LINEAR TECHNOLOGY LT3478, LT3478-1 Technical data

FEATURES

■

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:

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

■

Fixed Frequency Operation from 200kHz to 2.25MHz

■

Open LED Protection (Programmable OVP)

■

Accurate Shutdown/UVLO Threshold with

SENSE

(LT3478)

Programmable Hysteresis

■

16-Pin Thermally Enhanced TSSOP Package

U

APPLICATIO S

LT3478/LT3478-1

4.5A Monolithic LED

Drivers with True Color

PWM Dimming

U

DESCRIPTIO

The LT®3478/LT3478-1 are 4.5A step-up DC/DC convert­ers designed to drive LEDs with a constant current over
a wide programmable range. Series connection of the
LEDs provides identical LED currents for uniform bright­ness without the need for ballast resistors and expensive
factory calibration.

The LT3478-1 reduces external component count and cost
by integrating the LED current sense resistor. The LT3478
uses an external sense resistor to extend the maximum
programmable LED current beyond 1A and also to achieve
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
high power LED driver applications such as automotive TFT
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.

■

High Power LED Driver

■

Automotive Lighting

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

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

U

TYPICAL APPLICATIO

Automotive TFT LCD Backlight

LT3478-1

10µH

C

OUT

LED

R

T

0.1µF1µF

69.8k

4.7µF

45.3k

54.9k

130k

PWM DIMMING
CONTROL

8V TO 16V

V

IN

VINVSLSW

SHDN

V

REF

CTRL2

OVPSET

CTRL1

PWM SS V

10µF

0.1Ω
R

SENSE

(LT3478)

700mA
15W
6 LEDs
(WHITE)

3478 TA01

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.

Effi ciency vs V

100

I

= 700mA

LED

= 500kHz

f

OSC

PWM DUTY CYCLE = 100%

95

90

EFFICIENCY (%)

85

80

8

6 LEDs LUXEON III (WHITE)

VIN (V)

IN

14 16

1210

3478 TA01b

34781f

1

LT3478/LT3478-1

WW

W

U

ABSOLUTE AXI U RATI GS

(Note 1)

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

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

V

OUT

, VS, VL, ⎯S⎯H⎯D⎯N (Note 5) .......................................36V

V

IN

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

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

, VC, V

SS, R

T

Operating Junction Temperature Range

(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 ............................................2V

REF

UUW

PACKAGE/ORDER I FOR ATIO

TOP VIEW

1

SW

2

SW

3

V

IN

4

V

S

L

V

OUT

LED

OVPSET

16-LEAD PLASTIC TSSOP

T

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

JMAX

17

5

6

7

8

FE PACKAGE

= 125°C, θJA = 35°C/W

ORDER PART NUMBER FE PART MARKING

LT3478EFE
LT3478EFE-1
LT3478IFE
LT3478IFE-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 specifi ed with wider operating temperature ranges.

SS

16

R

15

T

PWM

14

CTRL2

13

CTRL1

12

SHDN

11

V

10

REF

V

9

C

3478FE
3478FE-1
3478FE
3478FE-1

●

The

ELECTRICAL CHARACTERISTICS

temperature range, otherwise specifi cations are at T
PWM = CTRL1, CTRL2 = 1.25V, V

PARAMETER CONDITIONS MIN TYP MAX UNITS

Minimum Operating Voltage (Rising)

Operational Input Voltage V

Quiescent Current VC = 0V (No Switching) 6.1 mA

V

IN

Shutdown Current

V

IN

⎯S⎯H⎯D⎯

N Pin Threshold (V

⎯S⎯H⎯D⎯

N Pin Threshold (V

⎯S⎯H⎯D⎯

N Pin Current

Voltage I(V

V

REF

Line Regulation I(V

V

REF

Load Regulation 0 < I(V

V

REF

Frequency: f

Frequency: f

200kHz RT = 200k 0.18 0.2 0.22 MHz

OSC

1MHz RT = 31.6k

OSC

) (Micropower)

SD_µp

) (Switching) 1.3 1.4 1.5 V

SD_UVLO

= open, VC = open, RT = 31.6k.

REF

S

VIN (Note 5)

⎯S⎯H⎯D⎯

N = 0V 3 6 µA

⎯S⎯H⎯D⎯

N = V

⎯S⎯H⎯D⎯

SD_UVLO

N = V

SD_UVLO

) = 0µA, VC = 0V

REF

) = 0µA, 2.7V < VIN < 36V 0.005 0.015 %/V

REF

) < 100µA (Max) 8 12 mV

REF

= 25°C. SW = open, VIN = VS = L = V

A

– 50mV
+ 50mV

denotes the specifi cations which apply over the full operating

= ⎯S⎯H⎯D⎯N = 2.7V, LED = open, SS = open,

OUT

●

2.8

2.8

●

0.1 0.4 0.7 V

810012 µA

●

1.213 1.240 1.263 V

●

0.88 1.12 MHz

2.4 2.8 V

36
36

µA

34781f

V
V

2

LT3478/LT3478-1

The

ELECTRICAL CHARACTERISTICS

temperature range, otherwise specifi cations are at T
PWM = CTRL1, CTRL2 = 1.25V, V

= open, VC = open, RT = 31.6k.

REF

= 25°C. SW = open, VIN = VS = L = V

A

●

denotes the specifi cations which apply over the full operating

= ⎯S⎯H⎯D⎯N = 2.7V, LED = open, SS = open,

OUT

PARAMETER CONDITIONS MIN TYP MAX UNITS

Frequency: f

Line Regulation f

Nominal R

Maximum Duty Cycle R

LED Current to V

LED Current to V

to Switch Current Gain 13 A/V

V

C

Source Current (Out of Pin) CTRL1 = 0.4V, VC = 1V 40 µA

V

C

Sink Current CTRL1 = 0V, VC = 1V 40 µA

V

C

Switching Threshold 0.65 V

V

C

High Level (VOH) CTRL1 = 0.4V 1.5 V

V

C

Low Level (VOL) CTRL1 = 0V 0.2 V

V

C

Inductor Current Limit 2.7V < V

Switch Current Limit

Switch V

Switch Leakage Current SW = 42V, V

Overvoltage Protection (OVP)

V

OUT

(Rising)

Full Scale LED Current (LT3478-1) CTRL1 = V

700mA LED Current (LT3478-1) CTRL1 = 700mV, Current Out of LED Pin

2.25MHz RT = 9.09k 2 2.25 2.6 MHz

OSC

OSC

Pin Voltage 0.64 V

T

Current Gain (Note 6) 770 µA/A

C

Voltage Gain (Note 6) 400 V/A

C

SAT ISW = 4.5A 270 mV

CE

RT = 31.6k, 2.7V < VIN < 36V 0.05 0.2 %/V

= 31.6k

T

R

= 200k

T

R

= 9.09k

T

< 36V

S

= 0V 1 µA

C

OVPSET = 1V
OVPSET = 0.3V

, Current Out of LED Pin 1010 1050 1090 mA

REF

●

80 88

97
73

●

4.5 6 6.8 A

●

4.5 6.3 7.5 A

41

12.3

●

655 700 730 mA

%
%
%

350mA LED Current (LT3478-1) CTRL1 = 350mV, Current Out of LED Pin 325 350 375 mA

100mA LED Current (LT3478-1) CTRL1 = 100mV, Current Out of LED Pin 70 100 130 mA

Full Scale LED Current V

CTRL1 = 700mV, V

CTRL1 = 350mV, V

CTRL1 = 100mV, V

SENSE

SENSE

SENSE

(LT3478) CTRL1 = V

SENSE

(LT3478) CTRL1 = 700mV, V

(LT3478) CTRL1 = 350mV, V

(LT3478) CTRL1 = 100mV, V

CTRL1, 2 Input Currents CTRL1 = 100mV, CTRL2 = 1.25V or

REF

, V

SENSE

SENSE

SENSE

SENSE

= V

= V

= V

= V

VOUT

VOUT

VOUT

VOUT

– V

LED

– V

– V

– V

LED

LED

LED

101 105 109 mV

●

67 70.5 74 mV

33 35.5 38 mV

71013 mV

40 nA

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

OVPSET Input Current OVPSET = 1V, V

= 41V (Current Out of Pin) 200 nA

OUT

PWM Switching Threshold 0.8 1 1.2 V

Pin Current in PWM Mode VC = 1V, PWM = 0 1 50 nA

V

C

OUT Pin Current in PWM Mode PWM = 0 1 100 nA

SS Low Level (V

)I

OL

SS Reset Threshold V

SS High Level (V

)V

OH

Soft-Start (SS) Pin Charge Current SS = 1V, Current Out of Pin, V

Soft-Start (SS) Pin Discharge Current SS = 0.5V, V

= 20µA 0.15 V

(SS)

= 0V 0.25 V

C

= 0V 1.5 V

C

= 0V 12 µA

C

= 0V 350 µA

C

V
V

34781f

3

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.

Note 2: The LT3478EFE/LT3478EFE-1 are guaranteed to meet performance
specifi cations from 0°C to 125°C junction temperature. Specifi cations 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.

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

temperature will exceed 125°C when over-temperature protection is active.
Continuous operation above the specifi ed maximum operating junction
temperature may impair device reliability.

Note 4: For maximum operating ambient temperature, see the “Thermal
Calculations” section in the Applications Information section.

Note 5: The maximum operational voltage for V
effi ciency considerations. Power switch base current is delivered from V
and should therefore be driven from the lowest available power supply in
the system. See “Thermal Calculations” in the Applications Information
section.

Note 6: For LT3478, parameter scales • (R

to protect the device during momentary overload conditions. Junction

UW

TYPICAL PERFOR A CE CHARACTERISTICS

LED Current vs CTRL1 LED Current vs Temperature

1400

TA = 25°C
CTRL2 = V

REF

(FOR LT3478 SCALE BY 0.1Ω/R

1050

700

LT3478-1

SENSE

)

1400

(FOR LT3478 SCALE BY 0.1Ω/R

I

= 1050mA, CTRL1 = CTRL2 = V

1050

LED

700

SENSE

LT3478-1

is limited by thermal and

SENSE

IN

/0.1Ω).

IN

LED Current vs PWM Duty Cycle
Wide PWM Dimming Range
(3000:1)

1000

)

REF

TA = 25°C

= VS = 12V

V

IN

6 LEDS AT 500mA
PWM FREQ = 100Hz

100

CTRL1 = 0.5V
CTRL2 = V
F

= 1.6MHz

OSC

L = 2.2µH

10

REF

LED CURRENT (mA)

350

0

0

0.35 0.70 1.05
CTRL1 (V)

CTRL1 Pin Current vs
Temperature

50

40

30

20

CTRL2 = V
CTRL1 AND CTRL2 PINS
INTERCHANGEABLE

10

CTRL1 PIN CURRENT X (–1) (nA)

0

–50

REF

–25 0 25

JUNCTION TEMPERATURE (°C)

V

REF

1.40

3478 G01

CTRL1 = 0.1V

CTRL1 = 0.35V

CTRL1 = 0.7V

CTRL1 = 0.9V

50 75 100 125

3478 G04

LED CURRENT (mA)

350

0

–50

I

LED

CTRL2 = V

–25 0 25

JUNCTION TEMPERATURE (°C)

Switch V
Current

240

TA = 25°C

210

180

(SAT) (mV)

CE

120

SWITCH V

60

0

0.5 1.5 3.0 3.5 4.0

1.0 2.0 2.5 4.5

0.0

= 100mA, CTRL1 = 100mV,

REF

50 75 100 125

(SAT) vs Switch

CE

SWITCH CURRENT (A)

3478 G02

3478 G05

LED CURRENT (mA)

1

0

0.01 1 10 100

0.1
PWM DUTY CYCLE (%)

Switch and Inductor Peak Current
Limits vs Temperature

7.0

6.5

6.0

5.5

CURRENT LIMIT (A)

5.0

4.5
–50

SWITCH

INDUCTOR

–25 0 25

JUNCTION TEMPERATURE (°C)

50 75 100 125

3478 G03

3478 G06

4

34781f

UW

TYPICAL PERFOR A CE CHARACTERISTICS

1.28

V

vs Temperature

REF

1.60

⎯S⎯H⎯D⎯

N Threshold vs Temperature

LT3478/LT3478-1

⎯S⎯H⎯D⎯

N Pin (Hysteresis) Current vs

Temperature

15

1.26

1.24

(V)

REF

V

1.22

1.20

1.18
–50

–25 0 25

JUNCTION TEMPERATURE (°C)

Shutdown Current vs

V

IN

50 75 100 125

3478 G07

Temperature V

50

SHDN = 0V

40

30

20

CURRENT (µA)

IN

V

10

0

–50

VIN = 36V

VIN = 20V

VIN = 2.8V

–25 0 25

JUNCTION TEMPERATURE (°C)

50 75 100 125

3478 G10

1.50

1.40

SHDN (V)

1.30

1.20
–50

IN

14

12

10

8

6

CURRENT (mA)

IN

V

4

2

0

36 12

0

–25 0 25

JUNCTION TEMPERATURE (°C)

50 75 100 125

Quiescent Current vs V

18 272491521 303336

VIN (V)

3478 G08

IN

TA= 25°C

= 0V

V

C

3478 G11

JUST BEFORE PART TURNS ON

10

5

SHDN PIN CURRENT (µA)

0

–50

AFTER PART TURNS ON

–25 0 25

JUNCTION TEMPERATURE (°C)

VIN Quiescent Current vs
Temperature

14

12

10

8

6

CURRENT (mA)

IN

V

4

2

VIN = 2.8V

= 0V

V

C

0

–50

–25 0 25

JUNCTION TEMPERATURE (°C)

50 75 100 125

3478 G09

50 75 100 125

3478 G12

VS, L, SW Shutdown Currents vs
Temperature

4

SHDN = 0V

= L = SW = 36V

V

S

2

I(VS PIN) = I(L PIN)

PIN CURRENT (µA)

I(SW PIN)

0

–50

–25 0 25

JUNCTION TEMPERATURE (°C)

50 75 100 125

3478 G18

Switch Peak Current Limit
vs Duty Cycle

7

6

5

4

3

2

1

SWITCH PEAK CURRENT LIMIT (A)

TA= 25°C

0

20

0

40 60 80 100

DUTY CYCLE (%)

3478 G19

34781f

5

LT3478/LT3478-1

UW

TYPICAL PERFOR A CE CHARACTERISTICS

Switching Frequency vs R

10000

TA = 25°C

1000

SWITCHING FREQUENCY (kHz)

100

1

10 100 1000

RT (kΩ)

T

3478 G13

SS Pin Charge Current vs
Temperature

14

13

Switching Frequency vs
Temperature

1.20

RT = 31.6k

1.15

1.10

1.05

1.00

0.95

0.90

SWITCHING FREQUENCY (MHz)

0.85

0.80

–25 0 25

–50

JUNCTION TEMPERATURE (°C)

50 75 100 125

3478 G14

Pin Active and Clamp Voltages

V

C

vs Temperature

1.8

1.5

1.2

Open-Circuit Output Clamp
Voltage vs Temperature

43.0
OVPSET = 1V

42.5

42.0

41.5

41.0

CLAMP (V)

40.5

OUT

V

40.0

39.5

39.0

–25 0 25

–50

JUNCTION TEMPERATURE (°C)

VC CLAMP

50 75 100 125

3478 G15

12

11

SS PIN CURRENT (µA) (OUT OF PIN)

10

–50

–25 0 25

JUNCTION TEMPERATURE (°C)

50 75 100 125

3478 G16

(V)

C

V

0.9

0.6

0.3

0

–50

VC ACTIVE THRESHOLD

–25 0 25

JUNCTION TEMPERATURE (°C)

50 75 100 125

3478 G17

6

34781f

PI FU CTIO S

LT3478/LT3478-1

UUU

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.

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

V

IN

a capacitor to ground.

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

V

S

with a capacitor to ground. Can be shorted to V

if only

IN

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

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 connect­ing the inductor power supply directly to the L pin and
leaving the V
to GND on L pin; not V

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

V

OUT

pin open (requires local bypass capacitor

S

pin).

S

capacitor from this pin to ground. Internal circuitry moni­tors V

for protection against open LED faults.

OUT

LED (Pin 7): Connect the LED string from this pin to
ground. An internal (LT3478-1)/external (LT3478) resistor
between the V

and LED pins senses LED current for

OUT

accurate control.

OVPSET (Pin 8): Programs V

overvoltage protection

OUT

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.

(Pin 9): Output of the transconductance error amplifi er

V

C

and compensation pin for the converter regulation loop.

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

V

REF

supply up to 100µA. Can be used to program CTRL1,
CTRL2, OVPSET pin voltages using resistor dividers to
ground.

⎯S⎯H⎯D⎯

N (Pin 11): The ⎯S⎯H⎯D⎯N 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

hysteresis allows programming of undervoltage lockout

⎯S⎯H⎯D⎯

(UVLO) hysteresis.
and removes a 10µA sink current from the pin.
reduces V
to V

IN

current < 3µA. ⎯S⎯H⎯D⎯N can be directly connected

IN

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

N above 1.4V turns the part on

⎯S⎯H⎯D⎯

N = 0V

CTRL1 (Pin 12): CTRL1 pin voltage is used to program
maximum LED current (CTRL2 = V
can be set by a resistor divider from V

). CTRL1 voltage

REF

or an external

REF

voltage source. Maximum LED current is given by:

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

()

LT Max LED Current

3478

(,.)•

Min CTRL

105

01

R

SE

(linear for 0.1V < CTRL1< 0.95V ; CTRL2 = V
mum LED current, short CTRL1 and CTRL2 pins to V

.

NNSE

=

Amps

) For maxi-

REF

REF

.

CTRL2 (Pin 13): The CTRL2 pin is available for program­ming a decrease in LED current versus temperature
(setting temperature breakpoint and slope). This feature
allows the output LED(s) to be programmed for maximum
allowable current without damage at higher temperatures.
This maximizes LED usage and increases reliability. A
CTRL2 voltage with negative temperature coeffi cient is
created using an external resistor divider from V

REF

with
temperature dependant resistance. If not used, CTRL2
should be tied to V

REF

.

PWM (Pin 14): Input pin for PWM dimming control. Above
1V allows converter switching and below 1V disables
switching with V

pin level maintained. With an external

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
be connected to V

(Pin 15): A resistor to ground programs switching

R

T

REF

.

frequency between 200kHz and 2.25MHz.

SS (Pin 16): Soft-Start Pin. Placing a capacitor here pro­grams soft-start timing to limit inductor inrush current
during start-up due to the converter. When inductor current

34781f

7

LT3478/LT3478-1

UUU

PI FU CTIO S

exceeds 6A or V

exceeds OVP, an internal soft-start

OUT

latch is set, the power NPN is immediately turned off and
the SS pin is discharged. The soft-start latch is also set

and/or ⎯S⎯H⎯D⎯N do not meet their turn on thresholds.

if V

IN

The SS pin only recharges when all faults are removed
and the pin has been discharged below 0.25V.

W

BLOCK DIAGRA

SHDN

11

10µA

4

–

UVLO

+
–

1.4V

+

V

IN

3

V

REF

10

REF

1.24V

V

S

5

9.5mΩ

57mV

INRUSH
CURRENT
PROTECTION

PWM

DETECT

L

V

C

–

+

OSC SQ Q1

SS

16

SOFT-START

R

Exposed Pad (Pin 17): The ground for the IC and the con­verter. The FE package has an Exposed Pad underneath the
IC which is the best path for heat out of the package. Pin 17
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.

SW

OVERVOLTAGE

DETECT

OVPSET

1, 2

100Ω R

SENSE

0.1Ω
(INTERNAL FOR

LT3478-1)

V

OUT

6

LED

7

R

SENSE

(EXTERNAL FOR

LT3478)

LED

CTRL1

CTRL2

–

12

13

8

OVPSET

1.05V

TO OVERVOLTAGE
DETECT CIRCUIT

PWM

+
+
+
–

1000Ω

SLOPE

COMP

Q2

Σ

+

–

+

R

–

15

R

T

EXPOSED PAD

S

17

(GND)

+

GM

–

1V

PWM

+

14

9

V

C

3478 F01

LED

LED

LED

Figure 1

34781f

8