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PDF LT3497 Data sheet ( Hoja de datos )
| Número de pieza | LT3497 | |
| Descripción | Dual Full Function White LED Driver | |
| Fabricantes | Linear | |
| Logotipo |  |
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FEATURES
■ Drives Up to 12 White LEDs (6 in Series per
Converter) from a 3V Supply
■ Two Independent Boost Converters Capable of
Driving Asymmetric LED Strings
■ Independent Dimming and Shutdown Control of the
Two LED Strings
■ High Side Sense Allows “One Wire Current Source”
per Converter
■ Internal Schottky Diodes
■ Open LED Protection (32V)
■ 2.3MHz Switching Frequency
■ ±5% Reference Accuracy
■ VIN Range: 2.5V to 10V
■ Dual Wide 250:1 True Color PWMTM Dimming
■ Requires Only 1µF Output Capacitor per Converter
■ Available in a 3mm × 2mm 10-Pin DFN Package
APPLICATIONS
■ Cellular Phones
■ PDAs, Handheld Computers
■ Digital Cameras
■ MP3 Players
■ GPS Receivers
LT3497
Dual Full Function White
LED Driver with Integrated
Schottky Diodes
DESCRIPTION
The LT®3497 is a dual full function step-up DC/DC con-
verter specifically designed to drive up to 12 white LEDs
(6 white LEDs in series per converter) from a Li-Ion cell. Se-
ries connection of the LEDs provides identical LED currents
resulting in uniform brightness and eliminating the need
for ballast resistors and expensive factory calibration.
The two independent converters are capable of driving
asymmetric LED strings. Accurate LED dimming and
shutdown of the two LED strings can also be controlled
independently. The LT3497 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 1-wire
LED connection. Traditional LED drivers use a grounded
resistor to sense LED current, requiring a 2-wire connec-
tion to the LED string.
The 2.3MHz switching frequency allows the use of tiny
inductors and capacitors. Few external components are
needed for the dual white LED Driver: open-LED protection
and the Schottky diodes are all contained inside the 3mm
× 2mm DFN package. With such a high level of integra-
tion, the LT3497 provides a high efficiency dual white LED
driver solution in the smallest of spaces.
, LT, LTC and LTM are registered trademarks of Linear Technology Corporation.
True Color PWM is a trademark of Linear Technology Corporation.
All other trademarks are the property of their respective owners.
TYPICAL APPLICATION
Li-Ion Power Driver for 4/4 White LEDs
VIN
3V TO 5V
15µH
15µH
1µF
SW1 VIN SW2
CAP1
CAP2
10Ω LT3497
10Ω
1µF 1µF
LED1
LED2
CTRL1 GND CTRL2
OFF ON
SHUTDOWN
AND DIMMING
CONTROL 1
OFF ON
SHUTDOWN
AND DIMMING
CONTROL 2
3497 TA01a
80
VIN = 3.6V
4/4LEDs
75
Efficiency
70
65
60
55
50
0
5 10 15 20
LED CURRENT (mA)
3497 TA01b
3497f
1
1 page  
LT3497
TYPICAL PERFORMANCE CHARACTERISTICS (TA = 25°C unless otherwise specified)
Quiescent Current
7
6
5
4
125°C
25°C
–50°C
3
2
1
0
0 2 4 6 8 10
VIN (V)
3497 G09
Current Limit vs Temperature
500
450
400
350
300
–50 –25
0 25 50 75
TEMPERATURE (°C)
100 125
3497 G11
Schottky Leakage Current vs
Temperature (–50°C to 125°C)
3
2
24V
1
16V
0
–50 –25
0 25 50 75
TEMPERATURE (°C)
100 125
3497 G12
Open-Circuit Output Clamp Voltage
vs Temperature (–50°C to 125°C)
36
34
32
30
28
–50 –25
0 25 50 75
TEMPERATURE (°C)
100 125
3497 G13
Input Current in Output Open
Circuit vs Temperature
(–50°C to 125°C)
30
VIN = 3V
25
20
15
10
5
0
–50 –25
0 25 50 75
TEMPERATURE (°C)
100 125
3497 G14
Switching Frequency vs
Temperature
2.60
VIN = 3.6V
2.50
2.40
2.30
2.20
2.10
2.00
1.90
1.80
–50 –25
0 25 50 75
TEMPERATURE (°C)
100 125
3497 G15
Sense Voltage (VCAP – VLED)
vs VCAP
208
Sense Voltage vs Temperature
206
204
200 125°C
25°C
196 –50°C
192
202
198
194
188
5
10 15 20 25 30
VCAP (V)
3497 G16
190
–50 –25
0 25 50 75
TEMPERATURE (°C)
100 125
3497 G17
3497f
5
5 Page 
LT3497
APPLICATIONS INFORMATION
Table 4: RSENSE Value Selection for 200mV Sense
ILED (mA)
RSENSE (Ω)
5 40
10 20
15 13.3
20 10
PWM
10kHz TYP
R1
100k
LT3497
CTRL1,2
C1
0.1µF
3497 F05
Figure 5. Dimming Control Using a Filtered PWM Signal
DIMMING CONTROL
There are three different types of dimming control circuits.
The LED current can be set by modulating the CTRL pin
with a DC voltage, a filtered PWM signal or directly with
a PWM signal.
Using a DC Voltage
For some applications, the preferred method of brightness
control is a variable DC voltage to adjust the LED current.
The CTRL pin voltage can be modulated to set the dim-
ming of the LED string. As the voltage on the CTRL pin
increases from 0V to 1.5V, the LED current increases from
0 to ILED. As the CTRL pin voltage increases beyond 1.5V,
it has no effect on the LED current.
The LED current can be set by:
ILED
≈
200mV
RSENSE
when VCTRL > 1.5V
ILED
≈
VCTRL
6.25 •RSENSE
when VCTRL < 1.25V
Feedback voltage variation versus control voltage is given
in the Typical Performance Characteristics.
Using a Filtered PWM Signal
A filtered PWM can be used to control the brightness of
the LED string. The PWM signal is filtered (Figure 5) by a
RC network and fed to the CTRL1, CTRL2 pins.
The corner frequency of R1, C1 should be much lower
than the frequency of the PWM signal. R1 needs to be
much smaller than the internal impedance in the CTRL
pins which is 10MΩ (typ).
Direct PWM Dimming
Changing the forward current flowing in the LEDs not only
changes the intensity of the LEDs, it also changes the color.
The chromaticity of the LEDs changes with the change in
forward current. Many applications cannot tolerate any
shift in the color of the LEDs. Controlling the intensity of
the LEDs with a direct PWM signal allows dimming of the
LEDs without changing the color. In addition, direct PWM
dimming offers a wider dimming range to the user.
Dimming the LEDs via a PWM signal essentially involves
turning the LEDs on and off at the PWM frequency. The
typical human eye has a limit of ~60 frames per second.
By increasing the PWM frequency to ~80Hz or higher,
the eye will interpret that the pulsed light source is con-
tinuously on. Additionally, by modulating the duty cycle
(amount of “on time”) the intensity of the LEDs can be
controlled. The color of the LEDs remains unchanged in
this scheme since the LED current value is either zero or
a constant value.
Figure 6 shows a Li-ion powered 4/4 white LED driver. Direct
PWM dimming method requires an external NMOS tied
between the cathode of the lowest LED in the string and
ground as shown in Figure 6. Si2318DS MOSFETs can be
used since its sources are connected to ground. The PWM
signal is applied to the (CTRL1 and CTRL2) control pins of
the LT3497 and the gate of the MOSFET. The PWM signal
should traverse between 0V to 5V to ensure proper turn
on and off of the converters and the NMOS transistors (Q1
and Q2). When the PWM signal goes high, LEDs are con-
nected to ground and a current of ILED = (200mV/RSENSE)
flows through the LEDs. When the PWM signal goes low,
the LEDs are disconnected and turn off. The low PWM
input applied to the LT3497 ensures that the respective
3497f
11
11 Page | ||
| Páginas | Total 20 Páginas | |
| PDF Descargar | [ Datasheet LT3497.PDF ] | |
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