LT3591 - White LED Driver with Integrated Schottky in 3mm x 2mm DFN

LT3591
White LED Driver with
Integrated Schottky in
3mm × 2mm DFN
DESCRIPTION
FEATURES
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Drives Up to Ten White LEDs from a 3V Supply
High Side Sense Allows “One Wire Current Source”
Internal Schottky Diode
One Pin Dimming and Shutdown
80:1 True Color PWMTM Dimming Range
42V Open LED Protection
1MHz Switching Frequency
±5% Reference Accuracy
VIN Range: 2.5V to 12V
Requires Only 2.2µF Output Capacitor
Low Profile 8-Lead DFN Package
(3mm × 2mm × 0.75mm)
APPLICATIONS
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Cellular Phones
PDAs, Handheld Computers
Digital Cameras
MP3 Players
GPS Receivers
The LT®3591 is a fixed frequency step-up DC/DC converter
specifically designed to drive up to ten 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 high 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 a low profile 3mm × 2mm
DFN package.
, 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 Driver for Ten White LEDs
Conversion Efficiency
80
SHUTDOWN AND
DIMMING CONTROL
75
CTRL
VIN
CAP
RSENSE
10Ω
LT3591
22µH
SW
EFFICIENCY (%)
VIN
3V TO 5V
VIN = 3.6V
10 LEDs
LED
GND
70
65
2.2µF
1µF
60
55
0
5
10
15
20
LED CURRENT (mA)
3591 TA01b
3591 TA01a
3591f
1
LT3591
ABSOLUTE MAXIMUM RATINGS
PACKAGE/ORDER INFORMATION
(Note 1)
TOP VIEW
Input Voltage (VIN) ................................................... 12V
CTRL Voltage ........................................................... 12V
SW Voltage .............................................................. 45V
CAP Voltage ............................................................. 45V
LED Voltage ............................................................. 45V
Operating Junction Temperature Range
(Note 2) ...............................................–40°C to 85°C
Maximum Junction Temperature ........................ 125°C
Storage Temperature Range...................–65°C to 150°C
VIN 1
GND 2
NC 3
8 CTRL
9
SW 4
7 LED
6 NC
5 CAP
DDB PACKAGE
8-LEAD (3mm × 2mm) PLASTIC DFN
TJMAX = 125°C, θJA = 76°C/W
EXPOSED PAD (PIN 9) SHOULD BE CONNECTED TO PCB GROUND
ORDER PART NUMBER
DDB PART MARKING
LT3591EDDB
LCPG
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 the specifications which apply over the full operating
temperature range, otherwise specifications are at TA = 25°C. VIN = 3V, VCTRL = 3V, unless otherwise specified.
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
200
210
mV
2.5
Minimum Operating Voltage
●
VCAP = 24V, ISW = 300mA
CAP Pin Bias Current
VCAP = 36V, VLED = 35.8V
40
80
µA
LED Pin Bias Current
VCAP = 36V, VLED = 35.8V
20
40
µA
Supply Current
VCAP = 24V, VLED = 23V
CTRL = 0V
4
9
5
11
mA
µA
0.75
1
1.2
MHz
92
94
%
500
Switching Frequency
Maximum Duty Cycle
●
190
V
LED Current Sense Voltage (VCAP – VLED)
800
mA
Switch VCESAT
ISW = 300mA
200
mV
Switch Leakage Current
VSW = 24V
0.1
VCTRL for Full LED Current
VCAP = 44V
Switch Current Limit
●
5
1.5
V
VCTRL to Shut Down IC
50
●
VCTRL to Turn On IC
100
CTRL Pin Bias Current
●
Schottky Forward Drop
ISCHOTTKY = 200mA
Schottky Leakage Current
VR = 30V
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.
40
mV
mV
100
CAP Pin Overvoltage Protection
µA
42
nA
44
0.8
V
V
4
µA
Note 2: The LT3591E is guaranteed to meet performance specifications
from 0°C to 85°C operating junction temperature range. Specifications
over the –40°C to 85°C operating junction temperature range are assured
by design, characterization and correlation with statistical process controls.
3591f
2
LT3591
TYPICAL PERFORMANCE CHARACTERISTICS
Switch Saturation Voltage
(VCESAT)
Schottky Forward Voltage Drop
25°C
400
300
125°C
200
– 50°C
100
500
SHUTDOWN CURRENT (µA)
SCHOTTKY FORWARD CURRENT (mA)
125°C
400
300
25°C
200
– 50°C
100
0
0
100 200 300 400 500 600 700 800
SWITCH CURRENT (mA)
200
400
600
800 1000
SCHOTTKY FORWARD DROP (mV)
Sense Voltage (VCAP – VLED)
vs VCTRL
– 50°C
25°C
6
125°C
3
1200
0
3
6
9
12
VIN (V)
3591 G03
Open-Circuit Output
Clamp Voltage
Input Current in Output
Open Circuit
8
45
– 50°C
25°C
125°C
7
OUTPUT CLAMP VOLTAGE (V)
200
9
3591 G02
3591 G01
240
12
0
0
0
160
120
80
40
44
INPUT CURRENT (mA)
SWITCH SATURATION VOLTAGE (mV)
Shutdown Current (VCTRL = 0V)
15
600
500
SENSE VOLTAGE (mV)
TA = 25°C, unless otherwise specified.
25°C
43
125°C
42
– 50°C
25°C
6
125°C
5
– 50°C
4
3
2
41
1
0
0
500
1000 1500 2000
VCTRL (mV)
2500
3000
40
0
3
0
6
9
12
0
VIN (V)
3591 G04
6
VIN (V)
9
12
3591 G06
3591 G05
Switching Waveform
Transient Response
VSW
20V/DIV
VCAP
5V/DIV
VCAP
50mV/DIV
VCTRL
5V/DIV
IL
200mA/DIV
IL
500mA/DIV
VIN = 3.6V
500ms/DIV
FRONT PAGE
APPLICATION CIRCUIT
3
3591 G07
VIN = 3.6V
1ms/DIV
FRONT PAGE
APPLICATION CIRCUIT
3591 G08
3591f
3
LT3591
TYPICAL PERFORMANCE CHARACTERISTICS
Quiescent Current
(VCTRL = 3V)
Schottky Leakage Current
vs Temperature
Current Limit vs Temperature
6
15
25°C
SCHOTTKY LEAKAGE CURRENT (µA)
1000
5
QUIESCENT CURRENT (mA)
TA = 25°C, unless otherwise specified.
800
CURRENT LIMIT (mA)
125°C
4
– 50°C
3
2
600
400
200
1
0
0
3
6
VIN (V)
9
0
12
–50
–25
0
25
50
75
TEMPERATURE (°C)
100
3591 G09
8
INPUT CURRENT (mA)
42
41
6
5
4
3
2
1100
1050
1000
950
900
850
800
50
25
75
0
TEMPERATURE (°C)
100
3591 G12
750
–50 –25
125
204
204
90
–50 –25
SENSE VOLTAGE (mV)
98
SENSE VOLTAGE (mV)
208
MAXIMUM DUTY CYCLE (%)
208
25°C
200
125°C
196
– 50°C
100
125
188
5
10
15
20
25
30
35
VCAP (V)
3591 G15
200
196
192
192
50
25
75
0
TEMPERATURE (°C)
125
Sense Voltage (VCAP – VLED)
vs Temperature
100
92
100
3591 G14
Sense Voltage (VCAP – VLED)
vs VCAP
94
50
25
75
0
TEMPERATURE (°C)
3591 G13
Maximum Duty Cycle
vs Temperature
125
1150
0
–50 –25
125
96
100
1200
VIN = 3V
1
100
50
25
75
0
TEMPERATURE (°C)
Switching Frequency
vs Temperature
SWITCHING FREQUENCY (kHz)
OUTPUT CLAMP VOLTAGE (V)
3
3591 G11
7
50
25
0
75
TEMPERATURE (°C)
6
0
–50 –25
125
44
–25
9
Input Current in Output Open
Circuit vs Temperature
45
40
–50
12
3591 G10
Open-Circuit Output Clamp
Voltage vs Temperature
43
VR = 10V
VR = 16V
VR = 20V
3591 G16
188
–50
–25
50
25
0
75
TEMPERATURE (°C)
100
125
3591 G17
3591f
4
LT3591
PIN FUNCTIONS
VIN (Pin 1): Input Supply Pin. Must be locally bypassed.
CTRL (Pin 8): 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.
GND (Pin 2): Ground Pin. Should be tied directly to local
ground plane.
SW (Pin 4): Switch Pin. Minimize trace area at this pin to
minimize EMI. Connect the inductor at this pin.
Exposed Pad (Pin 9): Ground. The Exposed Pad must
be soldered to PCB ground to achieve the rated thermal
performance.
CAP (Pin 5): 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.
LED (Pin 7): Connection Point for the Anode of the First
LED and the Sense Resistor. The LED current can be
programmed by :
ILED =
200mV
RSENSE
BLOCK DIAGRAM
1
4
SW
VIN
PWM
COMP
–
CAP
5
DRIVER
A2
R
+
S
Q1
Q
OVERVOLTAGE
PROTECTION
+
Σ
R
A3
–
RAMP
GENERATOR
+
OSCILLATOR
A1
RC
START-UP
CONTROL
+
+
–
VREF
1.25V
SHDN
A = 6.25
–
LED
7
CC
CTRL
8
GND
2
3591 F01
Figure 1. Block Diagram
3591f
5
LT3591
OPERATION
The LT3591 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.
the LED current. The LT3591 enters into shutdown when
CTRL is pulled lower than 50mV.
At power-up, the capacitor at the CAP pin is charged up
to VIN (input supply voltage) through the inductor and
the 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 VCAP and VLED voltage and 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 LT3591 can drive a 2-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 two white LEDs at 2mA load. Peak inductor current is less than 40mA 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.
Minimum Output Current
IL
20mA/DIV
VSW
5V/DIV
VIN = 4.2V
ILED = 2mA
2 LEDs
500ns/DIV
3591 F02
Figure 2. Switching Waveforms
3591f
6
LT3591
APPLICATIONS INFORMATION
INDUCTOR SELECTION
CAPACITOR SELECTION
A 22µH inductor is recommended for most LT3591 applications. Although small size and high efficiency are
major concerns, the inductor should have low core losses
at 1MHz 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.
The small size of ceramic capacitors make them ideal for
LT3591 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 50V, 2.2µF output capacitor are sufficient
for most applications.
Table 1. Recommended Inductors
PART
CURRENT
L
RATING
(µH)
(mA)
VENDOR
Table 2. Recommended Output Capacitors
4 × 3.8 × 1.2 TDK
www.tdk.com
VLF4012AT220MR51
22
VLCF4018T220MR49-2
22
490
4.1 × 4.1 × 1.8
VLCF4020T220MR56
22
560
4.1 × 4.1 × 2
LQH43CN220K03
22
420
4.8 × 3.4 × 2.8 Murata
www.murata.com
GRM31MR71H105KA88
4.2 × 4.2 × 1.8 Taiyo Yuden
www.t-yuden.com
4.2 × 4.2 × 1.2
GRM31CR71H225KA88
NR4018T220M
22
590
NR4012T220M
22
510
CDRH3D18220NC
22
600
B82470-A1223-M
22
480
VOLTAGE CASE SIZE
C
(µF)
PART
GRM21BR71H105KA12L
4 × 4 ×2
85
1
TEMP.
50V
X7R
Sumida
www.sumida.com
4.8 × 4.8 × 1.2 Epcos
www.epcos.com
1
2.2
GRM31CR71H475KA12L 4.7
UMK316BJ475KL-T
4.7
HEIGHT
(mm)
0805
Murata
1.25 ± 0.15 www.murata.com
50V
1206
X7R
1.15 ± 0.1
50V
1206
X7R
1.6 ± 0.2
50V
1206
X7R
1.6 ± 0.2
50V
1206
X7R
VENDOR
Taiyo Yuden
1.6 ± 0.2 www.t-yuden.com
VIN = 3.6V
10 LEDs
80
75
EFFICIENCY (%)
510
MAX
DIMENSION
L×W×H
(mm)
A limited number of manufacturers produce small 50V
capacitors. Table 2 shows a list of several recommended
50V capacitors. Consult the manufacturer for detailed
information on their entire selection of ceramic parts.
70
TAIYO YUDEN NR4018T220M
TDK VLCF4018T-220MR49-2
TAIYO YUDEN NR4012T220M
TDKVLCF4012AT-220MR51
MURATA LQH43CN220K03
TDK VLCF4020T-220MR56
SUMIDA CDRH3D18-220NC
EPCOS B82470-A1223-M
65
60
55
50
0
5
10
LED CURRENT (mA)
15
20
3591 F03
Figure 3. Efficiency Comparison of Different Inductors
3591f
7
LT3591
APPLICATIONS INFORMATION
SCHOTTKY DIODE
The LT3591 has a built-in Schottky diode. The internal
schottky saves board space in space constrained applications. In less space sensitive applications, an external
schottky diode connected between the SW node and the
CAP node increases efficiency one to two percent. It is
important to use a properly rated schottky diode that can
handle the peak switch current of the LT3591. In addition,
the schottky diode must have a breakdown voltage of at least
40V along with a low forward voltage in order to achieve
higher efficiency. One recommended external schottky
diode for the LT3591 is the Phillips PMEG4005AEA.
OVERVOLTAGE PROTECTION
The LT3591 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,
VCAP is clamped at 42V (typ). The LT3591 will then switch
at a very low frequency to minimize input current. The VCAP
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.
For low DCR inductors, which is usually the case for this
application, the peak inrush current can be simplified as
follows:
V – 0.6
⎛ α π⎞
• exp ⎜ – • ⎟
IPK = IN
⎝ ω 2⎠
L•ω
α=
r
2 •L
r2
1
–
L • C 4 • L2
ω=
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)
COUT (µF)
IP (A)
4.2
0.3
22
2.2
1.06
4.2
0.71
22
2.2
0.96
4.2
0.58
15
1
0.83
4.2
1.6
15
1
0.68
PROGRAMMING LED CURRENT
IL
500mA/DIV
The feedback resistor (RSENSE) and the sense voltage
(VCAP – VLED) control the LED current.
VCAP
20V/DIV
VIN = 3.6V
CIRCUIT OF
FRONT PAGE
APPLICATION
500µs/DIV
3591 F04
LEDs DISCONNECTED
AT THIS INSTANT
Figure 4. Output Open-Circuit Waveform
INRUSH CURRENT
The LT3591 has a built-in Schottky diode. When supply
voltage is applied to the VIN pin, an inrush current flows
through the inductor and the Schottky diode and charges
up the CAP voltage. The Schottky diode inside the LT3591
can sustain a maximum current of 1A.
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 RSENSE selection
are shown below.
RSENSE =
200mV
ILED
3591f
8
LT3591
APPLICATIONS INFORMATION
ILED (mA)
RSENSE (Ω)
5
40
10
20
15
13.3
20
10
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 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:
200mV
, when VCTRL > 1.5V
ILED ≈
RSENSE
ILED ≈
VCTRL
, when VCTRL < 1.25V
6.225 • RSENSE
Feedback voltage variation versus control voltage is given
in the Typical Performance Characteristics.
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).
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 continuously 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 driver for ten white LEDs.
Direct PWM dimming method requires an external NMOS
tied between the cathode of the lowest LED in the string
VIN
3V TO
5V
L1
22µH
C1
1µF
Using a Filtered PWM Signal
VIN
PWM
10kHz TYP
LT3591
C1
0.1µF
C2
2.2µF
CTRL
5V
0V
CTRL
RSENSE
10Ω
LED
GND
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.
R1
100k
CAP
SW LT3591
PWM
FREQ
Q1
Si2308
100k
3591 F06
Table 4. RSENSE Value Selection for 200mV Sense
3591 F05
Figure 5. Dimming Control Using a Filtered PWM Signal
Figure 6. Li-Ion to Ten White LEDs with Direct PWM Dimming
3591f
9
LT3591
APPLICATIONS INFORMATION
PWM
5V/DIV
The calculations show that for a 100Hz signal the dimming
range is 83 to 1. In addition, the minimum PWM duty cycle
of 1.2% ensures that the LED current has enough time
to settle to its final value. Figure 8 shows the dimming
range achievable for different frequencies with a settling
time of 120µs.
10000
PWM DIMMING RANGE
and ground as shown in Figure 6. A Si2308 MOSFET can
be used since its source is connected to ground. The PWM
signal is applied to the CTRL pin of the LT3591 and the gate
of the MOSFET. The PWM signal should traverse between
0V to 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
ILED = 200mV/RSENSE flows through the LEDs. When the
PWM signal 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.
1000
IL
500mA/DIV
PULSING MAY BE VISIBLE
100
10
1
10
100
1000
PWM DIMMING FREQUENCY (Hz)
10000
3591 F08
ILED
20mA/DIV
Figure 8. Dimming Range vs Frequency
2ms/DIV
Figure 7. Direct PWM Dimming Waveforms
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 120µs for a
3.6V input voltage. The achievable dimming range for this
application and 100Hz PWM frequency can be determined
using the following method.
Example:
ƒ = 100Hz, t SETTLE = 120µs
1
1
tPERIOD = =
= 0.01s
ƒ 100
t
0.01s
= 83 : 1
Dim Range = PERIOD =
t SETTLE 120µs
Min Duty Cycle =
t SETTLE
120µs
• 100 =
• 100 = 1.2%
tPERIOD
0.01s
Duty Cycle Range = 100% → 1.2% at 100Hz
10
In addition to extending the dimming range, PWM dimming
improves the efficiency of the converter for LED currents
below 20mA. Figure 9 shows the efficiency for traditional
analog dimming of the front page application and PWM
dimming of the application in Figure 6.
80
PWM DIMMING
75
EFFICIENCY (%)
VIN = 3.6V
10 LEDs
3591 F07
70
65
ANALOG DIMMING
60
VIN = 3.6V
10 LEDs
55
0
5
10
15
20
LED CURRENT (mA)
3591 F09
Figure 9. PWM vs Analog Dimming Efficiency
3591f
LT3591
APPLICATIONS INFORMATION
LOW INPUT VOLTAGE APPLICATIONS
BOARD LAYOUT CONSIDERATIONS
The LT3591 can be used in low input voltage applications.
The input supply voltage to the LT3591 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 LT3591.
The LEDs can be driven straight from the battery, resulting
in higher efficiency.
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 COUT
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.
Figure 10 shows six 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.
CIN
SHUTDOWN AND
DIMMING CONTROL
3.3V
C1
1µF
VIN
CTRL
2 AA CELLS
2V TO 3.2V
CTRL
8
1
CAP
2
RSENSE
10Ω
LT3591
L1
15µH
VIN
LED
L1
C2
4.7µF
SW
3
GND
9
7
LED
6
SW
5 CAP
4
GND
COUT
C1
1µF
RSENSE
3591 F11
3591 F10
C1: TAIYO YUDEN EMK107BJ105MA
C2: MURATA GRM31CR71H475KA12L
L1: TAIYO YUDEN NR4018T150M
Figure 11. Recommended Component Placement
Figure 10. 2 AA Cells to Six White LEDs
3591f
11
LT3591
TYPICAL APPLICATIONS
Li-Ion Driver for Ten White LEDs
Efficiency
D1
L1
22µH
85
*OPTIONAL
VIN = 3.6V
10 LEDs
80
SW
VIN
NO SCHOTTKY
CAP
RSENSE
10Ω
LT3591
SHUTDOWN
AND DIMMING
CONTROL
CTRL
LED
GND
C2
2.2µF
C1
1µF
EFFICIENCY (%)
VIN
3V TO 5V
75
70
EXTERNAL SCHOTTKY
65
60
55
5
3591 TA02a
0
C1:TAIYO YUDEN EMK107BJ105MA
C2: MURATA GRM31CR71H225KA88
3491 TA02b
Efficiency
SHUTDOWN AND
DIMMING CONTROL
80
CTRL
L1
10µH
RSENSE
3.92Ω
LT3591
SW
LED
GND
C2
4.7µF
3591 TA03a
C1
1µF
C1:TAIYO YUDEN EMK107BJ105MA
C2: MURATA GRM31CR71H475KA12L
L1: MURATA LQH32CN100K53
VIN = 3.6V
4 LEDs
75
CAP
EFFICIENCY (%)
VIN
20
L1: TAIYO YUDEN NR4018T220M
D1: PHILLIPS PMEG4005AEA
Li-Ion Driver for Four White LEDs at 50mA
VIN
3V TO 5V
15
10
LED CURRENT (mA)
70
65
60
0
10
20
30
LED CURRENT (mA)
40
50
3591 TA03b
3591f
12
LT3591
TYPICAL APPLICATIONS
24V to Four White LEDs at 100mA
Efficiency
95
C2
4.7µF
CAP
VIN
3V
90
RSENSE
2Ω
C3
1µF
LED
L1
22µH
VIN
C1
1µF
SHUTDOWN
AND
DIMMING
CONTROL
EFFICIENCY (%)
PVIN
24V
85
80
LT3591
75
CTRL
SW
3591 TA05a
GND
70
20
0
C1: TAIYO YUDEN EMK107BJ105MA
C2: MURATA GRM31CR71H475KA12L
C3: MURATA GRM21BR71H105KA12L
L1: TAIYO YUDEN NR4018T220M
95
RSENSE
2Ω
90
LED
L1
22µH
VIN
SHUTDOWN
AND
DIMMING
CONTROL
C1
1µF
LT3591
CTRL
SW
3591 TA06a
EFFICIENCY (%)
CAP
VIN
3V
100
Efficiency
C2
4.7µF
C3
1µF
80
3591 TA05b
24V to Five White LEDs at 100mA
PVIN
24V
40
60
LED CURRENT (mA)
85
80
75
GND
70
C1: TAIYO YUDEN EMK107BJ105MA
C2: MURATA GRM31CR71H475KA12L
C3: MURATA GRM21BR71H105KA12L
L1: TAIYO YUDEN NR4018T220M
0
20
40
60
LED CURRENT (mA)
80
100
3591 TA06b
3591f
13
LT3591
TYPICAL APPLICATIONS
Li-Ion Driver for Seven White LEDs
Conversion Efficiency
SHUTDOWN AND
DIMMING CONTROL
85
VIN = 3.6V
7 LEDs
80
CTRL
VIN
L1
22µH
C1
1µF
75
CAP
RSENSE
10Ω
LT3591
SW
LED
C2
2.2µF
GND
EFFICIENCY (%)
VIN
3V TO 5V
70
65
60
55
50
0
5
10
15
LED CURRENT (mA)
20
3591 TA07b
3591 TA07a
C1: TAIYO YUDEN EMK107BJ105MA
C2: MURATA GRM31CR71H225KA88
L1: TAIYO YUDEN NR4018T220M
Li-Ion Driver for Eight White LEDs
Conversion Efficiency
SHUTDOWN AND
DIMMING CONTROL
85
VIN = 3.6V
8 LEDs
80
VIN
3V TO 5V
VIN
L1
22µH
C1
1µF
CAP
RSENSE
10Ω
LT3591
SW
LED
C2
2.2µF
GND
EFFICIENCY (%)
CTRL
75
70
65
60
55
0
5
10
15
LED CURRENT (mA)
20
3591 TA08b
3591 TA08a
C1: TAIYO YUDEN EMK107BJ105MA
C2: MURATA GRM31CR71H225KA88
L1: TAIYO YUDEN NR4018T220M
3591f
14
LT3591
PACKAGE DESCRIPTION
DDB Package
8-Lead Plastic DFN (3mm × 2mm)
(Reference LTC DWG # 05-08-1702 Rev B)
0.61 ±0.05
(2 SIDES)
0.70 ±0.05
2.55 ±0.05
1.15 ±0.05
PACKAGE
OUTLINE
0.25 ± 0.05
0.50 BSC
2.20 ±0.05
(2 SIDES)
RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS
3.00 ±0.10
(2 SIDES)
R = 0.115
TYP
5
R = 0.05
TYP
0.40 ± 0.10
8
2.00 ±0.10
(2 SIDES)
PIN 1 BAR
TOP MARK
(SEE NOTE 6)
0.56 ± 0.05
(2 SIDES)
0.200 REF
0.75 ±0.05
0 – 0.05
4
0.25 ± 0.05
1
PIN 1
R = 0.20 OR
0.25 × 45°
CHAMFER
(DDB8) DFN 0905 REV B
0.50 BSC
2.15 ±0.05
(2 SIDES)
BOTTOM VIEW—EXPOSED PAD
NOTE:
1. DRAWING CONFORMS TO VERSION (WECD-1) IN JEDEC PACKAGE OUTLINE M0-229
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
3591f
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.
15
LT3591
TYPICAL APPLICATION
Li-Ion Driver for Nine White LEDs
Conversion Efficiency
85
SHUTDOWN AND
DIMMING CONTROL
VIN = 3.6V
9 LEDs
80
VIN
3V TO 5V
VIN
L1
22µH
CAP
SW
C1
1µF
RSENSE
10Ω
LT3591
C2
2.2µF
LED
GND
EFFICIENCY (%)
CTRL
75
70
65
60
55
0
5
20
15
10
LED CURRENT (mA)
3591 TA09b
3591 TA09a
C1: TAIYO YUDEN EMK107BJ105MA
C2: MURATA GRM31CR71H225KA88
L1: TAIYO YUDEN NR4018T220M
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ThinSOT is a trademark of Linear Technology Corporation
3591f
16 Linear Technology Corporation
LT 0207 • PRINTED IN USA
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900 ● FAX: (408) 434-0507
●
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© LINEAR TECHNOLOGY CORPORATION 2007