12 14 16 18 20 22 24

0.40

0.42

0.44

0.46

0.48

0.50

0.52

0.54

0.56

0.58

0.60

ISET

Vol

tage

(V)

Input Voltage(V)

Figure 8. No switching current vs. Input voltage Figure 9. ISET voltage vs. Input voltage

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Typical Performance Curves (Continued)

8 10 12 14 16 18 20 22 244.8

4.9

5.0

5.1

5.2

V REF

Vol

tage

(V)

Input Voltage(V)

50 55 60 65 70 75 80 85 90 95 10012

14

16

18

20

22

24

26

28

30

I OU

T(mA

)RISET(KΩ)

Figure 10. VREF Voltage vs. Input Voltage Figure 11. The LED Output Current vs. RISET

0 20 40 60 80 1000

5

10

15

20

Dimming frequency=100Hz

LED

Cur

rent

I OU

T(m

A)

DIM Duty Cycle(%) 0 20 40 60 80 100

0

5

10

15

20

Dimming frequency=20kHz

LED

Cur

rent

I OU

T(m

A)

DIM Duty Cycle(%)

Figure 12. The LED Output Current vs. DIM Duty cycle Figure 13. The LED Output Current vs. DIM Duty Cycle

8 10 12 14 16 18 20 22 24

50

60

70

80

90

100

8strings*9LEDS DIM=100% each channel=20mA

Effic

ienc

y%

Input Voltage (V) 8 10 12 14 16 18 20 22 24

50

60

70

80

90

100

8strings*8LEDS DIM=100% each channel=20mA

Effic

ienc

y%

Input Voltage (V)

Figure 14. Efficiency vs. Input Voltage Figure 15. Efficiency vs. Input Voltage

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Typical Performance Curves (Continued)

30 60 90 120 150 180 210 240

200

400

600

800

1000

1200

1400

1600

1800

2000

Freq

uenc

y(K

Hz)

RRT(KΩ)

Figure 16. Operation Frequency Vs. RRT Figure 17. Startup waveform

Figure 18. Switching Waveform at Vin=19V with 9*6LEDs Figure 19. Switching Waveform at Vin=9V with 9*6LEDs

Figure 20. Dimming Controlling (200Hz, Duty=10%) Figure 21. Dimming Controlling (200Hz, Duty=90%)

VSW

VOUT

ISW

VIN

VSW

VOUT

VSW

VOUT

VSW

DIM

ILED

VSW

DIM

ILED

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Application Information Operation

The FP6742A is a step-up DC-DC converter which is designed primarily for LED backlight of liquid crystal display (LCD) applications. It features a PWM converter operating with a fixed switching frequency and uses internal N-MOS to provide maximum efficiency. The converter provides eight LED current sense input to drive up to 10 LEDs each strings. The LED current into ISEN will always be actively matched according to the minimum value of the enable ISEN voltage. FP6742A also consists of a precision 5V reference, logic shutdown, PWM dimming, current-limited, soft-start, over-voltage protection and thermal shutdown.

Step Up Converter

The step up converter operates in fast transient response, current-mode PWM. Figure 3 shows block diagram of FP6742A. On the rising edge of the internal clock, the control and driver logic block sets internal flip-flop when the output voltage is too low, which turns on the N-MOS. The external inductor current ramps up linearly, storing energy in a magnetic filed. Once peak current of inductor over trans-conductance output level, the N-MOS turns off, the flip-flop resets, and external schottky diode turns on. This forces the current through the inductor to ramp back down; transferring the energy stored in the magnetic field to the output capacitor and LED strings. To add higher flexibility to the selection of external component values, the device uses external loop compensation

Programmable Operating Frequency

The operating frequency is programmable from 300kHz to 2MHz by connected a resistor from RT pin to GND. The operating frequency setting is calculated approximated by the formula

)k(R105)kHz(F

T

4

SW Ω×

≈

where RT is the switching frequency setting resistor.

Digital Enable

The FP6742A features a chip enable input pin that allows on/off control of the regulator. When ENA=Low, chip shutdown occurs and at that time almost no quiescent current (<1uA) flows. The chip enable input is TTL/CMOS compatible. Connect ENA to high level for normal operation.

Under Voltage Lockout

The under voltage lockout (UVLO) circuit provides the save operation to keeps the device from turning on when VREF is smaller than typically 3.9V (typ.).

Soft-Start

Soft-start allows a gradual incensement of the internal current-limit level for the step-up converter during power-up to reduce input surge currents. As the error amplifier output charges the external soft-start capacitor at SSTCMP, the peak N-MOS current is limited by the voltage on the SSTCMP.

Current Limit Protection

The FP6742A provides cycle-by-cycle over-current protection. Current limit is accomplished by sensing voltage drop across the drain to source of power switch. If the current sense amplifier output voltage is larger than current-limited threshold level, it will be immediately turned off power MOS. The current-limit feature protects over current fault at the output.

LED Current Setting

The LED current is specified by resistor from the ISET pin to ground. The full scale current setting is calculated approximated by the formula

)k(R1500)mA(I

SETLED Ω

≈

where RSET is a resistor of setting LED current.

PWM Dimming Control

The FP6742A can receive an external Pulse Width Modulation (PWM) signal for the dimming control function to precisely adjust LED brightness. PWM dimming control is achieved by applying an external PWM signal to PWM pin. Typically, a 0.1kHz to 30kHz PWM signal is used. Varying the PWM duty cycle from 0% to 100% controls the LED brightness. But the audio noise may appear when the PWM frequency is smaller than 20kHz. The PWM pin is initial high when it floating.

Thermal Overload Protection Thermal-overload protection limits total power dissipation in the FP6742. When the junction temperature exceeds 160°C, a thermal sensor activates the thermal protection, which shuts down the IC, allowing the IC to cool. Once the device cools

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Application Information (Continued)

down by 25°C, IC will automatically recover normal operation. For continuous operation, do not exceed the allowable operation junction temperature rating of 125°C.

Adjustable Over Voltage Protection The over-voltage function monitors the output voltage by OVP pin to protection the converter against all the LED strings open. This will cause N-MOS to switch with a maximum duty cycle and come out output over-voltage. This may cause the SW pin voltage to exceed its maximum voltage rating to damage built-in N-MOS. In the state, the OVP protection circuitry stops the internal N-MOS and clamp OVP pin at 2.75V (typ.). When OVP falls below 250mV (OVP Hysteresis), IC will automatically recover normal operation. The output over voltage protection threshold, VOVP-OUT, can be set by external divider resistors. The formula is as follow:

OVP2OV

1OVOUTOVP V)1

RR

()V(V ×+=−

where VOVP is the over voltage protection threshold, 2.75V.

LED Current Sources Matching FP6742A provides nearly no variation in LED current to maintain uniform LED brightness with a bank of eight matched current sources. These precision current sources are accurate to within ±1.0% (Typ.) respective LED strings.

The minimum voltage drop across each current source is approximately 650mV when the LED current is 20mA. The low-voltage drop helps reduce dissipation of IC.

Open LED String Protection If one or more strings are open, the respective ISEN pins are pulled to ground. The corresponding current source is disabled when any ISEN pin is lower than 200mV. The current regulator allows the unaffected others LED strings to operate normally.

Inductor Selection A 6.8uH to 15uH is recommended for general used. The value of inductor depends on the operating frequency. Higher frequency allows smaller inductor and capacitor but increase internal switching loss.

Two inductor parameters should be considered, current rating and DCR. The DCR of inductor affects the efficiency of the converter. The inductor with lowest DCR is chosen for highest efficiency. The inductor value can be calculated as:

D1I

I

2I

II

MAINLAVG

LLAVGLpeak

−=

Δ+=

where IL is the inductor peak-to-peak current ripple and is decided by:

OSC

INL f

DL

VI ×=Δ

where fOSC is the switching frequency. D is the MOSFET turn on ratio and is decided by:

O

INOV

VVD

−=

The inductor should be chosen to be able to handle this current and inductor saturation current rating should be greater than IPEAK.

Capacitor Selection

The FP6742A is permissible in using ceramic capacitor for LED backlight of LCD panel application. The value of capacitor depends on acceptable voltage ripple.

The input capacitor can reduced peak current and noise at power source. It should have 10uF at least and can be increased for better input voltage filtering. Select the input capacitor to meet the input ripple current and voltage rating. When selecting an output capacitor, consider the output ripple voltage and the ripple current. The ESR of capacitor is a major factor to the output ripple. For best performance, a low ESR output capacitor is required. The ripple voltage is given by:

)Co*f*8

1ESR(IV LO +Δ=Δ

The common aluminum-electrolytic capacitors have high ESR and should be avoided. Ceramic capacitors have the lowest ESR in general. It uses 22uF ceramic output capacitors for the FP6742A.

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Application Information (Continued)

Diode Selection

For diode selection, both forward voltage and diode capacitance need to be considered. The output diode should be rated to the output voltage and peak switch current. Schottky diodes, with their low forward voltage drop and fast reverse recovery, are the ideal choices for FP6742A applications. Make sure the diode’s peak current rating is at least IPK and its breakdown voltage exceeds VOUT.

Layout Consideration

Careful printed circuit layout is extremely important to avoid causing parasitical capacitance and line inductance. The following layout guidelines are recommended to achieve optimum performance. • Place the converter diode and inductor close to

the SW pin and no via. • Place ceramic bypass capacitors near the VREF

pin with a wide trace. • Place COUT next to schottky diode as possible. • Use wide traces and trace length is short as

possible to the SW node. • Keep the over-voltage detection-divider resistors

as close to the OVP and AGND pin with short wire and should be far away the noise source to reduce noise coupling into OVP pin.

• The analog ground (AGND and CGND) and the power ground (PGND) need to be separated, using via connection between them.

• For the best thermal performance, the exposed pad has to mount in wide copper directly with ground pin. If multilayer PC board is acceptable, place as more vias as possible is the better way to improve heat dissipation.

• If high duty ratio application is needed (low VIN drives many LEDs), the power dissipation of the device should be cared. Figure 23 and 24 show the recommend layout for FP6742A. Below references of surface temperature are given with 2oz. copper of demoboard.

Figure 22. Surface Temperature of TQFN

40

45

50

55

60

65

70

75

80

5 10 15 20 25Vin (V)

Surfa

ce T

empe

ratu

re (°

C)

6*8 LEDs8*8 LEDs10*8 LEDs

Figure 23. Top Layer of Demoboard

Figure 24. Bottom Layer of Demoboard

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Reference Application Circuit

FP6742A

VIN

L1

CREF1µF

ISEN5

CGND

ISEN3

ENA

VREF

VIN

AGND

RT

ON/OFF

CIN

10µF

RT 50K

RC1.2K

CC10n

ISEN4

ISEN6

10µH

PWMDIMMING

COUT10µF

COUT110µF

ROV11M

ROV2

75K

RSET75K

D1

CP1n

RO3,0Ω

RO2,0Ω

RO1,0Ω

RO4,0Ω

RO6,0Ω

RO5,0Ω

RO7,0Ω

CO81n

CO61n

1nCO7

CO51n

CO41nCO3

1n

CO21n

CO11n

RIN

10Ω

CIN1

1µF

RO8,0Ω

Figure 25. Reference Application Circuit of FP6742A

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Outline Information

TSSOP-20(Exposed Pad) Package (Unit: mm)

DIMENSION IN MILLIMETERSYMBOLS

UNIT MIN MAX A 0.80 1.20

A1 0.00 0.15

A2 0.80 1.05

b 0.19 0.30

D 6.40 6.60

E1 4.30 4.50

E 6.20 6.60

e 0.55 0.75

L 0.45 0.75

D1 3.00 4.50

E2 2.00 3.00

Note :Followed From JEDEC MO-153-F.

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Outline Information (Continued)

TQFN-20, 4mm x 4mm Package (Unit: mm)

DIMENSION IN MILLIMETER SYMBOL

S UNIT MIN MAX A 0.70 0.80 A1 0.00 0.05 C 0.18 0.30 E 3.95 4.05 E1 2.65 2.75 D 3.95 4.05 D1 2.65 2.75 L 0.35 0.45 b 0.20 0.30 e 0.45 0.55

Note :Followed From JEDEC MO-220-J

Life Support Policy Fitipower’s products are not authorized for use as critical components in life support devices or other medical systems

Eight-String White LED Driver with Dynamic Output Voltage...

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