Linear LT3755-2 36v, 2a synchronous step-up led driver Datasheet

LT3922
36V, 2A Synchronous
Step-Up LED Driver
Features
Description
±2% LED Current Regulation
nn ±2% Output Voltage Regulation
nn 5000:1 PWM Dimming at 100Hz
nn 128:1 Internal PWM Dimming
nn Spread-Spectrum Frequency Modulation
nn Silent Switcher® Architecture for Low EMI
nn Operates in Boost, Buck Mode and Buck-Boost Mode
nn 2.8V to 36V Input Voltage Range
nn Up to 34V LED String Voltage
nn 2A, 40V Internal Switches
nn 200kHz to 2MHz with SYNC Function
nn Analog or Duty Cycle LED Current Control
nn Open/Short LED Protection and Fault Indication
nn Thermally Enhanced 28-Pin (4mm × 5mm) QFN
The LT®3922 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 will
maintain ±2% current regulation through an external sense
resistor over a wide range of output voltages.
nn
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 also includes a driver for an external highside PFET for PWM dimming and an internal PWM signal
generator for analog control of PWM dimming when an
external signal is not available.
Applications
nn
nn
Automotive and Industrial Lighting
Machine Vision
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.
L, LT, LTC, LTM, Linear Technology, the Linear logo and Silent Switcher are registered
trademarks of Linear Technology Corporation. All other trademarks are the property of their
respective owners. Protected by U.S. Patents, including 7199560, 7321203, and other patents
pending.
Typical Application
2MHz, 93% Efficient 10W (30V, 333mA) Boost LED Driver
L1
4.7µH
Efficiency vs VIN
100
0.1µF
VLED = 30V
fSW = 2MHz
95
SW
VIN
1M
4.7µF
BST
VOUT
0.47µF
GND
EN/UVLO
EFFICIENCY (%)
VIN
8V TO 27V
0.47µF
365k
VOUT
OVLO
LT3922
59.0k
1M
FB
75
100% PWM Duty Cycle
70
ISP
100k
CTRL
PWM DIM
PWM
80
4.7µF
33.2k
ANALOG DIM
85
GND
VREF
1µF
90
300mΩ
6
9
12
15
18 21
VIN (V)
24
27
30
3922 TA01b
ISN
SYNC/SPRD
INTVCC
100k
PWMTG
FAULT
SS RT
2.2µF
10nF
RP
45.3k
2MHz
M1
ISMON
VC
10k
30V
333mA
LED
1nF
3922 TA01a
3922f
For more information www.linear.com/LT3922
1
LT3922
Absolute Maximum Ratings
Pin Configuration
(Note 1)
Order Information
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
9 10 11 12 13 14
ISN
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.............................................................................40V
BST............................................................................43V
BST – SW....................................................................3V
INTVCC, VREF, ISMON, PWMTG, RT, and RP....... (Note 2)
Operating Junction Temperature Range (Notes 3, 4)
LT3922E/LT3922I................................... –40 to 125°C
LT3922H................................................. –40 to 150°C
Storage Temperature Range.......................–60 to 150°C
UFD PACKAGE
28-LEAD (4mm × 5mm) PLASTIC QFN
θJA = 34°C/W
EXPOSED PAD (PIN 29) IS GND, MUST BE SOLDERED TO PCB
http://www.linear.com/product/LT3922#orderinfo
LEAD FREE FINISH
TAPE AND REEL
PART MARKING*
PACKAGE DESCRIPTION
TEMPERATURE RANGE
LT3922EUFD#PBF
LT3922EUFD#TRPBF
3922
28-Lead (4mm × 5mm) Plastic QFN
–40°C to 125°C
LT3922IUFD#PBF
LT3922IUFD#TRPBF
3922
28-Lead (4mm × 5mm) Plastic QFN
–40°C to 125°C
LT3922HUFD#PBF
LT3922HUFD#TRPBF
3922
28-Lead (4mm × 5mm) Plastic QFN
–40°C to 150°C
Consult LTC Marketing for parts specified with wider operating temperature ranges. *The temperature grade is identified by a label on the shipping container.
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/. Some packages are available in 500 unit reels through
designated sales channels with #TRMPBF suffix.
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
Input Voltage Range
VIN Pin Quiescent Current
MIN
2.8
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
MAX
VEN/UVLO = 1.2V
36
V
4
1
mA
µA
1.330
1.400
Input OVLO Falling Hysteresis
1.205
µA
1.265
50
VOVLO = 1.0V
–100
V
mV
2
1.145
UNITS
2.9
25
Input OVLO Threshold (Rising)
OVLO Pin Current
TYP
V
mV
100
nA
3922f
2
For more information www.linear.com/LT3922
LT3922
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
UNITS
1.97
1.985
2
2
2.03
2.015
V
V
Reference
VREF Voltage
IVREF = 0µA
IVREF = 500µA
VREF Pin Current Limit
VREF = 0V, Current Out of Pin
l
3.2
mA
LED Current Regulation
CTRL-Off Threshold (Falling)
l
200
CTRL-Off Rising Hysteresis
210
220
15
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
−100
l
l
l
98
48.5
4
100
50
5
mV
mV
100
nA
102
51.5
6
mV
mV
mV
ISP Pin Current
VISP = 24.1V, VISN = 24V, VCTRL = 2V
75
µA
ISN Pin Current
VISP = 24.1V, VISN = 24V , VCTRL = 2V
75
µA
Current Error Amplifier Transconductance
VISP = 24V
140
µA/V
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
mV
mV
mV
0.4
V
200
kHz
1.6
V
CTRL Pulse Input Low (VIL)
CTRL Pulse Input Frequency Range
10
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
3.3
20
V
mA
Power Stage
Peak Current Limit
Bottom Switch Minimum Off-Time
l
2.0
2.3
2.6
A
15
25
35
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
210
1880
175
1.4
SYNC/SPRD Falling Hysteresis
0.2
VSYNC/SPRD = 5V
kHz
kHz
2650
306
kHz
kHz
75
SYNC/SPRD Threshold (Rising)
SYNC/SPRD Pin Current
2120
245
−100
µA
1.5
V
V
100
nA
3922f
For more information www.linear.com/LT3922
3
LT3922
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
UNITS
Soft-Start
SS Pin Charging Current
VSS = 1V
SS Pin Discharging Current
VSS = 2V
20
µA
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
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
1.140
1.163
V
50
mV
150
mV
0.8
mA
−100
100
nA
Overvoltage Protection
FB Overvoltage Threshold (Rising)
l
1.240
FB Overvoltage Falling Hysteresis
1.266
1.292
22
V
mV
LED Current Monitor
ISMON Voltage
VISP − VISN = 100mV (100%), VISP = 24V
VISP − VISN = 10mV (10%), VISP = 24V
l
l
0.980
80
1.000
100
1.020
120
9
10
11
V
mV
PWM Driver
PWMTG Gate Drive (VOUT – VPWMTG)
VOUT = 20V, VPWM = 1.5V
PWM Threshold (Rising)
1.4
PWM Falling Hysteresis
V
0.2
PWM Pin Current
VPWM = 2V
−100
PWM to PWMTG Propagation Delay
Turn-On
Turn-Off
CPWMTG = 2.1nF (Connected from VOUT to PWMTG)
VOUT = 20V
V
V
100
110
140
nA
ns
ns
Internal PWM Dimming
PWM Voltage for 100% PWM Dimming
RP = 28.7k, VREF = 2V
2.00
V
PWM Voltage for 0% PWM Dimming
RP = 28.7k, VREF = 2V
0.99
V
PWM Dimming Accuracy
RP = 28.7k, VREF = 2V, VPWM = 1.1V
RP = 28.7k, VREF = 2V, VPWM = 1.9V
7.5
89
10.5
92
13.5
95
%
%
PWM Dimming Frequency
RP = 28.7k, RT = 45.3k, VSYNC/SPRD = 0V
RP = 332k, RT = 45.3k, VSYNC/SPRD = 0V
7.34
115
7.81
122
8.28
129
kHz
Hz
RP Pin Current Limit
VRP = 0V, Current Out of Pin
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 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
65
µA
LT3922I is guaranteed to meet performance specifications over the
−40°C to 125°C operating junction temperature range. The LT3922H 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.
3922f
4
For more information www.linear.com/LT3922
LT3922
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
2.50
2.00
1.50
1.00
0.50
3.60
5
1.00
0.50
3.2
VIN = 36V
VIN = 12V
VIN = 2.7V
3.20
3.00
2.80
2.60
2.40
5
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
3922 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
3922 G06
2.01
VREF VOLTAGE (V)
INTVCC VOLTAGE (V)
3.05
2.95
2.9
3922 G05
INTVCC Load Regulation
3.00
3.0
2.5
–50 –25
30 55 80 105 130 155
TEMPERATURE (°C)
3922 G04
3.10
VIN = 36V
VIN = 12V
VIN = 2.7V
2.6
2.00
–45 –20
30 55 80 105 130 155
TEMPERATURE (°C)
30 55 80 105 130 155
TEMPERATURE (°C)
INTVCC Voltage
3.1
2.20
0
–45 –20
5
3922 G03
INTVCC VOLTAGE (V)
1.50
1.15
1.05
–45 –20
30 55 80 105 130 155
TEMPERATURE (°C)
VIN Quiescent Current
3.40
VIN QUIESCENT CURRENT (mA)
VIN SHUTDOWN CURRENT (µA)
2.00
1.20
3922 G02
VIN Shutdown Current
VIN = 36V
VIN = 12V
VIN = 2.7V
1.25
1.10
3922 G01
2.50
RISING
FALLING
1.30
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)
3922 G08
1.97
0
200
400
600
800
VREF LOAD CURRENT (µA)
1000
3922 G09
3922f
For more information www.linear.com/LT3922
5
LT3922
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
SS 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)
3922 G16
8320
INTERNAL PWM FREQUENCY (Hz)
2100
RT = 45.3k
RT = 499k
5
30 55 80 105 130 155
TEMPERATURE (°C)
3922 G15
Internal PWM Frequency
260
5
SS HIGH
SS LOW
3922 G14
2200
1400
–45 –20
1.2
0
–45 –20
30 55 80 105 130 155
TEMPERATURE (°C)
3922 G13
1500
30 55 80 105 130 155
TEMPERATURE (°C)
SS Threshold Voltages
25
70
5
3922 G12
80
CURRENT (µA)
MAXIMUM PIN CURRENT (µA)
10
3922 G11
RT and RP Pin Current Limits
SWITCHING FREQUENCY (kHz)
15
0
–45 –20
30 55 80 105 130 155
TEMPERATURE (°C)
3922 G10
20
5
INTVCC UVLO
VREF UVLO
Internal PWM Duty Cycle
RT = 45.3k
7936
134
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)
3922 G17
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
10.2
10.1
10.0
–45 –20
5
30 55 80 105 130 155
TEMPERATURE (°C)
3922 G18
3922f
6
For more information www.linear.com/LT3922
LT3922
Typical Performance Characteristics
VISP – VISN vs VCTRL
125
75
50
25
0
PULSE FREQUENCY AT CTRL PIN = 20kHz
75
50
25
0
0.25 0.50 0.75 1 1.25 1.50 1.75
VCTRL (V)
0
2
0
150
100
120
80
40
0
98.6
99.3
100
100.7
ISP–ISN VOLTAGE (mV)
VCTRL = 2V
VISP = 24V
3922 G22
400
fSW = 2MHz
L = 4.7µH
ILED (mA)
2.3
2.2
2.1
400
PEAK SW CURRENT LIMITED
1.9
300
300
250
250
200
10
20
30
40 50 60 70
DUTY CYCLE (%)
80
90 100
3922 G25
50
fSW = 2MHz
RSNS = 0.3Ω
11 LEDs (VOUT ~33V)
0
3
6
9
20
12 15 18 21 24 27 30
VIN (V)
3922 G26
40
60
VISP – VISN (mV)
80
100
3922 G24
ILED vs VOUT
VFB OVERVOLTAGE PROTECTION LIMITED
200
VIN = 8V
fSW = 2MHz
RSNS = 0.3Ω
RFB_TOP = 1M
RFB_BOT = 34.8k
150
100
12 LEDs (VOUT ~ 36V)
6 LEDs (VOUT ~ 18V)
0
350
150
2.0
400
3922 G23
ILED vs VIN
350
2.4
1.22
3922 G21
600
0
5.8
ILED (mA)
2.5
4.6
5
5.4
ISP–ISN VOLTAGE (mV)
VCTRL = 0.3V
VISP = 24V
Peak SW Current Limit
2.6
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)
NUMBER OF UNITS
NUMBER OF UNITS
200
1.18
ISMON Voltage
1000
150°C
25°C
–40°C
160
50
PEAK SW CURRENT (A)
0
1.17
12.5 25 37.5 50 62.5 75 87.5 100
DCTRL (%)
5% Current Regulation
N = 355
250
50
25
200
150°C
25°C
–40°C
300
75
3922 G20
100% Current Regulation
350
VISP – VISN vs VFB
100
3922 G19
1.8
125
100
VISP – VISN (mV)
VISP – VISN (mV)
100
VISP – VISN vs DCTRL
VISP – VISN (mV)
125
VIN = 12V, TA = 25°C, unless otherwise noted.
100
50
9
15
21
27
VOUT (V)
33
39
3922 G27
3922f
For more information www.linear.com/LT3922
7
LT3922
Typical Performance Characteristics
RISING
FALLING
1.10
1.05
1.00
0.95
0.90
–45 –20
5
VISP – VISN SHORTLED Threshold
170
160
150
140
130
120
–45 –20
30 55 80 105 130 155
TEMPERATURE (°C)
5
ILED = 400mA
11 LEDs (VOUT ~ 33V)
96
94
94
88
86
84
80
6
50
92
90
88
86
80
8 10 12 14 16 18 20 22 24 26 28 30
VIN (V)
PWMTG ON Voltage
0
200
400
600
ILED (mA)
800
1.21
1.20
1.18
–45 –20
1000
FB OVLO Threshold
1.33
TURN OFF
TURN ON
PROPAGATION DELAY (ns)
8
30 55 80 105 130 155
TEMPERATURE (°C)
3922 G33
PWM Driver Propagation Delay
9
5
3922 G32
200
12
10
30 55 80 105 130 155
TEMPERATURE (°C)
1.19
6 LED (VOUT ~ 18V)
9 LED (VOUT ~ 27V)
12 LED (VOUT ~ 36V)
82
11
5
1.22
3922 G31
VOUT – VPWMTG (V)
100
Regulated FB Voltage
VIN = 12V
fSW = 2MHz
84
fSW = 400kHz
fSW = 2MHz
82
150
1.23
FB VOLTAGE (V)
96
90
200
3922 G30
Efficiency vs ILED
98
EFFICIENCY (%)
EFFICIENCY (%)
100
92
250
3922 G29
Efficiency vs VIN
98
TOP SWITCH
BOTTOM SWITCH
300
0
–45 –20
30 55 80 105 130 155
TEMPERATURE (°C)
3922 G28
100
Power Switch On-Resistance
350
FB OVLO THRESHOLD VOLTAGE (V)
1.15
VISP – VISN SHORTLED THRESHOLD (mV)
FB OPENLED THRESHOLD (V)
180
POWER SWITCH ON–RESISTANCE (mΩ)
FB OPENLED Threshold
1.20
VIN = 12V, TA = 25°C, unless otherwise noted.
160
120
80
RISING
FALLING
1.30
1.27
1.24
1.21
CPWMTG = 2.2nF (C0G type)
7
–45 –20
5
30 55 80 105 130 155
TEMPERATURE (°C)
3922 G34
40
–45 –20
5
30 55 80 105 130 155
TEMPERATURE (°C)
3922 G35
1.18
–45 –20
5
30 55 80 105 130 155
TEMPERATURE (°C)
3922 G36
3922f
8
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LT3922
Typical Performance Characteristics
C/10 Threshold
Case Temperature Rise
35
14
12
10
8
6
–45 –20
5
30 55 80 105 130 155
TEMPERATURE (°C)
25
15
10
5
0
250
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%)
150°C
25°C
–40°C
DC2247A DEMO BOARD
VLED = 30V
I LED = 333mA
TROOM = 25°C
30
3922 G37
300
Internal PWM Duty Cycle (10%)
350
NUMBER OF UNITS
RISING
FALLING
16
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
3922 G39
3922 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
5ms/DIV
3922 G41
3922 G42
FRONT PAGE APPLICATION
6.5V to 18V INPUT VOLTAGE TRANSIENT
VLED = 30V
ILED = 333mA
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
3922 G40
Turn ON and OFF Performance
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
3922 G43
3922 G44
5ms/DIV
FRONT PAGE APPLICATION WITH PWM = 1.1V
VLED = 30V
ILED = 333mA
3922 G45
5ms/DIV
FRONT PAGE APPLICATION WITH PWM = 1.5V
VLED = 30V
ILED = 333mA
3922f
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9
LT3922
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 100nF 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 can be set in two different fault modes for the shorted LED conditions: hiccup
(no resistor) and latchoff (100k). Refer to the Application
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.
3922f
10
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LT3922
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) > 0.7V, 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
Application 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 Application Information
section for the recommended capacitor placements.
GND (Pin 22, 23, 25, 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.
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11
LT3922
Block Diagram
VIN
4.7µH
10µF
2.2µF
3
VIN
2
INTVCC
0.1µF
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
SYNC/SPRD
17
–
V/I
CONVERTER
+
+
1.4V
0.47µF
VOUT
200kHz TO
2MHz
OSCILLATOR
VOUT – 10V
REGULATOR
PWMTG
S/H
PWMTG
DRIVER
gm = 140µA/V
CURRENT
REGULATION
AMPLIFIER
200mΩ
21
–
45.3k
4.7µF
23
+
–
+
1µF
0.47µF
22
20
S/H
8
+
+
–
A/D
DETECTOR
+
–
CONTROL
BUFFER
PWM
19
2.5k
2.5k
ISMON
INTERNAL
PWM SIGNAL
14
25k
10×
gm = 1000µA/V
VOLTAGE
REGULATION
AMPLIFIER
+
0.25V
–
CTRL
+
–
1.25V
ISN
10
ISP
+
+
–
9
12
1.205V
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
3922 BD
10k
1nF
3922f
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LT3922
Operation
The LT3922 is a step-up LED driver that utilizes a fixedfrequency peak current control to accurately regulate the
current through a string of LEDs. It includes two power
switches, their drivers, and a diode for providing power
to the top switch driver. The switches connect an external
inductor at 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 by
adjusting the duty ratio of the power switches 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), which reduces radiated and
conducted electromagnetic interference (EMI).
The bottom switch is turned off by the peak current comparator which waits during the on-time for the increasing
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 target for 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. After each pulse,
when the PMOS switch opens, the LT3922 preserves the
voltages of the capacitors at VC and VOUT to ensure a rapid
recovery for the next pulse.
Applications Information
The following is a guide to selecting the external components and configuring the LT3922 according to the
requirements of an application.
Programming LED Current with the CTRL Pin
The primary function of the LT3922 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
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13
LT3922
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
3922 F02
Figure 2. Duty Cycle CTRL Range
0
0.25V
0.75V
1.25V
1.5V
VCTRL
VREF
VREF
3922 F01
RCTRL1
Figure 1. Analog CTRL Range
LT3922
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 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 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
CTRL
CTRL
RCTRL2
RCTRL2
3922 F03
RNTC
Figure 3. Setting CTRL with NTC Resistors
Setting Switching Frequency with the RT Pin
The switching frequency of the LT3922 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.
3922f
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LT3922
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 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,
like all switching regulators, emits at the switching frequency and its harmonics. This feature is designed to
help devices that include the LT3922 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
3922 F04
Figure 4. Typical Conducted Peak EMI of the LT3922 with 2MHz
Switching Frequency
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.
VOUT =
VIN
(1–D)
(1)
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 typical minimum
off-time of the LT3922 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 limits the inductor peak current to a minimum
of 2A 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 2A. 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 2A is:
(
)
 VIN(MIN) • VOUT – VIN(MIN) 


2 • VOUT • fSW


LMIN =
VOUT • ILED


 2 – V

IN(MIN) • EFFICIENCY 
(2)
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15
LT3922
Applications Information
Using this equation gives an inductance of about 2µH 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 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 ESRlow 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 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 ceramic capacitors as they typically
retain their capacitance better than other capacitor types
over wide voltage and temperature ranges.
The LT3922 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’s
VOUT and GND pins. COUT3 should be larger in size and
value. A 1206-4.7µF ceramic capacitor is recommended
for typical applications.
3922f
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LT3922
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 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
3922 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 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.
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17
LT3922
Applications Information
However, the LT3922 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.
PWM Dimming with Very Long Off Times
To enhance PWM dimming, the VOUT and VC pins are driven
when the PWM pulse (internal or external) is at a logic low
to maintain the charge on the capacitors at those pins.
Consequently, when PWM returns to a logic high state,
the LED current can quickly reach the regulated level even
if PWM was low for a very long time.
Monitoring LED Current
Table 5. Internal PWM Dimming Frequencies
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 are a couple of exceptions to the above rules for
PWM dimming. First, once initiated, the PWM on-time
will last at least four switching cycles regardless of the
signal at the PWM pin and the resistor at the RP pin. This
ensures that the current regulation loop has enough time
to reach equilibrium but still allows for a 5000:1 dimming
ratio when the external PWM frequency is 100Hz and the
switching frequency is 2MHz. Second, 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 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.
LT3922
RMON
ISMON
ISMON(FILTERED)
CMON
3922 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
RFB2
FB
RFB1
 R 
VOUT(MAX) = 1.2V •  1+ FB2 
 RFB1 
3922 F07
Figure 7. FB Resistor Configuration
3922f
18
For more information www.linear.com/LT3922
LT3922
Applications Information
This network forms part of a voltage regulation loop when
FB is near 1.2V. In this case, the LT3922 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 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
LT3922
RFAULT
Understanding FB Overvoltage Lockout
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 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 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.
3922 F08
Figure 8. FAULT Resistor Configuration
Shorted LED Fault Detection and Responses
The LT3922 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, or
3) VOUT < (VIN – 2V)
If the LT3922 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.
3922f
For more information www.linear.com/LT3922
19
LT3922
Applications Information
Soft-Start and Fault Modes
SS PIN (V)
The LT3922’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
TIME
DETECTED LED SHORT
SS
CSS
(a) Latchoff Mode
3922 F09
Figure 9. SS Capacitor and Resistor Configuration
generate a linear ramp voltage. This voltage ramp at the
SS pin forces the LT3922 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 will latch
off until the user forces a reset by toggling the EN/UVLO
pin. Without the RSS, the LT3922 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 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 to enable
SS PIN (V)
INTVCC (3V)
1.7V
0.2V
TIME
DETECTED LED SHORT
FAULT CLEARED
3922 F10
(b) Hiccup Mode
Figure 10. Fault Responses: (a) Latchoff and (b) Hiccup
or disable the LT3922. 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.
The internal hysteresis is 25mV, but the user can program
FALLING THRESHOLD
VIN
LT3922
R1
 R1
VIN(UVLO) = 1.33V •  1+ 
 R2 
EN/UVLO
RISING HYSTERESIS
R2
3922 F11
 R1
VHYST(UVLO) = 25mV •  1+  +R1• 2µA
 R2 
Figure 11. EN/UVLO Threshold and Hysteresis Voltages
3922f
20
For more information www.linear.com/LT3922
LT3922
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. The internal hysteresis of the
OVLO pin is 50mV.
VIN
RISING THRESHOLD
LT3922
R3
OVLO
R4
 R3 
VIN(OVLO) = 1.205V •  1+ 
 R4 
FALLING HYSTERESIS
3922 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
OVLO
R7
3922 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 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.
Designing the Printed Circuit Board (PCB)
The output capacitors COUT1 and COUT2 of the LT3922
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 100nF 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 for more information.
3922f
For more information www.linear.com/LT3922
21
LT3922
Typical Applications
333mA Boost LED Driver Using External PWM and Strobe
L1
2.2µH
VIN
8V TO 27V
0.1µF
SW
VIN
1M
4.7µF
BST VOUT
0.47µF
GND
EN/UVLO
0.47µF
365k
VOUT
OVLO
LT3922
59.0k
1M
FB
10µF
33.2k
GND
VREF
CTRL
PWM
SYNC/SPRD
1µF
ISP
300mΩ
ISN
INTVCC
100k
PWMTG
FAULT
SS RT
2.2µF
10nF
RP
45.3k
2MHz
M1
ISMON
VC
30V
333mA
LED
51k
1nF
3922 TA02a
L1: WURTH 74437324022
M1: VISHAY Si2319CDS
5000:1 External PWM Dimming
100:1 External PWM Dimming
ILED
100mA/DIV
ILED
100mA/DIV
VPWM
2V/DIV
VPWM
2V/DIV
500ns/DIV
INFINITE PERSISTENCE
VIN = 18V
fPWM = 100Hz
3922 TA02b
20µs/DIV
INFINITE PERSISTENCE
VIN = 18V
fPWM = 100Hz
100µs Strobe/1s Period
100µs Strobe/100s Period
ILED
100mA/DIV
ILED
100mA/DIV
VPWM
2V/DIV
VPWM
2V/DIV
20µs/DIV
INFINITE PERSISTENCE
VIN = 18V
fPWM = 1Hz
3922 TA02c
3922 TA02d
20µs/DIV
INFINITE PERSISTENCE
VIN = 18V
fPWM = 0.01Hz
3922 TA02e
3922f
22
For more information www.linear.com/LT3922
LT3922
Typical Applications
333mA Boost LED Driver Using Internal PWM and Analog CTRL Dimming
L1
4.7µH
VIN
8V TO 27V
0.1µF
SW
VIN
BST VOUT
1M
4.7µF
0.47µF
GND
EN/UVLO
0.47µF
365k
VOUT
OVLO
1M
LT3922
59.0k
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
30V
333mA
LED
10k
1nF
3922 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
3922 TA03b
200µs/DIV
VCTRL = 2V
VIN = 12V
fPWM = 122Hz
3922 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
VCTRL = 0.3V
VIN = 12V
fPWM = 122Hz
3922 TA03d
200µs/DIV
3922 TA03e
VCTRL = 0.3V
VIN = 12V
fPWM = 122Hz
3922f
For more information www.linear.com/LT3922
23
LT3922
Typical Applications
333mA Boost LED Driver Using External PWM Dimming and SSFM
L1
4.7µH
VIN
8V TO 27V
0.1µF
SW
VIN
1M
4.7µF
BST VOUT
0.47µF
GND
EN/UVLO
0.47µF
365k
VOUT
OVLO
LT3922
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
3922 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
3922 TA04b
3922f
24
For more information www.linear.com/LT3922
LT3922
Typical Applications
Low EMI 400kHz, 96% Efficient 10W (30V, 333mA) Boost LED Driver with SSFM
Efficiency vs VIN
L1
22µH
0.1µF
VIN
8V TO
27V
10µF
0.1µF
0402
SW
VIN
FB1
4.7µF
BST VOUT
1M
0.47µF
365k
VOUT
OVLO
LT3922
59.0k
1M
FB
4.7µF
33.2k
1µF
90
85
GND
VREF
VLED = 30V
fSW = 400kHz
95
0.47µF
GND
EN/UVLO
33µF
100
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)
3922 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
3922 TA05a
Average Radiated EMI Performance (CISPR25)
AMPLITUDE (dBµV/m)
AMPLITUDE (dBµV/m)
Peak Radiated EMI Performance (CISPR25)
50
45
40
35
30
25
20
15
10
5
0
–5
–10
–15
CLASS 5 PEAK LIMIT
LT3922 400kHz fSW 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 400kHz fSW AVERAGE EMI
0
100
3922 TA05c
200
300
400
500
600
FREQUENCY (MHz)
700
800
900
1000
3922 TA05d
3922f
For more information www.linear.com/LT3922
25
LT3922
Typical Applications
500mA Boost LED Driver Using Pulse Duty Cycle CTRL Input
L1
6.8µH
VIN
8V TO 20V
0.1µF
SW
VIN
4.7µF
1M
BST VOUT
0.47µF
GND
EN/UVLO
0.47µF
348k
VOUT
OVLO
LT3922
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%
3922 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
3922 TA06b
1ms/DIV
3922 TA06c
3922f
26
For more information www.linear.com/LT3922
LT3922
Typical Applications
2MHz, 95% Efficient 15W (15V, 1A) Buck Mode LED Driver
L1
4.7µH
0.1µF
VIN
20V TO 36V
VIN
20V TO 36V
BST
VIN
SW
VOUT
1M
33µF
EN/UVLO
0.47µF
51.1k
VOUT
1µF
OVLO
249k
LT3922
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
3922 TA07a
Efficiency vs VIN
100
EFFICIENCY (%)
95
90
85
80
75
70
16
20
24
28
VIN (V)
32
36
40
3922 TA07b
3922f
For more information www.linear.com/LT3922
27
LT3922
Typical Applications
500mA, 5V to 12V Boost Converter with Accurate Input Current Limit
L1
4.7µH
56mΩ
VIN
5V
2.2µF
0.1µF
SW
VIN
BST VOUT
604k
0.47µF
GND
EN/UVLO
Efficiency
VOUT
12V
500mA
100
95
0.47µF
84.5k
VOUT
182k
90
549k
LT3922
FB
EFFICIENCY (%)
OVLO
10µF
60.4k
GND
VREF
1µF
ISN
100k
85
80
75
70
ISP
ANALOG DIM
CTRL
PWM DIM
PWM
65
SYNC/SPRD
60
INTVCC
PWMTG
100k
RP
200
300
ILOAD (mA)
400
500
3922 TA08b
VC
10k
45.3k
2MHz
10nF
100
ISMON
FAULT
SS RT
2.2µF
0
1nF
L1: WURTH 744316470
3922 TA08a
Short-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
0.1µF
GND
EN/UVLO
226k
0.47µF
VPWMTG
20V/DIV
0.47µF
VSS
2V/DIV
VOUT
OVLO
LT3922
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
3922 TA09b
RP
45.3k
2MHz
M1 D1
ISMON
VC
332k
122Hz
10k
1nF
VPWMTG
20V/DIV
VSS
2V/DIV
VFAULTB
2V/DIV
21V
500mA
LED
ILED
1A/DIV
3922 TA09a
100ms/DIV
3922 TA09c
3922f
28
For more information www.linear.com/LT3922
LT3922
Package Description
Please refer to http://www.linear.com/product/LT3922#packaging for the most recent package drawings.
UFD Package
28-Lead Plastic QFN (4mm × 5mm)
(Reference LTC DWG # 05-08-1712 Rev B)
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)
0.75 ±0.05
PIN 1 NOTCH
R = 0.20 OR 0.35
× 45° CHAMFER
2.50 REF
R = 0.115
TYP
R = 0.05
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 0506 REV B
0.25 ±0.05
0.200 REF
0.50 BSC
0.00 – 0.05
BOTTOM VIEW—EXPOSED PAD
NOTE:
1. DRAWING PROPOSED TO BE MADE A JEDEC PACKAGE OUTLINE MO-220 VARIATION (WXXX-X).
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
3922f
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.
For more
information
www.linear.com/LT3922
29
LT3922
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)
0.1µF
INPUT EMI
FILTER
0.47µF
365k
VOUT
OVLO
1M
LT3922
59.0k
FB
2.2µF
33.2k
GND
VREF
1µF
ISP
100k
ANALOG DIM
CTRL
PWM DIM
PWM
300mΩ
2.2µF
ISN
50
45
40
35
30
25
20
15
10
5
0
–5
–10
–15
CLASS 5 PEAK LIMIT
LT3922 2MHz fSW PEAK EMI
0
100
200
300
SYNC/SPRD
400
500
600
FREQUENCY (MHz)
700
800
900
1000
3922 TA10b
INTVCC
PWMTG
SS
2.2µF
M1
ISMON
FAULT
RT
100nF
RP
45.3k
2MHz
VC
332k
122Hz
24k
0402
0.1µF
D1
Average Radiated EMI Performance (CISPR25)
FB2
OUTPUT
EMI FILTER
0.22nF
L1: WURTH 74437324047
M1: VISHAY Si2319CDS
FB1: WURTH 7427920415
FB2: WURTH 742792097
D1: NXP PMEG4010 CEJ
30V
333mA
LED
3922 TA10a
AMPLITUDE (dBµV/m)
100k
50
45
40
35
30
25
20
15
10
5
0
–5
–10
–15
CLASS 5 AVERAGE LIMIT
LT3922 2MHz fSW AVERAGE EMI
0
100
200
300
400
500
600
FREQUENCY (MHz)
700
800
900
1000
3922 TA10c
Related Parts
PART NUMBER
DESCRIPTION
COMMENTS
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,
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Dimming with PMOS Disconnect FET Driver
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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
VIN: 6V to 60V, VOUT: 0V to VIN –2V, ±6% Current Accuracy, TSSOP-28
LT3763
60V, 1MHz Synchronous Buck LED Controller
LT3744
36V, 1MHz Synchronous Buck LED Controller with VIN: 3.3V to 36V, VOUT: 0V to 36V, ±2% Current Accuracy, Fast Four-State
Current Control, 5mm × 6mm QFN-36
Four-State Control
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
3922f
30 Linear Technology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417
For more information www.linear.com/LT3922
(408) 432-1900 ● FAX: (408) 434-0507
●
www.linear.com/LT3922
LT 0816 • PRINTED IN USA
 LINEAR TECHNOLOGY CORPORATION 2016