LINER LT3598IUF-TRPBF 6-string 30ma led driver with 1.5% current matching Datasheet

LT3598
6-String 30mA LED Driver
with 1.5% Current Matching
DESCRIPTION
FEATURES
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True Color PWM™ Dimming Delivers Up to 3000:1
Dimming Ratio
Drives Six Strings of LEDs at Up to 30mA
1.5% Accurate LED Current Matching
Wide Input Voltage Range: 3.2V to 30V
Output Voltage Up to 44V
Regulates Current Even When VIN > VOUT
Disconnects LEDs in Shutdown
Programmable Open LED Protection (Regulated)
OPENLED Alert Pin
Programmable LED Current Derating
Adjustable Frequency: 200kHz to 2.5MHz
Synchronizable to an External Clock
Parallel Channels for Higher Current per LED String
Thermally Enhanced 4mm × 4mm QFN Package
APPLICATIONS
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Notebook Computer Display
Medium Size Displays
Automotive LCD Display
The LT®3598 is a fixed frequency step-up DC/DC converter
designed to drive up to six strings of LEDs at an output
voltage up to 44V. LED dimming can be achieved with
analog dimming on the CTRL pin, and with pulse width
modulation dimming on the PWM pin. The LT3598 accurately regulates LED current even when the input voltage
is higher than the LED output voltage. The switching frequency is programmable from 200kHz to 2.5MHz through
an external resistor.
Additional features include programmable overvoltage
protection, switching frequency synchronization to an
external clock, LED current derating based on junction
temperature and/or LED temperature, LED string disable
control, OPENLED alert pin and output voltage limiting
when all LED strings are disconnected. The LT3598 is
available in a thermally enhanced 24-pin (4mm × 4mm)
QFN 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. Protected by U.S. Patents, including 7199560, 7321203.
TYPICAL APPLICATION
LED Current Matching
90% Efficient LED Driver for 60 White LEDs
PVIN
8V TO 40V
10μH
1.5
2.2μF
SW
VOUT
1.00M
100k
OPENLED
SHDN
SHDN
PWM
PWM
SYNC
SYNC
ALL SIX LED STRINGS
1.0
VO_SW
VIN
MATCHING (%)
VIN
5V
2.2μF
4.7μF
FB
30.9k
LT3598
0.5
0
–0.5
RT
51.1k
–1.0
VREF
10k
CTRL
100k
TSET ISET SS
60.4k
47pF
100k
14.7k
–1.5
–50
LED1
LED2
LED3
LED4
LED5
LED6
GND VC
10nF
–25
0
25
50
75
100
125
TEMPERATURE (°C)
20mA
3598 TA01b
2.61k
15nF
3598 TA01a
3598f
1
LT3598
ABSOLUTE MAXIMUM RATINGS
PIN CONFIGURATION
(Note 1)
GND
SHDN
VIN
VOUT
VO_SW
SW
TOP VIEW
SHDN ................................................................VIN + 3V
VIN, OPENLED ...........................................................30V
SW Voltage ...............................................................45V
VOUT, VO_SW Voltage .................................................44V
LED1 to LED6............................................................44V
PWM, SYNC, CTRL, RT, SS, VC ...................................6V
VREF, FB Voltage..........................................................6V
ISET, TSET.....................................................................6V
Operating Junction Temperature Range
(Note 2).................................................. –40°C to 125°C
Maximum Junction Temperature........................... 125°C
Storage Temperature Range................... –65°C to 150°C
24 23 22 21 20 19
LED1 1
18 VREF
LED2 2
17 SS
LED3 3
16 RT
25
LED4 4
15 PWM
13 NC
TSET
9 10 11 12
FB
8
VC
OPENLED
7
ISET
14 SYNC
LED6 6
CTRL
LED5 5
UF PACKAGE
24-LEAD (4mm × 4mm) PLASTIC QFN
TJMAX = 125°C, θJA = 37°C/W
EXPOSED PAD (PIN 25) IS GND, MUST BE SOLDERED TO PCB
ORDER INFORMATION
LEAD FREE FINISH
TAPE AND REEL
PART MARKING*
PACKAGE DESCRIPTION
TEMPERATURE RANGE
LT3598EUF#PBF
LT3598EUF#TRPBF
3598
24-Lead (4mm × 4mm) Plastic QFN
–40°C to 125°C
LT3598IUF#PBF
LT3598IUF#TRPBF
3598
24-Lead (4mm × 4mm) Plastic QFN
–40°C to 125°C
Consult LTC Marketing for parts specified with wider operating temperature ranges. *The temperature grade is identified by a label on the shipping container.
Consult LTC Marketing for information on non-standard lead based finish parts.
For more information on lead free part marking, go to: http://www.linear.com/leadfree/
For more information on tape and reel specifications, go to: http://www.linear.com/tapeandreel/
ELECTRICAL CHARACTERISTICS
The l denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at TA = 25°C. VIN = 5V, VSHDN = VIN unless otherwise noted. (Note 2)
PARAMETER
CONDITIONS
MIN
Minimum Operating Voltage
TYP
MAX
3
3.2
V
30
V
1.230
1.260
1.260
V
V
0.01
0.03
%/V
Maximum Operating Voltage
Reference Voltage
l
1.216
1.210
UNITS
Reference Voltage Line Regulation
3 < VIN < 30V, VC = 0.3V
Maximum VREF Pin Current
Out of Pin
FB Pin Bias Current
VFB = 1.230V (Note 3)
100
FB Error Amp Transconductance
ΔI = 5μA
300
μmhos
600
V/V
200
FB Error Amp Voltage Gain
FB Pin Voltage
Current Loop Amp Transconductance
1.22
μA
1.24
21
250
1.26
nA
V
μmhos
3598f
2
LT3598
ELECTRICAL CHARACTERISTICS
The l denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at TA = 25°C. VIN = 5V, VSHDN = VIN unless otherwise noted. (Note 2)
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
Current Loop Amp Voltage Gain
80
V/V
VC Sink Current
10
μA
Quiescent Current
VSHDN = 5V, PWM = 0V, Not Switching
Quiescent Current in Shutdown
VSHDN = 0V
ISET Voltage
VCTRL = 1.5V, VTSET = 1.5V, RISET = 14.7kΩ
LED Current
RISET = 14.7kΩ
LED String Current Matching
20mA LED Current
0.985
19.5
l
LED Open Detection Threshold
OPENLED Sink Current
3.5
5
mA
0
1
μA
1.000
1.015
V
20
20.7
mA
±0.5
±1.5
%
0.2
0.25
V
2
Minimum LED Regulation Voltage
mA
0.8
V
LED1-6 Leakage Current
VLED1-6 = 1V, VOUT = 5V, PWM = 0V
VLED1-6 = 42V, VOUT = 44V, PWM = 0V
0.1
0.2
1
2
μA
μA
CTRL Pin Bias Current
VCTRL = 0.8V (Note 4)
50
125
nA
Switching Frequency
RT = 309kΩ
RT = 51.1kΩ
RT = 14.7kΩ
190
1
2.5
209
1.1
2.75
kHz
MHz
MHz
171
0.9
2.25
TSET Voltage
Maximum Switch Duty Cycle
RT = 309kΩ
RT = 51.1kΩ
RT = 14.7kΩ
l
90
87
80
mV
95
90
86
%
%
%
Switch Current Limit
(Note 5)
Switch VCESAT
ISW = 0.5A
0.12
Switch Leakage Current
VSW = 40V
0.2
5
μA
SHDN Pin Current
VSHDN = 0V, VIN = 0V
VSHDN = 5V, VIN = 5V
0.1
30
1
60
μA
μA
SHDN Voltage High
1.5
602
2
VSS = 0.1V
PWM Input High Voltage
5
V
10
PWM = 3.3V
SYNC Input High Voltage
V
15
μA
V
0.1
0.4
V
1
μA
1.5
V
SYNC Input Low Voltage
SYNC Pin Bias Current
0.4
1
PWM Input Low Voltage
PWM Pin Bias Current
SYNC = 0V
SYNC = 3.3V
VO_SW Switch Resistance
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 LT3598E is guaranteed to meet performance specifications
from 0°C to 125°C junction temperature. Specifications over the –40°C
to 125°C operating junction temperature range are assured by design,
A
V
1.6
SHDN Voltage Low
Soft-Start Charging Current
2.5
25
0.1
0.4
V
50
1
μA
μA
1000
Ω
characterization and correlation with statistical process controls. The
LT3598I is guaranteed over the full –40°C to 125°C operating junction
temperature range.
Note 3: Current flows out of FB pin.
Note 4: Current flows out of CTRL pin.
Note 5: Current limit guaranteed by design and/or correlation to static test.
Current limit is independent of duty cycle and is guaranteed by design.
3598f
3
LT3598
TYPICAL PERFORMANCE CHARACTERISTICS
SHDN Pin Turn-On Threshold
SHDN Pin Current
1.5
Quiescent Current
50
6
125°C
1.4
1.3
–50°C
40
25°C
QUIESCENT CURRENT (mA)
SHDN PIN CURRENT (μA)
SHDN THRESHOLD (V)
45
35
30
25
20
15
10
5
4
3
5
1.2
–50
–25
0
25
50
75
100
0
125
0
5
10
TEMPERATURE (°C)
20
15
VSHDN (V)
25
2
–50
35
Reference Voltage
Oscillator Frequency
VIN = 5V
1.225
1.220
50
75
100
125
Current Limit
2.8
2.5MHz
2.5
2.4
2.0
CURRENT (A)
VIN = 30V
OSCILLATOR FREQUENCY (MHz)
VIN = 40V
25
3598 G03
3.0
1.230
0
3598 G02
1.240
1.235
–25
TEMPERATURE (°C)
3598 G01
REFERENCE VOLTAGE (V)
30
1.5
1MHz
1.0
2.0
1.6
1.2
0.5
0.8
200kHz
1.215
–50
–25
0
25
50
75
100
0
–50
125
–25
0
25
50
75
100
0.4
–50
125
–25
TEMPERATURE (°C)
TEMPERATURE (°C)
0
25
50
75
3598 G05
3598 G04
125
3598 G06
Soft-Start Pin Current
VCESAT
0.40
100
TEMPERATURE (°C)
Feedback Pin Voltage
15.0
1.24
12.5
1.23
0.35
0.25
ISS (μA)
VCESAT (V)
FEEDBACK VOLTAGE (V)
25°C
0.30
125°C
0.20
–50°C
0.15
0.10
10.0
7.5
VC = 1.5V
1.22
VC = 1V
1.21
0.05
0
0
0.25
0.50
0.75
1.00
1.25
1.5O
SWITCH CURRENT (A)
5.0
–50
–25
0
25
50
75
100
125
–25
0
25
50
75
100
125
TEMPERATURE (°C)
TEMPERATURE (°C)
3598 G07
1.20
–50
3598 G08
3598 G09
3598f
4
LT3598
TYPICAL PERFORMANCE CHARACTERISTICS
LED Current vs PWM Duty Cycle
LED Current vs Temperature
100
1
0.1
0.01
20
LED CURRENT (mA)
LED CURRENT (mA)
LED CURRENT (mA)
25
20.2
10
20.1
20.0
19.9
0.1
1
10
PWM DUTY CYCLE (%)
19.7
–50
100
15
10
5
19.8
0.001
0.01
–25
0
25
50
75
100
125
0
TEMPERATURE (°C)
3.0
1.0
2.5
OPENLED CURRENT (mA)
1.5
0.0
–0.5
–1.5
–50
–25
0
25
50
75
100
125
1.0
1.2
PWM
5V/DIV
SW
20V/DIV
2.0
1.5
IL
1A/DIV
ILED1
50mA/DIV
1.0
20μs/DIV
0
–50
–25
0
25
50
75
100
3598 G15
125
TEMPERATURE (°C)
TEMPERATURE (°C)
3598 G14
3598 G13
LED Current Waveforms
(0.1% PWM)
LED Current Waveforms
(90% PWM)
LED Current Waveforms
(90% PWM)
PWM
5V/DIV
PWM
5V/DIV
PWM
5V/DIV
SW
20V/DIV
SW
20V/DIV
SW
20V/DIV
IL
1A/DIV
ILED1
50mA/DIV
IL
1A/DIV
ILED1
50mA/DIV
IL
1A/DIV
ILED1
50mA/DIV
2μs/DIV
0.4
0.8
0.6
CTRL VOLTAGE (V)
LED Current Waveforms
(0.1% PWM)
0.5
–1.0
0.2
3598 G12
OPENLED Sink Current
LED Current vs Temperature
0.5
0
3598 G11
3598 G10
MATCHING (%)
LED Current vs CTRL Voltage
20.3
3598 G16
100μs/DIV
3598 G17
5μs/DIV
3598 G18
3598f
5
LT3598
PIN FUNCTIONS
LED1-6 (Pins 1, 2, 3, 4, 5, 6): LED String Output. Connect
the bottom cathode of each LED string to these pins. Tie
pins to VOUT if the string is not used.
driving the PWM pin provides accurate dimming control.
The PWM signal can be driven from 0V to 5V. If unused,
the pin should be connected to VREF.
OPENLED (Pin 7): Open LED Flag When Any LED String
Opens. The output is open-collector. Tie a resistor to other
supply for open LED flag function.
RT (Pin 16): A resistor to ground programs switching
frequency between 200kHz and 2.5MHz. For SYNC function, choose the resistor to program a frequency 20%
slower than the SYNC pulse frequency. Do not leave this
pin open.
ISET (Pin 8): Programs LED Current for Each String. Connect a 14.7k resistor between ISET and GND to program
each LED string current to 20mA. A 47pF capacitor on the
ISET pin reduces current ripple in each LED string.
CTRL (Pin 9): LED Current Control. If the CTRL pin is not
used, tie this pin to VREF through a 10k to 20k resistor.
VC (Pin 10): Error Amplifier Output Pin. Tie the external
compensation network to this pin.
FB (Pin 11): Feedback Pin for Overvoltage Protection.
Reference voltage is 1.230V. Connect the resistive divider
tap here. Minimize trace area at FB. Set VOUT according
to VOUT = 1.230(1 + R2/R1) when overvoltage protection
occurs (see Figure 2).
TSET (Pin 12): An external resistor divider from VREF programs a decrease in LED current versus internal junction
temperature (setting temperature breakpoint and slope).
If the TSET pin is not used, tie this pin to VREF.
NC (Pin 13): No Connection.
SYNC (Pin 14): Frequency Synchronization Pin. This
input allows for synchronizing the operating frequency
to an external clock. The RT resistor should be chosen to
program a switching frequency 20% slower than SYNC
pulse frequency. This pin should be grounded if this
feature is not used.
PWM (Pin 15): Input Pin for PWM Dimming Control. Above
1V allows converter switching and below 1V disables
switching with VC pin level maintained. A PWM signal
SS (Pin 17): Soft-Start Pin. Place a soft-start capacitor
here. Upon start-up, a 10μA current charges the capacitor.
Use a larger capacitor for slower start-up. Leave open if
not used.
VREF (Pin 18): Bandgap Voltage Reference. Internally set
to 1.230V. This pin can supply up to 100μA. Can be used
to program the CTRL pin voltage using resistor dividers
to ground.
GND (Pin 19): Ground. Tie directly to local ground
plane.
SHDN (Pin 20): Shutdown Pin. Tie to 1.6V or more to enable the device. Tie below 0.4V or less to disable device.
Do not float this pin.
VIN (Pin 21): Input Supply Pin. Must be locally bypassed
with a capacitor to ground.
SW (Pin 22): Switch Pin. This is the collector of the internal NPN power switch. Minimize the metal trace area
connected to this pin to minimize EMI.
VOUT (Pin 23): Output Pin. This pin provides power to
all LEDs.
VO_SW (Pin 24): Drain of an Internal PMOS. The internal
PMOS disconnects the feedback resistors from the VOUT
pin during shutdown and the PWM transitioned to low.
Exposed Pad (Pin 25): Ground. The Exposed Pad must
be soldered to the PCB.
3598f
6
LT3598
BLOCK DIAGRAM
20
SHDN
21
VIN
16
RT
14
SYNC
22
SW
OSCILLATOR
1.230VREF
SLOPE
3
VC
+
Q1
Q
–
10
S
A2
R
+
SS
17
A3
GND
–
GND
VOUT
PWM
15
PWM DIMMING
LOGIC
VO_SW
+
19
25
23
24
OVP gm
VREF
FB
–
18
+
11
0.8V
LED gm
–
CTRL
9
OPENLED
OPENLED DETECTION
TSET
12
VPTAT
–
+
VOUT
A1
LED1
LED2
LED
DRIVE
CIRCUITRY
1
2
LED3
LED
3
DISABLE
LED4
DETECTION
4
LED5
ISET
8
7
LED6
5
6
3598 F01
Figure 1. Block Diagram
3598f
7
LT3598
OPERATION
The LT3598 uses a constant-frequency, peak current mode
control scheme to provide excellent line and load regulation.
Each string can drive up to 30mA with 1.5% matching accuracy between strings. Operation can be best understood
by referring to the Block Diagram in Figure 1.
LT3598 has a built-in boost converter which converts the
input voltage to a higher output voltage to drive LEDs.
The LED strings are connected to current sources where
the current level is set with an external resistor on the
ISET pin. The LED1 to LED6 voltages are monitored for
output voltage regulation. During normal operation, when
all LEDs are used, the lowest LED pin voltage (LED1 to
LED6) is used to regulate the output voltage to ensure all
LED strings have enough voltage to run the programmed
current.
For any unused LED strings, tie their LED pins to VOUT.
An unused LED string is no longer in the regulation loop,
nor does it affect open LED detection. Never allow unused
LED strings to be left open.
The basic loop uses a pulse from an internal oscillator
to set the SR latch and turn on the internal power NPN
switch Q1. The signal at the noninverting input of the PWM
comparator (A2 slope) is proportional to the sum of the
switch current and oscillator ramp. When slope exceeds
VC (the output of the gm amplifier), the PWM comparator
resets the latch. The switch is then turned off, causing the
inductor current to lift the SW pin and turn on an external
Schottky diode connected to the output. Inductor current
flows via the Schottky diode charging the output capacitor. The switch is turned on again at the next reset cycle
of the internal oscillator. During normal operation, the VC
voltage controls the peak switch current limit and, hence,
the inductor current available to the output LEDs.
Dimming of the LEDs is accomplished by pulsing the LED
current using the PWM pin. When the PWM pin is low,
switching is disabled and the error amplifier is turned off
so that it does not drive the VC pin. Also, all internal loads
on the VC pin are disabled so that the state of the VC pin
is maintained on the external compensation capacitor.
This feature reduces transient recovery time. When the
PWM input again transitions high, the peak switch current
returns to the correct value.
The LT3598 uses the FB pin to provide overvoltage protection when all LED strings are open. There is an internal
PMOS switch between VOUT and VO_SW that is controlled
by the PWM signal. During the PWM off-period, this
PMOS is turned off, allowing for higher dimming range
and lower current during shutdown. A resistor divider is
connected between the VO_SW pin and ground, which sets
the overvoltage protection voltage.
If the LED1-6 pin voltage is below 0.2V (for a certain delay
after 80% of the programmed output voltage is reached),
the string is treated as an open LED string. As a result,
OPENLED lag is set. If a LED string is open in the middle
of the operation, the regulation will continue.
OPENLED detection is disabled during the start-up phase
to avoid erratic flag generation. An LED string that is disabled by connecting its LED pin to VOUT is not an open
LED condition. During normal operation, if an LED string is
open and has the lowest LED pin voltage, the output voltage will regulate itself to find another LED string that has
the lowest LED pin voltage at about 0.8V. If the open LED
string has an LED voltage above 0.8V, the output voltage
will remain the same. When the LED string is open, it is
no longer in the regulation loop. The OPENLED detection
is active only when the PWM signal is enabled. To avoid
spurious OPENLED detection and high PWM dimming
ratio, more output capacitance is recommended to allow
less voltage drop on VOUT.
During start-up, 10μA of current charges the external
soft-start capacitor. The SS pin directly limits the rate
of voltage rise on the VC pin, which in turn, limits the
peak switch current. Soft-start also enables the switching frequency foldback to provide a clean start-up for
the LT3598. Current limit protects the power switch and
external components.
3598f
8
LT3598
APPLICATIONS INFORMATION
Inductor Selection
Table 1 lists several inductors that work well with the
LT3598, however, there are many other manufacturers and
devices that can be used. Consult each manufacturer for
detailed information on their entire range of parts. Ferrite
core inductors should be used to obtain the best efficiency.
Choose an inductor that can handle the necessary peak
current without saturating. Also, ensure that the inductor
has a low DCR (copper wire resistance) to minimize I2R
power losses. Values between 4.7μH and 22μH will suffice
for most applications.
Inductor manufacturers specify the maximum current
rating as the current where inductance falls by a given
percentage of its nominal value. An inductor can pass a
current greater than its rated value without damaging it.
Consult each manufacturer to determine how the maximum
inductor current is measured and how much more current
the inductor can reliably conduct.
Table 1. Recommended Inductors
L
(μH)
MAX
DCR
(Ω)
CURRENT
RATING
(A)
B1015AS-100M
#817FY-4R7M
1123AS-4R7M
10
4.7
4.7
0.07
0.06
0.12
2.2
2.26
1.90
TOKO
www.toko.com
74454068
74454010
7447745100
6.8
10
10
0.055
0.065
0.12
2.2
2
1.7
Würth Electronics
www.we-online.com
CDH74NP-120L
CDH74NP-150L
CDRH6D38-100
12
15
10
0.065
0.083
0.038
2.45
2.10
2.00
Sumida
www.sumida.com
IHLP-2525BD-01
10
0.129
2.5
Vishay
www.vishay.com
SD25-4R7-R
4.7
0.056
1.83
Cooper
www.cooperet.com
LPS4018-472ML
4.7
0.200
1.8
Coilcraft
www.coilcraft.com
PART
VENDOR
Capacitor Selection
Low ESR (equivalent series resistance) ceramic capacitors should be used at the output to minimize the output
ripple voltage. Use only X5R or X7R dielectrics, as these
materials retain their capacitance over wider voltage and
temperature ranges than other dielectrics. A 4.7μF to 10μF
output capacitor is sufficient for most high output current
designs. Table 2 lists some suggested manufacturers.
Consult the manufacturers for detailed information on
their entire selection of ceramic parts.
Table 2. Recommended Ceramic Capacitor Manufacturers
Taiyo Yuden
408-573-4150
www.t-yuden.com
AVX
843-448-9411
www.avxcorp.com
Murata
770-436-1300
www.murata.com
Kemet
408-986-0424
www.kemet.com
United Chemi-Con
847-696-2000
www.chemi-con.com
Diode Selection
Schottky diodes, with their low forward voltage drop and
fast switching speed, must be used for all LT3598 applications. Do not use P-N diodes. Table 3 lists several Schottky
diodes that work well. The diode’s average current rating
must exceed the application’s average output current.
The diode’s maximum reverse voltage must exceed the
application’s output voltage. A 2A diode is sufficient for
most designs. For PWM dimming applications, be aware
of the reverse leakage current of the diode. Lower leakage
current will drain the output capacitor less, allowing for
higher dimming range. The companies below offer Schottky
diodes with high voltage and current ratings.
Table 3. Suggested Diodes
PART
MAX
MAX REVERSE
CURRENT
VOLTAGE
(A)
(V)
MANUFACTURER
B250A
B240A
SBR140S3
SBM340, PDS340
2
2
1
3
50
40
40
40
Diodes, Inc.
www.diodes.com
HSM150G
HSM150J
1
1
50
50
Microsemi
www.microsemi.com
SS3H9
3
90
Vishay
www.vishay.com
3598f
9
LT3598
APPLICATIONS INFORMATION
Overvoltage Protection
The output voltage should be set higher than the normal
LED string operating voltage. Under normal operation,
LED1 to LED6 pin voltages are monitored and provide
feedback information to the converter for output voltage regulation given the programmed LED current. The
maximum output regulation loop is activated only when
all LEDs are open.
The LT3598 uses the FB pin to provide regulated overvoltage
protection when all LED strings are open. A resistor divider
is connected between the VO_SW pin and ground (Figure
2). There is an internal PMOS switch between VOUT and
VO_SW, which is controlled by the PWM signal. The PMOS
switch addition prevents the feedback resistor divider from
draining the output capacitor during PWM off-period, allowing for a higher dimming range without falsely tripping
the OPENLED flag. It also reduces the system current in
shutdown. This PMOS has about 1k resistance, so select
FB resistors taking this resistance into account.
Programming Maximum LED Current
Maximum LED current is programmed by placing a resistor between the ISET pin and ground (RISET). The ISET pin
resistor can be selected from 10k to 100k.
The LED current is programmed according to the following equation:
To set the maximum output voltage, select the values
of R1 and R2 (see Figure 2) according to the following
equation:
ILED ≈
⎛ R2 ⎞
VOUT(MAX ) = 1 . 230 V ⎜ 1 + ⎟
⎝
R1⎠
LT3598
294V
RISET
See Table 4 and Figure 3 for resistor values and corresponding programmed LED current.
LED current can also be adjusted by programming the
CTRL pin voltage.
VOUT
VO_SW
Table 4. RISET Value Selection for LED Current
R2
FB
LED CURRENT (mA)
RESISTOR ON ISET PIN (k)
3mA
97.6
10mA
29.4
20mA
14.7
30mA
9.76
R1
3598 F02
Figure 2. Overvoltage Protection
Voltage Feedback Connections
30
LED CURRENT (mA)
25
20
15
10
5
0
0
20
40
60
80
100
RISET (k)
3598 F03
Figure 3. RISET Value Selection for LED Current
10
3598f
LT3598
APPLICATIONS INFORMATION
LED Current Dimming
Two different types of dimming control can be used with
the LT3598. The LED current can be set by modulating
the CTRL pin or the PWM pin.
For some applications, a variable DC voltage that adjusts
the LED current is the preferred method of brightness
control. The CTRL pin voltage can be modulated to set
the dimming of the LED string (see Figures 4 and 5). As
the voltage on the CTRL pin increases from 0V to 1.0V,
the LED current increases from 0 to the programmed LED
current level. As the CTRL pin voltage increases beyond
1V, it has no effect on the LED current.
35
For wide PWM dimming range, higher switching frequency
and lower PWM frequency configuration are needed. Special considerations are required for component selection
and compensation network. Please contact factory for
optimized components selection if very high dimming
ratio is desired.
RISET = 9.76k
True Color PWM is a registered trademark of Linear Technology Corporation.
30
LED CURRENT (mA)
For True Color PWMTM dimming, the LT3598 provides up
to a 3000:1 PWM dimming range. This is achieved by
allowing the duty cycle of the PWM pin (connected to
the IC and an internal switch in series with the LED(s)),
to be reduced from 100% to as low as 0.1% for a PWM
frequency of 100Hz (Figure 6). PWM duty cycle dimming
allows for constant LED color to be maintained over the
entire dimming range.
25
20
VREF
15
R2
10
LT3598
CTRL
5
R1
0
3598 F05
0
0.2
0.4
0.6
0.8
1
1.2
CTRL (V)
1.4
1.6
3598 F04
Figure 4. LED Current vs CTRL Voltage
Figure 5. LED Current vs CTRL
TPWM
TONPWM
(= 1/fPWM)
PWM
INDUCTOR
CURRENT
LED
CURRENT
MAX ILED
3598 F06
Figure 6. LED Current Using PWM Dimming
3598f
11
LT3598
APPLICATIONS INFORMATION
LED Current Derating Using the CTRL Pin
A useful feature of the LT3598 is its ability to program a
derating curve for maximum LED current versus temperature. LED data sheets provide curves of maximum-allowable
LED current versus temperature to warn against exceeding this current limit and damaging the LED. The LT3598
allows the output LEDs to be programmed for maximum
allowable current while still protecting the LEDs from
excessive currents at high temperature. This is achieved
by programming a voltage at the CTRL pin with a negative temperature coefficient using a resistor divider with
temperature dependent resistance (Figure 7). As ambient
temperature increases, the CTRL voltage will fall below the
internal 1V voltage reference, causing LED currents to be
controlled by the CTRL pin voltage. The LED current curve
breakpoint and slope versus temperature is defined by the
choice of resistor ratios and use of temperature-dependent
resistance in the divider for the CTRL pin.
Table 5 shows a list of manufacturers/distributors of NTC
resistors. There are several other manufacturers available and the chosen supplier should be contacted for
more detailed information. If an NTC resistor is used to
indicate LED temperature, it is effective only if the resistor
is connected as closely as possible to the LED strings.
LED derating curves shown by manufacturers are listed
for ambient temperature. The NTC resistor should have
the same ambient temperature as the LEDs. Since the
temperature dependency of an NTC resistor can be nonlinear over a wide range of temperatures, it is important
to obtain a resistor’s exact value over temperature from
the manufacturer. Hand calculations of CTRL voltage can
then be performed at each given temperature, resulting
in the CTRL versus temperature plotted curve. Several
iterations of resistor value calculations may be required
to achieve the desired breakpoint and slope of the LED
current derating curve.
Table 5. NTC Resistor Manufacturers/Distributors
Murata Electronics North America
770-436-1300
www.murata.com
TDK Corporation
516-535-2600
www.tdk.com
Digi-Key
800-344-4539
www.digikey.com
If calculating the CTRL voltage at various temperatures
gives a downward slope that is too strong, alternative
resistor networks can be chosen (B, C, D in Figure 7)
which use temperature independent resistance to reduce
the effects of the NTC resistor overtemperature.
Murata Electronics provides a selection of NTC resistors
with complete data over a wide range of temperatures.
In addition, a software tool is available which allows the
user to select from different resistor networks and NTC
resistor values, and then simulate the exact output voltage curve (CTRL behavior) overtemperature. Referred
to as the “Murata Chip NTC Thermistor Output Voltage
Simulator,” users can log onto www.murata.com and
download the software followed by instructions for creating an output voltage VOUT (CTRL) from a specified VCC
supply (VREF).
RY
VREF
R2
RY
LT3598
CTRL
RNTC
RNTC
RX RNTC
RNTC
RX
R1
(OPTION A TO D)
A
B
C
D
3598 F07
Figure 7 . LED Current Derating vs Temperature Using NTC Resistor
3598f
12
LT3598
APPLICATIONS INFORMATION
Using the TSET Pin for Thermal Protection
The LT3598 contains a special programmable thermal
regulation loop that limits the internal junction temperature
of the part. Since the LT3598 topology consists of a single
boost converter with six linear current sources, any LED
string voltage mismatch will cause additional power to be
dissipated in the package. This topology provides excellent
current matching between LED strings and allows a single
power stage to drive a large number of LEDs, but at the
price of additional power dissipation inside the part (which
means a higher junction temperature). Being able to limit
the maximum junction temperature allows the benefits of
this topology to be fully realized. This thermal regulation
feature provides important protection at high ambient temperatures, and allows a given application to be optimized
for typical, not worst-case, ambient temperatures with the
assurance that the LT3598 will automatically protect itself
and the LED strings under worst-case conditions.
The operation of the thermal loop is simple. As the ambient temperature increases, so does the internal junction
temperature of the part. Once the programmed maximum
junction temperature is reached, the LT3598 begins to
linearly reduce the LED current, as needed, to try and
maintain this temperature. This can only be achieved
when the ambient temperature stays below the desired
maximum junction temperature. If the ambient temperature continues to rise past the programmed maximum
junction temperature, the LEDs current will be reduced
to approximately 5% of the full LED current.
While this feature is intended to directly protect the LT3598,
it can also be used to derate the LED current at high temperatures. Since there is a direct relationship between the
LED temperature and LT3598 junction temperature, the
TSET function also provides some LED current derating
at high temperatures.
Two external resistors program the maximum IC junction
temperature using a resistor divider from the VREF pin, as
shown in Figure 8. Choose the ratio of R1 and R2 for the
desired junction temperature. Figure 9 shows the relationship of TSET voltage to junction temperature, and Table 6
shows commonly used values for R1 and R2.
Table 6. TSET Junction Temperature
TJ (°C)
R1
R2
90
100k
67.7k
100
100k
63.3k
110
100k
59k
120
100k
54.9k
900
VREF
850
LT3598
TSET
R1
3598 F08
Figure 8. Programming the TSET Pin
VTSET THRESHOLD (mV)
R2
800
750
700
650
600
550
500
0
25
50
75
100
125
JUNCTION TEMPERATURE (°C)
150
3598 F09
Figure 9. TSET Pin Threshold
3598f
13
LT3598
APPLICATIONS INFORMATION
Programming Switching Frequency
Switching Frequency Synchronization
The switching frequency of the LT3598 should be programmed between 200kHz and 2.5MHz by an external
resistor connected between the RT pin and ground. Do not
leave this pin open. See Table 7 and Figure 10 for resistor
values and corresponding frequencies.
The nominal operating frequency of the LT3598 is programmed using a resistor from the RT pin to ground
and can be controlled over a 200kHz to 2.5MHz range. In
addition, the internal oscillator can be synchronized to an
external clock applied to the SYNC pin. The synchronizing
clock signal input to the LT3598 must have a frequency
between 240kHz and 3MHz, a duty cycle between 20%
and 80%, a low state below 0.4V and a high state above
1.5V. Synchronization signals outside of these parameters
will cause erratic switching behavior. For proper operation,
an RT resistor should be chosen to program a switching
frequency 20% slower than the SYNC pulse frequency.
Synchronization occurs at a fixed delay after the rising
edge of SYNC.
Selecting the optimum switching frequency depends
on several factors. Inductor size is reduced with higher
frequency, but efficiency drops slightly due to higher
switching losses. In addition, some applications require
very high duty cycles to drive a large number of LEDs from
a low supply. Low switching frequency allows a greater
operational duty cycle and, hence, a greater number of
LEDs to be driven. In each case, the switching frequency
can be tailored to provide the optimum solution. When
programming the switching frequency, the total power
losses within the IC should be considered.
Table 7. Switching Frequency
SWITCHING FREQUENCY (MHz)
RT (k)
2.5
14.7
2
20.5
1.5
29.4
1
51.1
0.5
105
0.2
301
The SYNC pin should be grounded if the clock synchronization feature is not used. When the SYNC pin is grounded,
the internal oscillator generates switching frequency to
the converter.
Soft-Start and Switching Frequency Foldback
For many applications, it is necessary to minimize the
inrush current at start-up. The LT3598’s soft-start circuit
significantly reduces the start-up current spike and output
voltage overshoot. Before the SS pin voltage reaches 1V,
the switching frequency will also fold back proportional
to the SS pin voltage. A typical value for the soft-start
capacitor is 10nF.
SWITCHING FREQUENCY (MHz)
2.5
2.0
1.5
1.0
0.5
0
10
100
RT (k)
1000
3598 F10
Figure 10. Switching Frequency
3598f
14
LT3598
APPLICATIONS INFORMATION
OPENLED FLAG
The OPENLED pin is an open-collector output and needs
an external resistor tied to a supply (see Figure 11). If any
LED string is open during normal operation, the OPENLED
pin will be pulled down.
The OPENLED flag is only activated after the first PWM
edge. The open LED detection is enabled only when the
PWM signal is enabled. There is a delay for OPENLED
flag generation when the PWM signal is enabled to avoid
generating a spurious flag signal. The maximum current
the OPENLED can sink is typically 2mA.
During start-up (see the Operation section), the open LED
detection is disabled. If an LED string is not used and tied
to VOUT, the string will not be in any fault detection.
LT3598
R1
OPENLED
3598 F11
Figure 11. OPENLED Connection
Loop Compensation
The LT3598 has an internal transconductance error amplifier for LED current regulation whose VC output compensates the control loop. During an open LED event where
all LED strings are open, the VC node also compensates
the control loop. The external inductor, output capacitor,
and the compensation resistor and capacitor determine
the loop stability. The inductor and output capacitor are
chosen based on performance, size and cost. The compensation resistor and capacitor at VC are selected to optimize
control loop stability. For typical LED applications, a 15nF
compensation capacitor in series with a 3k resistor at VC
is adequate.
Thermal Considerations
The LT3598 provides six channels for LED strings with
internal NPN devices serving as constant-current sources.
When LED strings are regulated, the lowest LED pin voltage
is typically 0.8V. The higher the programmed LED current,
the more power dissipation in the LT3598. For 30mA LED
programming current with a 100% PWM dimming ratio,
at least 144mW is dissipated within the IC due to current
sources. If the forward voltages of the six LED strings are
very dissimilar, there can be significant power dissipation.
Thermal calculations shall include the power dissipation
on current sources in addition to conventional switch DC
loss, switch AC loss and input quiescent loss. For best
efficiency, it is recommended that all channels have the
same number of LEDs, and each string has a similar voltage drop across the LEDs.
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). Always use a ground plane under
the switching regulator to minimize interplane coupling.
Good grounding is essential in LED fault detection.
3598f
15
LT3598
TYPICAL APPLICATIONS
LED Driver for 40 White LEDs with Two Channels Unused
L1
10μH
PVIN
6V TO 40V
C1
2.2μF
VIN
5V
C3
2.2μF
R6
100k
D1
VIN
SW
C2
4.7μF
VOUT
VO_SW
R4
1.00M
OPENLED
SHDN
SHDN
PWM
PWM
SYNC
SYNC
FB
R5
30.9k
LT3598
RT
R1
51.1k
VREF
RHOT
10k
LED1
LED2
LED3
LED4
LED5
LED6
GND VC
CTRL
RNTC
100k
R8
60.4k
TSET ISET
R7
100k
C5
47pF
SS
R3
14.7k
C4
0.1μF
20mA
3598 TA02a
RC
2.61k
CC
15nF
C1: TAIYO YUDEN GMK325BJ225ML
C2: MURATA GRM32ER71H475K
C3: TAIYO YUDEN LMK212BJ225MG
D1: DIODES, INC. B240A
L1: WÜRTH ELEKTRONIK 744777410
RNTC : MURATA NCP18WF104J03RB
Efficiency (PWM Dimming)
95
PVIN = 25V
90
EFFICIENCY (%)
85
PVIN = 12V
80
75
70
65
60
55
50
0
10
20
30 40 50 60 70
TOTAL LED CURRENT (mA)
80
90
3598 TA02b
3598f
16
LT3598
TYPICAL APPLICATIONS
LED Driver for 30 White LEDs with 60mA Each String
L1
10μH
PVIN
6V TO 40V
C1
2.2μF
VIN
5V
C3
2.2μF
R6
100k
SHDN
CTRL
PWM
SYNC
VIN
D1
SW
C2
4.7μF
VOUT
VO_SW
OPENLED
SHDN
CTRL
PWM
SYNC
RT
R4
1.00M
FB
R5
30.9k
LT3598
R1
51.1k
VREF
RHOT
10k
CTRL
RNTC
100k
R8
60.4k
TSET ISET
R7
100k
C5
47pF
SS
LED1
LED2
LED3
LED4
LED5
LED6
GND VC
C4
0.1μF
R3
9.76k
60mA
RC
2.61k
CC
15nF
3598 TA03a
C1: TAIYO YUDEN GMK325BJ225ML
C2: MURATA GRM32ER71H475KA88L
C3: TAIYO YUDEN LMK212BJ225MG
D1: VISHAY SS3H9
L1: WÜRTH ELEKTRONIK 744777410
RNTC : MURATA NCP18WF104J03RB
Efficiency (PWM Dimming)
Dimming Range (1000:1 PWM) at 125°C
Junction Temperature, 10ms Period
EFFICIENCY (%)
100
95
PVIN = 25V
90
PVIN = 12V
PWM
5V/DIV
85
SW
20V/DIV
80
75
ILED1
100mA/DIV
70
2μs/DIV
65
3598
60
0
20
40
60 80 100 120 140 160 180
TOTAL LED CURRENT (mA)
3598 TA03b
3598f
17
LT3598
TYPICAL APPLICATIONS
Auto Battery Powered Driver for 20 LEDs with 90mA Each String
L1
4.7μH
PVIN
6V TO 40V
C1
2.2μF
VIN
5V
C3
2.2μF
VIN
D1
SW
C2
4.7μF
VOUT
VO_SW
100k
R4
1.00M
OPENLED
SHDN
CTRL
PWM
SHDN
CTRL
PWM
SYNC
RT
FB
R5
30.9k
LT3598
R1
51.1k
VREF
R10
20k
CTRL
R8
60.4k
TSET ISET
R7
100k
C5
56pF
SS
LED1
LED2
LED3
LED4
LED5
LED6
GND VC
C4
0.1μF
R3
9.76k
90mA
RC
5.11k
CC
6.8nF
3598 TA04a
C1: NIPPON CHEMI-CON KTS500B225M32NOTOO
C2: MURATA GRM32ER71H475KA88L
C3: TAIYO YUDEN LMK212BJ225MG
D1: VISHAY SS3H9
L1: WÜRTH ELEKTRONIK 7447785004
Dimming Range 1000:1 PWM,
10ms Period (125°C Junction Temperature)
Efficiency
95
EFFICIENCY (%)
90
PWM
5V/DIV
85
80
ILED1
100mA/DIV
75
70
65
5μs/DIV
3598 TA04c
60
0
20
40
60 80 100 120 140 160 180
TOTAL LED CURRENT (mA)
3598 TA04b
3598f
18
LT3598
PACKAGE DESCRIPTION
UF Package
24-Lead Plastic QFN (4mm × 4mm)
(Reference LTC DWG # 05-08-1697)
0.70 ±0.05
4.50 ± 0.05
2.45 ± 0.05
3.10 ± 0.05 (4 SIDES)
PACKAGE OUTLINE
0.25 ±0.05
0.50 BSC
RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS
4.00 ± 0.10
(4 SIDES)
BOTTOM VIEW—EXPOSED PAD
0.75 ± 0.05
R = 0.115
TYP
PIN 1 NOTCH
R = 0.20 TYP OR
0.35 × 45° CHAMFER
23 24
0.40 ± 0.10
PIN 1
TOP MARK
(NOTE 6)
1
2
2.45 ± 0.10
(4-SIDES)
(UF24) QFN 0105
0.200 REF
0.00 – 0.05
0.25 ± 0.05
0.50 BSC
NOTE:
1. DRAWING PROPOSED TO BE MADE A JEDEC PACKAGE OUTLINE MO-220 VARIATION (WGGD-X)—TO BE APPROVED
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, IF PRESENT
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
3598f
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.
19
LT3598
TYPICAL APPLICATION
LED Current Matching vs Temperature
90% Efficient LED Driver for 60 White LEDs
PVIN
8V TO 40V
100
VIN
SW
C2
4.7μF
VOUT
SHDN
PWM
PWM
SYNC
SYNC
90
R4
1.00M
OPENLED
SHDN
FB
R5
30.9k
LT3598
RT
95
VIN = 25V
VO_SW
100k
EFFICIENCY (%)
C1
2.2μF
D1
L1, 10μH
VIN
5V
C3
2.2μF
R1
51.1k
VIN = 16V
85
80
75
70
65
VREF
RHOT
10k
CTRL
RNTC
100k
R8
60.4k
TSET ISET
R7
100k
C5
47pF
R3
14.7k
SS
60
LED1
LED2
LED3
LED4
LED5
LED6
GND VC
C4
10nF
0
20
40
60
80
100
TOTAL LED CURRENT (mA)
3598 TA05b
20mA
RC
2.61k
CC
15nF
120
3598 TA04a
RELATED PARTS
PART NUMBER
DESCRIPTION
COMMENTS
LT3474/
LT3474-1
36V, 1A (ILED), 2MHz, Step-Down LED Driver
VIN: 4V to 36V, VOUT(MAX) = 13.5V, True Color PWM Dimming = 400:1,
ISD < 1μA, TSSOP-16E Package
LT3475/
LT3475-1
Dual 1.5A (ILED), 36V, 2MHz, Step-Down LED Driver
VIN: 4V to 36V, VOUT(MAX) = 13.5V, True Color PWM Dimming = 3000:1,
ISD < 1μA, TSSOP-20E Package
LT3476
Quad Output 1.5A, 2MHz High Current LED Driver with
1000:1 Dimming
VIN: 2.8V to 16V, VOUT(MAX) = 36V, True Color PWM Dimming = 1000:1,
ISD < 10μA, 5mm × 7mm QFN-10 Package
LT3477
3A, 42V, 3MHz Boost, Buck-Boost, Buck LED Driver
VIN: 2.5V to 25V, VOUT(MAX) = 40V, Dimming = Analog/PWM, ISD < 1μA,
QFN and TSSOP20E Packages
LT3478/LT3478-1 4.5A, 42V, 2.5MHz High Current LED Driver with 3000:1 VIN: 2.8V to 36V, VOUT(MAX) = 42V, True Color PWM Dimming = 3000:1,
Dimming
ISD < 3μA, TSSOP16E Package
LT3486
Dual 1.3A, 2MHz High Current LED Driver
VIN: 2.5V to 24V, VOUT(MAX) = 36V, True Color PWM Dimming = 1000:1,
ISD < 1μA, 5mm × 3mm DFN and TSSOP-16E Packages
LT3496
45V, 2.1MHz 3-Channel (ILED = 1A) Full Featured
LED Driver
VIN: 3V to 30V (40VMAX), VOUT(MAX) = 45V, True Color PWM Dimming =
3000:1, ISD < 1μA, 4mm × 3mm QFN-28 Package
LT3497
Dual 2.3MHz, Full Function LED Driver with Integrated
Schottkys and 250:1 True Color PWM Dimming
VIN: 2.5V to 10V, VOUT(MAX) = 32V, IQ = 6mA, ISD < 12μA, 2mm × 3mm
DFN-10 Package
LT3498
2.3MHz, 20mA LED Driver and OLED Driver with
Integrated Schottky
VIN: 2.5V to 12V, VOUT(MAX) = 32V, IQ = 1.65mA, ISD < 9μA, 2mm × 3mm
DFN-10 Package
LT3518/LT3517
2.3A/1.3A 45V, 2.5MHz Full Featured LED Driver with
True Color PWM Dimming
VIN: 3V to 30V (40VMAX), VOUT(MAX) = 42V, True Color PWM Dimming =
3000:1, ISD < 5μA, 4mm × 4mm QFN-16 Package
LT3590
48V, 850kHz 50mA Buck Mode LED Driver
VIN: 4.5V to 55V, Dimming = 200:1 True Color PWM, ISD < 15μA,
2mm × 2mm DFN-6 and SC70 Packages
LT3592
36V, 2.2MHz, 500mA Buck Mode LED Driver
VIN: 3.6V to 36V, True Color PWM Dimming = 10:1, ISD < 1μA, 2mm × 3mm
DFN-10 and MSOP-10E Packages
LT3595
45V, 2.5MHz 16-Channel Full Featured LED Driver
VIN: 4.5V to 55V, VOUT(MAX) = 45V, True Color PWM Dimming = 5000:1,
ISD < 1μA, 5mm × 9mm QFN-56 Package
3598f
20 Linear Technology Corporation
LT 0908 • PRINTED IN USA
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900 ● FAX: (408) 434-0507
●
www.linear.com
© LINEAR TECHNOLOGY CORPORATION 2008