FEATURES
■
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
LT3491
White LED Driver with
Integrated Schottky in SC70
and 2mm × 2mm DFN
U
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 external 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.
TYPICAL APPLICATIO
Li-Ion Driver for Four White LEDs
SHUTDOWN AND
DIMMING CONTROL
V
3V TO 5V
IN
L1
10µH
C1
1µF
C1: TAIYO YUDEN LMK212BJ105MD
C2: TAIYO YUDEN GMK316BJ105ML
L1: MURATA LQH32CN100
V
SW
CTRL
IN
CAP
LT3491
LED
GND
U
R
SENSE
10Ω
3491 TA01a
C2
1µF
80
VIN = 3.6V
4 LEDs
75
70
65
60
55
EFFICIENCY (%)
50
45
40
0
Efficiency
5
10
LED CURRENT (mA)
15
20
3491 TA01b
3491fa
1
LT3491
WWWU
ABSOLUTE AXI U RATI GS
(Note 1)
Input Voltage (VIN) ................................................. 12V
SW Voltage ............................................................. 32V
CAP Voltage ............................................................ 32V
CTRL Voltage .......................................................... 12V
UU
W
PACKAGE/ORDER I FOR ATIO
TOP VIEW
6
1
V
IN
GND
2
SW
3
DC PACKAGE
6-LEAD (2mm × 2mm) PLASTIC DFN
T
= 125°C, θJA = 102°C/W, θJC = 20°C/ W
EXPOSED PAD (PIN 7) SHOULD BE CONNECTED TO PCB GROUND
JMAX
ORDER PART NUMBER DC PART MARKING
LT3491EDC
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.
CTRL
5
7
LED
CAP
4
LCHJ
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
TOP VIEW
SW 1
GND 2
GND 3
GND 4
SC8 PACKAGE
8-LEAD PLASTIC SC70
T
= 125°C, θJA = 270°C/ W
JMAX
8 CAP
7 LED
6 CTRL
5 V
IN
ORDER PART NUMBER DC PART MARKING
LT3491ESC8
LBXQ
ELECTRICAL CHARACTERISTICS
The ● denotes specifications which apply over the full operating temperature range, otherwise specifications are TA = 25°C.
= 3V, V
V
IN
PARAMETER CONDITIONS MIN TYP MAX UNITS
Minimum Operating Voltage 2.5 V
LED Current Sense Voltage (V
CAP, LED Pin Bias Current V
V
, V
CAP
LED
Supply Current V
Switching Frequency 1.8 2.3 2.8 MHz
Maximum Duty Cycle
Switch Current Limit
Switch V
Switch Leakage Current VSW = 16V 0.1 5 µA
V
for Full LED Current V
CTRL
V
to Shut Down IC 50 mV
CTRL
V
to Turn On IC
CTRL
CTRL Pin Bias Current 100 nA
CAP Pin Overvoltage Protection
Schottky Forward Drop I
Schottky Leakage Current VR = 20V 4 µA
= 3V, unless otherwise specified.
CTRL
– V
CAP
)V
LED
= 30V
CAP
= 16V, V
CAP
●
190 200 210 mV
= 16V 20 40 µA
LED
Common Mode Minimum Voltage 2.5 V
= 16V, V
CAP
= 15V, CTRL = 3V 2.6 4 mA
LED
CTRL = 0V 8 10 µA
●
88 92 %
●
260 350 mA
CESAT
ISW = 200mA 200 mV
= 30V
CAP
SCHOTTKY
= 100mA 0.8 V
●
1.5 V
●
100 mV
●
26 27 28 V
3491fa
2
ELECTRICAL CHARACTERISTICS
LT3491
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.
UW
TYPICAL PERFOR A CE CHARACTERISTICS
Switch Saturation Voltage (V
350
300
250
200
150
100
50
SWITCH SATURATION VOLTAGE (mV)
0
0
50 100
200 400
150 300
SWITCH CURRENT (mA)
250
CESAT
350
3491 G01
)
Schottky Forward Voltage Drop
400
350
300
250
200
150
100
SCHOTTKY FORWARD CURRET (mA)
50
0
200 400 800
0
SCHOTTKY FORWARD DROP (mV)
600
1000
= 25°C unless otherwise specified)
(T
A
Shutdown Current (V
15
12
9
6
SHUTDOWN CURRENT (µA)
3
0
3491 G02
1200
0
3
VIN (V)
= 0V)
CTRL
6
9
12
3491 G03
Sense Voltage (V
vs V
CTRL
240
200
160
120
80
SENSE VOLTAGE (mV)
40
0
500
0
1000
V
CTRL
V
SW
10V/DIV
V
CAP
50mV/DIV
100mA/DIV
– V
LED
1500
)
2000
3491 G04
CAP
(mV)
Switching Waveform
I
L
V
= 3.6V
IN
FRONT PAGE
APPLICATION CIRCUIT
200ns/DIV
Open-Circuit Output Clamp
Voltage
30
29
28
27
OUTPUT CLAMP VOLTAGE (V)
26
25
0
3
VIN (V)
3491 G07
6
9
V
CAP
5V/DIV
V
CTRL
5V/DIV
200mA/DIV
INPUT CURRENT (mA)
12
3491 G05
Transient Response
I
L
V
= 3.6V
IN
FRONT PAGE
APPLICATION CIRCUIT
1ms/DIV
Input Current in Output Open
Circuit
6
5
4
3
2
1
0
0
36 912
VIN (V)
3491 G08
3491 G06
3491fa
3
LT3491
TEMPERATURE (°C)
–50 –25
1.95
SWITCH FREQUENCY (MHz)
2.05
2.20
0
50
75
3419 G14
2.00
2.15
2.10
25
100
125
TEMPERATURE (°C)
–50 –25
192
SENSE VOLTAGE (mV)
200
212
0
50
75
3491 G17
196
208
204
25
100
125
UW
TYPICAL PERFOR A CE CHARACTERISTICS
(TA = 25°C unless otherwise specified)
Quiescent Current (V
3.0
2.5
2.0
1.5
1.0
QUIESCENT CURRENT (mA)
0.5
0
0
36 912
VIN (V)
CTRL
= 3V)
Open-Circuit Output Clamp Voltage
vs Temperature
30
29
28
27
26
OUTPUT CLAMP VOLTAGE (V)
3491 G09
Switching Current Limt
vs Duty Cycle
450
400
350
300
250
200
150
CURRENT LIMIT (mA)
100
50
0
40
30
25°C
50
60 90
DUTY CYCLE (%)
70
Input Current in Output Open Circuit
vs Temperature
6
VIN = 3V
5
4
3
2
INPUT CURRENT (mA)
1
80
3491 G10
Schottky Leakage Current
vs Temperature
15
12
9
6
3
SCHOTTKY LEAKAGE CURRENT (µA)
0
–50
02550
–25
TEMPERATURE (°C)
Switching Frequency
vs Temperature
VR = 10V
= 16V
V
R
V
= 20V
R
75 100
3491 G11
25
–50 –25
Sense Voltage (V
vs V
240
200
160
120
80
SENSE VOLTAGE (mV)
40
0
0
4
0
TEMPERATURE (°C)
CAP
– V
75
LED
50
25
CTRL
500 1000 1500 2000
V
(mV)
CTRL
100
)
3491 G12
–50°C
25°C
85°C
3491 G15
125
0
–50
–25 0
Sense Voltage (V
vs V
CAP
212
208
204
200
SENSE VOLTAGE (mV)
196
192
5
10
50 100 125
25 75
TEMPERATURE (°C)
– V
CAP
15
V
(V)
CAP
LED
20
)
3491 G13
3491 G16
Sense Voltage (V
CAP
– V
LED
)
vs Temperature
25
3491fa
LT3491
U
PI FU CTIO S
UU
(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
(Pin 5/Pin 1): Input Supply Pin. Must be locally
IN
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
( = 200mV/R
LED
). The CTRL pin must not
SENSE
be left floating.
W
BLOCK DIAGRA
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 :
LED
=
200
R
SENSE
I
mV
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
–
A2
+
Σ
RAMP
GENERATOR
V
SHDN
START-UP
CONTROL
PIN NUMBERS CORRESPOND TO THE 8-PIN SC70 PACKAGE
REF
1.25V
OSCILLATOR
R
PWM
COMP
C
C
C
1
SW
CAP
DRIVER
Q
R
S
A3
Q1
OVERVOLTAGE
+
PROTECTION
R
8
–
–
+
A1
+
GND
CTRL
PINS 2, 3, 4
6
A = 6.25
+
LED
–
7
3491 F01
Figure 1. Block Diagram
3491fa
5
LT3491
OPERATIO
U
The LT3491 uses a constant frequency, current mode
control scheme to provide excellent line and load regulation. Operation can be best understood by referring to the
Block Diagram in Figure 1.
At power up, the capacitor at the CAP pin is charged up to
(input supply voltage) through the inductor and the
V
IN
internal Schottky diode. If CTRL is pulled higher than
100mV, the bandgap reference, the start-up bias and the
oscillator are turned on. At the start of each oscillator
cycle, the power switch Q1 is turned on. A voltage proportional to the switch current is added to a stabilizing ramp
and the resulting sum is fed into the positive terminal of the
PWM comparator, A2. When this voltage exceeds the level
at the negative input of A2, the PWM logic turns off the
power switch. The level at the negative input of A2 is set by
the error amplifier A1, and is simply an amplified version
of the difference between the V
CAP
and V
voltage and
LED
the bandgap reference. In this manner the error amplifier,
A1, sets the correct peak current level in inductor L1 to
keep the output in regulation. The CTRL pin is used to
adjust the LED current. The LT3491 enters into shutdown
when CTRL is pulled lower than 50mV.
Minimum Output Current
The LT3491 can drive a 3-LED string at 2mA LED current
without pulse skipping using the same external components shown in the application circuit on the front page of
this data sheet. As current is further reduced, the device
will begin skipping pulses. This will result in some low
frequency ripple, although the average LED current remains regulated down to zero. The photo in Figure 2
details circuit operation driving three white LEDs at 2mA
load. Peak inductor current is less than 60mA and the
regulator operates in discontinuous mode, meaning the
inductor current reaches zero during the discharge phase.
After the inductor current reaches zero, the SW pin
exhibits ringing due to the LC tank circuit formed by the
inductor in combination with the switch and the diode
capacitance. This ringing is not harmful; far less spectral
energy is contained in the ringing than in the switch
transitions.
6
50mA/DIV
V
SW
10V/DIV
I
L
V
= 4.2V
IN
= 2mA
I
LED
3 LEDs
Figure 2. Switching Waveforms
200ns/DIV
3491 F02
3491fa
WUUU
APPLICATIO S I FOR ATIO
LT3491
INDUCTOR SELECTION
A 10µH inductor is recommended for most LT3491 appli-
cations. Although small size and high efficiency are major
concerns, the inductor should have low core losses at
2.3MHz and low DCR (copper wire resistance). Some
small inductors in this category are listed in Table 1. The
efficiency comparison of different inductors is shown in
Figure 3.
Table 1. Recommended Inductors
CURRENT
L DCR RATING
PART (µH) (Ω) (mA) VENDOR
LQH32CN100K53 10 0.3 450 Murata
LQH2MCN100K02 10 1.2 225 www.murata.com
SD3112-100 10 0.446 550 Cooper
www.cooperet.com
1001AS-100M 10 0.48 460 Toko
(TYPE D312C) www.toko.com
CDRH2D11 10 0.5375 280 Sumida
CDRH2D14 10 0.294 700 www.sumida.com
80
75
70
65
60
55
50
EFFICIENCY (%)
45
40
35
30
0
Figure 3. Efficiency Comparison of Different Inductors
VIN = 3.6V
4 LEDs
FRONT PAGE
APPLICATION CIRCUIT
MURATA LQH2MCN100K02
MURATA LQH32CN100K53
TOKO 10001AS-100M
SUMIDA CDRH2D11
SUMIDA CDRH2D14
5
10
LED CURRENT (mA)
15
20
3491 F03
Table 2 shows a list of several ceramic capacitor manufacturers. Consult the manufacturers for detailed information
on their entire selection of ceramic parts.
Table 2. Recommended Ceramic Capacitor Manufacturers
Taiyo Yuden (800) 368-2496
www.t-yuden.com
AVX (803) 448-9411
www.avxcorp.com
Murata (714) 852-2001
www.murata.com
OVERVOLTAGE PROTECTION
The LT3491 has an internal open-circuit protection circuit.
In the cases of output open circuit, when the LEDs are
disconnected from the circuit or the LEDs fail open circuit,
is clamped at 27V (typ). The LT3491 will then switch
V
CAP
at a very low frequency to minimize input current. The V
CAP
and input current during output open circuit are shown in
the Typical Performance Characteristics. Figure 4 shows
the transient response when the LEDs are disconnected.
I
L
200mA/DIV
V
CAP
10V/DIV
V
= 3.6V
IN
CIRCUIT OF
FRONT PAGE
APPLICATION
500µs/DIV
LEDs DISCONNECTED
AT THIS INSTANT
Figure 4. Output Open-Circuit Waveform
3491 F04
CAPACITOR SELECTION
The small size of ceramic capacitors make them ideal for
LT3491 applications. Use only X5R and X7R types because they retain their capacitance over wider temperature
ranges than other types such as Y5V or Z5U. A 1µF input
capacitor and a 1µF output capacitor are sufficient for
most applications.
INRUSH CURRENT
The LT3491 has a built-in Schottky diode. When supply
voltage is applied to the V
pin, an inrush current flows
IN
through the inductor and the Schottky diode and charges
up the CAP voltage. The Schottky diode inside the LT3491
can sustain a maximum current of 1A.
3491fa
7
LT3491
WUUU
APPLICATIO S I FOR ATIO
For low DCR inductors, which is usually the case for this
application, the peak inrush current can be simplified as
follows:
I
α
ω
PK
=
=
=
2
V
IN
•
L
r
•
L
1
•
LC
• exp – •
ω
2
rr
–
4•
L
αωπ
⎛
⎜
⎝
2
⎞
⎟
⎠
2
06
–.
where L is the inductance, r is the DCR of the inductor and
C is the output capacitance.
Table 3 gives inrush peak currents for some component
selections.
Table 3. Inrush Peak Currents
VIN (V) r (Ω)L (µH) C
4.2 0.3 10 1.0 1.06
4.2 1.2 10 1.0 0.86
4.2 0.58 15 1.0 0.83
4.2 1.6 15 1.0 0.68
(µF) IP (A)
OUT
Table 4. R
Value Selection for 200mV Sense
SENSE
I
(mA) R
LED
540
10 20
15 13.3
20 10
SENSE
(Ω)
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 dimming
of the LED string. As the voltage on the CTRL pin increases
from 0V to 1.5V, the LED current increases from 0 to I
LED
.
As the CTRL pin voltage increases beyond 1.5V, it has no
effect on the LED current.
The LED current can be set by:
PROGRAMMING LED CURRENT
The feedback resistor (R
– V
(V
CAP
) control the LED current.
LED
) and the sense voltage
SENSE
The CTRL pin controls the sense reference voltage as
shown in the Typical Performance Characteristics. For
CTRL higher than 1.5V, the sense reference is 200mV,
which results in full LED current. In order to have accurate
LED current, precision resistors are preferred (1% is
recommended). The formula and table for R
SENSE
selec-
tion are shown below.
mV
R
SENSE
200
=
I
LED
200
mV
I
≈ >
LED
R
I
≈
LED
6
,.
when V V
SENSE
V
CTRL
.
225
•
R
SENSE
CTRL
,.
when V V
15
CTRL
<
125
Feedback voltage variation versus control voltage is given
in the Typical Performance Characteristics.
3491fa
8
WUUU
APPLICATIO S I FOR ATIO
LT3491
Using a Filtered PWM Signal
A filtered PWM signal 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 CTRL pin.
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 of the CTRL pin which
is 10MΩ (typ).
C1
0.1µF
LT3491
CTRL
3491 F05
R1
100k
PWM
10kHz TYP
Figure 5. Dimming Control Using a Filtered PWM Signal
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.
level Si2302 MOSFET can be used since its source is
connected to ground. The PWM signal is applied to the
CTRL pin of the LT3491 and the gate of the MOSFET. The
PWM signal should traverse between 0V to 2.5V, to ensure
proper turn on and off of the driver and the NMOS
transistor Q1. When the PWM signal goes high, the LEDs
are connected to ground and a current of I
R
flows through the LEDs. When the PWM signal
SENSE
= 200mV/
LED
goes low, the LEDs are disconnected and turn off. The
MOSFET ensures that the LEDs quickly turn off without
discharging the output capacitor which in turn allows the
LEDs to turn on faster. Figure 7 shows the PWM dimming
waveforms for the circuit in Figure 6.
V
IN
3V TO 5V
L1
10µH
C1
1µF
2.5V
PWM
FREQ
0V
SW
GND
V
IN
LT3491
CTRL
100k
CAP
LED
R
SENSE
10Ω
Q1
Si2302
3491 F06
C2
1µF
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 continuously on.
Additionally, by modulating the duty cycle (amount of “ontime”), 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 driver for four white
LEDs. 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. A simple logic
Figure 6. Li-Ion to Four White LEDs with Direct PWM Dimming
I
LED
20mA/DIV
I
L
200mA/DIV
PWM
5V/DIV
= 3V
V
IN
4 LEDs
2ms/DIV
3491 F07
Figure 7. Direct PWM Dimming Waveforms
3491fa
9
LT3491
WUUU
APPLICATIO S I FOR ATIO
The time it takes for the LED current to reach its programmed value sets the achievable dimming range for a
given PWM frequency. For example, the settling time of
the LED current in Figure 7 is approximately 30µs for a 3V
input voltage. The achievable dimming range for this
application and 100Hz PWM frequency can be determined
using the following method.
Example:
ƒƒ==
ts
PERIOD
D
Hz t µs
100 30
,
SETTLE
11
== =
100
t
iim Range
PERIOD
t
SETTLE
001
.
001
30
.
s
:===
300 1
µs
Min
t
SETTLE
==100
t
PERIOD
= →
•
100 0 3 100
%.%Duty Cycle Range at Hz
µs
30
.
001
•Duty Cycle
1000 0 3
s
=
.%
down to 100mV. The use of both techniques together
allows the average LED current for the four LED application to be varied from 20mA down to less than 20µA.
Figure 9 shows the application for dimming using both
analog dimming and PWM dimming. A potentiometer
must be added to ensure that the gate of the NMOS
receives a logic-level signal, while the CTRL signal can be
adjusted to lower amplitudes.
100Hz
1kHz
10kHz
1 10 100 1000
PWM DIMMING RANGE
3491 F08
The calculations show that for a 100Hz signal the dimming
range is 300 to 1. In addition, the minimum PWM duty
cycle of 0.3% ensures that the LED current has enough
time to settle to its final value. Figure 8 shows the dimming
range achievable for three different frequencies with a
settling time of 30µs.
The dimming range can be further extended by changing
the amplitude of the PWM signal. The height of the PWM
signal sets the commanded sense voltage across the
sense resistor through the CTRL pin. In this manner both
analog dimming and direct PWM dimming extend the
dimming range for a given application. The color of the
LEDs no longer remains constant because the forward
current of the LED changes with the height of the CTRL
signal. For the four LED application described above, the
LEDs can be dimmed first, modulating the duty cycle of the
PWM signal. Once the minimum duty cycle is reached, the
height of the PWM signal can be decreased below 1.5V
Figure 8. Dimming Range Comparison
of Three PWM Frequencies
V
IN
3V TO 5V
L1
10µH
C1
1µF
Figure 9. Li-Ion to Four White LEDs with Both
PWM Dimming and Analog Dimming
SW
GND
V
IN
LT3491
CTRL
2.5V
CAP
LED
PWM
FREQ
0V
100k
R
SENSE
10Ω
Q1
Si2302
3491 F09
C2
1µF
10
3491fa
WUUU
APPLICATIO S I FOR ATIO
LT3491
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.
SHUTDOWN AND
DIMMING CONTROL
3.3V
2 AA CELLS
2V TO 3.2V
C1
0.1µF
10µH
C1
1µF
V
IN
L1
SW
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 essential. 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 C
next to the CAP pin. Always use a ground plane under the
switching regulator to minimize interplane coupling. Recommended component placement is shown in Figure 11.
CTRL
CAP
R
LT3491
LED
GND
SENSE
10Ω
C2
2.2µF
OUT
LED
(A) SC70 PACKAGE
R
SENSE
CTRL
CAP
C1: TAIYO YUDEN LMK212BJ105MD
C2: TAIYO YUDEN GMK325BJ225ML
L1: MURATA LQH32CN100
3491 F10
Figure 10. 2 AA Cells to Three White LEDs
C
IN
V
IN
5
6
7
8
C
OUT
GND
GND
SW
C
IN
V
IN
1
GND
2
3
L1
7
CTRL
6
5
LED
4
CAP
R
C
OUT
SENSE
3491 F11
4
3
2
1
L1
SW
(B) DFN PACKAGE
Figure 11. Recommended Component Placement
3491fa
11
LT3491
TYPICAL APPLICATIO S
Li-Ion Driver for One White LED Efficiency
C2
1µF
R
SENSE
10Ω
V
3V TO 5V
V
3V TO 5V
IN
L1
10µH
C1
1µF
C1: TAIYO YUDEN LMK212BJ105MD
C2: TAIYO YUDEN GMK316BJ105ML
L1: MURATA LQH32CN100
Li-Ion Driver for Two White LEDs Efficiency
IN
L1
10µH
C1
1µF
C1: TAIYO YUDEN LMK212BJ105MD
C2: TAIYO YUDEN GMK316BJ105ML
L1: MURATA LQH32CN100
LED
LT3491
GND
C2
1µF
LED
LT3491
GND
CAP
3491 TA07a
R
SENSE
10Ω
CAP
3491 TA08a
V
IN
SW CTRL
V
IN
SW CTRL
U
SHUTDOWN
AND
DIMMING
CONTROL
SHUTDOWN
AND
DIMMING
CONTROL
60
V
= 3.6V
IN
55
50
45
40
35
30
EFFICIENCY (%)
25
20
15
10
0
70
V
= 3.6V
IN
65
60
55
50
45
EFFICIENCY (%)
40
35
30
25
0
5
5
10
LED CURRENT (mA)
10
LED CURRENT (mA)
15
15
20
3491 TA07b
20
3491 TA08b
2-Cell Li-Ion Driver for Torch and Flash Mode LED Control
C2
4.7µF
V
IN
V
CTRL
680mV
FLASH MODE
I
= 200mA
LED
TORCH MODE
= 100mA
I
LED
6V TO 9V
V
1.5V
CTRL
C1
1µF
C1: TAIYO YUDEN LMK212BJ105MD
C2: TAIYO YUDEN LMK212BJ475MG
D1: AOT-2015 HPW1751B
L1: MURATA LQH32CN100
R
SENSE
1Ω
CAP LED
V
IN
LT3491
CTRL SW
GND
3491 TA09a
D1
12
L1
10µH
80
I
= 100mA
LED
75
70
65
EFFICIENCY (%)
60
55
50
6
6.5 7
Efficiency
7.5 8.5
VIN (V)
89
3491 TA09b
3491fa
TYPICAL APPLICATIO S
12V to One White LED at 200mA Efficiency
LT3491
U
PV
IN
12V
V
IN
3V
SHUTDOWN
AND
DIMMING
CONTROL
C1, C3: TAIYO YUDEN LMK212BJ105MD
C2: TAIYO YUDEN LMK316BJ475ML
D1: LUXEON EMITTER LXHL-BWO2
L1: MURATA LQH32CN150
C3
1µF
C1
1µF
R
CAP LED
V
IN
LT3491
CTRL SW
12V to Two White LEDs at 200mA
PV
IN
12V
V
IN
3V
SHUTDOWN
AND
DIMMING
CONTROL
C3
1µF
C1
1µF
C1, C3: TAIYO YUDEN LMK212BJ105MD
C2: TAIYO YUDEN LMK316BJ475ML
D1: LUXEON EMITTER LXHL-BWO2
L1: MURATA LQH32CN150
R
SENSE
CAP LED
V
IN
LT3491
CTRL SW
GND
SENSE
1Ω
GND
1Ω
C2
4.7µF
C2
4.7µF
3491 TA03a
3491 TA02a
80
75
D1
L1
15µH
70
65
EFFICIENCY (%)
60
55
50
40 80 120 160
200 60 100 140 180
LED CURRENT (mA)
200
3491 TA02b
Efficiency
90
85
D1
L1
15µH
80
75
EFFICIENCY (%)
70
65
60
40 80 120 160
200 60 100 140 180
LED CURRENT (mA)
200
3491 TA03b
3491fa
13
LT3491
TYPICAL APPLICATIO S
U
3V TO 5V
3V TO 5V
Li-Ion Driver for Three White LEDs
SHUTDOWN AND
DIMMING CONTROL
V
IN
L1
10µH
C1
1µF
C1: TAIYO YUDEN LMK212BJ105MD
C2: TAIYO YUDEN GMK316BJ105ML
L1: MURATA LQH32CN100
V
SW
CTRL
IN
LT3491
GND
Li-Ion Driver for Five White LEDs Efficiency
SHUTDOWN AND
DIMMING CONTROL
V
IN
L1
10µH
C1
1µF
C1: TAIYO YUDEN LMK212BJ105MD
C2: TAIYO YUDEN GMK316BJ105ML
L1: MURATA LQH32CN100
V
SW
CTRL
IN
LT3491
GND
CAP
LED
CAP
LED
R
SENSE
10Ω
R
SENSE
10Ω
3491 TA05a
3491 TA04a
C2
1µF
C2
1µF
80
VIN = 3.6V
3 LEDs
75
70
65
60
55
EFFICIENCY (%)
50
45
40
35
2 4 6 8 10 20
0
80
VIN = 3.6V
5 LEDs
75
70
65
60
55
EFFICIENCY (%)
50
45
40
35
2 4 6 8 10 20
0
Efficiency
12 14 16 18
LED CURRENT (mA)
3491 TA04b
12 14 16 18
LED CURRENT (mA)
3491 TA05b
14
3491fa
PACKAGE DESCRIPTIO
LT3491
U
SC8 Package
8-Lead Plastic SC70
(Reference LTC DWG # 05-08-1639 Rev Ø)
2.8 BSC
GAUGE PLANE
0.15 BSC
0.30
MAX
1.8 REF
RECOMMENDED SOLDER PAD LAYOUT
0.10 – 0.40
NOTE:
1. DIMENSIONS ARE IN MILLIMETERS
2. DRAWING NOT TO SCALE
3. DIMENSIONS ARE INCLUSIVE OF PLATING
4. DIMENSIONS ARE EXCLUSIVE OF MOLD FLASH AND METAL BURR
5. MOLD FLASH SHALL NOT EXCEED 0.254mm
6. DETAILS OF THE PIN 1 IDENTIFIER ARE OPTIONAL,
BUT MUST BE LOCATED WITHIN THE INDEX AREA
7. EIAJ PACKAGE REFERENCE IS EIAJ SC-70 AND JEDEC MO-203 VARIATION BA
PER IPC CALCULATOR
0.50
0.26 – 0.46
REF
1.00 REF
0.10 – 0.18
(NOTE 3)
1.80 – 2.40
1.15 – 1.35
(NOTE 4)
1.00 MAX
0.50 BSC
0.80 – 1.00
PIN 1
PIN 8
1.80 – 2.20
(NOTE 4)
INDEX AREA
(NOTE 6)
0.15 – 0.27
8 PLCS (NOTE 3)
0.00 – 0.10
REF
SC8 SC70 0905 REV Ø
2.50 ±0.05
1.15 ±0.05
DC Package
6-Lead DFN (2mm × 2mm)
(Reference LTC DWG # 05-08-1703)
0.675 ±0.05
0.61 ±0.05
(2 SIDES)
1.42 ±0.05
RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS
NOTE:
1. DRAWING TO BE MADE A JEDEC PACKAGE OUTLINE M0-229 VARIATION OF (WCCD-2)
2. DRAWING NOT TO SCALE
3. ALL DIMENSIONS ARE IN MILLIMETERS
4. DIMENSIONS OF EXPOSED PAD ON BOTTOM OF PACKAGE DO NOT INCLUDE
MOLD FLASH. MOLD FLASH, IF PRESENT, SHALL NOT EXCEED 0.15mm ON ANY SIDE
5. EXPOSED PAD SHALL BE SOLDER PLATED
6. SHADED AREA IS ONLY A REFERENCE FOR PIN 1 LOCATION ON THE
TOP AND BOTTOM OF PACKAGE
(2 SIDES)
0.50 BSC
PACKAGE
OUTLINE
0.25 ± 0.05
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 representation that the interconnection of its circuits as described herein will not infringe on existing patent rights.
PIN 1 BAR
TOP MARK
(SEE NOTE 6)
0.200 REF
2.00 ±0.10
(4 SIDES)
0.75 ±0.05
0.00 – 0.05
R = 0.115
0.56 ± 0.05
(2 SIDES)
TYP
BOTTOM VIEW—EXPOSED PAD
3
1.37 ±0.05
(2 SIDES)
64
1
0.50 BSC
0.38 ± 0.05
PIN 1
CHAMFER OF
EXPOSED PAD
0.25 ± 0.05
(DC6) DFN 1103
3491fa
15
LT3491
TYPICAL APPLICATIO
Li-Ion Driver for Six White LEDs Efficiency
U
80
VIN = 3.6V
6 LEDs
75
70
65
60
55
EFFICIENCY (%)
50
45
40
0
5
10
LED CURRENT (mA)
15
20
3491 TA06b
V
3V TO 5V
SHUTDOWN AND
DIMMING CONTROL
IN
L1
10µH
C1
1µF
C1: TAIYO YUDEN LMK212BJ105MD
C2: TAIYO YUDEN GMK316BJ105ML
L1: MURATA LQH32CN100
V
SW
CTRL
LT3491
GND
CAP
LED
R
SENSE
10Ω
C2
1µF
3491 TA06a
IN
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Q
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ThinSOT is a trademark of Linear Technology Corporation.
OUT(MAX)
= 34V,
OUT(MAX)
= 34V,
= 34V,
= 34V,
= 39V,
16
Linear Technology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900 ● FAX: (408) 434-0507
●
www.linear.com
3491fa
LT 0406 • PRINTED IN THE USA
© LINEAR TECHNOLOGY CORPORATION 2006
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