LT3478IFE#PBF_技术文档

LT3478/LT3478-1

4.5A Monolithic LED

Drivers with True Color

PWM Dimming

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DESCRIPTIO

FEATURES

The LT^®3478/LT3478-1 are 4.5A step-up DC/DC convert-

ers designed to drive LEDs with a constant current over

a wide programmable range. Series connection of the

LEDs provides identical LED currents for uniform bright-

ness without the need for ballast resistors and expensive

factory calibration.

■

True Color PWM™ Dimming Delivers Constant LED

Color with Up to 3000:1 Range

■

Wide Input Voltage Range: 2.8V to 36V

■

4.5A, 60mΩ, 42V Internal Switch

■

Drives LEDs in Boost, Buck-Boost or Buck Modes

■

Integrated Resistors for Inductor and LED Current

Sensing

Program LED Current:

TheLT3478-1reducesexternalcomponentcountandcost

by integrating the LED current sense resistor. The LT3478

uses an external sense resistor to extend the maximum

programmableLEDcurrentbeyond1Aandalsotoachieve

greater accuracy when programming low LED currents.

Operating frequency can be set with an external resistor

from 200kHz up to 2.25MHz. Unique circuitry allows a

PWM dimming range up to 3000:1 while maintaining

constant LED color. The LT3478/LT3478-1 are ideal for

highpowerLEDdriverapplicationssuchasautomotiveTFT

LCD backlights, courtesy lighting and heads-up displays.

One of two CTRL pins can be used to program maximum

LED current. The other CTRL pin can be used to program

a reduction in maximum LED current vs temperature to

maximize LED usage and improve reliability.

■

100mA to 1050mA (LT3478-1)

(10mV to 105mV)/R

Program LED Current De-Rating vs Temperature

Separate Inductor Supply Input

Inrush Current Protection

Programmable Soft-Start

(LT3478)

SENSE

■

Fixed Frequency Operation from 200kHz to 2.25MHz

Open LED Protection (Programmable OVP)

Accurate Shutdown/UVLO Threshold with

Programmable Hysteresis

■

16-Pin Thermally Enhanced TSSOP Package

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APPLICATIO S

■

High Power LED Driver

Automotive Lighting

Additional features include inrush current protection,

programmable open LED protection and programmable

soft-start. Each part is available in a 16-pin thermally

enhanced TSSOP Package.

■

, LT, LTC and LTM are registered trademarks of Linear Technology Corporation.

All other trademarks are the property of their respective owners. Patents Pending.

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TYPICAL APPLICATIO

Automotive TFT LCD Backlight

Efﬁciency vs V

IN

100

95

90

85

80

10µH

V

IN

8V TO 16V

I

f

= 700mA

= 500kHz

PWM DUTY CYCLE = 100%

LED

OSC

10µF

4.7µF

V

IN

V

S

L

SW

SHDN

OUT

V

REF

0.1Ω

SENSE

(LT3478)

R

45.3k

CTRL2

LT3478-1

LED

OVPSET

54.9k

130k

CTRL1

PWM

700mA

15W

SS

V

R

T

C

6 LEDs LUXEON III (WHITE)

12 14 16

(V)

6 LEDs

(WHITE)

8

10

1µF

0.1µF

69.8k

V

IN

3478 TA01b

PWM DIMMING

CONTROL

3478 TA01

34781f

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LT3478/LT3478-1

W W U W

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W

U

ABSOLUTE AXI U RATI GS

PACKAGE/ORDER I FOR ATIO

(Note 1)

TOP VIEW

SW............................................................................42V

SW

1

2

3

4

5

6

7

8

16

15

14

13

12

11

10

9

SS

V

IN

, LED..................................................................42V

OUT

R

T

V , V , V , SHDN (Note 5).......................................36V

S

L

V

IN

PWM

PWM.........................................................................15V

V

S

CTRL2

CTRL1

SHDN

17

CTRL1, 2.....................................................................6V

L

SS, R , V , V , OVPSET............................................2V

T

C

REF

V

OUT

Operating Junction Temperature Range

LED

V

REF

(Notes 2, 3, 4).................................... –40°C to 125°C

Storage Temperature Range................... –65°C to 150°C

Lead Temperature (Soldering, 10 Sec).................. 300°C

OVPSET

V

C

FE PACKAGE

16-LEAD PLASTIC TSSOP

= 125°C, θ = 35°C/W

EXPOSED PAD (PIN 17) IS PGND, MUST BE SOLDERED TO PCB.

T

JMAX

JA

ORDER PART NUMBER

FE PART MARKING

LT3478EFE

LT3478EFE-1

LT3478IFE

3478FE

3478FE-1

3478FE

LT3478IFE-1

3478FE-1

Order Options Tape and Reel: Add #TR

Lead Free: Add #PBF Lead Free Tape and Reel: Add #TRPBF

Lead Free Part Marking: http://www.linear.com/leadfree/

Consult LTC Marketing for parts speciﬁed with wider operating temperature ranges.

ELECTRICAL CHARACTERISTICS ^The

●

denotes the speciﬁcations which apply over the full operating

temperature range, otherwise speciﬁcations are at T = 25°C. SW = open, V = V = L = V = SHDN = 2.7V, LED = open, SS = open,

A

IN

S

OUT

PWM = CTRL1, CTRL2 = 1.25V, V

= open, V = open, R = 31.6k.

REF

C

CONDITIONS

(Rising)

T

PARAMETER

MIN

TYP

MAX

UNITS

●

Minimum Operating Voltage

Operational Input Voltage

2.4

2.8

V

S

V

2.8

36

V

(Note 5)

IN

V

Quiescent Current

Shutdown Current

V = 0V (No Switching)

6.1

3

mA

µA

V

IN

C

SHDN = 0V

(Micropower)

(Switching)

6

IN

●

SHDN Pin Threshold (V

SHDN Pin Current

)

0.1

1.3

8

0.4

1.4

0.7

1.5

12

SD_µp

)

V

SD_UVLO

SHDN = V

– 50mV

+ 50mV

10

0

µA

SD_UVLO

V

Voltage

I(V ) = 0µA, V = 0V

1.213

1.240

0.005

8

1.263

0.015

12

V

%/V

mV

REF

C

Line Regulation

Load Regulation

I(V ) = 0µA, 2.7V < V < 36V

REF IN

0 < I(V ) < 100µA (Max)

REF

Frequency: f

200kHz

R = 200k

0.18

0.88

0.2

0.22

1.12

MHz

OSC

T

●

1MHz

R = 31.6k

T

34781f

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LT3478/LT3478-1

ELECTRICAL CHARACTERISTICS ^The

●

denotes the speciﬁcations which apply over the full operating

temperature range, otherwise speciﬁcations are at T = 25°C. SW = open, V = V = L = V = SHDN = 2.7V, LED = open, SS = open,

A

IN

S

OUT

PWM = CTRL1, CTRL2 = 1.25V, V

= open, V = open, R = 31.6k.

REF

C

T

PARAMETER

CONDITIONS

MIN

TYP

2.25

0.05

0.64

MAX

2.6

UNITS

MHz

%/V

V

Frequency: f

2.25MHz

R = 9.09k

T

2

OSC

Line Regulation f

R = 31.6k, 2.7V < V < 36V

T

0.2

OSC

IN

Nominal R Pin Voltage

T

●

Maximum Duty Cycle

R = 31.6k

80

88

97

73

%

T

R = 200k

T

R = 9.09k

T

LED Current to V Current Gain

(Note 6)

770

400

13

µA/A

V/A

A/V

µA

V

C

LED Current to V Voltage Gain

C

V to Switch Current Gain

C

V Source Current (Out of Pin)

C

CTRL1 = 0.4V, V = 1V

40

C

V Sink Current

C

CTRL1 = 0V, V = 1V

40

C

V Switching Threshold

C

0.65

1.5

0.2

6

V High Level (V

)

OH

CTRL1 = 0.4V

CTRL1 = 0V

V

C

V Low Level (V

)

OL

V

C

●

Inductor Current Limit

Switch Current Limit

2.7V < V < 36V

4.5

6.8

7.5

A

S

6.3

270

1

A

Switch V SAT

I

= 4.5A

SW

mV

µA

CE

Switch Leakage Current

SW = 42V, V = 0V

C

V

Overvoltage Protection (OVP)

OVPSET = 1V

OVPSET = 0.3V

41

12.3

V

OUT

(Rising)

Full Scale LED Current (LT3478-1)

700mA LED Current (LT3478-1)

350mA LED Current (LT3478-1)

100mA LED Current (LT3478-1)

CTRL1 = V , Current Out of LED Pin

1010

655

325

70

1050

700

350

100

105

70.5

35.5

10

1090

730

375

130

109

74

mA

mV

nA

REF

●

CTRL1 = 700mV, Current Out of LED Pin

CTRL1 = 350mV, Current Out of LED Pin

CTRL1 = 100mV, Current Out of LED Pin

Full Scale LED Current V

(LT3478) CTRL1 = V , V

= V

– V

LED

101

67

SENSE

REF SENSE

VOUT

CTRL1 = 700mV, V

CTRL1 = 350mV, V

CTRL1 = 100mV, V

(LT3478)

CTRL1 = 700mV, V

CTRL1 = 350mV, V

CTRL1 = 100mV, V

= V

– V

SENSE

VOUT

LED

(LT3478)

= V

– V

33

38

7

13

CTRL1, 2 Input Currents

CTRL1 = 100mV, CTRL2 = 1.25V or

CTRL2 = 100mV, CTRL1 = 1.25V (Current Out of Pin)

40

OVPSET Input Current

OVPSET = 1V, V = 41V (Current Out of Pin)

200

1

nA

V

OUT

PWM Switching Threshold

0.8

1.2

50

V Pin Current in PWM Mode

C

V = 1V, PWM = 0

C

1

nA

V

OUT Pin Current in PWM Mode

PWM = 0

1

100

SS Low Level (V

)

OL

I

= 20µA

0.15

0.25

1.5

12

(SS)

SS Reset Threshold

SS High Level (V

V = 0V

C

V

)

OH

V = 0V

C

V

Soft-Start (SS) Pin Charge Current

Soft-Start (SS) Pin Discharge Current

SS = 1V, Current Out of Pin, V = 0V

µA

C

SS = 0.5V, V = 0V

350

C

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LT3478/LT3478-1

ELECTRICAL CHARACTERISTICS

Note 1: Stresses beyond those listed under Absolute Maximum Ratings

may cause permanent damage to the device. Exposure to any Absolute

Maximum Rating condition for extended periods may affect device

reliability and lifetime.

temperature will exceed 125°C when over-temperature protection is active.

Continuous operation above the speciﬁed maximum operating junction

temperature may impair device reliability.

Note 4: For maximum operating ambient temperature, see the “Thermal

Note 2: The LT3478EFE/LT3478EFE-1 are guaranteed to meet performance

speciﬁcations from 0°C to 125°C junction temperature. Speciﬁcations over

the –40°C to 125°C operating junction temperature range are assured by

design, characterization and correlation with statistical process controls.

The LT3478IFE/LT3478IFE-1 are guaranteed over the full –40°C to 125°C

operating junction temperature range.

Calculations” section in the Applications Information section.

Note 5: The maximum operational voltage for V is limited by thermal and

IN

efﬁciency considerations. Power switch base current is delivered from V

IN

and should therefore be driven from the lowest available power supply in

the system. See “Thermal Calculations” in the Applications Information

section.

Note 3: This IC includes over-temperature protection that is intended

to protect the device during momentary overload conditions. Junction

Note 6: For LT3478, parameter scales • (R /0.1Ω).

SENSE

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TYPICAL PERFOR A CE CHARACTERISTICS

LED Current vs PWM Duty Cycle

Wide PWM Dimming Range

(3000:1)

LED Current vs CTRL1

LED Current vs Temperature

1000

100

10

1

1400

1050

700

350

0

1400

1050

700

350

0

T

= 25°C

T

= 25°C

(FOR LT3478 SCALE BY 0.1Ω/R

)

A

V

A

SENSE

= V = 12V

CTRL2 = V

IN

S

REF

6 LEDS AT 500mA

PWM FREQ = 100Hz

CTRL1 = 0.5V

(FOR LT3478 SCALE BY 0.1Ω/R

)

SENSE

I

= 1050mA, CTRL1 = CTRL2 = V

REF

LED

CTRL2 = V

REF

LT3478-1

F

= 1.6MHz

OSC

LT3478-1

L = 2.2µH

I

= 100mA, CTRL1 = 100mV,

LED

V

CTRL2 = V

REF

0

0.01

0.1

1

10

100

–50 –25

0

25

50

75 100 125

0

0.35

0.70

1.05

1.40

PWM DUTY CYCLE (%)

JUNCTION TEMPERATURE (°C)

CTRL1 (V)

3478 G03

3478 G01

3478 G02

CTRL1 Pin Current vs

Temperature

Switch V (SAT) vs Switch

Switch and Inductor Peak Current

Limits vs Temperature

CE

Current

240

210

180

120

60

7.0

6.5

6.0

5.5

5.0

4.5

50

40

30

20

10

0

T

= 25°C

A

CTRL1 = 0.1V

SWITCH

INDUCTOR

CTRL1 = 0.35V

CTRL1 = 0.7V

CTRL2 = V

CTRL1 AND CTRL2 PINS

INTERCHANGEABLE

REF

CTRL1 = 0.9V

0

–50 –25

0

25

50

75 100 125

–50 –25

0

25

50

75 100 125

JUNCTION TEMPERATURE (°C)

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5

SWITCH CURRENT (A)

JUNCTION TEMPERATURE (°C)

3478 G04

3478 G05

3478 G06

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LT3478/LT3478-1

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TYPICAL PERFOR A CE CHARACTERISTICS

SHDN Pin (Hysteresis) Current vs

Temperature

V

vs Temperature

SHDN Threshold vs Temperature

REF

1.28

1.26

1.24

1.22

1.20

1.18

1.60

1.50

1.40

1.30

1.20

15

JUST BEFORE PART TURNS ON

10

5

AFTER PART TURNS ON

25 50 75 100 125

0

–50 –25

0

25

50

75 100 125

–50 –25

0

25

50

75 100 125

–50 –25

0

JUNCTION TEMPERATURE (°C)

3478 G07

3478 G08

3478 G09

V

Shutdown Current vs

V

Quiescent Current vs

IN

Temperature

V

IN

Quiescent Current vs V

Temperature

IN

50

40

30

20

10

0

14

12

10

8

14

SHDN = 0V

12

10

8

6

V

= 36V

= 20V

IN

4

2

V

= 2.8V

IN

T = 25°C

C

V

= 2.8V

= 0V

A

V

IN

C

= 0V

0

–50 –25

0

25

50

75 100 125

0

3

6

9

12 15 18 21 24 27 30 33 36

(V)

–50 –25

0

25

50

75 100 125

JUNCTION TEMPERATURE (°C)

V

JUNCTION TEMPERATURE (°C)

IN

3478 G10

3478 G11

3478 G12

V , L, SW Shutdown Currents vs

Switch Peak Current Limit

vs Duty Cycle

S

Temperature

4

2

0

7

6

5

4

3

2

1

0

SHDN = 0V

S

V

= L = SW = 36V

I(V PIN) = I(L PIN)

S

I(SW PIN)

T = 25°C

A

–50 –25

0

25

50

75 100 125

0

20

40

60

80

100

JUNCTION TEMPERATURE (°C)

DUTY CYCLE (%)

3478 G19

3478 G18

34781f

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LT3478/LT3478-1

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TYPICAL PERFOR A CE CHARACTERISTICS

Switching Frequency vs

Temperature

Open-Circuit Output Clamp

Voltage vs Temperature

Switching Frequency vs R

T

10000

1000

100

1.20

1.15

1.10

1.05

1.00

0.95

0.90

0.85

0.80

43.0

42.5

42.0

41.5

41.0

40.5

40.0

39.5

39.0

R

= 31.6k

OVPSET = 1V

T

= 25°C

T

A

–50 –25

0

25

50

75 100 125

–50 –25

0

25

50

75 100 125

1

10

100

1000

JUNCTION TEMPERATURE (°C)

R

(kΩ)

T

3478 G13

3478 G14

3478 G15

SS Pin Charge Current vs

Temperature

V Pin Active and Clamp Voltages

C

vs Temperature

14

13

12

11

10

1.8

1.5

1.2

0.9

0.6

0.3

0

V

CLAMP

C

V

ACTIVE THRESHOLD

C

0

–50 –25

0

25

50

75 100 125

–50 –25

25

50

75 100 125

JUNCTION TEMPERATURE (°C)

3478 G16

3478 G17

34781f

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LT3478/LT3478-1

U

PI FU CTIO S

SW (Pins 1, 2): Switch Pin. Collector of the internal NPN

power switch. Both pins are fused together inside the IC.

Connect the inductor and diode here and minimize the

metal trace area connected to this pin to minimize EMI.

hysteresis allows programming of undervoltage lockout

(UVLO) hysteresis. SHDN above 1.4V turns the part on

and removes a 10µA sink current from the pin. SHDN = 0V

reducesV current<3µA.SHDNcanbedirectlyconnected

IN

to V . If left open circuit the part will be turned off.

IN

V (Pin 3): Input Supply. Must be locally bypassed with

IN

a capacitor to ground.

CTRL1 (Pin 12): CTRL1 pin voltage is used to program

maximum LED current (CTRL2 = V ). CTRL1 voltage

REF

V (Pin 4): Inductor Supply. Must be locally bypassed

S

can be set by a resistor divider from V

or an external

REF

with a capacitor to ground. Can be shorted to V if only

IN

voltage source. Maximum LED current is given by:

one supply is available (see L (Pin 5) function).

(LT3478-1) Max LED Current = Min(CTRL1, 1.05) Amps

L (Pin 5): Inductor Pin. An internal resistor between V

S

and L pins monitors inductor current to protect against

inrush current. Exceeding 6A immediately turns off the

internal NPN power switch and discharges the soft-start

pin. Input current monitoring can be disabled by connect-

ing the inductor power supply directly to the L pin and

(LT3478)Max LED Current =

0.1

R_SENSE

Min(CTRL, 1.05)•

Amps

(linear for 0.1V < CTRL1< 0.95V ; CTRL2 = V ) For maxi-

mum LED current, short CTRL1 and CTRL2 pins to V

REF

leaving the V pin open (requires local bypass capacitor

S

.

REF

to GND on L pin; not V pin).

S

CTRL2 (Pin 13): The CTRL2 pin is available for program-

ming a decrease in LED current versus temperature

(setting temperature breakpoint and slope). This feature

allows the output LED(s) to be programmed for maximum

allowablecurrentwithoutdamageathighertemperatures.

This maximizes LED usage and increases reliability. A

CTRL2 voltage with negative temperature coefﬁcient is

V

(Pin 6): Output voltage of the converter. Connect a

OUT

capacitor from this pin to ground. Internal circuitry moni-

tors V for protection against open LED faults.

OUT

LED (Pin 7): Connect the LED string from this pin to

ground.Aninternal(LT3478-1)/external(LT3478)resistor

between the V

accurate control.

and LED pins senses LED current for

OUT

created using an external resistor divider from V with

REF

temperature dependant resistance. If not used, CTRL2

OVPSET (Pin 8): Programs V

overvoltage protection

OUT

should be tied to V

.

REF

level (OVP) to protect against open LED faults. OVP =

(OVPSET • 41)V. OVPSET range is 0.3V to 1V for an OVP

range of typically 12.3V to 41V.

PWM(Pin14):InputpinforPWMdimmingcontrol.Above

1V allows converter switching and below 1V disables

switching with V pin level maintained. With an external

C

V (Pin 9): Output of the transconductance error ampliﬁer

C

MOSFET placed in series with the ground side of the LED

string, a PWM signal driving the PWM pin and MOSFET

gate provides accurate dimming control. The PWM signal

can be driven from 0V to 15V. If unused, the pin should

and compensation pin for the converter regulation loop.

V

REF

(Pin 10): Bandgap Voltage Reference. This pin can

supply up to 100µA. Can be used to program CTRL1,

CTRL2, OVPSET pin voltages using resistor dividers to

ground.

be connected to V

.

REF

R (Pin 15): A resistor to ground programs switching

T

SHDN (Pin 11): The SHDN pin has an accurate 1.4V

threshold and can be used to program an undervoltage

lockout (UVLO) threshold for system input supply using a

resistor divider from supply to ground. A 10µA pin current

frequency between 200kHz and 2.25MHz.

SS (Pin 16): Soft-Start Pin. Placing a capacitor here pro-

grams soft-start timing to limit inductor inrush current

duringstart-upduetotheconverter.Wheninductorcurrent

34781f

7

LT3478/LT3478-1

U

PI FU CTIO S

exceeds 6A or V

exceeds OVP, an internal soft-start

Exposed Pad (Pin 17): The ground for the IC and the con-

verter.TheFEpackagehasanExposedPadunderneaththe

ICwhichisthebestpathforheatoutofthepackage. Pin17

should be soldered to a continuous copper ground plane

under the device to reduce die temperature and increase

the power capability of the LT3478/LT3478-1.

OUT

latch is set, the power NPN is immediately turned off and

the SS pin is discharged. The soft-start latch is also set

if V and/or SHDN do not meet their turn on thresholds.

IN

The SS pin only recharges when all faults are removed

and the pin has been discharged below 0.25V.

W

BLOCK DIAGRA

SHDN

V

S

L

SS

16

SW

1, 2

11

4

5

V

10µA

OUT

V

C

6

OVERVOLTAGE

DETECT

–

+

9.5mΩ

–

+

–

57mV

OVPSET

1.4V

100Ω

R

SENSE

(EXTERNAL FOR

LT3478)

SENSE

0.1Ω

INRUSH

SOFT-START

(INTERNAL FOR

LT3478-1)

CURRENT

V

IN

PROTECTION

UVLO

REF

1.24V

3

LED

7

PWM

DETECT

V

REF

OSC

S

Q

R

Q1

10

LED

PWM

CTRL1

CTRL2

1.05V

+

12

13

SLOPE

COMP

+

–

GM

Q2

–

+

1V

Σ

PWM

14

+

–

1000Ω

R

S

TO OVERVOLTAGE

DETECT CIRCUIT

8

15

17

9

3478 F01

OVPSET

R

T

EXPOSED PAD

(GND)

V

C

Figure 1

34781f

8

LT3478/LT3478-1

U

OPERATIO

The LT3478/LT3478-1 are high powered LED drivers with

a 42V, 4.5A internal switch and the ability to drive LEDs

withupto1050mAforLT3478-1andupto105mV/R

for LT3478.

the V voltage controls the peak switch current limit and

C

hence the inductor current available to the output LED(s).

As with all current mode converters, slope compensation

is added to the control path to ensure stability.

SENSE

The LT3478/LT3478-1 work similarly to a conventional

currentmodeboostconverterbutuseLEDcurrent(instead

of output voltage) as feedback for the control loop. The

Block Diagram in Figure 1 shows the major functions of

the LT3478/LT3478-1.

The CTRL1 pin is used to program maximum LED current

via Q2. The CTRL2 pin can be used to program a decrease

inLEDcurrentversustemperatureformaximumreliability

andutilizationoftheLED(s).ACTRL2voltagewithnegative

temperature coefﬁcient can be created using an external

resistor divider from V

with temperature dependant

REF

For the part to turn on, the V pin must exceed 2.8V and

IN

resistance. Unused CTRL2 is tied to V

.

REF

the SHDN pin must exceed 1.4V. The SHDN pin threshold

allows programming of an undervoltage lockout (UVLO)

threshold for the system input supply using a simple

resistor divider. A 10µA current ﬂows into the SHDN pin

before part turn on and is removed after part turn on. This

current hysteresis allows programming of hysteresis for

theUVLOthreshold.See“ShutdownPinandProgramming

Undervoltage Lockout” in the Applications Information

Section. For micropower shutdown the SHDN pin at 0V

For True Color PWM dimming, the LT3478/LT3478-1

provide up to a 3000:1 wide PWM dimming range by al-

lowing the duty cycle of the PWM pin (connected to the

IC and an external N-channel MOSFET in series with the

LED(s)) to be reduced from 100% to as low as 0.033%

for a PWM frequency of 100Hz. Dimming by PWM duty

cycle, allows for constant LED color to be maintained over

the entire dimming range.

reduces V supply current to approximately 3µA.

IN

Forrobustoperation,theLT3478/LT3478-1monitorsystem

Each LED driver is a current mode step-up switch-

ing regulator. A regulation point is achieved when the

performance for any of the following faults : V or SHDN

IN

pin voltages too low and/or inductor current too high

and/or boosted output voltage too high. On detection of

any of these faults, the LT3478/LT3478-1 stop switching

immediately and a soft-start latch is set discharging the

SS pin (see Timing Diagram for SS pin in Figure 11). All

faults are detected internally and do not require external

components. When all faults no longer exist, an internal

12µAsupplychargestheSSpinwithatimingprogrammed

using a single external capacitor. A gradual ramp up of SS

pin voltage limits switch current during startup.

boosted output voltage V

across the output LED(s) is

OUT

high enough to create current in the LED(s) equal to the

programmed LED current. A sense resistor connected in

series with the LED(s) provides feedback of LED current

to the converter loop.

Thebasicloopusesapulsefromaninternaloscillatortoset

the RS ﬂip-ﬂop and turn on the internal power NPN switch

Q1 connected between the switch pin, SW, and ground.

Current increases in the external inductor until switch

current limit is exceeded or until the oscillator reaches

its maximum duty cycle. The switch is then turned off,

causing inductor current to lift the SW pin and turn on an

external Schottky diode connected to the output. Inductor

current ﬂows via the Schottky diode charging the output

capacitor. The switch is turned back on at the next reset

cycle of the internal oscillator. During normal operation

For optimum component sizing, duty cycle range and ef-

ﬁciencytheLT3478/LT3478-1allowforaseparateinductor

supply V and for switching frequency to be programmed

S

from 200kHz up to 2.25MHz using a resistor from the R

T

pintoground.Theadvantagesoftheseoptionsarecovered

in the Applications Informations section.

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Inductor Selection

Capacitor Selection

Low ESR (equivalent series resistance) ceramic capaci-

tors should be used at the output to minimize the output

ripple voltage. Use only X5R or X7R dielectrics, as these

materials retain their capacitance over wider voltage and

temperature ranges than other dielectrics. A 4.7µF to

10µF output capacitor is sufﬁcient for most high output

current designs. Some suggested manufacturers are

listed in Table 2.

SeveralinductorsthatworkwellwiththeLT3478/LT3478-1

are listed in Table 1. However, there are many other manu-

facturers and inductors that can be used. Consult each

manufacturerformoredetailedinformationandtheirentire

range of parts. Ferrite cores should be used to obtain the

best efﬁciency. Choose an inductor that can handle the

necessary peak current without saturating. Also ensure

that the inductor has a low DCR (copper-wire resistance)

2

to minimize I R power losses. Values between 4.7µH and

Diode Selection

22µH will sufﬁce for most applications.

Schottky diodes, with their low forward voltage drop and

fast switching speed, are ideal for LT3478/LT3478-1 ap-

plications. Table 3 lists several Schottky diodes that work

well. The diode’s average current rating must exceed the

application’saverageoutputcurrent.Thediode’smaximum

reverse voltage must exceed the application’s output volt-

age. A 4.5A diode is sufﬁcient for most designs. For PWM

dimming applications, be aware of the reverse leakage

current of the diode. Lower leakage current will drain the

output capacitor less, allowing for higher dimming range.

The companies below offer Schottky diodes with high

voltage and current ratings.

Inductor manufacturers specify the maximum current

rating as the current where inductance falls by a given

percentage of its nominal value. An inductor can pass a

current greater than its rated value without damaging it.

Aggressive designs where board space is precious will

exceedthemaximumcurrentratingoftheinductortosave

space. Consult each manufacturer to determine how the

maximum inductor current is measured and how much

more current the inductor can reliably conduct.

Table 1. Suggested Inductors

MANUFACTURER PART NUMBER

IDC (A)

INDUCTANCE (µH)

MAX DCR (mΩ)

L × W × H (mm)

MANUFACTURER

CDRH104R-100NC

CDRH103RNP-4R7NC-B

CDRH124R-100MC

CDRH104R-5R2NC

3.8

10

4.7

10

35

30

28

22

10.5 × 10.3 × 4.0

10.5 × 10.3 × 3.1

12.3 × 12.3 × 4.5

10.5 × 10.3 × 4.0

Sumida

www.sumida.com

4

4.5

5.5

5.2

FDV0630-4R7M

4.2

4.7

49

7.0 × 7.7 × 3.0

Toko

www.toko.com

UP4B-220

7.6

22

34

22 × 15 × 7.9

Cooper

www.cooperet.com

Table 2. Ceramic Capacitor Manufacturers

MANUFACTURER

Taiyo Yuden

AVX

PHONE NUMBER

WEB

(408) 573-4150

(803) 448-9411

(714) 852-2001

www.t-yuden.com

www.avxcorp.com

www.murata.com

Murata

Table 3. Suggested Diodes

MANUFACTURER PART NUMBER

UPS340

MAX CURRENT (A)

MAX REVERSE VOLTAGE

WEB

3

40

Microsemi

www.microsemi.com

B520C

B530C

B340A

B540C

PDS560

5

3

5

30

40

60

Diodes, Inc.

www.diodes.com

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Shutdown and Programming Undervoltage Lockout

Programming Switching Frequency

Theswitchingfrequencyisprogrammedusinganexternal

resistor(R )connectedbetweentheR pinandground.The

The LT3478/LT3478-1 have an accurate 1.4V shutdown

threshold at the SHDN pin. This threshold can be used in

conjunction with a resistor divider from the system input

supply to deﬁne an accurate undervoltage lockout (UVLO)

threshold for the system (Figure 2). SHDN pin current

hysteresis allows programming of hysteresis voltage for

this UVLO threshold. Just before part turn on, 10µA ﬂows

into the SHDN pin. After part turn on, 0µA ﬂows from the

SHDNpin.Calculationoftheon/offthresholdsforasystem

input supply using the LT3478/LT3478-1 SHDN pin can

be made as follows:

T

internal free-running oscillator is programmable between

200kHz and 2.25MHz. Table 4 shows the typical R values

T

required for a range of switching frequencies.

Selecting the optimum switching frequency depends

on several factors. Inductor size is reduced with higher

frequency but efﬁciency drops due to higher switching

losses.Inaddition,someapplicationsrequireveryhighduty

cycles to drive a large number of LEDs from a low supply.

Lowswitchingfrequencyallowsagreateroperationalduty

cycle and hence a greater number of LEDs to be driven.

In each case the switching frequency can be tailored to

provide the optimum solution. When programming the

switching frequency the total power losses within the IC

should be considered. See “Thermal Calculations” in the

Applications Information section.

V

OFF = 1.4 [1 + R1/R2)]

SUPPLY

ON = V

OFF + (10µA • R1)

SUPPLY

An open drain transistor can be added to the resistor

divider network at the SHDN pin to independently control

the turn off of the LT3478/LT3478-1.

V

SUPPLY

10000

T

= 25°C

A

R1

R2

SHDN

11

–

+

1.4V

1000

100

OFF ON

10µA

3478 F02

1

10

100

1000

Figure 2. Programming Undervoltage Lockout (UVLO)

with Hysteresis

R

(kΩ)

T

3478 F03

Figure 3. Switching Frequency vs R Resistor Value

T

With the SHDN pin connected directly to the V pin, an

IN

internal undervoltage lockout threshold exists for the V

IN

Table 4. Switching Frequencies vs R Values

T

pin (2.8V max). This prevents the converter from operat-

ing in an erratic mode when supply voltage is too low.

The LT3478/LT3478-1 provide a soft-start function when

SWITCHING FREQUENCY (MHz)

R (kΩ)

T

2.25

1

9.09

31.6

200

recovering from such faults as SHDN <1.4V and/or V

IN

0.2

<2.8V. See details in the Applications Information section

“Soft-Start”.

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Programming Maximum LED current

maximum allowed LED current versus temperature to

warn against exceeding this current limit and damaging

the LED (Figure 6).

MaximumLEDcurrentcanbeprogrammedusingtheCTRL1

pin with CTRL2 tied to the V pin (see Figures 4 and 5).

REF

The maximum allowed LED current is deﬁned as:

Luxeon V (Maximum) and LT3478-1

(Programmed) Current Derating

Curves vs Temperature

(LT3478-1) Max LED Current = Min(CTRL1, 1.05) Amps

900

800

(LT3478)Max LED Current =

0.1

R_SENSE

Min(CTRL1, 1.05)•

Amps

700

600

500

400

300

200

100

0

LUXEON V EMITTER

CURRENT DERATING

CURVE

LED current vs CTRL1 is linear for approximately

0.1V < CTRL1 < 0.95V

EXAMPLE

LT3478-1

PROGRAMMED LED

CURRENT DERATING CURVE

For maximum possible LED current, connect CTRL1 and

CTRL2 to the V pin.

REF

0

25

50

75

100

1400

T

= 25°C

A

T

AMBIENT TEMPERATURE (°C)

A

CTRL2 = V

REF

(FOR LT3478 SCALE

BY 0.1Ω/R

LUXEON V EMITTER

)

(GREEN, CYAN, BLUE, ROYAL BLUE)

SENSE

1050

700

350

0

θ

= 20°C/W

JA

3478 F06

LT3478-1

Figure 6. LED Current Derating Curve vs Ambient Temperature

Without the ability to back off LED current as temperature

increases, many LED drivers are limited to driving the

LED(s)atonly50%orlessoftheirmaximumratedcurrents.

This limitation requires more LEDs to obtain the intended

brightness for the application. The LT3478/LT3478-1 al-

low the output LED(s) to be programmed for maximum

allowable current while still protecting the LED(s) from

excessive currents at high temperature. This is achieved

by programming a voltage at the CTRL2 pin with a nega-

tive temperature coefﬁcient using a resistor divider with

temperature dependent resistance (Figures 7 and 8).

CTRL2voltageisprogrammedhigherthanCTRL1voltage.

This allows initial LED current to be deﬁned by CTRL1.

As temperature increases, CTRL2 voltage will fall below

CTRL1 voltage causing LED currents to be controlled by

CTRL2 pin voltage. The choice of resistor ratios and use

of temperature dependent resistance in the divider for the

CTRL2 pin will deﬁne the LED current curve breakpoint

and slope versus temperature (Figure 8).

V

REF

0

0.35

0.70

CTRL1 (V)

1.05

1.40

3478 F04

Figure 4. LED Current vs CTRL1 Voltage

LT3478/LT3478-1

(LT3478)

10

13

12

V

OUT

REF

R2

R1

R

SENSE

CTRL2

CTRL1

LED

3478 F05

Figure 5. Programming LED Current

Programming LED Current Derating vs Temperature

A useful feature of the LT3478/LT3478-1 is the ability

to program a derating curve for maximum LED current

versus temperature. LED data sheets provide curves of

AvarietyofresistornetworksandNTCresistorswithdiffer-

enttemperaturecoefﬁcientscanbeusedforprogramming

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CTRL2toachievethedesiredCTRL2curvevstemperature.

The current derating curve shown in Figure 6 uses the

resistor network shown in option C of Figure 7.

to obtain a resistor’s exact values over temperature from

the manufacturer. Hand calculations of CTRL2 voltage

can then be performed at each given temperature and the

resultingCTRL2curveplottedversustemperature.Several

iterations of resistor value calculations may be required

to achieve the desired breakpoint and slope of the LED

current derating curve.

10

V

REF

R2

R1

R4

LT3478/LT3478-1

13

12

CTRL2

CTRL1

Table 5. NTC Resistor Manufacturers/Distributors

MANUFACTURER

R3

OPTION A TO D

Murata Electronics North America

TDK Corporation

www.murata.com

www.tdk.com

R

Y

Digi-key

www.digikey.com

R

X

NTC

X

NTC

If calculation of CTRL2 voltage at various temperatures

gives a downward slope that is too strong, alternative

resistor networks can be chosen (B, C, D in Figure 7)

which use temperature independent resistance to reduce

the effects of the NTC resistor over temperature.

A

B

C

D

3478 F07

Figure 7. Programming LED Current Derating Curve

vs Temperature (R

Located on LEDs PCB)

NTC

1100

1000

900

800

700

600

500

400

300

200

Murata Electronics provides a selection of NTC resistors

with complete data over a wide range of temperatures. In

addition, a software tool is available which allows the user

toselectfromdifferentresistornetworksandNTCresistor

values and then simulate the exact output voltage curve

(CTRL2 behavior) over temperature. Referred to as the

‘Murata Chip NTC Thermistor Output Voltage Simulator’,

userscanlogontowww.murata.com/designlibanddown-

load the software followed by instructions for creating an

CTRL1

CTRL2

LED CURRENT = MINIMUM

OF CTRL1, CTRL2

R3 = OPTION C

100

0

output voltage V

(CTRL2) from a speciﬁed V supply

OUT

CC

0

25

50

75

100

(V ). At any time during selection of circuit parameters

T

AMBIENT TEMPERATURE (°C)

REF

A

3478 F08

the user can access data on the chosen NTC resistor by

clicking on a link to the Murata catalog.

Figure 8. CTRL1, 2 Programmed Voltages vs Temperature

The following example uses hand calculations to derive

the resistor values required for CTRL1 and CTRL2 pin

voltages to achieve a given LED current derating curve.

The resistor values obtained using the Murata simulation

tool are also provided and were used to create the derating

curve shown in Figure 6. The simulation tool illustrates

the non-linear nature of the NTC resistor temperature

coefﬁcient at temperatures exceeding 50°C ambient. In

addition, the resistor divider technique using an NTC

resistor to derive CTRL2 voltage inherently has a ﬂatten-

ing characteristic (reduced downward slope) at higher

Table 5 shows a list of manufacturers/distributors of NTC

resistors. There are several other manufacturers available

and the chosen supplier should be contacted for more

detailed information. To use an NTC resistor to indicate

LED temperature it is only effective if the resistor is con-

nected as close as possible to the LED(s). LED derating

curves shown by manufacturers are listed for ambient

temperature. The NTC resistor should be submitted to

the same ambient temperature as the LED(s). Since the

temperature dependency of an NTC resistor can be non-

linear over a wide range of temperatures it is important

temperatures.ToavoidLEDcurrentexceedingamaximum

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–1.026

allowed level at higher temperatures, the CTRL2 voltage

curve may require a greater downward slope between

25°C and 50°C to compensate for that loss of slope at

higher temperatures.

R

(50°C) = R

(25°C).e

NTC

(50°C) = 22k • 0.358

(50°C) = 7.9k

CTRL2(50°C) = 1.24/(1 + 16.9/7.9) = 395mV

Example:Calculatetheresistorvaluesrequiredforgenerat-

ing CTRL1 and CTRL2 from V based on the following

REF

CTRL2 slope (25°C to 50°C) = [CTRL2(50°C)

– CTRL2(25°C)]/25°C

requirements:

(a) I

(b) I

(c) I

= 700mA at 25°C

LED

= (395 – 701)/25

= –306mV/25°C

derating curve breakpoint occurs at 25°C

derating curve has a slope of –200mA/25°C be-

tween 25°C and 50°C ambient temperature

I

slope = –306mA/25°C

LED

The required I

slope is –200mA/25°C. To reduce the

LED

Step1: Choose CTRL1 = 700mV for I = 700mA

LED

slope of CTRL2 versus temperature it is easier to keep

the exact same NTC resistor value and B-constant (there

are limited choices) and simply adjust R4 and the type

of resistor network used for the CTRL2 pin. By changing

the resistor network to option C it is possible to place a

CTRL1 = V /(1 + R2/R1)

REF

R2 = R1 • [(V /CTRL1) – 1]

REF

For V = 1.24V and choosing R1 = 22.1k,

REF

temperature independent resistor in series with R

reduce the effects of R

temperature.

to

NTC

R2 = 22.1k [(1.24/0.7) – 1]

R2 = 17k (choose 16.9k)

on the CTRL2 pin voltage over

NTC

CTRL1 = 1.24/(1 + (16.9/22.1))

Step 4: Calculate the resistor value required for R in

Y

resistor network option (c) (Figure 7) to provide an I

LED

CTRL1 = 703mV (I

= 703mA)

LED

slope of –200mA/25°C between 25°C and 50°C ambient

temperature.

Step 2: Choose resistor network option A (Figure 7) and

CTRL2 = CTRL1 for 25°C breakpoint

CTRL2 (25°C) = 0.7V = 1.24/(1 + (R4/(R (25°C)+

NTC

start with R4 = R2 = 16.9k, R

available)

= 22k (closest value

NTC

R ))

Y

R4 = 0.77 (R (25°C) + R )

(a)

NTC

Y

CTRL2 = 701mV (I

701mA)

= Min(CTRL1, CTRL2) • 1A =

LED

for –200mA/25°C slope ≥ CTRL2(50°C) = 0.7 – 0.2 =

0.5

Step 3: Calculate CTRL2 slope between 25°C and 50°C

CTRL2 (T) = 1.24/(1 + R4/R (T))

CTRL2(50°C) = 0.5V = 1.24/(1 + (R4/(R

+ R ))

Y

NTC

R4 = 1.48 (R (50°C) + R )

(b)

NTC

Y

at T = T = 25°C, CTRL2 = 701mV

O

Equating (a) = (b) and knowing R (25°C) = 22k and

NTC

x

at T = 50°C, R

– 1/298)]

(T) = R

(T ).e , x = B [(1/(T + 273)

NTC

NTC O

R

(50°C) = 7.9k gives,

NTC

0.77 (22k + R ) = 1.48 (7.9k + R )

Y

(B=B-constant;linearoverthe25°Cto50°Ctemperature

range)

17k + 0.77 R = 11.7 k + 1.48 R

Y

R = (17k – 11.7k)/(1.48 – 0.77)

Y

For R

B-constant = 3950 and T = 50°C

NTC

R = 7.5k

Y

x = 3950 [(1/323) – 1/298] = –1.026

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The value for R4 can now be solved using equation (a)

where,

for the output LED(s) is programmed for a given bright-

ness/color and “chopped” over a PWM duty cycle range

(Figure 10) from 100% to as low as 0.033%.

R4 = 0.77 (R (25°C) + R ) = 0.77 (22k + 7.5k)

NTC

Y

D2

R4 = 22.7k (choose 22.6k)

C

OUT

I

slope can now be calculated from,

V

L

SW

LED

S

V

IN

V

OUT

slope = [CTRL2(50°C) – CTRL2(25°C)]/25°C

SHDN

where CTRL2 (50°C) = 1.24/(1 + 22.6/(7.9 + 7.5)) =

503mV

(LT3478)

V

REF

LT3478/

R

SENSE

LT3478-1

CTRL2

CTRL1

and CTRL2 (25°C) = 1.24/(1 + 39.2/(22 + 28.7)) =

699mV

OVPSET

LED

D1

R

V

PWM

T

C

giving I

slope (from 25°C to 50°C)

LED

= 503mV – 699mV/25°C

= –196mV/25°C => I slope = –196mA/25°C

PWM DIMMING

CONTROL

3478 F09

LED

Figure 9. PWM Dimming Control Using the LT3478/LT3478-1

Using the Murata simulation tool for the resistor network

and values in the above example shows a CTRL2 volt-

age curve that ﬂattens out as temperatures approach

100°C ambient. The ﬁnal resistor network chosen for the

derating curve in Figure 6 used option C network with

T

PWM

(= 1/f

)

PWM

TON

PWM

R4 = 19.3k, R

= 22k (NCP15XW223J0SRC) and R

NTC

Y

= 3.01k. Although the CTRL2 downward slope is greater

than –200mA/25°C initially, the slope is required to avoid

exceedingmaximumallowedLEDcurrentsathighambient

temperatures (see Figure 6).

INDUCTOR

CURRENT

MAX I

LED

CURRENT

LED

3478 F10

PWM Dimming

Figure 10. PWM Dimming Waveforms Using the

LT3478/LT3478-1

Many LED applications require an accurate control of the

brightness of the LED(s). In addition, being able to main-

tain a constant color over the entire dimming range can

be just as critical. For constant color LED dimming, the

LT3478/LT3478-1 provide a PWM pin and special internal

circuitry to allow up to a 3000:1 wide PWM dimming

range. With an N-channel MOSFET connected between

the LED(s) and ground and a PWM signal connected to

the gate of the MOSFET and the PWM pin (Figure 9), it

is possible to control the brightness of the LED(s) based

on PWM signal duty cycle only. This form of dimming is

superior to dimming control using an analog input voltage

(reducingCTRL1voltage)becauseitallowsconstantcolor

to be maintained during dimming. The maximum current

Some general guidelines for LED Current Dimming using

the PWM pin (see Figure 10):

(1) PWM Dimming Ratio (PDR) = 1/(PWM duty cycle) =

1/(TON

• f

)

PWM PWM

(2) Lower f

allows higher PWM Dimming Ratios

= 100Hz to avoid visible ﬂicker and

PWM

(use minimum f

PWM

to maximize PDR)

(3)Higherf valueimprovesPDR(allowslowerTON

)

PWM

OSC

but will reduce efﬁciency and increase internal heating. In

general, minimum operational TON = 3 • (1/f ).

PWM

OSC

(4) Lower inductor value improves PDR

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To limit inductor current overshoot to <0.5A when SS

charges past the V level required for loop control, the C

(5) Higher output capacitor value improves PDR

C

SS

(6) Choose the schottky diode (D2, Figure 9) for minimum

reverse leakage

capacitor should be chosen using the following formula:

C

= C (7.35 – 0.6(I • V /V ))

SS(MIN)

C

LED

OUT

S

See Typical Performance Characteristics graph “LED Cur-

rent vs PWM Duty Cycle”.

Example: V = 8V, V

= 16V, I

= 1.05A, C = 0.1µF,

S

OUT

LED C

C

= 0.1µF (7.35 – 0.6(1.05 • 16/8))

SS(MIN)

Soft-Start

= 0.612µF (choose 0.68µF).

To limit inrush current and output voltage overshoot dur-

ing startup/recovery from a fault condition, the LT3478/

LT3478-1 provide a soft-start pin SS. The SS pin is used

to program switch current ramp up timing using a ca-

pacitor to ground. The LT3478/LT3478-1 monitor system

High Inductor Current “Inrush” Protection

The LT3478/LT3478-1 provide an integrated resistor

between the V and L pins to monitor inductor current

S

(Figure 1). During startup or “hotplugging” of the induc-

tor supply, it is possible for inductor currents to exceed

the maximum switch current limit. When inductor current

exceeds 6A, the LT3478/LT3478-1 protect the internal

power switch by turning it off and triggering a soft-start

latch. Thisprotectionpreventstheswitchfromrepetitively

turning on during excessive inductor currents by delay-

ing switching until the fault has been removed. To defeat

inductor current sensing the inductor supply should be

parametersforthefollowingfaults:V <2.8V, SHDN<1.4,

IN

inductor current >6A and boosted output voltage >OVP.

On detection of any of these faults, the LT3478/LT3478-1

stop switching immediately and a soft-start latch is set

causing the SS pin to be discharged (see Timing Diagram

for the SS pin in Figure 11). When all faults no longer ex-

ist and the SS pin has been discharged to at least 0.25V,

the soft-start latch is reset and an internal 12µA supply

charges the SS pin. A gradual ramp up of SS pin voltage

is equivalent to a ramp up of switch current limit until SS

connected to the L pin and the V pin left open. See details

S

in the Applications Information section “Soft-Start”.

exceeds V .

C

LED Open Circuit Protection and Maximum PWM

Dimming Ratios

The ramp rate of the SS pin is given by:

ΔV /Δt = 12µA/C

SS

The LT3478/LT3478-1 LED drivers provide optimum pro-

tection from open LED faults by clamping the converter

output to a programmable overvoltage protection level

(OVP). In addition, the programmable OVP feature draws

zero current from the output during PWM = 0 to allow

higher PWM dimming ratios. This provides an advantage

overotherLEDdriverapplicationswhichconnectaresistor

SW

SS

FAULTS TRIGGERING

SOFT-START LATCH

WITH SW TURNED OFF

IMMEDIATELY:

0.65V (ACTIVE THRESHOLD)

0.25V (RESET THRESHOLD)

0.15V

divider directly from V

.

OUT

V

< 2.8V OR

SHDN < 1.4V OR

IN

An open LED fault occurs when the connection to the

LED(s) becomes broken or the LED(s) fails open. For an

LED driver using a step-up switching regulator, an open

circuit LED fault can cause the converter output to exceed

the voltage capabilities of the regulator’s power switch,

SOFT-START LATCH RESET:

SS < 0.25V AND

> 2.8V AND

V

I

> OVP OR

OUT

> 6A

SOFT-START

LATCH SET:

(INDUCTOR)

V

IN

SHDN > 1.4V AND

V

I

< OVP AND

< 6A

OUT

3478 F11

(INDUCTOR)

Figure 11. LT3478 Fault Detection and SS Pin Timing Diagram

causingpermanentdamage. WhenV

exceedsOVP, the

OUT

34781f

16

LT3478/LT3478-1

U

W U U

APPLICATIO S I FOR ATIO

LT3478/LT3478-1immediatelystopswitching,asoft-start

V = inductor supply input

S

latch is set and the SS pin is discharged. The SS latch can

D = switch duty cycle = (V + V – V )/(V + V – V )

SAT

OUT

F

S

OUT

F

only be reset when V

falls below OVP and the SS pin

OUT

V = forward voltage drop of external Schottky diode

F

hasbeendischargedbelow0.25V(Figure11).IftheLED(s)

simply go open circuit and are reconnected, however, the

OVP used to protect the switch might be too high for the

reconnectedLED(s).TheLT3478/LT3478-1thereforeallow

OVP to be programmable to protect both the LED driver

switch and the LED(s). (The minimum allowable OVP for

normal operation for a given LED string depends on the

number of LEDs and their maximum forward voltage rat-

ings.) OVP is programmed using the OVPSET pin (front

page), given by,

V

= I

• R

L(AVE) SW

SAT

(2) Switch AC loss = P

SW(AC)

= t (1/2)I

(V

+ V )(F

)

EFF

L(AVE) OUT

F

OSC

t

= effective switch current and switch V voltage

CE

EFF

overlap time during turn on and turn off = 2 • (t

+

ISW

t

f

)

VSW

ISW

VSW

OSC

= I

rise/fall time = I

• 2ns

SWITCH

L(AVE)

OVP = (OVPSET • 41)V

= SW fall/rise time = (V

+ V ) • 0.7ns

OUT F

wheretheprogrammablerangefortheOVPSETpinis0.3V

to 1V resulting in an OVP range of 12.3V to 41V.

= switching frequency

(3) Current sensing loss = P

=

SENSE

TheOVPSETpincanbeprogrammedwithasingleresistor

P

+ P

SENSE(IL)

SENSE(ILED)

2

by tapping off of the resistor divider from V

used to

REF

P

= I

• 9.5mΩ

SENSE(IL)

L(AVE)

program CTRL1. If both CTRL1 and CTRL2 are connected

2

= I

• 0.1Ω

directlytoV (maximumLEDcurrentsetting)thenOVP-

SENSE(ILED)

LED

REF

SET requires a simple 2 resistor divider from V

.

REF

(4) Input quiescent loss = P = V • I where

Q

IN

Q

I = (6.2mA + (100mA • D))

Q

Thermal Calculations

Example (Using LT3478-1):

To maximize output power capability in an application

withoutexceedingtheLT3478/LT3478-1125°Cmaximum

operational junction temperature, it is useful to be able

to calculate power dissipation within the IC. The power

dissipation within the IC comes from four main sources:

switch DC loss, switch AC loss, Inductor and LED cur-

rent sensing and input quiescent current. These formulas

assume a boost converter architecture, continuous mode

operation and no PWM dimming.

For V = V = 8V, I

= 700mA, V

= 24.5V (7 LEDs),

IN

S

LED

= 0.2Mhz,

OUT

V = 0.5V and f

F

OSC

η = 0.89 (initial assumption)

= (24.5 • 0.7)/(0.89 • 8) = 2.41A

I

L(AVE)

D = (24.5 + 0.5 – 8)/(24.5 + 0.5 – 0.17) = 0.684

= 2 • ((2.41 • 2)ns + (24.5 + 0.5) • 0.7)ns = 45ns

T

EFF

(1) Switch DC loss = P

Total Power Dissipation:

SW(DC)

2

= (R • I

• D)

P = P

+ P

SW L(AVE)

IC

SW(DC)

SW(AC)

2

SENSE Q

R

= switch resistance = 0.07Ω (at T = 125°C)

P

= 0.07 • (2.41) • 0.684 = 0.278W

SW

J

SW(DC)

SW(AC)

I

= P /(η • V )

= 45ns • 0.5 • 2.41 • 25 • 0.2MHz = 0.271W

L(AVE)

OUT

S

2

P

= V

• I

= ((2.41) • 0.0095) + ((0.7) • 0.1) = 0.104W

SENSE

OUT

OUT LED

η = converter efﬁciency = P /(P

+ P

)

P = 8 • (6.2mA + (100mA • 0.684)) = 0.597W

Q

OUT OUT

LOSS

P = 0.278 + 0.271 + 0.104 + 0.597 = 1.25W

IC

34781f

17

LT3478/LT3478-1

U

W U U

APPLICATIO S I FOR ATIO

LocalheatingfromthenearbyinductorandSchottkydiode

will also add to the ﬁnal junction temperature of the IC.

Based on empirical measurements, the effect of diode and

inductor heating on the LT3478-1 junction temperature

can be approximated as:

If an application is built, the inductor current can be mea-

suredandanewvalueforjunctiontemperatureestimated.

Ideally a thermal measurement should be made to achieve

the greatest accuracy for T .

J

Note: The junction temperature of the IC can be reduced

ΔT (LT3478-1) = 5°C/W • (P

+ P

)

if a lower V supply is available – separate from the

J

DIODE

= (1 – D) • V • I

F L(AVE)

INDUCTOR

IN

inductor supply V . In the above example, driving V

S

IN

P

DIODE

from an available 3V source (instead of V = 8V) reduces

S

1 – D = 0.316

V = 0.5V

input quiescent losses in item(4) from 0.597W to 0.224W,

resulting in a reduction of T from 118°C to 105°C.

J

F

I

= 2.41

Layout Considerations

L(AVE)

P

= 0.316 • 0.5 • 2.41 = 0.381W

As with all switching regulators, careful attention must be

given to PCB layout and component placement to achieve

optimal thermal,electrical and noise performance (Figure

12). The exposed pad of the LT3478/LT3478-1 (Pin 17)

is the only GND connection for the IC. The exposed pad

should be soldered to a continuous copper ground plane

underneath the device to reduce die temperature and

maximize the power capability of the IC. The ground path

DIODE

2

= I

• DCR

INDUCTOR

L(AVE)

DCR = inductor DC resistance (assume 0.05Ω)

2

P

= (2.41) • 0.05 = 0.29W

INDUCTOR

The LT3478/LT3478-1 use a thermally enhanced FE pack-

age. With proper soldering to the Exposed Pad on the

undersideofthepackagecombinedwithafullcopperplane

for the R resistor and V capacitor should be taken from

T

C

underneath the device, thermal resistance (θ ) will be

JA

A

nearby the analog ground connection to the exposed pad

(near Pin 9) separate from the power ground connection

to the exposed pad (near Pin 16). The bypass capacitor

about 35°C/W. For an ambient temperature of T = 70°C,

the junction temperature of the LT3478-1 for the example

application described above, can be calculated as:

for V should be placed as close as possible to the V

IN

T (LT3478-1)

pinandtheanaloggroundconnection. SWpinvoltagerise

and fall times are designed to be as short as possible for

maximumefﬁciency.Toreducetheeffectsof bothradiated

and conducted noise, the area of the SW trace should be

kept as small as possible. Use a ground plane under the

switching regulator to minimize interplane coupling. The

schottky diode and output capacitor should be placed as

close as possible to the SW node to minimize this high

frequencyswitchingpath.TominimizeLEDcurrentsensing

errors for the LT3478, the terminals of the external sense

J

= T + θ (P ) + 5(P

+ P

)

A

JA TOT

DIODE

INDUCTOR

= 70 + 35(1.25) + 5(0.671)

= 70 + 44 + 4

= 118°C

In the above example, efﬁciency was initially assumed to

be η = 0.89. A lower efﬁciency (η) for the converter will

increase I

for T . η can be calculated as:

and hence increase the calculated value

L(AVE)

J

resistor R

should be tracked to the V

and LED

SENSE

OUT

pins separate from any high current paths.

η = P /(P

+ P

)

OUT OUT

LOSS

P

= V

• I

= 17.15W

OUT

OUT LED

(estimated) = P + P

+ P

= 1.92W

LOSS

IC

DIODE

INDUCTOR

η = 17.15/(17.15 + 1.92) = 0.9

34781f

18

LT3478/LT3478-1

U

W U U

APPLICATIO S I FOR ATIO

(CONNECT MULTIPLE GROUND PLANES

THROUGH VIAS UNDERNEATH THE IC)

V

S

C

VS

V

OUT

GND

OUTPUT CAPACITOR

V

IN

C

SCHOTTKY

DIODE

VIN

SOLDER THE EXPOSED PAD (PIN 17)

TO THE ENTIRE COPPER GROUND PLANE

UNDERNEATH THE DEVICE

LT3478/LT3478-1

POWER GND

C

SW

1

2

3

4

5

6

7

8

16

15

14

13

12

11

10

9

SS

SW

INDUCTOR

R

T

V

PWM

R

IN

V

S

CTRL2

CTRL1

SHDN

R

L

V

OUT

R

EXPOSED PAD

PIN 17

SENSE

LED

V

REF

(LT3478 ONLY)

C

R

OVPSET

V

C

F

ANALOG GND

C

V

BYPASS CAP

IN

R

3252 F08

Figure 12. Recommended Layout for LT3478/LT3478-1 (Boost Conﬁguration)

U

TYPICAL APPLICATIO S

15W, 6 LEDs at 700mA, Boost LED Driver

L1

10µH

D1

V

IN

LT3478-1 PWM Dimming

Waveforms

8V TO 16V

C1

4.7µF

25V

C2

10µF

V

L

SW

IN

S

25V

SHDN

OUT

PWM

5V/DIV

V

REF

R1

45.3k

f

= 100Hz

PWM

700mA

CTRL2

LT3478-1

LED

INDUCTOR

CURRENT

1A/DIV

OVPSET

R4

54.9k

I

LED

CTRL1

PWM

0.5A/DIV

3478 TA02b

R2

130k

SS

V

R

T

C

2µs/DIV

PWM DIMMING RATIO = 1000:1

(SEE EFFICIENCY ON PAGE 1)

R

C

0.1µF

T

SS

L1: CDRH104R-100NC

D1: PDS560

69.8k

1µF

Q1: Si2318DS

f

= 500kHz

OSC

LEDs: LUXEON III (WHITE)

3.3V

0V

Q1

PWM

DIMMING RATIO = 1000:1

100Hz

R3

10k

3478 TA02a

34781f

19

LT3478/LT3478-1

U

TYPICAL APPLICATIO S

17W, 15 LEDs at 350mA, Boost LED Driver plus LT3003

V

S

8V TO 14V

C1

4.7µF

16V

L1

5.2µH

D1

V

OUT

Efﬁciency vs Input V

V

S

IN

3.3V

C3

3.3µF

10V

C2

3.3µF

25V

90

85

80

75

70

V

L

SW

IN

S

V

= 3.3V

= 350mA

= 1MHz

IN

I

f

LED

OSC

SHDN

OUT

PWM DUTY CYCLE = 100%

V

REF

1.05A

CTRL2

CTRL1

R1

24k

LT3478-1

LED

OVPSET

PWM

R2

100k

SS

V

R

T

C

V

SS

1µF

C

R

T

15 LEDs

(5 SERIES x 3 CHANNELS)

LUXEON I (WHITE)

C

31.6k

L1: CDRH104R-5R2

D1: PDS560

LEDs: LUXEON I (WHITE)

0.1µF

f

= 1MHz

OSC

8

10

12

14

V

(V)

S

3478 TA03b

3.3V

LED1

MAX

LED2

LED3

OT1

OT2

V

OUT

0V

LT3003

100Hz

PWM

DIMMING RATIO = 3000:1

V

IN

V

C

PWM

GND

V

EE

3478 TA03a

16W, 12 LEDs at 350mA, Buck-Boost Mode LED Driver plus LT3003

V

S

12V TO 16V

C1

4.7µF

25V

L1

8.2µH

D1

V

OUT

Efﬁciency vs Input V

V

S

IN

5V

C3

3.3µF

10V

C2

10µF

50V

90

85

80

75

70

65

60

55

50

V

L

SW

IN

S

V

= 5V

= 350mA

= 500kHz

IN

I

f

LED

OSC

SHDN

OUT

PWM DUTY CYCLE = 100%

V

REF

1.05A

CTRL2

CTRL1

R1

24k

LT3478-1

LED

OVPSET

PWM

R2

100k

SS

V

R

T

C

V

SS

1µF

C

R

T

12 LEDs

(4 SERIES x 3 CHANNELS)

LUXEON I (WHITE)

C

69.8k

L1: CDRH105R-8R2

D1: PDS560

0.1µF

D2: 7.5V ZENER

f

= 500kHz

OSC

D2

12

13

14

(V)

15

16

LEDs: LUXEON I (WHITE)

C4

1µF

V

S

OUT

3478 TA04b

3.3V

0V

LED1

MAX

LED2

LED3

OT1

OT2

V

LT3003

100Hz

PWM

DIMMING RATIO = 200:1

IN

V

C

PWM

GND

V

EE

3478 TA04a

34781f

20

LT3478/LT3478-1

U

TYPICAL APPLICATIO S

4W, 1 LED at 1A, Buck-Boost Mode LED Driver

V

IN

3.8V TO 6.5V

C1

L1

6.8µH

D1

NiMH 4× 10µF

Efﬁciency vs V

IN

10V

80

75

70

65

60

55

50

C2

I

f

= 1A

LED

OSC

V

L

SW

IN

S

4.7µF

= 500kHz

16V

ON OFF

PWM DUTY CYCLE = 100%

SHDN

OUT

V

REF

Q2

1A

CTRL2

CTRL1

R1

LT3478-1

LED

100k

R4

510Ω

OVPSET

PWM

R2

34k

L1: CDRH105R-6R8

D1: B320

Q1: Si2302ADS

Q2: Si2315BDS

LED: LUXEON III (WHITE)

SS

V

R

T

C

SS

1µF

R

T

R5

510Ω

SINGLE LED

C

69.8k

LUXEON III (WHITE)

0.1µF

3

4

5

6

7

3.3V

0V

f

= 500kHz

OSC

V

(V)

IN

3478 TA06b

Q1

PWM

DIMMING RATIO = 200:1

1kHz

R3

10k

3478 TA06a

34781f

21

LT3478/LT3478-1

U

TYPICAL APPLICATIO S

24W, 4 LEDs at 1.5A, Buck Mode LED Driver

PV

IN

32V

C1

R

3.3µF

SENSE

0.068Ω

50V

1.5A

4 LEDs

R4

365Ω

TYPICAL EFFICIENCY = 90%

FOR CONDITIONS/COMPONENTS SHOWN

(PWM DUTY CYCLE = 100%, T =25°C)

C3

A

10µF

25V

Q2

L1

10µH

R5

510Ω

V

IN

3.3V

C2

4.7µF

10V

D1

V

L

OUT LED SW

PWM

IN

S

Q1

SHDN

R3

10k

V

REF

LT3478

CTRL2

CTRL1

R1

24k

PWM

DIMMING RATIO = 3000:1

OVPSET

L1: CDRH105R-100

D1: PDS560

3.3V

R2

100k

Q1: 2N7002

SS

V

C

R

T

0V

100Hz

Q2: Si2319DS

LEDs: LXK2 (WHITE)

C

SS

R

T

1µF

C

69.8k

0.1µF

f

= 500kHz

OSC

3478 TA07a

34781f

22

LT3478/LT3478-1

U

PACKAGE DESCRIPTIO

FE Package

16-Lead Plastic TSSOP (4.4mm)

(Reference LTC DWG # 05-08-1663)

Exposed Pad Variation BC

4.90 – 5.10*

(.193 – .201)

3.58

(.141)

3.58

(.141)

16 1514 13 12 1110

9

6.60 0.10

4.50 0.10

2.94

(.116)

6.40

2.94

SEE NOTE 4

(.252)

(.116)

BSC

0.45 0.05

1.05 0.10

0.65 BSC

5

7

8

1

2

3

4

6

RECOMMENDED SOLDER PAD LAYOUT

1.10

(.0433)

MAX

4.30 – 4.50*

(.169 – .177)

0.25

REF

0° – 8°

0.65

(.0256)

BSC

0.09 – 0.20

(.0035 – .0079)

0.50 – 0.75

(.020 – .030)

0.05 – 0.15

(.002 – .006)

FE16 (BC) TSSOP 0204

0.195 – 0.30

(.0077 – .0118)

TYP

NOTE:

1. CONTROLLING DIMENSION: MILLIMETERS 4. RECOMMENDED MINIMUM PCB METAL SIZE

FOR EXPOSED PAD ATTACHMENT

*DIMENSIONS DO NOT INCLUDE MOLD FLASH. MOLD FLASH

SHALL NOT EXCEED 0.150mm (.006") PER SIDE

MILLIMETERS

(INCHES)

2. DIMENSIONS ARE IN

3. DRAWING NOT TO SCALE

34781f

Information furnished by Linear Technology Corporation is believed to be accurate and reliable.

However, no responsibility is assumed for its use. Linear Technology Corporation makes no representa-

tion that the interconnection of its circuits as described herein will not infringe on existing patent rights.

23

LT3478/LT3478-1

U

TYPICAL APPLICATIO

6W, 6 LEDs at 250mA, Boost LED Driver

V

S

8V TO 16V

C1

4.7µF

25V

L1

10µH

D1

Efﬁciency vs Input V

V

S

IN

3.3V

C3

3.3µF

10V

C2

3.3µF

25V

100

95

90

85

80

75

70

65

60

V

L

SW

IN

S

V

= 3.3V

= 250mA

= 2MHz

IN

I

f

LED

OSC

SHDN

OUT

R

SENSE

PWM DUTY CYCLE = 100%

V

REF

0.42Ω

CTRL2

CTRL1

R1

8.25k

LT3478

250mA

LED

OVPSET

PWM

R2

10k

SS

V

R

T

C

SS

R

T

1µF

C

10k

L1: CDRH6D28

D1: ZLLS1000

Q1: Si2318DS

0.1µF

6 LEDs = LUXEON I (WHITE)

12 14 16

(V)

f

= 2MHz

OSC

8

10

LEDs: LUXEON I (WHITE)

V

S

3478 TA05b

3.3V

0V

Q1

PWM

DIMMING RATIO = 1000:1

100Hz

R3

10k

3478 TA05a

RELATED PARTS

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COMMENTS

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IN

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PWM Dimming

V : 2.5V to 25V, V

= 40V, I <1µA, QFN, TSSOP20E

SD

IN

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3A, 3.5MHz Full Featured DC/DC Converter with Soft-Start and

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V : 2.5V to 24V, V

= 40V, I <1µA, 4mm × 3mm DFN,

SD

IN

TSSOP16E Packages

Dual 1.3A , 2MHz High Current LED Driver with 1,000:1 True Color

PWM Dimming

V : 2.5V to 24V, V

= 36V, I <1µA, 5mm × 3mm DFN,

SD

IN

OUT(MAX)

TSSOP16E Packages

High Current LED Controller with 3,000:1 True Color PWM Dimming V : 3V to 36V, V

= Ext FET, I <20µA, 5mm × 4mm

SD

IN

OUT(MAX)

DFN, TSSOP16E Packages

34781f

LT 0107 • PRINTED IN USA

LinearTechnology Corporation

1630 McCarthy Blvd., Milpitas, CA 95035-7417

24

●

(408) 432-1900 FAX: (408) 434-0507 www.linear.com


型号：	LT3478IFE#PBF
厂家：	Linear
描述：	LT3478 - 4.5A Monolithic LED Drivers with True Color PWM Dimming; Package: TSSOP; Pins: 16; Temperature Range: -40°C to 85°C 驱动光电二极管接口集成电路
文件：	总24页 (文件大小：272K)
中文：	中文翻译
下载：	下载PDF数据表文档文件

LT3478IFE#PBF [Linear]

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