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PDF LT3491 Data sheet ( Hoja de datos )
| Número de pieza | LT3491 | |
| Descripción | White LED Driver | |
| Fabricantes | Linear Technology | |
| Logotipo |  |
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LT3491
White LED Driver with
Integrated Schottky in SC70
and 2mm × 2mm DFN
FEATURES
DESCRIPTIO
■ 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
■ VIN Range: 2.5V to 12V
■ Requires Only 1µF Output Capacitor
■ Wide 300:1 True Color PWMTM Dimming Range
■ 8-Lead SC70 Package
■ Low Profile 6-Lead DFN Package
(2mm × 2mm × 0.75mm)
U
APPLICATIO S
■ Cellular Phones
■ PDAs, Handheld Computers
■ Digital Cameras
■ MP3 Players
■ GPS Receivers
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.
www.DataSheet4U.com
TYPICAL APPLICATIO
VIN
3V TO 5V
Li-Ion Driver for Four White LEDs
SHUTDOWN AND
DIMMING CONTROL
L1
10µH
C1
1µF
CTRL
VIN CAP
LT3491
SW LED
GND
RSENSE
10Ω
C2
1µF
C1: TAIYO YUDEN LMK212BJ105MD
C2: TAIYO YUDEN GMK316BJ105ML
L1: MURATA LQH32CN100
3491 TA01a
Efficiency
80
VIN = 3.6V
75 4 LEDs
70
65
60
55
50
45
40
0
5 10 15
LED CURRENT (mA)
20
3491 TA01b
3491fa
1
1 page  
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.
VIN (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 ILED ( = 200mV/RSENSE). 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 :
ILED
=
200mV
RSENSE
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.
BLOCK DIAGRA
5
VIN
–
PWM
COMP
A2
+
www.DataSheet4U.com
SHDN
VREF
1.25V
START-UP
CONTROL
Σ
RAMP
GENERATOR
OSCILLATOR
RC
CC
PIN NUMBERS CORRESPOND TO THE 8-PIN SC70 PACKAGE
RSQ
A1
1
SW
DRIVER
+
A3
–
Q1
OVERVOLTAGE
PROTECTION
R
CAP
8
+
A = 6.25
–
LED
7
CTRL
6
GND
PINS 2, 3, 4
3491 F01
Figure 1. Block Diagram
3491fa
5
5 Page 
LT3491
APPLICATIO S I FOR ATIO
LOW INPUT VOLTAGE APPLICATIONS
The LT3491 can be used in low input voltage applications.
The input supply voltage to the LT3491 must be 2.5V or
higher. However, the inductor can be run off a lower
battery voltage. This technique allows the LEDs to be
powered off two alkaline cells. Most portable devices have
a 3.3V logic supply voltage which can be used to power the
LT3491. The LEDs can be driven straight from the battery,
resulting in higher efficiency.
Figure 10 shows three LEDs powered by two AA cells. The
battery is connected to the inductor and the chip is
powered off a 3.3V logic supply voltage.
BOARD LAYOUT CONSIDERATIONS
As with all switching regulators, careful attention must be
paid to the PCB board layout and component placement.
To prevent electromagnetic interference (EMI) problems,
proper layout of high frequency switching paths is essen-
tial. Minimize the length and area of all traces connected to
the switching node pin (SW). Keep the sense voltage pins
(CAP and LED) away from the switching node. Place COUT
next to the CAP pin. Always use a ground plane under the
switching regulator to minimize interplane coupling. Rec-
ommended component placement is shown in Figure 11.
3.3V
C1
0.1µF
2 AA CELLS
2V TO 3.2V
SHUTDOWN AND
DIMMING CONTROL
L1
10µH
C1
1µF
CTRL
VIN CAP
LT3491
SW LED
GND
RSENSE
10Ω
C2
2.2µF
www.DataSheet4U.com
C1: TAIYO YUDEN LMK212BJ105MD
C2: TAIYO YUDEN GMK325BJ225ML
L1: MURATA LQH32CN100
3491 F10
Figure 10. 2 AA Cells to Three White LEDs
CTRL
LED
RSENSE
CIN
VIN
5
6
7
8
GND
4
3
2
1
COUT
CAP
GND
L1
SW
CIN
VIN
L1
SW
1
2
3
CTRL
6
GND 5 LED
7
4 CAP
COUT
RSENSE
(A) SC70 PACKAGE
(B) DFN PACKAGE
Figure 11. Recommended Component Placement
3491 F11
3491fa
11
11 Page | ||
| Páginas | Total 16 Páginas | |
| PDF Descargar | [ Datasheet LT3491.PDF ] | |
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