LT3491
White LED Driver with
Integrated Schottky in SC70
and 2mm
×
2mm DFN
DESCRIPTIO
The LT
®
3491 is a fixed frequency step-up DC/DC converter
specifically designed to drive up to six white LEDs in series
from a Li-Ion cell. Series connection of the LEDs provides
identical LED currents resulting in uniform brightness and
eliminating the need for ballast resistors. The device
features a unique high side LED current sense that enables
the part to function as a “one wire current source;” one
side of the LED string can be returned to ground anywhere,
allowing a simpler one wire LED connection. Traditional
LED drivers use a grounded resistor to sense LED current,
requiring a 2-wire connection to the LED string.
The 2.3MHz switching frequency allows the use of tiny
inductors and capacitors. A single pin performs both
shutdown and accurate LED dimming control. Few exter-
nal components are needed: open-LED protection and the
Schottky diode are all contained inside the tiny SC70 and
2mm
×
2mm DFN packages. With such a high level of
integration, the LT3491 provides a high efficiency LED
driver solution in the smallest of spaces.
, LTC, LT and LTM are registered trademarks of Linear Technology Corporation.
All other trademarks are the property of their respective owners.
FEATURES
■
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■
■
■
■
■
■
■
■
■
Drives Up to Six White LEDs from a 3V Supply
High Side Sense Allows “One Wire Current Source”
Internal Schottky Diode
One Pin Dimming and Shutdown
27V Open LED Protection
2.3MHz Switching Frequency
±5%
Reference Accuracy
V
IN
Range: 2.5V to 12V
Requires Only 1µF Output Capacitor
Wide 300:1 True Color PWM
TM
Dimming Range
8-Lead SC70 Package
Low Profile 6-Lead DFN Package
(2mm
×
2mm
×
0.75mm)
APPLICATIO S
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Cellular Phones
PDAs, Handheld Computers
Digital Cameras
MP3 Players
GPS Receivers
TYPICAL APPLICATIO
Li-Ion Driver for Four White LEDs
SHUTDOWN AND
DIMMING CONTROL
CTRL
EFFICIENCY (%)
V
IN
3V TO 5V
L1
10µH
SW
GND
C1
1µF
V
IN
LT3491
LED
C2
1µF
CAP
R
SENSE
10Ω
80
75
70
65
60
55
50
45
40
0
3491 TA01a
V
IN
= 3.6V
4 LEDs
C1: TAIYO YUDEN LMK212BJ105MD
C2: TAIYO YUDEN GMK316BJ105ML
L1: MURATA LQH32CN100
U
5
U
U
Efficiency
10
LED CURRENT (mA)
15
20
3491 TA01b
3491fa
1
LT3491
ABSOLUTE
AXI U
RATI GS
(Note 1)
12V
32V
32V
12V
LED Voltage ............................................................ 32V
Operating Temperature Range (Note 2) .. – 40°C to 85°C
Maximum Junction Temperature ......................... 125°C
Storage Temperature Range ................ – 65°C to 150°C
Lead Temperature (Soldering, 10sec, SC-70) ......... 300°C
Input Voltage (V
IN
) .................................................
SW Voltage .............................................................
CAP Voltage ............................................................
CTRL Voltage ..........................................................
PACKAGE/ORDER I FOR ATIO
TOP VIEW
V
IN
1
GND 2
SW 3
7
6 CTRL
5 LED
4 CAP
DC PACKAGE
6-LEAD (2mm
×
2mm) PLASTIC DFN
T
JMAX
= 125°C,
θ
JA
= 102°C/W,
θ
JC
= 20°C/ W
EXPOSED PAD (PIN 7) SHOULD BE CONNECTED TO PCB GROUND
ORDER PART NUMBER
LT3491EDC
DC PART MARKING
LCHJ
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 specifications which apply over the full operating temperature range, otherwise specifications are T
A
= 25°C.
V
IN
= 3V, V
CTRL
= 3V, unless otherwise specified.
PARAMETER
Minimum Operating Voltage
LED Current Sense Voltage (V
CAP
– V
LED
)
CAP, LED Pin Bias Current
V
CAP
, V
LED
Common Mode Minimum Voltage
Supply Current
Switching Frequency
Maximum Duty Cycle
Switch Current Limit
Switch V
CESAT
Switch Leakage Current
V
CTRL
for Full LED Current
V
CTRL
to Shut Down IC
V
CTRL
to Turn On IC
CTRL Pin Bias Current
CAP Pin Overvoltage Protection
Schottky Forward Drop
Schottky Leakage Current
I
SCHOTTKY
= 100mA
V
R
= 20V
●
●
●
●
CONDITIONS
V
CAP
= 30V
V
CAP
= 16V, V
LED
= 16V
V
CAP
= 16V, V
LED
= 15V, CTRL = 3V
CTRL = 0V
●
I
SW
= 200mA
V
SW
= 16V
V
CAP
= 30V
●
2
U
U
W
W W
U
W
TOP VIEW
SW 1
GND 2
GND 3
GND 4
8 CAP
7 LED
6 CTRL
5 V
IN
SC8 PACKAGE
8-LEAD PLASTIC SC70
T
JMAX
= 125°C,
θ
JA
= 270°C/ W
ORDER PART NUMBER
DC PART MARKING
LT3491ESC8
LBXQ
MIN
2.5
190
TYP
200
20
2.6
8
MAX
210
40
2.5
4
10
2.8
UNITS
V
mV
µA
V
mA
µA
MHz
%
mA
mV
1.8
88
260
2.3
92
350
200
0.1
5
50
µA
V
mV
mV
nA
1.5
100
100
26
27
0.8
4
28
V
V
µA
3491fa
LT3491
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 LT3491E is guaranteed to meet performance specifications
from 0°C to 85°C. Specifications over the –40°C to 85°C operating
temperature range are assured by design, characterization and correlation
with statistical process controls.
TYPICAL PERFOR A CE CHARACTERISTICS
(T
A
= 25°C unless otherwise specified)
Switch Saturation Voltage (V
CESAT
)
350
SWITCH SATURATION VOLTAGE (mV)
SCHOTTKY FORWARD CURRET (mA)
300
250
200
150
100
50
0
0
50
100 150 200 250 300 350 400
SWITCH CURRENT (mA)
3491 G01
300
250
200
150
100
50
0
SHUTDOWN CURRENT (µA)
Sense Voltage (V
CAP
– V
LED
)
vs V
CTRL
240
200
SENSE VOLTAGE (mV)
OUTPUT CLAMP VOLTAGE (V)
INPUT CURRENT (mA)
160
120
80
40
0
0
500
1000
V
CTRL
(mV)
1500
Switching Waveform
V
SW
10V/DIV
V
CAP
50mV/DIV
I
L
100mA/DIV
V
IN
= 3.6V
200ns/DIV
FRONT PAGE
APPLICATION CIRCUIT
3491 G07
U W
3491 G04
Schottky Forward Voltage Drop
400
350
12
15
Shutdown Current (V
CTRL
= 0V)
9
6
3
0
0
200
800 1000
600
SCHOTTKY FORWARD DROP (mV)
400
1200
0
3
6
V
IN
(V)
9
12
3491 G03
3491 G02
Open-Circuit Output Clamp
Voltage
30
6
5
4
3
2
1
0
Input Current in Output Open
Circuit
29
28
27
26
25
2000
0
3
6
V
IN
(V)
9
12
3491 G05
0
3
6
V
IN
(V)
9
12
3491 G06
Transient Response
V
CAP
5V/DIV
V
CTRL
5V/DIV
I
L
200mA/DIV
V
IN
= 3.6V
1ms/DIV
FRONT PAGE
APPLICATION CIRCUIT
3491 G08
3491fa
3
LT3491
TYPICAL PERFOR A CE CHARACTERISTICS
(T
A
= 25°C unless otherwise specified)
Quiescent Current (V
CTRL
= 3V)
3.0
2.5
450
400
SCHOTTKY LEAKAGE CURRENT (µA)
QUIESCENT CURRENT (mA)
CURRENT LIMIT (mA)
2.0
1.5
1.0
0.5
0
0
3
6
V
IN
(V)
9
12
3491 G09
Open-Circuit Output Clamp Voltage
vs Temperature
30
OUTPUT CLAMP VOLTAGE (V)
4
3
2
1
0
–50 –25
SWITCH FREQUENCY (MHz)
29
INPUT CURRENT (mA)
28
27
26
25
–50
–25
50
25
0
75
TEMPERATURE (°C)
Sense Voltage (V
CAP
– V
LED
)
vs V
CTRL
240
200
SENSE VOLTAGE (mV)
160
120
80
40
0
0
500
1000
V
CTRL
(mV)
1500
–50°C
25°C
85°C
2000
3491 G15
SENSE VOLTAGE (mV)
SENSE VOLTAGE (mV)
4
U W
100
125
3491 G12
Switching Current Limt
vs Duty Cycle
15
25°C
Schottky Leakage Current
vs Temperature
V
R
= 10V
V
R
= 16V
V
R
= 20V
350
300
250
200
150
100
50
0
30
40
50
60
70
DUTY CYCLE (%)
80
90
12
9
6
3
0
–50
–25
0
25
50
TEMPERATURE (°C)
75
100
3491 G11
3491 G10
Input Current in Output Open Circuit
vs Temperature
6
5
2.20
Switching Frequency
vs Temperature
V
IN
= 3V
2.15
2.10
2.05
2.00
50
25
75
0
TEMPERATURE (°C)
100
125
1.95
–50
–25
50
25
0
75
TEMPERATURE (°C)
100
125
3491 G13
3419 G14
Sense Voltage (V
CAP
– V
LED
)
vs V
CAP
212
212
Sense Voltage (V
CAP
– V
LED
)
vs Temperature
208
208
204
204
200
200
196
196
192
5
10
15
V
CAP
(V)
20
25
3491 G16
192
–50
–25
50
25
0
75
TEMPERATURE (°C)
100
125
3491 G17
3491fa
LT3491
PI FU CTIO S
(SC70/DFN)
SW (Pin 1/Pin 3):
Switch Pin. Minimize trace area at this
pin to minimize EMI. Connect the inductor at this pin.
GND (Pins 2, 3, 4/Pin 2):
Ground Pins. All three pins
should be tied directly to local ground plane.
V
IN
(Pin 5/Pin 1):
Input Supply Pin. Must be locally
bypassed.
CTRL (Pin 6/Pin 6):
Dimming and Shutdown Pin. Connect
this pin below 50mV to disable the driver. As the pin
voltage is ramped from 0V to 1.5V, the LED current ramps
from 0 to I
LED
( = 200mV/R
SENSE
). The CTRL pin must not
be left floating.
LED (Pin 7/Pin 5):
Connection Point for the Anode of the
First LED and the Sense Resistor. The LED current can be
programmed by :
BLOCK DIAGRA
Σ
RAMP
GENERATOR
V
REF
1.25V
SHDN
OSCILLATOR
A1
R
C
START-UP
CONTROL
C
C
PIN NUMBERS CORRESPOND TO THE 8-PIN SC70 PACKAGE
Figure 1. Block Diagram
+
+
–
+
–
W
U
U
U
I
LED
=
200mV
R
SENSE
CAP (Pin 8/Pin 4):
Output of the Driver. This pin is
connected to the cathode of internal Schottky. Connect the
output capacitor to this pin and the sense resistor from
this pin to the LED pin.
EXPOSED PAD (NA/Pin 7):
The Exposed Pad should be
soldered to the PCB ground to achieve the rated thermal
performance.
5
V
IN
PWM
COMP
DRIVER
A2
R
S
Q
1
SW
CAP
Q1
OVERVOLTAGE
PROTECTION
R
8
+
A3
–
+
A = 6.25
–
LED
7
CTRL
6
GND
PINS 2, 3, 4
3491 F01
3491fa
5
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