LT3922-1
36V, 2.3A Synchronous Step-Up
LED Driver with 25,000:1 PWM Dimming
FEATURES
DESCRIPTION
±2.5% LED Current Regulation
n ±2% Output Voltage Regulation
n 25,000:1 PWM Dimming at 100Hz
n 128:1 Internal PWM Dimming
n Spread Spectrum Frequency Modulation
n Silent Switcher® Architecture for Low EMI
n Operates in Boost, Buck Mode and Buck-Boost Mode
n 2.8V to 36V Input Voltage Range
n Up to 34V LED String Voltage
n 2.3A, 40V Internal Switches
n 200kHz to 2MHz Switching Frequency with SYNC
n Analog or Duty Cycle LED Current Control
n Open/Short LED Protection and Fault Indication
n Thermally Enhanced 28-Lead (4mm × 5mm) QFN
n AEC-Q100 Qualified for Automotive Applications
The LT®3922-1 is a monolithic, synchronous, step-up DC/
DC converter that utilizes fixed-frequency, peak current
control and provides PWM dimming for a string of LEDs.
The LED current is programmed by an analog voltage or
the duty cycle of pulses at the CTRL pin. The LT3922-1
will maintain ±2.5% current regulation through an external
sense resistor over a wide range of output voltages.
n
The switching frequency is programmable from 200kHz
to 2MHz by an external resistor at the RT pin or by an
external clock applied at the SYNC/SPRD pin. With the
optional spread spectrum frequency modulation enabled,
the frequency varies from 100% to 125% to reduce EMI.
The LT3922-1 also includes a driver for an external high
side PMOS for PWM dimming and an internal PWM signal
generator for analog control of PWM dimming. When an
external signal is available, the LT3922-1 can perform
25,000:1 PWM dimming with 100Hz PWM pulses.
APPLICATIONS
Additional features include an accurate external reference
voltage for use with the CTRL and PWM pins, an LED
current monitor, an accurate EN/UVLO pin threshold,
open-drain fault reporting for open-circuit and short-circuit
load conditions, and thermal shutdown.
Automotive and Industrial Lighting
n Machine Vision
n
All registered trademarks and trademarks are the property of their respective owners. Protected
by U.S. patents, including 7199560, 7321203, and other patents pending.
TYPICAL APPLICATION
30V, 333mA Boost LED Driver with 25,000:1 PWM Dimmming
L1, 2.2µH
VIN
8V TO 27V
SW
VIN
4.7µF
33µF
1M
BST
VOUT
0.47µF
GND
EN/UVLO
0.47µF
365k
VOUT
OVLO
LT3922-1
59.0k
ILED
100mA/DIV
1M
10µF
FB
33.2k
1µF
VREF
GND
CTRL
ISP
PWM
300mΩ
SYNC/SPRD
INTVCC
ISN
100k
PWMTG
FAULT
SS RT
2.2µF
10nF
L1: WURTH 74437324022
M1: VISHAY Si2319CDS
VPWM
2V/DIV
RP
45.3k
2MHz
ISMON
VC
51k
M1
INFINITE PERSISTENCE
VIN = 12V
fPWM = 100Hz
200ns/DIV
39221 TA01b
30V
333mA
LED
1nF
39221 TA01a
Rev. A
Document Feedback
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1
LT3922-1
ABSOLUTE MAXIMUM RATINGS
PIN CONFIGURATION
(Note 1)
VIN and EN/UVLO.......................................................40V
ISP, ISN, and VOUT.....................................................40V
ISP – ISN..................................................................0.3V
CTRL and FB.............................................................3.3V
OVLO, PWM, SYNC/SPRD, and FAULT.........................6V
SS and VC.................................................................3.3V
SW, BST, INTVCC, VREF, ISMON, PWMTG, RT, and RP (Note 2)
Operating Junction Temperature Range (Notes 3, 4)
LT3922E-1/LT3922I-1............................. –40 to 125°C
LT3922H-1.............................................. –40 to 150°C
Storage Temperature Range.......................–60 to 150°C
GND
VOUT
NC
NC
SW
SW
TOP VIEW
28 27 26 25 24 23
SW 1
22 GND
BST 2
21 VOUT
INTVCC 3
20 PWMTG
VIN 4
19 PWM
29
GND
EN/UVLO 5
18 RP
OVLO 6
17 SYNC/SPRD
VREF 7
16 RT
CTRL 8
15 FAULT
ISMON
SS
FB
VC
ISP
ISN
9 10 11 12 13 14
UFD PACKAGE
28-LEAD (4mm × 5mm) PLASTIC QFN
θJA = 25°C/W (AS MEASURED ON DEMO BOARD DC2247A), θJC = 3.4°C/W
EXPOSED PAD (PIN 29) IS GND, MUST BE SOLDERED TO PCB
ORDER INFORMATION
LEAD FREE FINISH
TAPE AND REEL
PART MARKING*
PACKAGE DESCRIPTION
TEMPERATURE RANGE
LT3922EUFD-1#PBF
LT3922EUFD-1#TRPBF
39221
28-Lead (4mm × 5mm) Plastic QFN
–40°C to 125°C
LT3922IUFD-1#PBF
LT3922IUFD-1#TRPBF
39221
28-Lead (4mm × 5mm) Plastic QFN
–40°C to 125°C
LT3922HUFD-1#PBF
LT3922HUFD-1#TRPBF
39221
28-Lead (4mm × 5mm) Plastic QFN
–40°C to 150°C
LT3922EUFD-1#WPBF
LT3922EUFD-1#WTRPBF
39221
28-Lead (4mm × 5mm) Plastic QFN
–40°C to 125°C
LT3922IUFD-1#WPBF
LT3922IUFD-1#WTRPBF
39221
28-Lead (4mm × 5mm) Plastic QFN
–40°C to 125°C
LT3922HUFD-1#WPBF
LT3922HUFD-1#WTRPBF 39221
28-Lead (4mm × 5mm) Plastic QFN
–40°C to 150°C
AUTOMOTIVE PRODUCTS**
Contact the factory for parts specified with wider operating temperature ranges. *The temperature grade is identified by a label on the shipping container.
Tape and reel specifications. Some packages are available in 500 unit reels through designated sales channels with #TRMPBF suffix.
**Versions of this part are available with controlled manufacturing to support the quality and reliability requirements of automotive applications. These
models are designated with a #W suffix. Only the automotive grade products shown are available for use in automotive applications. Contact your local
Analog Devices account representative for specific product ordering information and to obtain the specific Automotive Reliability reports for these models.
Rev. A
2
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LT3922-1
ELECTRICAL CHARACTERISTICS
The l denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at TA = 25°C, VIN = 12V, VEN/UVLO = 2V unless otherwise noted.
PARAMETER
CONDITIONS
MIN
Input Voltage Range
VIN Pin Quiescent Current
VEN/UVLO = 1.5V, Not Switching
VEN/UVLO = 0.1V, Shutdown
EN/UVLO Threshold (Falling)
1.260
EN/UVLO Rising Hysteresis
EN/UVLO Pin Current
VEN/UVLO = 1.2V
Input OVLO Threshold (Rising)
1.145
Input OVLO Falling Hysteresis
OVLO Pin Current
TYP
2.8
MAX
V
2.9
4
1
mA
µA
1.330
1.400
V
25
mV
2
µA
1.205
1.265
50
VOVLO = 1.0V
UNITS
36
–100
V
mV
100
nA
2.03
2.015
V
V
Reference
VREF Voltage
IVREF = 10µA
IVREF = 500µA
VREF Pin Current Limit
VREF = 0V, Current Out of Pin
l
1.97
1.985
2
2
3.2
mA
LED Current Regulation
CTRL-Off Threshold (Falling)
l
200
CTRL-Off Rising Hysteresis
210
220
15
−100
mV
mV
CTRL Pin Current
VCTRL = 2V
Sense Voltage (VISP−VISN)
(Analog Input)
VCTRL = 2V (100%), VISP = 24V
VCTRL = 0.75V (50%), VISP = 24V
VCTRL = 0.3V (5%), VISP = 24V
ISP Pin Current
VISP = 24.1V, VISN = 24V, VCTRL = 2V
ISN Pin Current
VISP = 24.1V, VISN = 24V , VCTRL = 2V
75
µA
Current Error Amplifier Transconductance
VISP = 24V
140
µA/V
l
l
l
97.5
48
3.5
100
50
5
100
nA
102.5
52
6.5
mV
mV
mV
75
µA
Duty Cycle Control of LED Current
Sense Voltage (VISP−VISN)
(Duty Cycle Input)
CTRL Duty = 75% (100%), VISP = 24V
CTRL Duty = 37.5% (50%), VISP = 24V
CTRL Duty = 15% (5%), VISP = 24V
99
49
4
CTRL Pulse Input High (VIH)
100
50
5
101
51
6
1.6
V
CTRL Pulse Input Low (VIL)
CTRL Pulse Input Frequency Range
mV
mV
mV
10
0.4
V
200
kHz
Voltage Regulation
FB Regulation Voltage
VCTRL = 2V
FB Pin Current
FB in Regulation
l
1.175
1.200
−100
Voltage Error Amplifier Transconductance
1.225
V
100
nA
1000
µA/V
INTVCC Regulator
INTVCC Voltage
INTVCC Pin Current Limit
2.7
VINTVCC = 0V, Current Out of Pin
3
20
3.3
V
mA
Rev. A
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3
LT3922-1
ELECTRICAL CHARACTERISTICS
The l denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at TA = 25°C, VIN = 12V, VEN/UVLO = 2V unless otherwise noted.
PARAMETER
CONDITIONS
MIN
TYP
MAX
2.3
2.55
2.8
25
35
UNITS
Power Stage
Peak Current Limit
VIN = 3V
Bottom Switch Minimum Off-Time
l
15
A
ns
Bottom Switch On-Resistance
140
mΩ
Top Switch On-Resistance
155
mΩ
Oscillator
Programmed Switching Frequency (fSW)
RT = 45.3k, VSYNC/SPRD = 0V
RT = 499k, VSYNC/SPRD = 0V
Spread Spectrum Frequency Range
RT = 45.3k, VSYNC/SPRD = 3V
RT = 499k, VSYNC/SPRD = 3V
RT Pin Current Limit
VRT = 0V, Current Out of Pin
l
l
1880
175
2000
200
1880
175
kHz
kHz
2650
306
kHz
kHz
75
SYNC/SPRD Threshold (Rising)
1.4
SYNC/SPRD Falling Hysteresis
0.2
SYNC/SPRD Pin Current
2120
245
VSYNC/SPRD = 5V
−100
µA
1.5
V
V
100
nA
Soft-Start
SS Pin Charging Current
VSS = 1V
20
µA
SS Pin Discharging Current
VSS = 2V
2
µA
SS Lower Threshold
0.2
V
SS Higher Threshold
1.7
V
Fault Detection
Open-Circuit Threshold (FB Rising)
VISP = VISN = 20V
l
1.117
Open-Circuit Falling Hysteresis
1.140
1.163
50
LED Short-Circuit Threshold (VISP − VISN)
VISP = 20V
FAULT Pull-Down Current
VFAULT = 0.2V, VFB = 1.25V
FAULT Leakage Current
VFAULT = 3V, VFB = 0.7V
V
mV
150
mV
0.8
mA
−100
100
nA
1.292
V
Overvoltage Protection
FB Overvoltage Threshold (Rising)
l
1.240
FB Overvoltage Falling Hysteresis
1.266
22
mV
LED Current Monitor
ISMON Voltage
VISP − VISN = 100mV (100%), VISP = 24V
VISP − VISN = 10mV (10%), VISP = 24V
0.980
80
1.000
100
1.020
120
10
11
V
mV
PWM Driver
PWMTG Gate Drive (VOUT – VPWMTG)
VOUT = 20V, VPWM = 1.5V
V
PWM Threshold (Rising)
1.4
V
PWM Falling Hysteresis
0.2
V
PWM Pin Current
VPWM = 2V
PWM to PWMTG Propagation Delay
Turn-On
Turn-Off
CPWMTG = 2.1nF (Connected from VOUT to PWMTG)
VOUT = 20V
−100
100
110
140
nA
ns
ns
Rev. A
4
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LT3922-1
ELECTRICAL CHARACTERISTICS
PARAMETER
CONDITIONS
MIN
PWM Voltage for 100% PWM Dimming
RP = 28.7k, VREF = 2V
2.00
PWM Voltage for 0% PWM Dimming
RP = 28.7k, VREF = 2V
PWM Dimming Accuracy
RP = 28.7k, VREF = 2V, VPWM = 1.1V
RP = 28.7k, VREF = 2V, VPWM = 1.9V
7.5
87.5
PWM Dimming Frequency
RP = 28.7k, RT = 45.3k, VSYNC/SPRD = 0V
RP = 332k, RT = 45.3k, VSYNC/SPRD = 0V
7.34
115
RP Pin Current Limit
VRP = 0V, Current Out of Pin
TYP
MAX
UNITS
Internal PWM Dimming
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: Do not apply a positive or negative voltage source to these pins,
otherwise permanent damage may occur.
Note 3: The LT3922E-1 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,
characterization and correlation with statistical process controls. The
V
0.99
V
10.5
90.5
13.5
93.5
%
%
7.81
122
8.28
129
kHz
Hz
65
µA
LT3922I-1 is guaranteed to meet performance specifications over the
−40°C to 125°C operating junction temperature range. The LT3922H-1 is
guaranteed over the −40°C to 150°C operating junction temperature range.
Operating lifetime is derated at junction temperatures greater than 125°C.
Note 4: This IC includes overtemperature protection that is intended to
protect the device during momentary overload conditions. The maximum
rated junction temperature will be exceeded when this protection is active.
Continuous operation above the specified absolute maximum operating
junction temperature may impair device reliability or permanently damage
the device.
Rev. A
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5
LT3922-1
TYPICAL PERFORMANCE CHARACTERISTICS
EN/UVLO Thresholds
RISING
FALLING
EN/UVLO PIN CURRENT (µA)
1.45
EN/UVLO THRESHOLD (V)
EN/UVLO Pin Current
3.00
1.40
1.35
1.30
1.25
1.20
–45 –20
5
VEN/UVLO = 1.2V
2.00
1.50
1.00
5
0.50
3.2
3.20
3.00
2.80
2.60
2.40
5
2.00
–45 –20
30 55 80 105 130 155
TEMPERATURE (°C)
5
2.02
–40°C
25°C
125°C
150°C
2.85
2.80
VREF Voltage
0
2
6
8
10
INTVCC LOAD CURRENT (mA)
12
39221 G07
0
25 50 75 100 125 150
TEMPERATURE (°C)
2.00
1.99
1.97
–50 –25
VREF Load Regulation
2.01
1.98
4
2.7
2.02
VREF VOLTAGE (V)
2.90
2.8
39221 G06
2.01
VREF VOLTAGE (V)
INTVCC VOLTAGE (V)
3.05
2.95
2.9
39221 G05
INTVCC Load Regulation
3.00
3.0
2.5
–50 –25
30 55 80 105 130 155
TEMPERATURE (°C)
39221 G04
3.10
VIN = 36V
VIN = 12V
VIN = 2.7V
2.6
2.20
0
–45 –20
30 55 80 105 130 155
TEMPERATURE (°C)
INTVCC Voltage
3.1
INTVCC VOLTAGE (V)
1.00
5
39221 G03
VIN = 36V
VIN = 12V
VIN = 2.7V
3.40
1.50
1.15
1.05
–45 –20
30 55 80 105 130 155
TEMPERATURE (°C)
VIN Quiescent Current
3.60
VIN QUIESCENT CURRENT (mA)
VIN SHUTDOWN CURRENT (µA)
2.00
1.20
39221 G02
VIN Shutdown Current
VIN = 36V
VIN = 12V
VIN = 2.7V
1.25
1.10
0.50
39221 G01
2.50
RISING
FALLING
1.30
2.50
0
–45 –20
30 55 80 105 130 155
TEMPERATURE (°C)
OVLO Threshold
1.35
OVLO THRESHOLD (V)
1.50
VIN = 12V, TA = 25°C, unless otherwise noted.
2.00
1.99
–40°C
25°C
125°C
150°C
1.98
0
25 50 75 100 125 150
TEMPERATURE (°C)
39221 G08
1.97
0
200
400
600
800
VREF LOAD CURRENT (µA)
1000
39221 G09
Rev. A
6
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LT3922-1
TYPICAL PERFORMANCE CHARACTERISTICS
INTVCC and VREF UVLO Falling
Thresholds
VREF Line Regulation
2.008
THRESHOLD VOLTAGE (V)
2.006
2.002
2.000
1.998
1.996
1.994
2.7
25
2.4
2.1
1.8
1.5
1.2
1.992
1.990
30
0
3
6
0.9
–45 –20
9 12 15 18 21 24 27 30 33 36
VIN VOLTAGE (V)
5
39221 G10
10
0
–45 –20
30 55 80 105 130 155
TEMPERATURE (°C)
Soft-Start Currents
75
20
1.6
60
THRESHOLD VOLTAGE (V)
2.0
65
15
PULL-UP CURRENT
PULL-DOWN CURRENT
10
5
55
–45 –20
RT PIN
RP PIN
5
0
–45 –20
30 55 80 105 130 155
TEMPERATURE (°C)
5
Switching Frequency
0.8
0.4
250
2000
240
1900
230
1800
220
1700
210
1600
200
190
180
30 55 80 105 130 155
TEMPERATURE (°C)
8320
INTERNAL PWM FREQUENCY (Hz)
2100
RT = 45.3k
RT = 499k
5
30 55 80 105 130 155
TEMPERATURE (°C)
39221 G15
Internal PWM Frequency
260
5
SS HIGH
SS LOW
39221 G14
2200
1400
–45 –20
1.2
0
–45 –20
30 55 80 105 130 155
TEMPERATURE (°C)
39221 G13
1500
30 55 80 105 130 155
TEMPERATURE (°C)
Soft-Start Thresholds
25
70
5
39221 G12
80
CURRENT (µA)
MAXIMUM PIN CURRENT (µA)
15
39221 G11
RT and RP Pin Current Limits
SWITCHING FREQUENCY (kHz)
20
5
INTVCC UVLO
VREF UVLO
Internal PWM Duty Cycle
RT = 45.3k
134
7936
7552
128
7168
122
116
6784
6400
–45 –20
RP = 28.7k
RP = 332k
5
10.5
140
110
30 55 80 105 130 155
TEMPERATURE (°C)
39221 G17
39221 G16
INTERNAL PWM DUTY CYCLE (%)
VOLTAGE (V)
2.004
Minimum Off Time
3.0
MINIMUM OFF TIME (ns)
2.010
VIN = 12V, TA = 25°C, unless otherwise noted.
RT = 45.3k
RP = 332k
10.4
18.2k
1%
10.3
22.1k
1%
VREF
PWM
LT3922-1
10.2
10.1
10.0
–45 –20
5
30 55 80 105 130 155
TEMPERATURE (°C)
39221 G18
Rev. A
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7
LT3922-1
TYPICAL PERFORMANCE CHARACTERISTICS
LED Current (Digital CTRL)
100
100
VISP – VISN (mV)
125
VISP – VISN (mV)
125
75
50
PULSE FREQUENCY AT CTRL PIN = 20kHz
0
0.25 0.50 0.75 1 1.25 1.50 1.75
VCTRL (V)
75
50
0
2
0
LED Current (5% Regulation)
150
100
120
80
40
50
0
98.6
99.3
100
100.7
ISP–ISN VOLTAGE (mV)
VCTRL = 2V
VISP = 24V
39221 G22
2.9
4.6
5
5.4
ISP–ISN VOLTAGE (mV)
39221 G21
400
LED Current Line Regulation
400
PEAK SW CURRENT LIMITED
350
0
20
39221 G23
400
fSW = 2MHz
L = 4.7µH
1.22
600
0
5.8
VCTRL = 0.3V
VISP = 24V
Peak SW Current Limit
3.0
1.21
200
0
4.2
101.4
1.19
1.20
VFB (V)
–40°C
25°C
125°C
800
N = 355
ISMON (mV)
200
1.18
ISMON Voltage
1000
150°C
25°C
–40°C
160
N = 355
NUMBER OF UNITS
NUMBER OF UNITS
0
1.17
12.5 25 37.5 50 62.5 75 87.5 100
DCTRL (%)
200
150°C
25°C
–40°C
250
50
39221 G20
LED Current (100% Regulation)
300
75
25
39221 G19
350
VCTRL = 2V
100
25
25
0
LED Voltage Limit
125
VISP – VISN (mV)
LED Current (Analog CTRL)
VIN = 12V, TA = 25°C, unless otherwise noted.
40
60
VISP – VISN (mV)
80
100
39221 G24
LED Current vs VOUT
VFB OVERVOLTAGE PROTECTION LIMITED
350
300
300
2.6
250
250
2.5
2.4
2.3
200
150
2.2
20
30
40 50 60 70
DUTY CYCLE (%)
80
90 100
39221 G25
50
200
VIN = 8V
fSW = 2MHz
RSNS = 0.3Ω
RFB_TOP = 1M
RFB_BOT = 34.8k
150
fSW = 2MHz
RSNS = 0.3Ω
11 LEDs (VOUT ~33V)
100
10 LEDs (VOUT ~ 30V)
5 LEDs (VOUT ~ 15V)
2.1
2.0
10
ILED (mA)
2.7
ILED (mA)
PEAK SW CURRENT (A)
2.8
0
3
6
9
12 15 18 21 24 27 30
VIN (V)
39221 G26
100
50
9
15
21
27
VOUT (V)
33
39
39221 G27
Rev. A
8
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LT3922-1
TYPICAL PERFORMANCE CHARACTERISTICS
VISP – VISN SHORTLED Threshold
FB OPENLED Threshold
1.10
1.05
1.00
0.95
5
170
160
150
140
130
120
–45 –20
30 55 80 105 130 155
TEMPERATURE (°C)
5
39221 G28
96
94
94
90
88
86
80
fSW = 400kHz
fSW = 2MHz
6
92
90
88
86
80
PWMTG ON Voltage
0
200
800
1.20
1.18
–45 –20
1000
9
8
120
80
CPWMTG = 2.2nF (C0G type)
30 55 80 105 130 155
TEMPERATURE (°C)
39221 G34
5
30 55 80 105 130 155
TEMPERATURE (°C)
39221 G33
FB OVLO Threshold
1.33
160
40
–45 –20
5
39221 G32
TURN OFF
TURN ON
PROPAGATION DELAY (ns)
VOUT – VPWMTG (V)
400
600
ILED (mA)
200
10
30 55 80 105 130 155
TEMPERATURE (°C)
1.21
PWM Driver Propagation Delay
12
11
5
1.19
6 LED (VOUT ~ 18V)
9 LED (VOUT ~ 27V)
12 LED (VOUT ~ 36V)
39221 G31
5
50
1.22
82
8 10 12 14 16 18 20 22 24 26 28 30
VIN (V)
7
–45 –20
100
Regulated FB Voltage
VIN = 12V
fSW = 2MHz
84
84
82
150
1.23
FB VOLTAGE (V)
96
92
200
39221 G30
Efficiency vs ILED
98
EFFICIENCY (%)
EFFICIENCY (%)
100
ILED = 400mA
11 LEDs (VOUT ~ 33V)
98
250
39221 G29
Efficiency vs VIN
100
TOP SWITCH
BOTTOM SWITCH
300
0
–45 –20
30 55 80 105 130 155
TEMPERATURE (°C)
30 55 80 105 130 155
TEMPERATURE (°C)
39221 G35
FB OVLO THRESHOLD VOLTAGE (V)
0.90
–45 –20
POWER SWITCH ON–RESISTANCE (mΩ)
RISING
FALLING
1.15
Power Switch On-Resistance
350
180
VISP – VISN SHORTLED THRESHOLD (mV)
FB OPENLED THRESHOLD (V)
1.20
VIN = 12V, TA = 25°C, unless otherwise noted.
RISING
FALLING
1.30
1.27
1.24
1.21
1.18
–45 –20
5
30 55 80 105 130 155
TEMPERATURE (°C)
39221 G36
Rev. A
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9
LT3922-1
TYPICAL PERFORMANCE CHARACTERISTICS
C/10 Threshold
Case Temperature Rise
35
16
14
12
10
8
6
–45 –20
5
30 55 80 105 130 155
TEMPERATURE (°C)
25
15
10
5
0
150°C
25°C
–40°C
8
12
16
20
VIN (V)
24
28
N = 359
250
200
150
100
50
fSW = 2MHz
fSW = 400kHz
4
150°C
25°C
–40°C
300
20
Internal PWM Duty Cycle (90%)
250
DC2247A DEMO BOARD
VLED = 30V
I LED = 333mA
TROOM = 25°C
30
39221 G37
300
Internal PWM Duty Cycle (10%)
350
NUMBER OF UNITS
RISING
FALLING
CASE TEMPERATURE RISE (°C)
VISP – VISN C/10 THRESHOLD (mV)
18
VIN = 12V, TA = 25°C, unless otherwise noted.
32
0
8.6
9.4
10.2
11.0
PWM DUTY CYCLE (%)
11.8
39221 G39
39221 G38
Input Voltage Transient Response
Input Voltage Transient Response
N = 359
VIN
5V/DIV
VIN
5V/DIV
ILED
100mA/DIV
ILED
100mA/DIV
200
150
100
5ms/DIV
50
0
90.2 90.6
91.4
92.2
PWM DUTY CYCLE (%)
93.0 93.4
FRONT PAGE APPLICATION
18V to 6.5V INPUT VOLTAGE TRANSIENT
VLED = 30V
ILED = 333mA
Start-Up with 10% Internal PWM
Start-Up with 50% Internal PWM
VOUT
10V/DIV
VIN
10V/DIV
VOUT
10V/DIV
VIN
10V/DIV
ILED
100mA/DIV
VIN
10V/DIV
ILED
100mA/DIV
ILED
100mA/DIV
500µs/DIV
FRONT PAGE APPLICATION
VLED = 30V
ILED = 333mA
39221 G43
39221 G42
FRONT PAGE APPLICATION
6.5V to 18V INPUT VOLTAGE TRANSIENT
VLED = 30V
ILED = 333mA
39221 G40
Turn ON and OFF Performance
5ms/DIV
39221 G41
39221 G44
5ms/DIV
FRONT PAGE APPLICATION WITH PWM = 1.1V
RP = 332k TO GND
VLED = 30V
ILED = 333mA
39221 G45
5ms/DIV
FRONT PAGE APPLICATION WITH PWM = 1.5V
RP = 332k TO GND
VLED = 30V
ILED = 333mA
Rev. A
10
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LT3922-1
PIN FUNCTIONS
SW: Switch Pins. These pins are internally connected to
the power devices and drivers. They should always be
tied together. In normal operation, the voltage of these
pins will switch between the output voltage and zero at
the programmed frequency. Do not force any voltage on
these pins.
BST: Boost Pin. This pin supplies the top power switch
GATE driver. Connect a 33nF capacitor between this pin
and SW close to the package. An internal diode from
INTVCC to BST will charge the capacitor when the SW pin
switches low.
INTVCC: Internally Regulated, Low-Voltage Supply Pin.
This pin provides the power for the converter switch GATE
drivers. Do not force any voltage on this pin. Place a 2.2µF
bypass capacitor to GND close to the package.
VIN: Input Voltage Pin. This pin supplies power to the
internal, high-performance analog circuitry. Connect a
bypass capacitor between this pin and GND.
EN/UVLO: Enable and Undervoltage Lockout Pin. A voltage at this pin greater than 1.33V will enable switching,
and a voltage less than 0.1V is guaranteed to shut down
the internal current bias and sub-regulators. A resistor
network between this pin and ground can be used to set
the pin voltage and automatically lockout the part when
VIN is below a certain level. No internal components pull up
or down on this pin, so it requires an external voltage bias
for normal operation. This pin may be tied directly to VIN.
OVLO: Input Overvoltage Lockout Pin. When the voltage at
this pin rises above 1.205V, the system disables switching and resets the soft-start capacitor. Do not leave this
pin open. Tie this pin to GND when the OVLO function is
not used.
VREF: Reference Voltage Pin. This pin provides a buffered
2V reference capable of 3mA drive. It can be used to supply
resistor networks for setting the voltages at the CTRL and
PWM pins. Bypass with a 1μF capacitor to GND.
CTRL: Control Pin. An analog voltage from 250mV to
1.25V at this pin programs the regulated voltage between
ISP and ISN (and therefore, the regulated current supplied
to the load). Alternatively, a digital pulse at this pin with
duty cycle from 12.5% to 62.5% can be used to program
the regulated voltage. Below 200mV or 10% duty cycle,
the CTRL pin voltage disables switching. For more detail,
see Typical Performance Characteristics and Applications
Information sections.
ISP: Positive Current Sense Pin. This pin is one of the inputs
to the internal current sense error amplifier. It should be
connected to the positive side of the external sense resistor. Use Kelvin connection for accurate current sensing.
ISN: Negative Current Sense Pin. This pin is one of the
inputs to the internal current sense error amplifier. It
should be connected to the negative side of the external
sense resistor. Use Kelvin connection for accurate current sensing.
VC: Compensation Pin. A resistor and capacitor connected
in series from this pin to GND stabilize the current and
voltage regulation. Typical resistor and capacitor values
are from 0k to 100k and from 0.1nF to 10nF, respectively.
FB: Feedback Pin. When the voltage at this pin is near 1.2V
the regulated current is automatically reduced from the
programmed value. A resistor network between this pin
and VOUT can be used to set a limit for the output voltage.
If the voltage at the FB pin reaches 1.266V, a FB overvoltage lockout comparator disables switching.
SS: Soft-Start Pin. At startup and recovery from fault
conditions, a 20μA current charges the capacitor and the
FB voltage tracks the rising voltage at this pin until the load
current reaches its programmed level. Typical values for
the capacitor are 10nF to 100nF. Using a single resistor
from SS to INTVCC, the LT3922-1 can be set in two different fault modes for the shorted LED conditions: hiccup
(no resistor) and latchoff (100k). Refer to the Applications
Information section for a detailed explanation.
ISMON: Output Current Monitoring Pin. This pin provides
a buffered voltage output equal to 10mV for every 1mV
between ISP and ISN.
Rev. A
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11
LT3922-1
PIN FUNCTIONS
FAULT: Fault Pin. Connect to INTVCC through a resistance
of 100k. An internal switch pulls this pin low when any of
following conditions happen:
1. Open LED: VFB > 1.14V and (VISP – VISN) < 10mV
2. Shorted LED:
(VISP – VISN) > 150mV for more than 300us, or
(VISP – VISN) > 700mV (typical), or
VOUT < (VIN – 2V)
RT: Timing Resistor Pin. A resistor from this pin to GND
programs the switching frequency between 200kHz and
2MHz. Do not leave this pin open.
SYNC/SPRD: Synchronization Pin. To override the programmed switching frequency, drive this pin with an
external clock having a frequency between 200kHz and
2MHz. Even when using the external clock, select an RT
resistor that corresponds to the desired switching frequency. Tie the pin to INTVCC to enable Spread Spectrum
Frequency Modulation. This pin should be tied to GND
when not in use.
RP: PWM Resistor Pin. Connect a resistor from this pin to
GND to set the frequency of the internal PWM signal. Do not
use a resistor larger than 1MΩ. If using an external PWM
pulse for LED dimming, tie this pin to GND. Refer to the
Applications Information section for a detailed explanation.
PWM: PWM Input Pin. With the RP pin tied to GND, drive
this pin with a digital pulse to control PWM dimming of
the LEDs. Alternatively, when using a resistor on the RP
pin to GND, set the voltage of this pin between 1V and
2V to generate an internal pulse with duty cycle between
0% and 100%. When using an analog signal, place a 1µF
bypass capacitor between this pin and GND. Tie this pin
high when PWM dimming is not required.
PWMTG: PWM Driver Output Pin. This pin can drive the
gate of an external high-side PMOS device for PWM dimming of LEDs. Do not force any voltage on this pin.
VOUT: Output Pins. Connect to the output and place output
capacitors between these pins and GND as close as possible to the package. Refer to the Applications Information
section for the recommended capacitor placements.
GND (Pin 22, 23, Exposed Pad Pin 29): Ground Pins. All
GND pins must be soldered to the board ground plane.
NC: No Connect Pins. These pins can be left open or connected to the ground.
Rev. A
12
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LT3922-1
BLOCK DIAGRAM
VIN
4.7µH
10µF
2.2µF
3
VIN
2
INTVCC
33nF
BST
SW
28
27
EN/UVLO
5
356k
INTERNAL VCC
REGULATOR
MTSW
S
UVLO AND
OVLO
OVLO
R
6
PEAK CURRENT
COMPARATOR
VREF
2V REFERENCE
VOUT
Q
24
SYNCHRONOUS
CONTROLLER
59.0k
7
29
GND
(EXPOSED PAD)
4
1M
1
GND
MBSW
RT
16
45.3k
1.4V
SYNC/SPRD
17
–
0.47µF
VOUT
200kHz TO
2MHz
OSCILLATOR
4.7µF
23
+
–
+
1µF
0.47µF
22
200mΩ
21
VOUT – 10V
REGULATOR
PWMTG
V/I
CONVERTER
+
PWMTG
DRIVER
20
gm = 140µA/V
CURRENT
REGULATION
AMPLIFIER
+
–
PWM
19
2.5k
2.5k
ISMON
INTERNAL
PWM SIGNAL
14
25k
0.25V
10×
gm = 1000µA/V
VOLTAGE
REGULATION
AMPLIFIER
–
A/D
DETECTOR
CONTROL
BUFFER
+
8
+
+
–
ISN
10
ISP
9
+
+
–
CTRL
+
–
1.25V
12
1.2V
INTVCC
20µA
FAULT
SOFT-START
AND LED
FAULT CONTROL
2µA
SS
25
N/C
26
N/C
1M
FB
13
VC
11
10nF
33.2k
100k
15
RP
18
39221 BD
10k
1nF
Rev. A
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13
LT3922-1
OPERATION
The LT3922-1 is a step-up LED driver that utilizes a fixedfrequency peak current control to accurately regulate the
current through a string of LEDs. Low EMI performance is
realized with the LT3922-1's Silent Switcher architecture,
which employs magnetic field cancellation techniques to
minimize electromagnetic interference. The LT3922-1
includes two power switches, their drivers, and a diode
for providing power to the top switch driver. The switches
connect the external inductor terminal connected to the
SW pin alternately to the ground and then to the output
(VOUT). The inductor current rises and falls accordingly and
the peak current can be regulated through the combined
effect of the other circuit blocks.
The synchronous controller ensures the power switches
do not conduct at the same time, and a programmable
oscillator turns on the bottom switch at the beginning of
each switching cycle. The frequency of this oscillator is set
by an external resistor at the RT pin and can be overridden by external pulses at the SYNC/SPRD pin. The SYNC/
SPRD pin can also be used to command spread spectrum
frequency modulation (SSFM).
The bottom switch is turned off by the peak current comparator which waits during the on-time for the inductor
current to exceed the target set by the voltage at the VC
pin. This target is modified by a signal from the oscillator
which stabilizes the inductor current. A network of passive components at the VC pin is necessary to stabilize
this regulation loop.
The inductor current is derived from the desired LED
current programmed by the voltage at the CTRL pin. The
analog-to-digital detector and the control buffer convert
either a DC voltage or duty cycle of pulses at the CTRL
pin into the input for the current regulation amplifier. The
other input to this amplifier comes from the ISP and ISN
pin voltages. An external current sense resistor between
these pins should be placed in series with the string of
LEDs such that the voltage across it provides the feedback
to regulate the LED current. The current regulation amplifier then compares the actual LED current to the desired
LED current and adjusts VC as necessary.
The voltage regulation amplifier overrides the current
regulation amplifier when the FB pin voltage is higher than
an internal 1.2V reference. An external resistor network
from the LED string to the FB pin provides an indication
of the LED string voltage and allows the voltage amplifier
to prevent overvoltage of the LED string.
The ISP, ISN, and FB pin voltages are also monitored to
detect fault conditions like open and short circuits, which
are then reported by pulling FAULT pin low. The response
to a fault can be selected either to try hiccup restarts or to
latchoff by the choice of an external resistor connected to
the SS pin. Refer to the Applications Information section
for a detailed explanation of fault responses.
Finally, pulse-width modulation (PWM) of the LED current
is achieved by turning on and off an external PMOS switch
between the VOUT and the string of LEDs. An external pulse
at the PWM pin controls the state of the PWM driver or
a DC voltage at the PWM pin dictates the duty ratio of an
internal PWM pulse, whose frequency is programmed
with an external resistor at the RP pin. The proprietary
circuits of the LT3922-1 ensures a rapid recovery of the
LED current pulses for PWM.
APPLICATIONS INFORMATION
The following is a guide to selecting the external components and configuring the LT3922-1 according to the
requirements of an application.
Programming LED Current with the CTRL Pin
The primary function of the LT3922-1 is to regulate the
current in a string of LEDs. This current should pass
through a series current sense resistor. The voltage across
this resistor is sensed by the current regulation amplifier
through the ISP and ISN pins and regulated to a level programmed by the CTRL pin. The maximum resistor voltage
that can be programmed is 100mV which corresponds to
1A through the LED string when a 100mΩ current sense
resistor is used.
To allow for this maximum current, the CTRL pin may
be connected directly to the VREF pin which provides
Rev. A
14
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LT3922-1
APPLICATIONS INFORMATION
an accurate 2V reference. Lower current levels can be
programmed by DC CTRL voltages between 250mV and
1.25V as shown in Figure 1.
ILED
100mV
RSNS
DCTRL < 10%
CTRL-OFF
ILED
100mV
RSNS
50mV
RSNS
VCTRL < 200mV
CTRL-OFF
50mV
RSNS
0
12.5%
37.5%
62.5%
75%
DCTRL
39221 F02
Figure 2. Duty Cycle CTRL Range
0
0.25V
0.75V
1.25V
1.5V
VCTRL
VREF
VREF
39221 F01
RCTRL1
Figure 1. Analog CTRL Range
LT3922-1
RCTRL1
Below 250mV, the CTRL pin commands zero LED current,
and above 1.25V, it commands the maximum. When an
independent voltage source is not available, the intermediate CTRL voltages may be derived from the 2V reference
at the VREF pin using a resistor network or potentiometer
as long as the total current drawn from the VREF pin is
less than 1mA.
Additionally, the LT3922-1 is capable of interpreting a
digital pulse at the CTRL pin. The high level of the pulse
must be greater than 1.6V. The low level must be less
than 0.4V. The frequency must be greater than 10kHz and
less than 200kHz. Then the regulated voltage between
ISP and ISN will vary with the duty ratio of the pulse as
shown in Figure 2.
In this case, the LED current is zero for duty cycles less
than 12.5% and reaches its maximum above 62.5%. The
LT3922-1 will cease switching if the duty cycle of the
CTRL pin pulse is less than 10%, and also for DC CTRL
pin voltages less than 200mV.
RNTC
LT3922-1
CTRL
CTRL
RCTRL2
RCTRL2
39221 F03
RNTC
Figure 3. Setting CTRL with NTC Resistors
Setting Switching Frequency with the RT Pin
The switching frequency of the LT3922-1 is programmed
by a resistor connected between the RT pin and GND.
Values of the RT resistor from 45.3k up to 499k program
frequencies from 2MHz down to 200kHz as shown in
Table 1. Higher frequencies allow for smaller external
components but increase switching power losses and
radiated EMI.
Table 1. RT Resistance Range
SWITCHING FREQUENCY
RT
2.0 MHz
45.3k
1.6 MHz
57.6k
1.2 MHz
78.7k
1.0 MHz
95.3k
400 kHz
249k
200 kHz
499k
To reduce the LED current when the temperature of the
LEDs rises, use resistors with negative temperature coefficient (NTC) in the network from VREF to CTRL as shown
in Figure 3.
Rev. A
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15
LT3922-1
APPLICATIONS INFORMATION
Synchronizing Switching Frequency
Maximum Duty Cycle
The switching frequency can also be synchronized to
an external clock connected to the SYNC/SPRD pin. The
high-level of the external clock must be at least 1.5V, and
the frequency must be between 200kHz and 2MHz. The
RT resistor is still required in this case, and the resistance
should correspond to the frequency of the external clock.
If the external clock ever stops, the LT3922-1 will rely on
the RT resistor to set the frequency.
The choice of switching frequency should be made knowing that the maximum VOUT voltage of a boost converter
is determined by the maximum duty cycle for a given VIN
voltage as shown in the following equation:
Enabling Spread Spectrum Frequency Modulation
Connecting SYNC/SPRD to INTVCC will enable spread
spectrum frequency modulation (SSFM). The switching
frequency will vary from the frequency set by the RT resistor
to 125% of that frequency. If neither synchronization nor
SSFM is required, connect SYNC/SPRD to GND.
As shown in Figure 4, enabling SSFM can significantly attenuate the electromagnetic interference that the LT3922-1,
like all switching regulators, emits at the switching frequency and its harmonics. This feature is designed to help
devices that include the LT3922-1 perform better in the
various standard industrial tests related to interference.
80
SSFM ON
SSFM OFF
PEAK CONDUCTED EMI (dBµV)
70
60
50
40
30
20
10
0
–10
–20
0
5
10
15
20
FREQUENCY (MHz)
25
30
39221 F04
Figure 4. Typical Conducted Peak EMI of the LT3922-1 with
2MHz Switching Frequency
VOUT =
VIN
(1)
( 1– D )
where D is the duty cycle of the boost converter defined
as the ratio of the on-time of the bottom power switch to
the total switching period. The maximum duty cycle for a
given switching frequency is determined by the minimum
off-time of the bottom power switch. The longest minimum
off-time of the LT3922-1 is 35ns, so the maximum duty
cycle is 93% at 2MHz switching frequency. Therefore, if
an application requires higher duty cycle, the switching
frequency should be set lower to achieve the demanded
duty cycle.
Selecting an Inductor
The LT3922-1 limits the inductor peak current to a minimum of 2.3A over the duty cycle without sub-harmonic
oscillations. This current limit will override the CTRL input
command if the programmed LED current demands higher
inductor peak current than 2.3A. Therefore, it is important to
select the inductor value to ensure the peak inductor current
is below the limit over the desired input voltage range. The
following is an example of inductor value decision process
for the application where we want 300mA LED current at
30V output, while the input ranges from 8V to 25V and the
switching frequency is 2MHz. The maximum peak inductor
current can be derived by adding the half of the inductor
current ripple amplitude to the average inductor current
value, both values of which are determined by the input
and output voltages, switching frequency, efficiency and
the inductor values. Hence, the minimum inductor value
LMIN that ensures the peak inductor current below 2.3A is:
The attenuation varies depending on the chosen switching
frequency, the range of frequencies in which interference is
measured, and whether a test measures peak, quasi-peak,
or average emissions. The results of several other emission
measurements are with select typical application circuits.
(
)
⎛ VIN(MIN) • VOUT – VIN(MIN) ⎞
⎜
⎟
2 • VOUT • fSW
⎜⎝
⎟⎠
LMIN =
VOUT • ILED
⎛
⎞
⎜⎝ 2.3 – V
⎟
IN(MIN) • EFFICIENCY ⎠
(2)
Rev. A
16
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LT3922-1
APPLICATIONS INFORMATION
Using this equation gives an inductance of about 1.4µH
assuming 90% efficiency for the given conditions.
With this minimum inductor value guideline, choose an
inductor with low core loss and low DC resistance. Inductor must be able to handle the peak inductor current
without saturation. To minimize the radiated noise, use a
shielded inductor. The manufacturers featured in Table 2
are recommended sources of inductors.
Table 2. Inductor Manufacturers
MANUFACTURER
WEBSITE
Wurth Electronics
www.we-online.com
Coilcraft
www.coilcraft.com
Vishay Intertechnology
www.vishay.com
Selecting an Input Capacitor
The input capacitor supplies the inductor ripple current
and the transient current that occurs in PWM dimming
operations. A 10µF ceramic capacitor should be sufficient
to provide these non-steady state currents. Place the input capacitor close to the inductor. If possible, place an
additional 1µF ceramic capacitor close to the VIN pin for
better noise immunity. Use X7R or X5R ceramic capacitors
as they typically retain their capacitance better than other
capacitor types over wide voltage and temperature ranges.
If the input power source has high impedance, or there
is significant inductance due to long wires or cables, additional bulk electrolytic capacitance may be necessary. A
low ESR ceramic input capacitor combined with parasitic
inductances in the current paths can form a high-Q LC tank
circuit which can ring the capacitor voltage up to twice
the input voltage. A higher ESR electrolytic capacitor, on
the other hand, minimizes this ringing. Refer to the Linear
Technology Application Note 88 for more information.
Sources of quality ceramic and electrolytic capacitors are
listed in Table 3.
Table 3. Capacitor Manufacturers
MANUFACTURER
WEBSITE
Murata Manufacturing
www.murata.com
Garrett Electronics
www.garrettelec.com
Panasonic
www.industrial.panasonic.com
Nippon Chemi-Con
www.chemi-con.co.jp
Stabilizing the Regulation Loop
The LT3922-1 uses internal error amplifiers to regulate
the LED current and the output voltage to the user programmed values. The output impedance of the error
amplifiers and the external compensation capacitor, CC,
connected to VC pin create the dominant pole of the control
loop. The compensation resistor, RC, in series with CC
forms a left-half-plane (LHP) zero. This LHP zero allows
better regulation of LED current and output voltage during transient operations. For most LED applications, 1nF
and 10k would be good starting values for CC and RC,
respectively. Refer to the Linear Technology Application
Note 76 for more information.
Selecting and Placing Output Capacitors
The output capacitors need to have very low ESR to reduce the output ripple. Placing several low ESR ceramic
capacitors in parallel is an effective way to reduce ESR.
These output capacitors in a boost converter should have a
ripple current rating greater than the half of the maximum
SW pin current. Use X7R or X5R ceramic capacitors as
they typically retain their capacitance better than other
capacitor types over wide voltage and temperature ranges.
The LT3922-1 utilizes a proprietary architecture to reduce
EMI noise generated by switching. To best utilize this
feature, VOUT should be bypassed with three capacitors.
Figure 5 shows the VOUT capacitor placements for the
QFN package. COUT1 and COUT2 are 0402-0.47µF ceramic
capacitors placed as close as possible to the LT3922-1’s
VOUT and GND pins. COUT3 should be larger in size and
value. A 1206-(2.2µF to 22µF) ceramic capacitor is recommended for typical applications.
Rev. A
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17
LT3922-1
APPLICATIONS INFORMATION
The drain source voltage rating of the chosen PMOS
should be greater than the maximum output voltage. Typically the output voltage is a little higher than the sum of
the forward voltages of the LEDs in the string. However,
when the string is broken, the output voltage will begin to
increase due to the imbalance of inductor current and load
current. As described in detail later, the LT3922-1 will not
reduce the inductor current nor limit the output voltage
until the FB pin voltage approaches 1.2V. Therefore, the
maximum output voltage is ultimately determined by the
resistor network between FB and VOUT.
VOUT
VOUT
COUT3
1206
COUT1
0402
25
24
VOUT
23
22
COUT2
0402
21
GND
20
39221 F05
Figure 5. Placement of Output Capacitors
Selecting a MOSFET for PWM Dimming
Pulse-Width-Modulation (PWM) dimming of the LED current is an effective way to control the brightness of the
light without varying its color. The brightness can also
be adjusted with finer resolution this way than by varying
the current level.
The LT3922-1 features a PWMTG driver that is intended
for a high-voltage PMOS switch in position to effectively
PWM dim a string of LEDs from the output capacitor and
the current sense resistor. When the switch is open and
the string is disconnected, the LED current will be zero. In
contrast to a low-side NMOS driver, this feature eliminates
the need for a dedicated return path for the LED current
in automotive applications or other grounded chassis
systems.
The gate driver for this PMOS is supplied through the VOUT
pin. When the PWM pin voltage is greater than 1.4V, the
driver will pull the gate of the PMOS to a maximum of 10V
below the VOUT pin. If VOUT is below 10V, the gate drive
is necessarily reduced. For constant current applications,
leave PWMTG open, connect the load directly after the
current sense resistor, and connect PWM to INTVCC. In
these cases, analog dimming may be implemented with
the CTRL pin.
In most applications, the gate source voltage rating of the
PMOS should be at least 10V. The only exceptions to this
rule are applications for which the output voltage is always
less than 10V. The PWMTG driver will try to pull the gate
of the PMOS down to 10V below VOUT, but it cannot pull
the gate below GND. Therefore, when the maximum output
voltage is less than 10V, the PMOS gate source voltage
rating will be sufficient if it is merely equal to or greater
than the output voltage.
Finally, the drain current rating of the PMOS must exceed
the programmed LED current. Assuming this condition and
the conditions above are met, the only electrical parameter
to be considered is the on-resistance. Other parameters
such as gate charge are less important because PWM dimming frequencies are typically too low for efficiency to be
affected noticeably by gate charging loss or transition loss.
Table 4 lists recommended manufacturers of PMOS
devices.
Table 4. PMOS Manufacturers
MANUFACTURER
WEBSITE
Infineon
www.infineon.com
Vishay Intertechnology
www.vishay.com
Fairchild Semiconductor Corp.
www.fairchildsemi.com
NXP Semiconductors
www.nxp.com
Selecting an RP Resistor for Internal PWM Dimming
If the RP pin is tied to GND, an external pulse-width
modulated signal at the PWM pin will control PWM dimming of the LED load. The signal will enable the PWMTG
driver and turn on the external PMOS device when it is
higher than 1.4V.
Rev. A
18
For more information www.analog.com
LT3922-1
APPLICATIONS INFORMATION
However, the LT3922-1 is capable of PWM dimming even
when an external PWM signal is not available. In this case,
an internal PWM signal with frequency set by a resistor at
the RP pin and duty ratio set by a DC voltage at the PWM
pin will control the PWMTG driver. The RP resistor should
be one of the seven values listed in Table 5. For each of
these values, the PWM frequency is a unique ratio of the
switching frequency.
High PWM Dimming Ratio
Table 5. Internal PWM Dimming Frequencies
The ISMON pin provides an amplified and buffered monitor
of the voltage between the ISP and ISN pins. The gain of
the internal amplifier is ten, and the speed is fast enough
to track the pulse-width modulated LED current. However,
as shown in Figure 6, the ISMON voltage can be filtered
with a resistor-capacitor network to monitor the average
LED current instead.
SWITCHING FREQUENCY
RP
RATIO
2MHz
1MHz
200kHz
28.7k
28
7.81kHz
3.91kHz
781Hz
47.5k
29
3.91kHz
1.95kHz
391Hz
76.8k
210
1.95kHz
977Hz
195Hz
118k
211
977Hz
488Hz
97.7Hz
169k
212
488Hz
244Hz
48.8Hz
237k
213
244Hz
122Hz
24.4Hz
332k
214
122Hz
61Hz
12.2Hz
When using the internal PWM signal, set the voltage at
the PWM pin between 1V and 2V. The PWMTG driver will
stay off if PWM is below 1V, and it will stay on if PWM
is above 2V. Between 1V and 2V there are 128 evenly
spaced thresholds corresponding to 128 discrete PWM
duty ratios from 0% to 100%. This range of 1V to 2V has
been chosen so that the PWM voltage may be set using
a potentiometer or a resistor network and the 2V reference available at the VREF pin. Place a small 1µF ceramic
capacitor near PWM pin to ground.
There is one exception to the above rules for PWM dimming. To avoid excessive start-up times, after the first
PWM pulse, PWMTG will stay on until the SS pin voltage
reaches 1.7V or the LED current has reached approximately
10% of the full-scale current.
The LT3922-1 can drive regulated current pulses with a
duration as short as 400ns. This means the PWM dimming ratio can be 25,000:1 when the external PWM signal
frequency is 100Hz. The PWM dimming ratio can be even
higher as there are no limits on the maximum PWM period.
Monitoring LED Current
LT3922-1
RMON
ISMON
ISMON(FILTERED)
CMON
39221 F06
Figure 6. ISMON Filter Configuration
The resistor should be at least 10k. The capacitance can
be as large or small as needed without affecting the stability of the internal amplifier. For example, when the PWM
frequency is 200Hz, a 10μF capacitor combined with the
10k resistor would limit the ripple on ISMON to 1%.
Selecting the FB Resistors
Two resistors should be selected to form a network between
the output voltage and the FB pin as shown in Figure 7.
VOUT
LT3922-1
RFB2
! R $
VOUT(MAX) = 1.2V • # 1+ FB2 &
" RFB1 %
FB
RFB1
39221 F07
Figure 7. FB Resistor Configuration
Rev. A
For more information www.analog.com
19
LT3922-1
APPLICATIONS INFORMATION
This network forms part of a voltage regulation loop when
FB is near 1.2V. In this case, the LT3922-1 will override
the programmed LED current and adjust the inductor current to lower the output voltage and limit FB to 1.2V. This
resistor configuration therefore determines the maximum
output voltage.
In this way, the LT3922-1 can also be configured as a
voltage regulator instead of an LED driver. It will regulate
the output voltage near the programmed maximum as long
as the load current is less than the current programmed
by CTRL.
Note that this voltage limit may be reached inadvertently
if it is set too close to the typical output voltage and the
output capacitor is too small. To avoid interference with
the current regulation, the feedback resistors should be
chosen such that FB is below 1.14V when the LEDs are
conducting.
Open LED Fault Detection and Response
The resistor network formed by RFB1 and RFB2 also defines
the criteria for the open-LED fault condition. An open-LED
fault is detected when the FB pin voltage is greater than
1.14V and simultaneously the difference between ISP and
ISN pins is less than 10mV. The latter condition ensures
that the output current is low (as it should be in an open
circuit) not just that output voltage is high as it may be
when the LEDs are conducting a large current.
A fault is reported by an internal device pulling the voltage
at the FAULT pin low. There is nothing internal that pulls
this voltage high, so an external resistor between INTVCC
and FAULT is necessary as shown in Figure 8. This configuration allows multiple FAULT pins and similar pins on
other parts to be connected and share a single resistor.
INTVCC
RFAULT
Understanding FB Overvoltage Lockout
LT3922-1
FAULT
Despite the voltage regulation loop, the FB voltage can
temporarily exceed the 1.2V limit. If the output voltage
is near the maximum when the LED string opens, it may
take too long for the feedback loop to adjust the inductor
current and avoid overcharging the output. To quickly
respond to the overvoltage conditions, the LT3922-1 will
immediately stop switching, disconnect the LED string by
shutting the external PMOS off when the FB pin exceeds
the 1.266V FB overvoltage lockout threshold.
The FB overvoltage lockout threshold may be routinely
exceeded when the LT3922-1 is being operated as a voltage regulator if the load current decreases rapidly. In this
case, the pause in switching limits the output overshoot
and ensures that the voltage is back in regulation as
quickly as possible. For safe operation, choose RFB1 and
RFB2 values to ensure the output voltage is not greater
than 40V when the FB voltage is 1.266V.
39221 F08
Figure 8. FAULT Resistor Configuration
Shorted LED Fault Detection and Responses
The LT3922-1 prevents excessive currents that could damage the LED and the driver by three detection schemes
as follows:
1) (VISP – VISN) > 150mV for more than 300µs, or
2) (VISP – VISN) > 700mV (typical), or
3) VOUT < (VIN – 2V)
If the LT3922-1 detects any one of these events, it immediately stops switching, turns off the external PMOS
PWM switch, pulls down FAULT pin, and initiates a fault
response routine using the SS pin. Note that FAULT pin is
held low until the part successfully restarts.
Rev. A
20
For more information www.analog.com
LT3922-1
APPLICATIONS INFORMATION
Soft-Start and Fault Modes
SS PIN (V)
The LT3922-1’s soft-start (SS) pin has two functions. First,
it allows the user to program the output startup voltage
ramp rate through the SS pin. An internal 20µA current
pulls up the SS pin to INTVCC. As shown in Figure 9, connecting an external capacitor CSS at the SS pin to GND will
VINTVCC (3V)
1.7V
INTVCC
RSS
(OPTION FOR LATCH-OFF)
LT3922-1
TIME
DETECTED LED SHORT
SS
CSS
(a) Latchoff Mode
39221 F09
Figure 9. SS Capacitor and Resistor Configuration
generate a linear ramp voltage. This voltage ramp at the
SS pin forces the LT3922-1 to regulate the FB pin voltage
to track the SS pin voltage until VOUT is high enough to
drive the LED at the commanded current level.
The SS pin is also used as a fault timer. After a shorted
LED fault is detected, an internal 2µA current pulls down
the voltage on the SS pin. The user can configure two
different fault response routines by using or not using a
pull-up resistor, RSS, from the SS pin to INTVCC. Figures
10a and 10b illustrate corresponding waveforms of the
SS pin voltage for the two responses: latchoff and hiccup
mode. With a 470k or smaller RSS, the LT3922-1 will latch
off until the user forces a reset by toggling the EN/UVLO
pin. Without the RSS, the LT3922-1 enters a hiccup mode
operation. The 2µA pulls SS pin down to 0.2V, at which
point the 20µA pull-up current turns on again to raise the
SS pin voltage. If the fault condition has not been removed
until the SS pin reaches 1.7V, the 2µA pull-down current
source turns on again to start another cycle. This hiccup
mode will continue until the fault is cleared. A typical CSS
value is 10nF.
Programming EN/UVLO and OVLO Thresholds
The LT3922-1 will stop switching, disable the PWMTG
driver, and reset the soft-start when the voltage at the
EN/UVLO pin drops below 1.33V, or the voltage at the
OVLO pin rises above 1.205V. External voltage sources
can be used to set the voltage at EN/UVLO and OVLO pins
SS PIN (V)
INTVCC (3V)
1.7V
0.2V
TIME
DETECTED LED SHORT
FAULT CLEARED
39221 F10
(b) Hiccup Mode
Figure 10. Fault Responses: (a) Latchoff and (b) Hiccup
to enable or disable the LT3922-1. Alternatively, resistor
networks can be placed from VIN to these pins to set the
operating range of VIN voltage.
For instance, the VIN undervoltage lockout (UVLO) threshold can be accurately set by an external resistor divider.
Figure 11 illustrates how to set the falling EN/UVLO
threshold and the rising hysteresis voltages in LT3922-1.
The internal hysteresis is 25mV, but the user can program
VIN
FALLING THRESHOLD
R1
LT3922-1
! R1$
VIN(UVLO) = 1.33V • # 1+ &
" R2 %
EN/UVLO
RISING HYSTERESIS
R2
39221 F11
! R1$
VHYST(UVLO) = 25mV • # 1+ & +R1• 2µA
" R2 %
Figure 11. EN/UVLO Threshold and Hysteresis Voltages
Rev. A
For more information www.analog.com
21
LT3922-1
APPLICATIONS INFORMATION
additional hysteresis through the external resistor as the
EN/UVLO pin sinks 2µA current when the EN/UVLO pin
voltage is below the threshold.
On the other hand, the VIN overvoltage lockout (OVLO)
threshold can be accurately set by the external resistor
divider as well. Figure 12 illustrates how to set the rising
OVLO threshold in LT3922-1. The internal hysteresis of
the OVLO pin is 50mV.
VIN
RISING THRESHOLD
LT3922-1
R3
! R3 $
VIN(OVLO) = 1.205V • # 1+ &
" R4 %
OVLO
FALLING HYSTERESIS
R4
39221 F12
! R3 $
VHYST(OVLO) = 50mV • # 1+ &
" R4 %
Figure 12. OVLO Threshold and Hysteresis Voltages
Both EN/UVLO and OVLO can be set precisely using a
single resistor string consisting of three series resistors.
Figure 13 shows the resistor string and the threshold and
hysteresis voltages for EN/UVLO and OVLO.
VIN
R5 $
!
VIN(UVLO) = 1.33V • # 1+
" R6+R7 &%
R5
EN/UVLO
R6
LT3922-1
OVLO
R7
39221 F13
R5 $
!
VHYST(UVLO) = 25mV • # 1+
+R5 • 2µA
" R6+R7 &%
! R5+R6 $
VIN(OVLO) = 1.205V • # 1+
&
"
R7 %
! R5+R6 $
VHYST(OVLO) = 50mV • # 1+
&
"
R7 %
Figure 13. EN/UVLO–OVLO Threshold and Hysteresis Voltages
Tie EN/UVLO to VIN and tie OVLO to GND if they are not
used. Do not leave these pins open.
Planning for Thermal Shutdown
The LT3922-1 automatically stops switching when the
internal temperature is too high. The temperature limit is
guaranteed to be higher than the operational temperature
of the part. During thermal shutdown, all switching is terminated, SS is forced low, and the LEDs are disconnected
through the PWMTG driver.
The exposed pad on the bottom of the package must be
soldered to a ground plane. Vias placed directly under the
package are necessary to dissipate heat. Following these
guidelines, the official four-layer demo board DC2247A
reduces the thermal resistance, θJA to 25°C/W. With a
compromised board design, θJA could be 40°C/W or higher.
Designing the Printed Circuit Board (PCB)
The output capacitors COUT1 and COUT2 of the LT3922-1
bypass large switched currents from VOUT to GND (see
Figure 5). The loops travelled by these currents should
be made small as possible to these pins. These output
capacitors, along with the inductor and the input capacitors, should be placed on the same side of the PCB, and
their connections should be made on that layer.
Create a Kelvin ground network by keeping the ground
connection for all of the other components separate.
It should only join the ground for the input and output
capacitors and the return path for the LED current at the
exposed pad.
There are a few other aspects of the board design that
improve performance. An unbroken ground plane on
the second layer dissipates heat, but also reduces noise.
Likewise minimizing the area of the SW and BST nodes
reduces noise. The traces for FB and VC should be kept
short to lessen the susceptibility to noise of these highimpedance nodes. Matched Kelvin connections from the
external current sense resistor to the ISP and ISN pins
are essential for current regulation accuracy. The 2.2μF
INTVCC and 1µF VREF capacitors as well as the 33nF BST
capacitor should be placed as closely as possible to their
respective pins. Use bypass capacitors for the DC input
nodes such as VIN, CTRL, and PWM (for internal PWM) to
reduce noise. Keep the RT and RP nodes small and away
from noisy signals. Finally, a diode with anode connected
to ground and cathode to the drain of the PWMTG MOSFET can protect that device from overvoltage caused by
excessive inductance in the LED string. Please refer to the
demo board layout of the LT3922-1 for more information.
Rev. A
22
For more information www.analog.com
LT3922-1
TYPICAL APPLICATIONS
2MHz, 93% Efficient 10W (30V, 333mA) Boost LED Driver
L1, 4.7µH
VIN
8V TO 27V
33nF
SW
VIN
1M
4.7µF
BST
VOUT
0.47µF
GND
EN/UVLO
0.47µF
365k
VOUT
OVLO
LT3922-1
59.0k
1M
FB
4.7µF
33.2k
GND
VREF
1µF
ISP
100k
ANALOG DIM
PWM DIM
300mΩ
CTRL
PWM
ISN
SYNC/SPRD
INTVCC
100k
PWMTG
FAULT
SS RT
2.2µF
10nF
RP
M1
ISMON
VC
30V
333mA
LED
10k
45.3k
2MHz
1nF
39221 TA02a
Efficiency vs VIN
100
VLED = 30V
fSW = 2MHz
95
EFFICIENCY (%)
100:1 External PWM Dimming
90
ILED
100mA/DIV
85
80
VPWM
2V/DIV
75
100% PWM DUTY CYCLE
70
6
9
12
15
18 21
VIN (V)
24
27
20µs/DIV
30
39221 TA02b
INFINITE PERSISTENCE
VIN = 12V
fPWM = 100Hz
39221 TA02c
Rev. A
For more information www.analog.com
23
LT3922-1
TYPICAL APPLICATIONS
333mA Boost LED Driver Using Internal PWM and Analog CTRL Dimming
L1
4.7µH
VIN
8V TO 27V
33nF
SW
VIN
BST VOUT
1M
4.7µF
0.47µF
GND
EN/UVLO
0.47µF
365k
VOUT
OVLO
LT3922-1
59.0k
1M
FB
4.7µF
33.2k
VCTRL
GND
CTRL
VREF
100k
1µF
0.1µF
(OPTION)
PWM
ISP
97.6k
300mΩ
SYNC/SPRD
ISN
INTVCC
100k
PWMTG
FAULT
SS RT
2.2µF
10nF
RP
45.3k
2MHz
M1
ISMON
VC
332k
122Hz
10k
1nF
30V
333mA
LED
39221 TA03a
L1: WURTH 74437324047
M1: VISHAY Si2319CDS
128:1 Internal PWM Dimming
10:1 Internal PWM Dimming
ILED
100mA/DIV
ILED
100mA/DIV
10µs/DIV
39221 TA03b
200µs/DIV
VCTRL = 2V
VIN = 12V
fPWM = 122Hz
39221 TA03c
VCTRL = 2V
VIN = 12V
fPWM = 122Hz
128:1 Internal PWM with 20:1 Analog CTRL Dimming
ILED
10mA/DIV
10:1 Internal PWM with 20:1 Analog CTRL Dimming
ILED
10mA/DIV
10µs/DIV
39221 TA03d
VCTRL = 0.3V
VIN = 12V
fPWM = 122Hz
200µs/DIV
39221 TA03e
VCTRL = 0.3V
VIN = 12V
fPWM = 122Hz
Rev. A
24
For more information www.analog.com
LT3922-1
TYPICAL APPLICATIONS
333mA Boost LED Driver Using External PWM Dimming and SSFM
L1
4.7µH
VIN
8V TO 27V
33nF
SW
VIN
1M
4.7µF
BST VOUT
0.47µF
GND
EN/UVLO
0.47µF
365k
VOUT
OVLO
LT3922-1
59.0k
1M
FB
4.7µF
33.2k
GND
VREF
CTRL
PWM
1µF
ISP
300mΩ
SSFM
SYNC/SPRD
ISN
INTVCC
PWMTG
100k
FAULT
SS RT
2.2µF
10nF
RP
45.3k
2MHz
M1
ISMON
VC
30V
333mA
LED
10k
1nF
39221 TA04a
L1: WURTH 74437324047
M1: VISHAY Si2319CDS
122Hz 10:1 External PWM Dimming with
and without SSFM
ILED (SSFM)
200mA/DIV
ILED (NO SSFM)
200mA/DIV
2ms/DIV
39221 TA04b
Rev. A
For more information www.analog.com
25
LT3922-1
TYPICAL APPLICATIONS
Low EMI 400kHz, 96% Efficient 10W (30V, 333mA) Boost LED Driver with SSFM
Efficiency vs VIN
L1
22µH
33nF
VIN
8V TO
27V
10µF
0.1µF
0402
SW
VIN
FB1
4.7µF
100
BST VOUT
1M
0.47µF
365k
33µF
VOUT
OVLO
1M
LT3922-1
59.0k
FB
4.7µF
33.2k
1µF
90
85
GND
VREF
VLED = 30V
fSW = 400kHz
95
0.47µF
GND
EN/UVLO
EFFICIENCY (%)
INPUT EMI
FILTER
WITHOUT EMI FILTERS
WITH EMI FILTERS
ISP
100k
2.2µF 300mΩ
CTRL
ANALOG DIM
PWM DIM
80
ISN
PWM
6
8 10 12 14 16 18 20 22 24 26 28
VIN (V)
39221 TA05b
SYNC/SPRD
INTVCC
100k
PWMTG
FAULT
SS
2.2µF
RT
RP
0402
0.1µF
10k
249k
400kHz
10nF
M1
ISMON
VC
D1
FB2
OUTPUT
EMI FILTER
1nF
L1: COILCRAFT XAL5050-223MEB
M1: VISHAY Si2319CDS
FB1: WURTH 742792040
FB2: WURTH 742792097
D1: NXP PMEG4010CEJ
30V
333mA
LED
39221 TA05a
50
45
40
35
30
25
20
15
10
5
0
–5
–10
–15
Average Radiated EMI Performance (CISPR25)
AMPLITUDE (dBµV/m)
AMPLITUDE (dBµV/m)
Peak Radiated EMI Performance (CISPR25)
CLASS 5 PEAK LIMIT
LT3922-1 400kHz f SW PEAK EMI
0
100
200
300
400
500
600
FREQUENCY (MHz)
700
800
900
1000
50
45
40
35
30
25
20
15
10
5
0
–5
–10
–15
CLASS 5 AVERAGE LIMIT
LT3922-1 400kHz f SW AVERAGE EMI
0
100
39221 TA05c
200
300
400
500
600
FREQUENCY (MHz)
700
800
900
1000
39221 TA05d
Rev. A
26
For more information www.analog.com
LT3922-1
TYPICAL APPLICATIONS
500mA Boost LED Driver Using Pulse Duty Cycle CTRL Input
L1
6.8µH
VIN
8V TO 20V
33nF
SW
VIN
4.7µF
1M
BST VOUT
0.47µF
GND
EN/UVLO
0.47µF
348k
VOUT
OVLO
LT3922-1
84.5k
1M
FB
4.7µF
33.2k
1µF
3V, 10kHz PULSE
VREF
GND
PWM
CTRL
ISP
200mΩ
SYNC/SPRD
ISN
INTVCC
PWMTG
100k
FAULT
SS RT
2.2µF
10nF
RP
45.3k
2MHz
M1
ISMON
VC
10k
1nF
L1: WURTH 74437324068
M1: VISHAY Si2319CDS
VCTRL Duty Cycle Stepped from 15% to 75%
39221 TA06a
VCTRL Duty Cycle Stepped from 75% to 15%
ILED
200mA/DIV
ILED
200mA/DIV
VCTRL
2V/DIV
VCTRL
2V/DIV
1ms/DIV
24V
500mA
LED
39221 TA06b
1ms/DIV
39221 TA06c
Rev. A
For more information www.analog.com
27
LT3922-1
TYPICAL APPLICATIONS
2MHz, 95% Efficient 15W (15V, 1A) Buck Mode LED Driver
L1
4.7µH
33nF
VIN
22V TO 36V
VIN
20V TO 36V
BST
VIN
SW
VOUT
1M
33µF
EN/UVLO
0.47µF
51.1k
VOUT
1µF
OVLO
249k
LT3922-1
34.8k
20k
20k
Q1
FB
VREF
1µF
0.47µF
GND
22µF
33.2k
100k
GND
ANALOG DIM
CTRL
PWM DIM
PWM
ISP
100mΩ
SYNC/SPRD
ISN
INTVCC
100k
PWMTG
FAULT
SS RT
2.2µF
L1: WURTH 74437324047
M1: VISHAY Si2319CDS
Q1: ZETEX FMMT591A
RP
45.3k
2MHz
100nF
M1
ISMON
VC
4.7nF
15V
1A
LED
39221 TA07a
Efficiency vs VIN
100
EFFICIENCY (%)
95
90
85
80
75
70
16
20
24
28
VIN (V)
32
36
40
39221 TA07b
Rev. A
28
For more information www.analog.com
LT3922-1
TYPICAL APPLICATIONS
500mA, 5V to 12V Boost Converter with Accurate Input Current Limit
56mΩ
VIN
5V
2.2µF
L1
4.7µH
33nF
SW
VIN
499k
BST VOUT
0.47µF
GND
EN/UVLO
100
95
0.47µF
121k
VOUT
150k
90
549k
LT3922-1
FB
10µF
60.4k
GND
VREF
ISN
100k
EFFICIENCY (%)
OVLO
1µF
Efficiency
VOUT
12V
500mA
85
80
75
70
ISP
ANALOG DIM
CTRL
PWM DIM
PWM
65
SYNC/SPRD
60
INTVCC
PWMTG
100k
2.2µF
L1: WURTH 744316470
100
200
300
ILOAD (mA)
400
500
39221 TA08b
ISMON
FAULT
SS RT
10nF
0
RP
45.3k
2MHz
VC
10k
1nF
39221 TA08a
Rev. A
For more information www.analog.com
29
LT3922-1
TYPICAL APPLICATIONS
Shorted LED Robust Boost LED Driver
L1
4.7µH
VIN
8V TO 18V
SW
VIN
BST VOUT
1M
4.7µF
Shorted LED Protection without RSS:
Hiccup Mode
33nF
GND
EN/UVLO
226k
0.47µF
VPWMTG
20V/DIV
0.47µF
VSS
2V/DIV
VOUT
OVLO
LT3922-1
80.6k
1M
FB
VFAULTB
2V/DIV
4.7µF
33.2k
VREF
1µF
CTRL
PWM DIM
PWM
20ms/DIV
ISP
100k
ANALOG DIM
ILED
1A/DIV
GND
200mΩ
ISN
Shorted LED Protection with RSS:
Latchoff Mode
SYNC/SPRD
INTVCC
100k
RSS
100k
OPTIONAL
PWMTG
FAULT
SS RT
2.2µF 10nF
L1: WURTH 74437324047
M1: VISHAY Si2319CDS
D1: NXP PMEG4010CEJ
39221 TA09b
ISMON
VC
RP
45.3k
2MHz
M1 D1
332k
122Hz
10k
1nF
21V
500mA
LED
VPWMTG
20V/DIV
VSS
2V/DIV
VFAULTB
2V/DIV
39221 TA09a
ILED
1A/DIV
100ms/DIV
39221 TA09c
Rev. A
30
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LT3922-1
TYPICAL APPLICATIONS
2MHz, 88% Efficient 6.25W (12.5V, 500mA) Buck-Boost Mode LED Driver
L1
4.7µH
VIN
6V TO
18V
C4, 33nF
SW
VIN
C2
10µF
EN/UVLO
C3
1µF
VOUT
C11
0.47µF
GND
OVLO
C10
0.47µF
VOUT
LT3922-1
R3
249k
R4
R5
20k
20k
Q1
FB
VREF
C5
1µF
C9
10µF
R6
39.2k
GND
CTRL
ISP
R7
0.2Ω
SYNC/SPRD
PWM
ISN
INTVCC
R1
100k
PWMTG
SS
M1
ISMON
FAULT
C6
2.2µF
RT
RP
VC
R8
10k
R2
45.3k
2MHz
C7
10nF
C8
1nF
12.5V
500mA
LED
39221 TA10a
L1: WURTH 7443551470
M1: VISHAY Si2319DS
Q1: ZETEX FMMT591A
Efficiency vs VIN
100
95
90
EFFICIENCY (%)
C1
33µF
BST
85
80
75
70
65
60
4
6
8
10
12 14
VIN (V)
16
18
20
39221 TA10b
Rev. A
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31
LT3922-1
PACKAGE DESCRIPTION
UFD Package
28-Lead Plastic QFN (4mm × 5mm)
(Reference LTC DWG # 05-08-1712 Rev C)
0.70 ±0.05
4.50 ±0.05
3.10 ±0.05
2.50 REF
2.65 ±0.05
3.65 ±0.05
PACKAGE OUTLINE
0.25 ±0.05
0.50 BSC
3.50 REF
4.10 ±0.05
5.50 ±0.05
RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS
APPLY SOLDER MASK TO AREAS THAT ARE NOT SOLDERED
4.00 ±0.10
(2 SIDES)
R = 0.05
TYP
0.75 ±0.05
PIN 1 NOTCH
R = 0.20 OR 0.35
× 45° CHAMFER
2.50 REF
R = 0.115
TYP
27
28
0.40 ±0.10
PIN 1
TOP MARK
(NOTE 6)
1
2
5.00 ±0.10
(2 SIDES)
3.50 REF
3.65 ±0.10
2.65 ±0.10
(UFD28) QFN 0816 REV C
0.200 REF
0.00 – 0.05
0.25 ±0.05
0.50 BSC
BOTTOM VIEW—EXPOSED PAD
NOTE:
1. DRAWING PROPOSED TO BE MADE A JEDEC PACKAGE OUTLINE MO-220 VARIATION (WGHD-3).
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
Rev. A
32
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LT3922-1
REVISION HISTORY
REV
DATE
DESCRIPTION
A
02/21
Updated LED current regulation to 2.5% accuracy
PAGE NUMBER
1
Added AEC-Q100 Qualification and automotive model order information
1, 2
Changed BST pin capacitor to 33 nF
1, 10, 12, 21-29, 30, 32
Clarified Absolute Maximum Ratings
2
Clarified VREF Voltage conditions
Revised Electrical Characteristic values
3
3, 4
Rev. A
Information furnished by Analog Devices is believed to be accurate and reliable. However, no responsibility is assumed by Analog
Devices for its use, nor for any infringements of patents or other rights of third parties that may result from its use. Specifications
subject to change without notice. No license For
is granted
implication or
otherwise under any patent or patent rights of Analog Devices.
moreby
information
www.analog.com
33
LT3922-1
TYPICAL APPLICATION
Low EMI 2MHz, 333mA Boost LED Driver with SSFM
L1
4.7µH
0.1µF
0402
SW
VIN
FB1
33µF
4.7µF
BST VOUT
1M
0.47µF
GND
EN/UVLO
AMPLITUDE (dBµV/m)
VIN
8V TO
27V
Peak Radiated EMI Performance (CISPR25)
33nF
INPUT EMI
FILTER
0.47µF
365k
VOUT
OVLO
1M
LT3922-1
59.0k
FB
2.2µF
33.2k
GND
VREF
1µF
ISP
100k
ANALOG DIM
CTRL
PWM DIM
PWM
300mΩ
2.2µF
50
45
40
35
30
25
20
15
10
5
0
–5
–10
–15
ISN
CLASS 5 PEAK LIMIT
LT3922-1 2MHz f SW PEAK EMI
0
100
200
300
SYNC/SPRD
400
500
600
FREQUENCY (MHz)
700
800
900
1000
39221 TA11b
INTVCC
SS
2.2µF
M1
ISMON
FAULT
RT
100nF
RP
45.3k
2MHz
VC
332k
122Hz
24k
0402
0.1µF
D1
0.22nF
L1: WURTH 74437324047
M1: VISHAY Si2319CDS
FB1: WURTH 7427920415
FB2: WURTH 742792097
D1: NXP PMEG4010 CEJ
Average Radiated EMI Performance (CISPR25)
FB2
OUTPUT
EMI FILTER
30V
333mA
LED
39221 TA11a
AMPLITUDE (dBµV/m)
PWMTG
100k
50
45
40
35
30
25
20
15
10
5
0
–5
–10
–15
CLASS 5 AVERAGE LIMIT
LT3922-1 2MHz f SW AVERAGE EMI
0
100
200
300
400
500
600
FREQUENCY (MHz)
700
800
900
1000
39221 TA11c
RELATED PARTS
PART NUMBER
DESCRIPTION
COMMENTS
LT3922
36V, 2A Synchronous Step-Up LED Driver
VIN = 2.8V to 36V, VOUT(MAX) = 40V, 128:1 Internal Dimming and 5,000:1 External
Dimming, ISD = 1µA, 4mm × 5mm QFN-28
LT3932/LT3932-1 36V, 2A Synchronous Step-Down LED Driver
VIN = 3.6V to 36V, VOUT = 0V to 36V, 128:1 Internal Dimming and 5,000:1/10,000+:1
External Dimming, ISD = 1µA, 4mm × 5mm QFN-28
LT3952
60V, 4A LED Driver with 4000:1 PWM Dimming with VIN: 3V to 42V, VOUT(MAX) = 60V, 4000:1 PWM, 20:1 Analog, ISD < 1µA,
TSSOP-28E Package
Spread Spectrum
LT3518
2.3A, 2.5MHz High Current LED Driver with 3000:1 VIN: 3V to 30V, VOUT(MAX) = 45V, 3000:1 PWM Dimming, ISD < 1μA,
4mm × 4mm QFN-16 and TSSOP-16E Packages
Dimming with PMOS Disconnect FET Driver
LT3755/LT3755-1/ 40VIN, 75VOUT, 1MHz Non-Synchronous Boost LED VIN: 4.5V to 40V, VOUT: VIN to 75V, ±4% Current Accuracy, 3mm × 3mm
Controller
QFN-16 and MSE-16
LT3755-2
LT3761
60VIN, 80VOUT, 1MHz Non-Synchronous Boost LED VIN: 4.5V to 60V, VOUT: VIN to 80V, ±3% Current Accuracy, External and Internal
Controller with Internal PWM Generator
PWM Dimming, MSE-16
LT3763
60V, 1MHz Synchronous Buck LED Controller
LT3795
110V, 1MHz Non-Synchronous Boost LED Controller VIN: 4.5V to 110V, VOUT: VIN to 110V, ±3% Current Accuracy, Internal
Spread Spectrum, TSSOP-28
with Spread Spectrum Frequency Modulation
LT8391
60V, 650kHz Synchronous 4-Switch Buck-Boost LED VIN: 4V to 60V, VOUT: 0V to 60V, ±3% Current Accuracy, External and Internal
PWM Dimming, Spread Spectrum, TSSOP-28 and 4mm × 5mm QFN-28
Controller with Spread Spectrum
VIN: 6V to 60V, VOUT: 0V to VIN –2V, ±6% Current Accuracy, TSSOP-28
Rev. A
34
02/21
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