LT8611
42V, 2.5A Synchronous Step-Down
Regulator with Current Sense and 2.5µA
Quiescent Current
DESCRIPTION
FEATURES
Rail-to-Rail Current Sense Amplifier with Monitor
n Wide Input Voltage Range: 3.4V to 42V
n Ultralow Quiescent Current Burst Mode® Operation:
n 2.5μA I Regulating 12V to 3.3V
Q
IN
OUT
n Output Ripple < 10mV
P-P
n High Efficiency Synchronous Operation:
n 96% Efficiency at 1A, 5V
OUT from 12VIN
n 94% Efficiency at 1A, 3.3V
OUT from 12VIN
n Fast Minimum Switch-On Time: 50ns
n Low Dropout Under All Conditions: 200mV at 1A
n Allows Use of Small Inductors
n Low EMI
n Adjustable and Synchronizable: 200kHz to 2.2MHz
n Current Mode Operation
n Accurate 1V Enable Pin Threshold
n Internal Compensation
n Output Soft-Start and Tracking
n Small Thermally Enhanced 3mm × 5mm 24-Lead
QFN Package
n AEC-Q100 Qualified for Automotive Applications
n
APPLICATIONS
Automotive and Industrial Supplies
n General Purpose Step-Down
n CCCV Power Supplies
The LT®8611 is a compact, high efficiency, high speed
synchronous monolithic step-down switching regulator
that consumes only 2.5µA of quiescent current. Top and
bottom power switches are included with all necessary
circuitry to minimize the need for external components.
The built-in current sense amplifier with monitor and
control pins allows accurate input or output current
regulation and limiting. Low ripple Burst Mode operation
enables high efficiency down to very low output currents
while keeping the output ripple below 10mVP-P. A SYNC
pin allows synchronization to an external clock. Internal
compensation with peak current mode topology allows
the use of small inductors and results in fast transient
response and good loop stability. The EN/UV pin has an
accurate 1V threshold and can be used to program VIN
undervoltage lockout or to shut down the LT8611 reducing the input supply current to 1µA. A capacitor on the
TR/SS pin programs the output voltage ramp rate during
start-up. The PG flag signals when VOUT is within ±9% of
the programmed output voltage as well as fault conditions.
The LT8611 is available in a small 24-lead 3mm × 5mm
QFN package with exposed pad for low thermal resistance.
All registered trademarks and trademarks are the property of their respective owners.
n
TYPICAL APPLICATION
12VIN to 5VOUT Efficiency
5V Step-Down Converter with 2.5A Output Current Limit
4.7µF ON OFF
VIN
SYNC
IMON
0.1µF
LT8611
INTVCC
1µF
fSW = 700kHz
Document Feedback
60.4k
1µF
VOUT
5V
2.5A
ISP
ISN
PG
10pF
85
80
75
70
65
fSW = 700kHz
VIN = 12V
VIN = 24V
60
TR/SS
PGND
0.02Ω
SW
BIAS
RT
90
4.7µH
ICTRL
0.1µF
95
BST
EN/UV
EFFICIENCY (%)
VIN
5.5V TO 42V
100
GND
FB
55
1M
243k
47µF
8611 TA01a
50
0
0.5
1.5
1
LOAD CURRENT (A)
2
2.5
8611 G01
Rev. B
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1
LT8611
PIN CONFIGURATION
VIN, EN/UV, PG, ISP, ISN............................................42V
BIAS...........................................................................30V
BST Pin Above SW Pin................................................4V
FB, TR/SS, RT, INTVCC, IMON, ICTRL..........................4V
SYNC Voltage ..............................................................6V
Operating Junction Temperature Range (Note 2)
LT8611E.................................................. –40 to 125°C
LT8611I................................................... –40 to 125°C
Storage Temperature Range.......................–65 to 150°C
ISP
ICTRL
TOP VIEW
ISN
(Note 1)
IMON
ABSOLUTE MAXIMUM RATINGS
24 23 22 21
SYNC 1
20 FB
TR/SS 2
19 PG
RT 3
18 BIAS
EN/UV 4
17 INTVCC
25
GND
VIN 5
16 BST
VIN 6
PGND 7
15 SW
14 SW
PGND 8
13 SW
NC
NC
NC
NC
9 10 11 12
UDD PACKAGE
24-LEAD (3mm × 5mm) PLASTIC QFN
θJA = 40°C/W, θJC(PAD) = 5°C/W
EXPOSED PAD (PIN 25) IS GND, MUST BE SOLDERED TO PCB
ORDER INFORMATION
LEAD FREE FINISH
TAPE AND REEL
PART MARKING*
PACKAGE DESCRIPTION
TEMPERATURE RANGE
LT8611EUDD#PBF
LT8611EUDD#TRPBF
LGBR
24-Lead (3mm × 5mm) Plastic QFN
–40°C to 125°C
LT8611IUDD#PBF
LT8611IUDD#TRPBF
LGBR
24-Lead (3mm × 5mm) Plastic QFN
–40°C to 125°C
LT8611EUDD#WPBF
LT8611EUDD#WTRPBF
LGBR
24-Lead (3mm × 5mm) Plastic QFN
–40°C to 125°C
LT8611IUDD#WPBF
LT8611IUDD#WTRPBF
LGBR
24-Lead (3mm × 5mm) Plastic QFN
–40°C to 125°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. B
2
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LT8611
ELECTRICAL
CHARACTERISTICS
The
l denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at TA = 25°C.
PARAMETER
CONDITIONS
MIN
Minimum Input Voltage
VIN Quiescent Current
TYP
MAX
l
2.9
3.4
V
l
1.0
1.0
3
8
µA
µA
l
1.7
1.7
4
10
µA
µA
0.46
2
mA
24
210
50
350
µA
µA
0.970
0.970
0.973
0.984
V
V
0.004
0.02
%/V
20
nA
3.4
3.29
3.57
3.35
V
V
2.6
2.7
VEN/UV = 0V, VSYNC = 0V
VEN/UV = 2V, Not Switching, VSYNC = 0V
VEN/UV = 2V, Not Switching, VSYNC = 2V
VIN Current in Regulation
VOUT = 0.97V, VIN = 6V, Output Load = 100µA
VOUT = 0.97V, VIN = 6V, Output Load = 1mA
l
l
Feedback Reference Voltage
VIN = 6V, ILOAD = 0.5A
VIN = 6V, ILOAD = 0.5A
l
Feedback Voltage Line Regulation
VIN = 4.0V to 42V, ILOAD = 0.5A
l
Feedback Pin Input Current
VFB = 1V
–20
INTVCC Voltage
ILOAD = 0mA, VBIAS = 0V
ILOAD = 0mA, VBIAS = 3.3V
3.23
3.25
INTVCC Undervoltage Lockout
0.967
0.956
2.5
BIAS Pin Current Consumption
VBIAS = 3.3V, ILOAD = 1A, 2MHz
8.5
Minimum On-Time
ILOAD = 1A, SYNC = 0V
ILOAD = 1A, SYNC = 3.3V
l
l
Oscillator Frequency
RT = 221k, ILOAD = 1A
RT = 60.4k, ILOAD = 1A
RT = 18.2k, ILOAD = 1A
l
l
l
Top Power NMOS On-Resistance
VINTVCC = 3.4V, ISW = 1A
Top Power NMOS Current Limit
VINTVCC = 3.4V
Bottom Power NMOS On-Resistance
VINTVCC = 3.4V, ISW = 1A
Bottom Power NMOS Current Limit
VINTVCC = 3.4V
Minimum Off-Time
30
30
50
45
70
65
ns
ns
50
80
110
ns
180
665
1.85
210
700
2.00
240
735
2.15
kHz
kHz
MHz
3.5
SW Leakage Current
VIN = 42V, VSW = 0V, 42V
EN/UV Rising
4.8
mΩ
5.8
65
2.5
EN/UV Pin Threshold
0.94
EN/UV Pin Hysteresis
A
mΩ
3.3
4.8
A
1.5
µA
1.0
1.06
V
–1.5
l
V
mA
120
l
UNITS
40
–20
mV
EN/UV Pin Current
VEN/UV = 2V
20
nA
PG Upper Threshold Offset from VFB
VFB Falling
l
6
9.0
12
%
PG Lower Threshold Offset from VFB
VFB Rising
l
–6
–9.0
–12
%
PG Hysteresis
1.3
PG Leakage
VPG = 3.3V
–40
PG Pull-Down Resistance
VPG = 0.1V
SYNC Threshold
SYNC Falling
SYNC Rising
0.8
1.6
SYNC Pin Current
VSYNC = 2V
–40
l
TR/SS Source Current
TR/SS Pull-Down Resistance
l
Fault Condition, TR/SS = 0.1V
1.2
%
40
nA
680
2000
Ω
1.1
2.0
1.4
2.4
V
V
40
nA
3.2
µA
2
230
Ω
Rev. B
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3
LT8611
ELECTRICAL
CHARACTERISTICS
The
l denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at TA = 25°C.
PARAMETER
CONDITIONS
Current Sense Voltage (VISP-ISN)
VICTRL = 1.5V, VISN = 3.3V
VICTRL = 1.5V, VISN = 0V
VICTRL = 800mV, VISN = 3.3V
VICTRL = 800mV, VISN = 0V
VICTRL = 200mV, VISN = 3.3V
VICTRL = 200mV, VISN = 0V
IMON Monitor Pin Voltage
VISP-ISN = 50mV, VISN = 3.3V
VISP-ISN = 50mV, VISN = 0V
VISP-ISN = 10mV, VISN = 3.3V
VISP-ISN = 10mV, VISN = 0V
ISP, ISN Pin Bias Current
Note 1: Stresses beyond those listed under Absolute Maximum Ratings
may cause permanent damage to the device. Exposure to any Absolute
Maximum Rating condition for extended periods may affect device
reliability and lifetime.
Note 2: The LT8611E 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
LT8611I is guaranteed over the full –40°C to 125°C operating junction
MIN
TYP
MAX
UNITS
l
l
l
l
l
l
48
46
38
37
5
4
50
50.5
41
42
10
10.5
52
56
46
47
15
17
mV
mV
mV
mV
mV
mV
l
l
l
l
0.960
0.890
130
110
1.00
0.99
220
205
1.040
1.090
320
300
V
V
mV
mV
l
–20
20
µA
temperature range. High junction temperatures degrade operating
lifetimes. Operating lifetime is derated at junction temperatures greater
than 125°C.
Note 3: This IC includes overtemperature protection that is intended to
protect the device during overload conditions. Junction temperature will
exceed 150°C when overtemperature protection is active. Continuous
operation above the specified maximum operating junction temperature
will reduce lifetime.
Rev. B
4
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LT8611
TYPICAL PERFORMANCE CHARACTERISTICS
Efficiency at 3.3VOUT
Efficiency at 5VOUT
100
95
95
90
90
90
80
85
85
70
80
75
70
65
fSW = 700kHz
VIN = 12V
VIN = 24V
60
55
50
0
0.5
1.5
1
LOAD CURRENT (A)
2
EFFICIENCY (%)
100
EFFICIENCY (%)
EFFICIENCY (%)
Efficiency at 5VOUT
100
80
75
70
65
fSW = 700kHz
VIN = 12V
VIN = 24V
60
55
50
2.5
0.5
0
1.5
1
LOAD CURRENT (A)
2
Efficiency at 3.3VOUT
VIN = 12V
60
50
40
0.982
90
88
fSW = 700MHz
82
0.25
100 1000
LOAD CURRENT (mA)
VIN = 12V
VIN = 24V
0.75
0.973
0.970
0.967
0.964
0.961
1.25
2.25
1.75
SWITCHING FREQUENCY (MHz)
0.955
–55
0.20
1.02
0.15
0.99
VOUT = 3.3V
VIN = 12V
0.06
0.10
0.05
0
0.04
0.02
0
–0.02
–0.04
–0.10
EN FALLING
–0.15
–0.06
0.96
–0.20
–0.08
0.95
–55
–0.25
–25
5
35
65
95
TEMPERATURE (°C)
125
155
8611 G07
155
VOUT = 3.3V
ILOAD = 0.5A
0.08
–0.05
0.98
125
Line Regulation
0.10
CHANGE IN VOUT (%)
1.03
EN RISING
65
35
5
95
TEMPERATURE (°C)
–25
8611 G06
Load Regulation
0.25
CHANGE IN VOUT (%)
EN THRESHOLD (V)
EN Pin Thresholds
0.97
0.976
8611 G05
1.04
1.00
0.979
0.958
8611 G04
1.01
8611 G03
VOUT = 3.3V
84
1000 10000
Reference Voltage
86
20
10
fSW = 700MHz
0.1
1
10 100
LOAD CURRENT (mA)
0.985
92
30
1
0
0.001 0.01
REFERENCE VOLTAGE (V)
VIN = 24V
70
0.1
30
10
94
EFFICIENCY (%)
EFFICIENCY (%)
80
0
0.0001 0.001 0.01
50
40
Efficiency vs Frequency
96
10
60
8611 G02
100
90
VIN = 24V
20
2.5
8611 G01
VIN = 12V
0
0.5
1.5
2
1
LOAD CURRENT (A)
3
2.5
8611 G08
–0.10
0
5
10
15 20 25 30 35
INPUT VOLTAGE (V)
40
45
8611 G09
Rev. B
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5
LT8611
TYPICAL PERFORMANCE CHARACTERISTICS
No Load Supply Current
25
VOUT = 3.3V
IN REGULATION
4.5
20
INPUT CURRENT (µA)
INPUT CURRENT (µA)
4.0
3.5
3.0
2.5
2.0
1.5
1.0
Top FET Current Limit vs Duty Cycle
6.0
VOUT = 3.3V
VIN = 12V
IN REGULATION
5.5
CURRENT LIMIT (A)
No Load Supply Current
5.0
15
10
0
5
10
15 20 25 30 35
INPUT VOLTAGE (V)
40
0
–55
45
65
5
95
35
TEMPERATURE (°C)
–25
125
2.0
155
250
3.4
70% DC
3.5
3.0
3.2
3.0
2.8
–25
5
35
65
TEMPERATURE (°C)
95
2.4
–55
125
–25
5
35
65
TEMPERATURE (°C)
95
75
350
70
MINIMUM ON-TIME (ns)
400
TOP SW
200
BOT SW
150
100
50
1
1.5
2
SWITCH CURRENT (A)
2.5
3
BOT SW
–25
65
5
95
35
TEMPERATURE (°C)
ILOAD = 1A, VSYNC = 0V
ILOAD = 1A, VSYNC = 3V
ILOAD = 2.5A, VSYNC = 0V
ILOAD = 2.5A, VSYNC = 3V
65
155
8611 G15
95
60
55
50
45
30
–55
125
Minimum Off-Time
VIN = 3.3V
ILOAD = 0.5A
90
85
80
75
70
65
35
0.5
100
100
40
0
TOP SW
Minimum On-Time
80
250
150
0
–55
125
MINIMUM OFF-TIME (ns)
Switch Drop
300
SWITCH CURRENT = 1A
8611 G14
8611 G13
450
1.0
50
2.6
2.5
–55
0.8
200
SWITCH DROP (mV)
CURRENT LIMIT (A)
30% DC
4.0
0.4
0.6
DUTY CYCLE
Switch Drop
3.6
4.5
0.2
0
8611 G12
Bottom FET Current Limit
Top FET Current Limit
CURRENT LIMIT (A)
3.5
8611 G11
5.0
SWITCH DROP (mV)
4.0
2.5
8611 G10
0
4.5
3.0
5
0.5
0
5.0
–25
65
35
5
95
TEMPERATURE (°C)
125
8611 G16
155
8611 G17
60
–50 –25
95
65
35
TEMPERATURE (°C)
5
125
155
8611 G18
Rev. B
6
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LT8611
TYPICAL PERFORMANCE CHARACTERISTICS
Switching Frequency
700
730
600
500
400
300
200
RT = 60.4k
720
710
700
690
680
670
100
0
Burst Frequency
800
SWITCHING FREQUENCY (kHz)
740
SWITCHING FREQUENCY (kHz)
DROPOUT VOLTAGE (mV)
Dropout Voltage
800
1
0.5
0
1.5
2
2.5
LOAD CURRENT (A)
95
65
35
TEMPERATURE (°C)
5
125
Minimum Load to Full Frequency
(SYNC DC High)
VOUT = 5V
fSW = 700kHz
SWITCHING FREQUENCY (kHz)
20
20
25 30 35
INPUT VOLTAGE (V)
40
600
1.0
500
400
300
0
0.2
0
0.2
0.4
0.6
FB VOLTAGE (V)
0.8
11.5
–7.5
10.5
2.1
FB RISING
10.0
2.0
1.9
1.8
1.7
125
155
0.2
1.0
0.4 0.6 0.8
TR/SS VOLTAGE (V)
FB FALLING
9.5
9.0
1.4
–8.0
–8.5
–9.0
FB RISING
–9.5
FB FALLING
–10.0
8.5
–10.5
8.0
–11.0
7.5
7.0
–55
1.2
PG Low Thresholds
–7.0
PG THRESHOLD OFFSET FROM VREF (%)
2.2
0
8611 G24
12.0
PG THRESHOLD OFFSET FROM VREF (%)
SS PIN CURRENT (µA)
0
1
11.0
95
65
35
TEMPERATURE (°C)
0.4
PG High Thresholds
VSS = 0.5V
5
0.6
8611 G23
Soft-Start Current
1.6
–50 –25
0.8
200
45
2.3
200
Soft-Start Tracking
8611 G22
2.4
100
50
150
LOAD CURRENT (mA)
0
1.2
100
15
200
8611 G21
FB VOLTAGE (V)
LOAD CURRENT (mA)
40
10
300
0
155
VOUT = 3.3V
VIN = 12V
VSYNC = 0V
RT = 60.4k
700
60
5
400
Frequency Foldback
800
80
0
500
8611 G20
8611 G19
100
600
100
660
–55 –25
3
VIN = 12V
VOUT = 3.3V
700
–11.5
–25
65
35
5
95
TEMPERATURE (°C)
125
8611 G25
155
8611 G26
–12.0
–55
–25
65
35
5
95
TEMPERATURE (°C)
125
155
8611 G27
Rev. B
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7
LT8611
TYPICAL PERFORMANCE CHARACTERISTICS
RT Programmed Switching
Frequency
3.6
225
175
INPUT VOLTAGE (V)
RT PIN RESISTOR (kΩ)
200
150
125
100
75
VIN UVLO
Bias Pin Current
5.00
3.4
4.75
3.2
4.50
BIAS PIN CURRENT (mA)
250
3.0
2.8
2.6
2.4
50
2.2
25
0
0.2
0.6
1.4
1.8
1
SWITCHING FREQUENCY (MHz)
2.2
2.0
–55 –25
BIAS PIN CURRENT (mA)
4.00
3.75
3.50
95
65
35
TEMPERATURE (°C)
5
125
155
3.00
10
15
20 25 30 35
INPUT VOLTAGE (V)
40
45
8611 G30
Switching Waveforms
VBIAS = 5V
VOUT = 5V
VIN = 12V
ILOAD = 1A
5
8611 G29
Bias Pin Current
10
4.25
3.25
8611 G28
12
VBIAS = 5V
VOUT = 5V
ILOAD = 1A
fSW = 700kHz
Switching Waveforms
IL
200mA/DIV
IL
1A/DIV
8
VSW
5V/DIV
6
4
VSW
5V/DIV
500ns/DIV
12VIN TO 5VOUT AT 1A
500µs/DIV
12VIN TO 5VOUT AT 10mA
VSYNC = 0V
8611 G32
2
0
0
0.5
1
1.5
2
SWITCHING FREQUENCY (MHz)
8611 G33
2.5
8611 G31
Transient Response
Switching Waveforms
IL
1A/DIV
Transient Response
ILOAD
1A/DIV
ILOAD
1A/DIV
VOUT
100mV/DIV
VSW
10V/DIV
500ns/DIV
36VIN TO 5VOUT AT 1A
8611 G34
VOUT
200mV/DIV
50µs/DIV
0.5A TO 1.5A TRANSIENT
12VIN, 5VOUT
COUT = 47µF
8611 G35
50µs/DIV
0.5A TO 2.5A TRANSIENT
12VIN, 5VOUT
COUT = 47µF
8611 G36
Rev. B
8
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LT8611
TYPICAL PERFORMANCE CHARACTERISTICS
Transient Response
Start-Up Dropout Performance
IL
1A/DIV
ICTRL Voltage
40
30
20
10
55
54
54
53
53
52
VISP = 0V
51
50
VISP = 3V
49
48
47
0
500
1000
1500
ICTRL VOLTAGE (mV)
0
51
50
49
48
47
45
25 50 75 100 125 150
TEMPERATURE (°C)
IMON Voltage
0.5
2
1.5
1
2.5
3
ISP-ISN COMMON MODE (V)
1000
1000
800
800
3.5
8611 G42
IMON Voltage
1200
VSYNC = 3.3V
0
8611 G41
8611 G40
1200
52
46
46
–50 –25
2000
8611 G39
VISP-VISN Sense Voltage
55
MAX VISP-VISN VOLTAGE (mV)
50
MAX VISP-VISN VOLTAGE (mV)
MAX VISP-VISN VOLTAGE (mV)
100ms/DIV
20Ω LOAD
(250mA IN REGULATION)
8611 G38
VISP-VISN Sense Voltage
60
VOUT
VOUT
2V/DIV
100ms/DIV
2.5Ω LOAD
(2A IN REGULATION)
8611 G37
VIN
VIN
2V/DIV
VOUT
VOUT
2V/DIV
50µs/DIV
50mA TO 1A TRANSIENT
12VIN, 5VOUT
COUT = 47µF
0
VIN
VIN
2V/DIV
VOUT
200mV/DIV
Start-Up Dropout Performance
IMON Voltage
1.10
VSYNC = 0V
VISP-VISN = 50mV
600
IMON VOLTAGE (V)
VIMON (mV)
VIMON (mV)
1.05
600
400
400
200
200
1.00
0.95
0
0
10
20
30
VISP-VISN (mV)
40
50
8611 G43
0
0
10
20
30
VISP-VISN (mV)
50
40
8611 G44
0.90
0
0.5
2.5
3
1
1.5
2
ISP-ISN COMMON MODE (V)
3.5
8611 G45
Rev. B
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9
LT8611
PIN FUNCTIONS
SYNC (Pin 1): External Clock Synchronization Input.
Ground this pin for low ripple Burst Mode operation at low
output loads. Tie to a clock source for synchronization to
an external frequency. Apply a DC voltage of 3V or higher
or tie to INTVCC for pulse-skipping mode. When in pulseskipping mode, the IQ will increase to several hundred
µA. When SYNC is DC high or synchronized, frequency
foldback will be disabled. Do not float this pin.
TR/SS (Pin 2): Output Tracking and Soft-Start Pin. This
pin allows user control of output voltage ramp rate during
start-up. A TR/SS voltage below 0.97V forces the LT8611
to regulate the FB pin to equal the TR/SS pin voltage. When
TR/SS is above 0.97V, the tracking function is disabled
and the internal reference resumes control of the error
amplifier. An internal 2.2μA pull-up current from INTVCC on
this pin allows a capacitor to program output voltage slew
rate. This pin is pulled to ground with an internal 230Ω
MOSFET during shutdown and fault conditions; use a series
resistor if driving from a low impedance output. This pin
may be left floating if the tracking function is not needed.
RT (Pin 3): A resistor is tied between RT and ground to
set the switching frequency.
EN/UV (Pin 4): The LT8611 is shut down when this pin
is low and active when this pin is high. The hysteretic
threshold voltage is 1.00V going up and 0.96V going
down. Tie to VIN if the shutdown feature is not used. An
external resistor divider from VIN can be used to program
a VIN threshold below which the LT8611 will shut down.
VIN (Pins 5, 6): The VIN pins supply current to the LT8611
internal circuitry and to the internal topside power switch.
These pins must be tied together and be locally bypassed.
Be sure to place the positive terminal of the input capacitor as close as possible to the VIN pins, and the negative
capacitor terminal as close as possible to the PGND pins.
PGND (Pins 7, 8): Power Switch Ground. These pins are
the return path of the internal bottom-side power switch
and must be tied together. Place the negative terminal of
the input capacitor as close to the PGND pins as possible.
NC (Pins 9, 10, 11, 12): No Connect. These pins are not
connected to internal circuitry. It is recommended that
these be connected to GND so that the exposed pad GND
can be run to the top level GND copper to enhance thermal
performance.
SW (Pins 13, 14, 15): The SW pins are the outputs of the
internal power switches. Tie these pins together and connect them to the inductor and boost capacitor. This node
should be kept small on the PCB for good performance.
BST (Pin 16): This pin is used to provide a drive voltage,
higher than the input voltage, to the topside power switch.
Place a 0.1µF boost capacitor as close as possible to the IC.
INTVCC (Pin 17): Internal 3.4V Regulator Bypass Pin.
The internal power drivers and control circuits are powered from this voltage. INTVCC maximum output current is 20mA. Do not load the INTVCC pin with external
circuitry. INTVCC current will be supplied from BIAS if
VBIAS > 3.1V, otherwise current will be drawn from VIN.
Voltage on INTVCC will vary between 2.8V and 3.4V when
VBIAS is between 3.0V and 3.6V. Decouple this pin to power
ground with at least a 1μF low ESR ceramic capacitor
placed close to the IC.
BIAS (Pin 18): The internal regulator will draw current from
BIAS instead of VIN when BIAS is tied to a voltage higher
than 3.1V. For output voltages of 3.3V and above this pin
should be tied to VOUT. If this pin is tied to a supply other
than VOUT use a 1µF local bypass capacitor on this pin.
PG (Pin 19): The PG pin is the open-drain output of an
internal comparator. PG remains low until the FB pin is
within ±9% of the final regulation voltage, and there are
no fault conditions. PG is valid when VIN is above 3.4V,
regardless of EN/UV pin state.
FB (Pin 20): The LT8611 regulates the FB pin to 0.970V.
Connect the feedback resistor divider tap to this pin. Also,
connect a phase lead capacitor between FB and VOUT.
Typically, this capacitor is 4.7pF to 10pF.
Rev. B
10
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LT8611
PIN FUNCTIONS
ISP (Pin 21): Current Sense (+) Pin. This is the noninverting input to the current sense amplifier.
ISN (Pin 22): Current Sense (–) Pin. This is the inverting
input to the current sense amplifier.
IMON (Pin 23): Proportional-to-Current Monitor Output.
This pin sources a voltage 20 times the voltage between
the ISP and ISN pins such that:
VIMON = 20 • (VISP-VISN).
IMON can source 200µA and sink 10µA. Float IMON if
unused.
ICTRL (Pin 24): Current Adjustment Pin. ICTRL adjusts
the maximum ISP-ISN drop before the LT8611 reduces
output current. Connect directly to INTVCC or float for
full-scale ISP-ISN threshold of 50mV or apply values
between GND and 1V to modulate current limit. There is
an internal 1.4µA pull-up current on this pin. Float or tie
to INTVCC when unused.
GND (Exposed Pad Pin 25): Ground. The exposed pad
must be connected to the negative terminal of the input
capacitor and soldered to the PCB in order to lower the
thermal resistance.
Rev. B
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11
LT8611
BLOCK DIAGRAM
VIN
5, 6
CIN
R3
OPT
4
R4
OPT
19
EN/UV
PG
1V
+
–
SHDN
±9%
R2
CSS
(OPT)
R1
20
2
RT
3
1
FB
TR/SS
BIAS
3.4V
REG
SLOPE COMP
ERROR
AMP
+
+
–
VOUT
C1
–
+
INTERNAL 0.97V REF
INTVCC
OSCILLATOR
200kHz TO 2.2MHz
VC
BST
SWITCH
LOGIC
AND
ANTISHOOT
THROUGH
BURST
DETECT
SHDN
TSD
INTVCC UVLO
VIN UVLO
18
17
CVCC
6
CBST
M1
L
SW
13-15
M2
PGND
SHDN
TSD
VIN UVLO
2.2µA
RT
7, 8
+
–
+
+
–
VIN
SYNC
1.0V
R
ISP
R
ISN
21
CF RSEN
VOUT
22
COUT
20R
1×
1.4µA
GND
25
ICTRL
24
IMON
23
8611 BD
Rev. B
12
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LT8611
OPERATION
The LT8611 is a monolithic, constant frequency, current
mode step-down DC/DC converter. An oscillator, with
frequency set using a resistor on the RT pin, turns on
the internal top power switch at the beginning of each
clock cycle. Current in the inductor then increases until
the top switch current comparator trips and turns off the
top power switch. The peak inductor current at which
the top switch turns off is controlled by the voltage on
the internal VC node. The error amplifier servos the VC
node by comparing the voltage on the VFB pin with an
internal 0.97V reference. When the load current increases
it causes a reduction in the feedback voltage relative to
the reference leading the error amplifier to raise the VC
voltage until the average inductor current matches the new
load current. When the top power switch turns off, the
synchronous power switch turns on until the next clock
cycle begins or inductor current falls to zero. If overload
conditions result in more than 3.3A flowing through the
bottom switch, the next clock cycle will be delayed until
switch current returns to a safe level.
To optimize efficiency at light loads, the LT8611 operates
in Burst Mode operation in light load situations. Between
bursts, all circuitry associated with controlling the output
switch is shut down, reducing the input supply current to
1.7μA. In a typical application, 2.5μA will be consumed
from the input supply when regulating with no load. The
SYNC pin is tied low to use Burst Mode operation and can
be tied to a logic high to use pulse-skipping mode. If a
clock is applied to the SYNC pin the part will synchronize to
an external clock frequency and operate in pulse-skipping
mode. While in pulse-skipping mode the oscillator operates
continuously and positive SW transitions are aligned to
the clock. During light loads, switch pulses are skipped
to regulate the output and the quiescent current will be
several hundred µA.
The LT8611 includes a current control and monitoring
loop using the ISN, ISP, IMON and ICTRL pins. The ISP/
ISN pins monitor the voltage across an external sense
resistor such that the VISP-VISN does not exceed 50mV
by limiting the peak inductor current controlled by the VC
node. The current sense amplifier inputs (ISP/ISN) are railto-rail such that input, output, or other system currents
may be monitored and regulated. The IMON pin outputs
a ground-referenced voltage equal to 20 times the voltage
between the ISP-ISN pins for monitoring system currents.
The ICTRL pin can be used to override the internal 50mV
limit between the ISP, ISN pin to a lower set point for the
current control loop.
Comparators monitoring the FB pin voltage will pull the
PG pin low if the output voltage varies more than ±9%
(typical) from the set point, or if a fault condition is present.
To improve efficiency across all loads, supply current to
internal circuitry can be sourced from the BIAS pin when
biased at 3.3V or above. Else, the internal circuitry will draw
current from VIN. The BIAS pin should be connected to
VOUT if the LT8611 output is programmed at 3.3V or above.
The oscillator reduces the LT8611’s operating frequency
when the voltage at the FB pin is low. This frequency
foldback helps to control the inductor current when the
output voltage is lower than the programmed value which
occurs during start-up or overcurrent conditions. When
a clock is applied to the SYNC pin or the SYNC pin is
held DC high, the frequency foldback is disabled and the
switching frequency will slow down only during overcurrent conditions.
If the EN/UV pin is low, the LT8611 is shut down and draws
1µA from the input. When the EN/UV pin is above 1V, the
switching regulator will become active.
Rev. B
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13
LT8611
APPLICATIONS INFORMATION
Achieving Ultralow Quiescent Current
To enhance efficiency at light loads, the LT8611 operates
in low ripple Burst Mode operation, which keeps the output capacitor charged to the desired output voltage while
minimizing the input quiescent current and minimizing
output voltage ripple. In Burst Mode operation the LT8611
delivers single small pulses of current to the output capacitor followed by sleep periods where the output power is
supplied by the output capacitor. While in sleep mode the
LT8611 consumes 1.7μA.
As the output load decreases, the frequency of single current pulses decreases (see Figure 1a) and the percentage
of time the LT8611 is in sleep mode increases, resulting in
Burst Frequency
SWITCHING FREQUENCY (kHz)
800
VIN = 12V
VOUT = 3.3V
700
600
500
400
300
200
100
0
100
50
150
LOAD CURRENT (mA)
0
(1a)
200
8611 F01a
Minimum Load to Full Frequency (SYNC DC High)
100
much higher light load efficiency than for typical converters. By maximizing the time between pulses, the converter
quiescent current approaches 2.5µA for a typical application
when there is no output load. Therefore, to optimize the
quiescent current performance at light loads, the current
in the feedback resistor divider must be minimized as it
appears to the output as load current.
While in Burst Mode operation the current limit of the top
switch is approximately 400mA resulting in output voltage
ripple shown in Figure 2. Increasing the output capacitance
will decrease the output ripple proportionally. As load ramps
upward from zero the switching frequency will increase
but only up to the switching frequency programmed by
the resistor at the RT pin as shown in Figure 1a. The output load at which the LT8611 reaches the programmed
frequency varies based on input voltage, output voltage,
and inductor choice.
For some applications it is desirable for the LT8611 to
operate in pulse-skipping mode, offering two major differences from Burst Mode operation. First is the clock stays
awake at all times and all switching cycles are aligned to
the clock. In this mode much of the internal circuitry is
awake at all times, increasing quiescent current to several
hundred µA. Second is that full switching frequency is
reached at lower output load than in Burst Mode operation
(see Figure 1b). To enable pulse-skipping mode, the SYNC
pin is tied high either to a logic output or to the INTVCC
pin. When a clock is applied to the SYNC pin the LT8611
will also operate in pulse-skipping mode.
5VOUT
700kHz
LOAD CURRENT (mA)
80
IL
200mA/DIV
60
40
VOUT
10mV/DIV
20
0
VSYNC = 0V
5
10
15
20
25 30 35
INPUT VOLTAGE (V)
(1b)
40
8611 F02
Figure 2. Burst Mode Operation
45
8611 F01b
Figure 1. SW Frequency vs Load Information in
Burst Mode Operation (1a) and Pulse-Skipping Mode (1b)
14
5µs/DIV
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Rev. B
LT8611
APPLICATIONS INFORMATION
FB Resistor Network
The output voltage is programmed with a resistor divider
between the output and the FB pin. Choose the resistor
values according to:
⎛ V
⎞
R1= R2 ⎜ OUT – 1⎟
⎝ 0.970V ⎠
where RT is in kΩ and fSW is the desired switching frequency in MHz.
Table 1. SW Frequency vs RT Value
fSW (MHz)
RT (kΩ)
0.2
232
0.3
150
0.4
110
0.5
88.7
0.6
71.5
0.7
60.4
0.8
52.3
1.0
41.2
1.2
33.2
14
28.0
1.6
23.7
1.8
20.5
2.0
18.2
2.2
15.8
(1)
Reference designators refer to the Block Diagram. 1%
resistors are recommended to maintain output voltage
accuracy.
If low input quiescent current and good light-load efficiency
are desired, use large resistor values for the FB resistor
divider. The current flowing in the divider acts as a load
current, and will increase the no-load input current to the
converter, which is approximately:
⎛ V
⎞⎛ V ⎞⎛ 1⎞
IQ = 1.7µA + ⎜ OUT ⎟⎜ OUT ⎟⎜ ⎟
⎝ R1+R2 ⎠⎝ VIN ⎠⎝ n ⎠
(2)
where 1.7µA is the quiescent current of the LT8611 and
the second term is the current in the feedback divider
reflected to the input of the buck operating at its light
load efficiency n. For a 3.3V application with R1 = 1M and
R2 = 412k, the feedback divider draws 2.3µA. With VIN =
12V and n = 80%, this adds 0.8µA to the 1.7µA quiescent
current resulting in 2.5µA no-load current from the 12V
supply. Note that this equation implies that the no-load
current is a function of VIN; this is plotted in the Typical
Performance Characteristics section.
When using large FB resistors, a 4.7pF to 10pF phase-lead
capacitor should be connected from VOUT to FB.
Setting the Switching Frequency
The LT8611 uses a constant frequency PWM architecture
that can be programmed to switch from 200kHz to 2.2MHz
by using a resistor tied from the RT pin to ground. A table
showing the necessary RT value for a desired switching
frequency is in Table 1.
The RT resistor required for a desired switching frequency
can be calculated using:
RT =
46.5
fSW
– 5.2
(3)
Operating Frequency Selection and Trade-Offs
Selection of the operating frequency is a trade-off between
efficiency, component size, and input voltage range. The
advantage of high frequency operation is that smaller inductor and capacitor values may be used. The disadvantages
are lower efficiency and a smaller input voltage range.
The highest switching frequency (fSW(MAX)) for a given
application can be calculated as follows:
fSW(MAX) =
(
VOUT + VSW(BOT)
tON(MIN) VIN – VSW(TOP) + VSW(BOT)
)
(4)
where VIN is the typical input voltage, VOUT is the output
voltage, VSW(TOP) and VSW(BOT) are the internal switch
drops (~0.3V, ~0.15V, respectively at maximum load)
and tON(MIN) is the minimum top switch on-time (see the
Electrical Characteristics). This equation shows that a
slower switching frequency is necessary to accommodate
a high VIN/VOUT ratio.
For transient operation, VIN may go as high as the absolute maximum rating of 42V regardless of the RT value,
however the LT8611 will reduce switching frequency as
necessary to maintain control of inductor current to assure safe operation.
Rev. B
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15
LT8611
APPLICATIONS INFORMATION
The LT8611 is capable of a maximum duty cycle of greater
than 99%, and the VIN-to-VOUT dropout is limited by the
RDS(ON) of the top switch. In this mode the LT8611 skips
switch cycles, resulting in a lower switching frequency
than programmed by RT.
For applications that cannot allow deviation from the programmed switching frequency at low VIN/VOUT ratios use
the following formula to set switching frequency:
VIN(MIN) =
VOUT + VSW(BOT)
1– fSW • tOFF(MIN)
– VSW(BOT) + VSW(TOP) (5)
where ∆IL is the inductor ripple current as calculated in
Equation 9 and ILOAD(MAX) is the maximum output load
for a given application.
As a quick example, an application requiring 1A output
should use an inductor with an RMS rating of greater than
1A and an ISAT of greater than 1.3A. During long duration
overload or short-circuit conditions, the inductor RMS is
greater to avoid overheating of the inductor. To keep the
efficiency high, the series resistance (DCR) should be less
than 0.04Ω, and the core material should be intended for
high frequency applications.
where VIN(MIN) is the minimum input voltage without
skipped cycles, VOUT is the output voltage, VSW(TOP) and
VSW(BOT) are the internal switch drops (~0.3V, ~0.15V,
respectively at maximum load), fSW is the switching frequency (set by RT), and tOFF(MIN) is the minimum switch
off-time. Note that higher switching frequency will increase
the minimum input voltage below which cycles will be
dropped to achieve higher duty cycle.
The LT8611 limits the peak switch current in order to
protect the switches and the system from overload faults.
The top switch current limit (ILIM) is at least 3.5A at low
duty cycles and decreases linearly to 2.8A at DC = 0.8. The
inductor value must then be sufficient to supply the desired
maximum output current (IOUT(MAX)), which is a function
of the switch current limit (ILIM) and the ripple current.
Inductor Selection and Maximum Output Current
The LT8611 is designed to minimize solution size by
allowing the inductor to be chosen based on the output
load requirements of the application. During overload or
short-circuit conditions the LT8611 safely tolerates operation with a saturated inductor through the use of a high
speed peak-current mode architecture.
The peak-to-peak ripple current in the inductor can be
calculated as follows:
A good first choice for the inductor value is:
L=
VOUT + VSW(BOT)
fSW
(6)
where fSW is the switching frequency in MHz, VOUT is
the output voltage, VSW(BOT) is the bottom switch drop
(~0.15V) and L is the inductor value in μH.
To avoid overheating and poor efficiency, an inductor must
be chosen with an RMS current rating that is greater than
the maximum expected output load of the application. In
addition, the saturation current (typically labeled ISAT)
rating of the inductor must be higher than the load current
plus 1/2 of in inductor ripple current:
1
IL(PEAK) = ILOAD(MAX) + ΔIL
2
16
(7)
IOUT(MAX) = ILIM –
ΔIL =
VOUT
L • fSW
ΔIL
2
⎞
⎛
V
• ⎜⎜1– OUT ⎟⎟
⎝ VIN(MAX) ⎠
(8)
(9)
where fSW is the switching frequency of the LT8611, and
L is the value of the inductor. Therefore, the maximum
output current that the LT8611 will deliver depends on
the switch current limit, the inductor value, and the input
and output voltages. The inductor value may have to be
increased if the inductor ripple current does not allow
sufficient maximum output current (IOUT(MAX)) given the
switching frequency, and maximum input voltage used in
the desired application.
The optimum inductor for a given application may differ
from the one indicated by this design guide. A larger value
inductor provides a higher maximum load current and
reduces the output voltage ripple. For applications requiring smaller load currents, the value of the inductor may
be lower and the LT8611 may operate with higher ripple
Rev. B
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LT8611
APPLICATIONS INFORMATION
current. This allows use of a physically smaller inductor,
or one with a lower DCR resulting in higher efficiency. Be
aware that low inductance may result in discontinuous
mode operation, which further reduces maximum load
current.
For more information about maximum output current
and discontinuous operation, see Linear Technology’s
Application Note 44.
Finally, for duty cycles greater than 50% (VOUT/VIN > 0.5),
a minimum inductance is required to avoid sub-harmonic
oscillation. See Application Note 19.
Input Capacitor
Bypass the input of the LT8611 circuit with a ceramic capacitor of X7R or X5R type placed as close as possible to
the VIN and PGND pins. Y5V types have poor performance
over temperature and applied voltage, and should not be
used. A 4.7μF to 10μF ceramic capacitor is adequate to
bypass the LT8611 and will easily handle the ripple current.
Note that larger input capacitance is required when a lower
switching frequency is used. If the input power source has
high impedance, or there is significant inductance due to
long wires or cables, additional bulk capacitance may be
necessary. This can be provided with a low performance
electrolytic capacitor.
Step-down regulators draw current from the input supply in pulses with very fast rise and fall times. The input
capacitor is required to reduce the resulting voltage
ripple at the LT8611 and to force this very high frequency
switching current into a tight local loop, minimizing EMI.
A 4.7μF capacitor is capable of this task, but only if it is
placed close to the LT8611 (see the PCB Layout section).
A second precaution regarding the ceramic input capacitor
concerns the maximum input voltage rating of the LT8611.
A ceramic input capacitor combined with trace or cable
inductance forms a high quality (under damped) tank circuit. If the LT8611 circuit is plugged into a live supply, the
input voltage can ring to twice its nominal value, possibly
exceeding the LT8611’s voltage rating. This situation is
easily avoided (see Linear Technology Application Note 88).
Output Capacitor and Output Ripple
The output capacitor has two essential functions. Along
with the inductor, it filters the square wave generated
by the LT8611 to produce the DC output. In this role it
determines the output ripple, thus low impedance at the
switching frequency is important. The second function
is to store energy in order to satisfy transient loads and
stabilize the LT8611’s control loop. Ceramic capacitors
have very low equivalent series resistance (ESR) and
provide the best ripple performance. For good starting
values, see the Typical Applications section.
Use X5R or X7R types. This choice will provide low output
ripple and good transient response. Transient performance
can be improved with a higher value output capacitor and
the addition of a feedforward capacitor placed between
VOUT and FB. Increasing the output capacitance will also
decrease the output voltage ripple. A lower value of output
capacitor can be used to save space and cost but transient
performance will suffer and may cause loop instability. See
the Typical Applications in this data sheet for suggested
capacitor values.
When choosing a capacitor, special attention should be
given to the data sheet to calculate the effective capacitance
under the relevant operating conditions of voltage bias and
temperature. A physically larger capacitor or one with a
higher voltage rating may be required.
Rev. B
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17
LT8611
APPLICATIONS INFORMATION
The LT8611 is in shutdown when the EN pin is low and
active when the pin is high. The rising threshold of the EN
comparator is 1.0V, with 40mV of hysteresis. The EN pin
can be tied to VIN if the shutdown feature is not used, or
tied to a logic level if shutdown control is required.
Adding a resistor divider from VIN to EN programs the
LT8611 to regulate the output only when VIN is above a
desired voltage (see the Block Diagram). Typically, this
threshold, VIN(EN), is used in situations where the input
supply is current limited, or has a relatively high source
resistance. A switching regulator draws constant power
from the source, so source current increases as source
voltage drops. This looks like a negative resistance load
to the source and can cause the source to current limit or
latch low under low source voltage conditions. The VIN(EN)
threshold prevents the regulator from operating at source
voltages where the problems might occur. This threshold
can be adjusted by setting the values R3 and R4 such that
they satisfy the following equation:
⎛ R3 ⎞
VIN(EN) = ⎜ + 1⎟ • 1.0V
⎝ R4 ⎠
(10)
where the LT8611 will remain off until VIN is above VIN(EN).
Due to the comparator’s hysteresis, switching will not stop
until the input falls slightly below VIN(EN).
When operating in Burst Mode operation for light load
currents, the current through the VIN(EN) resistor network
can easily be greater than the supply current consumed
by the LT8611. Therefore, the VIN(EN) resistors should be
large to minimize their effect on efficiency at low loads.
Current Control Loop
In addition to regulating the output voltage the LT8611
includes a current regulation loop for setting the average
input or output current limit as shown in the Typical Applications section.
The LT8611 measures voltage drop across an external
current sense resistor using the ISP and ISN pins. This
resistor may be connected between the inductor and the
output capacitor to sense the output current or may be
placed between the VIN bypass capacitor and the input
power source to sense input current. The current loop
modulates the internal cycle-by-cycle switch current limit
such that the average voltage across ISP-ISN pins does
not exceed 50mV.
Care must be taken and filters should be used to assure
the signal applied to the ISN and ISP pins has a peak-topeak ripple of less than 30mV for accurate operation. In
addition to high crest factor current waveforms such as
the input current of DC/DC regulators, another cause of
high ripple voltage across the sense resistor is excessive
resistor ESL. Typically the problem is solved by using a
small ceramic capacitor across the sense resistor or using
a filter network between the ISP and ISN pins.
The ICTRL pin allows the ISP-ISN set point to be linearly
controlled from 50mV to 0mV as the ICTRL pin is ramped
from 1V down to 0V, respectively and as shown in Figure 3.
When this functionality is unused the ICTRL pin may be
tied to INTVCC or floated. In addition the ICTRL pin includes
a 2µA pull-up source such that a capacitor may be added
for soft-start functionality.
The IMON pin is a voltage output proportional to the voltage
across the current sense resistor such that VIMON = 20 •
(ISP-ISN) as shown in Figure 4. This output can be used
to monitor the input or output current of the LT8611 or
may be an input to an ADC for further processing.
60
MAX VISP-VISN VOLTAGE (mV)
Enable Pin
50
40
30
20
10
0
0
500
1000
1500
ICTRL VOLTAGE (mV)
2000
8611 F03
Figure 3. LT8611 Sense Voltage vs ICTRL Voltage
Rev. B
18
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LT8611
APPLICATIONS INFORMATION
1200
capacitor on TR/SS enables soft starting the output to prevent current surge on the input supply. During the soft-start
ramp the output voltage will proportionally track the TR/SS
pin voltage. For output tracking applications, TR/SS can
be externally driven by another voltage source. From 0V to
0.97V, the TR/SS voltage will override the internal 0.97V
reference input to the error amplifier, thus regulating the
FB pin voltage to that of TR/SS pin. When TR/SS is above
0.97V, tracking is disabled and the feedback voltage will
regulate to the internal reference voltage. The TR/SS pin
may be left floating if the function is not needed.
VSYNC = 3.3V
1000
VIMON (mV)
800
600
400
200
0
0
10
20
30
VISP-VISN (mV)
40
50
8611 G45
Figure 4. LT8611 Sense Voltage vs IMON Voltage
INTVCC Regulator
An internal low dropout (LDO) regulator produces the 3.4V
supply from VIN that powers the drivers and the internal
bias circuitry. The INTVCC can supply enough current for
the LT8611’s circuitry and must be bypassed to ground
with a minimum of 1μF ceramic capacitor. Good bypassing
is necessary to supply the high transient currents required
by the power MOSFET gate drivers. To improve efficiency
the internal LDO can also draw current from the BIAS
pin when the BIAS pin is at 3.1V or higher. Typically the
BIAS pin can be tied to the output of the LT8611, or can
be tied to an external supply of 3.3V or above. If BIAS is
connected to a supply other than VOUT, be sure to bypass
with a local ceramic capacitor. If the BIAS pin is below
3.0V, the internal LDO will consume current from VIN.
Applications with high input voltage and high switching
frequency where the internal LDO pulls current from VIN
will increase die temperature because of the higher power
dissipation across the LDO. Do not connect an external
load to the INTVCC pin.
Output Voltage Tracking and Soft-Start
The LT8611 allows the user to program its output voltage
ramp rate by means of the TR/SS pin. An internal 2.2μA
pulls up the TR/SS pin to INTV CC. Putting an external
An active pull-down circuit is connected to the TR/SS pin
which will discharge the external soft-start capacitor in
the case of fault conditions and restart the ramp when the
faults are cleared. Fault conditions that clear the soft-start
capacitor are the EN/UV pin transitioning low, VIN voltage
falling too low, or thermal shutdown.
Output Power Good
When the LT8611’s output voltage is within the ±9%
window of the regulation point, which is a VFB voltage in
the range of 0.883V to 1.057V (typical), the output voltage
is considered good and the open-drain PG pin goes high
impedance and is typically pulled high with an external
resistor. Otherwise, the internal pull-down device will pull
the PG pin low. To prevent glitching both the upper and
lower thresholds include 1.3% of hysteresis.
The PG pin is also actively pulled low during several fault
conditions: EN/UV pin is below 1V, INTVCC has fallen too
low, VIN is too low, or thermal shutdown.
Synchronization
To select low ripple Burst Mode operation, tie the SYNC pin
below 0.4V (this can be ground or a logic low output). To
synchronize the LT8611 oscillator to an external frequency
connect a square wave (with 20% to 80% duty cycle) to
the SYNC pin. The square wave amplitude should have valleys that are below 0.4V and peaks above 2.4V (up to 6V).
Rev. B
For more information www.analog.com
19
LT8611
APPLICATIONS INFORMATION
The LT8611 will not enter Burst Mode operation at low
output loads while synchronized to an external clock, but
instead will pulse skip to maintain regulation. The LT8611
may be synchronized over a 200kHz to 2.2MHz range. The
RT resistor should be chosen to set the LT8611 switching
frequency equal to or below the lowest synchronization
input. For example, if the synchronization signal will be
500kHz and higher, the RT should be selected for 500kHz.
The slope compensation is set by the RT value, while the
minimum slope compensation required to avoid subharmonic oscillations is established by the inductor size,
input voltage, and output voltage. Since the synchronization frequency will not change the slopes of the inductor
current waveform, if the inductor is large enough to avoid
subharmonic oscillations at the frequency set by RT, then
the slope compensation will be sufficient for all synchronization frequencies.
For some applications it is desirable for the LT8611 to
operate in pulse-skipping mode, offering two major differences from Burst Mode operation. First is the clock stays
awake at all times and all switching cycles are aligned to
the clock. Second is that full switching frequency is reached
at lower output load than in Burst Mode operation. These
two differences come at the expense of increased quiescent
current. To enable pulse-skipping mode, the SYNC pin is
tied high either to a logic output or to the INTVCC pin.
Frequency foldback behavior depends on the state of the
SYNC pin: If the SYNC pin is low the switching frequency
will slow while the output voltage is lower than the programmed level. If the SYNC pin is connected to a clock
source or tied high, the LT8611 will stay at the programmed
frequency without foldback and only slow switching if the
inductor current exceeds safe levels.
There is another situation to consider in systems where
the output will be held high when the input to the LT8611
is absent. This may occur in battery charging applications
or in battery-backup systems where a battery or some
other supply is diode ORed with the LT8611’s output. If
the VIN pin is allowed to float and the EN pin is held high
(either by a logic signal or because it is tied to VIN), then
the LT8611’s internal circuitry will pull its quiescent current
through its SW pin. This is acceptable if the system can
tolerate several μA in this state. If the EN pin is grounded
the SW pin current will drop to near 1µA. However, if the
VIN pin is grounded while the output is held high, regardless of EN, parasitic body diodes inside the LT8611 can
pull current from the output through the SW pin and
the VIN pin. Figure 5 shows a connection of the VIN and
EN/UV pins that will allow the LT8611 to run only when
the input voltage is present and that protects against a
shorted or reversed input.
The LT8611 does not operate in forced continuous mode
regardless of SYNC signal. Never leave the SYNC pin
floating.
Shorted and Reversed Input Protection
D1
VIN
VIN
LT8611
EN/UV
GND
8611 F05
The LT8611 will tolerate a shorted output. Several features
are used for protection during output short-circuit and
brownout conditions. The first is the switching frequency
will be folded back while the output is lower than the set
point to maintain inductor current control. Second, the
bottom switch current is monitored such that if inductor
current is beyond safe levels switching of the top switch
will be delayed until such time as the inductor current
falls to safe levels.
Figure 5. Reverse VIN Protection
Rev. B
20
For more information www.analog.com
LT8611
APPLICATIONS INFORMATION
PCB Layout
For proper operation and minimum EMI, care must be taken
during printed circuit board layout. Figure 6 shows the
recommended component placement with trace, ground
plane and via locations. Note that large, switched currents
flow in the LT8611’s VIN pins, PGND pins, and the input capacitor (C1). The loop formed by the input capacitor should
be as small as possible by placing the capacitor adjacent
to the VIN and PGND pins. When using a physically large
input capacitor the resulting loop may become too large
in which case using a small case/value capacitor placed
close to the VIN and PGND pins plus a larger capacitor
further away is preferred. These components, along with
the inductor and output capacitor, should be placed on
the same side of the circuit board, and their connections
should be made on that layer. Place a local, unbroken ground
plane under the application circuit on the layer closest to
the surface layer. The SW and BOOST nodes should be
as small as possible. Finally, keep the FB and RT nodes
small so that the ground traces will shield them from the
SW and BOOST nodes. The exposed pad on the bottom of
the package must be soldered to ground so that the pad
is connected to ground electrically and also acts as a heat
sink thermally. To keep thermal resistance low, extend the
ground plane as much as possible, and add thermal vias
under and near the LT8611 to additional ground planes
within the circuit board and on the bottom side.
ICTRL IMON
24
23
ISN
ISP
22
21
GND
VOUT
FB
1
20
TR/SS
2
19
RT
3
18
4
17 INTVCC
5
16
6
15
7
14
8
13
SYNC
EN/UV
VIN
GND
9
10
11
PG
BIAS
BST
SW
12
VOUT
VOUT LINE TO BIAS
VOUT LINE TO ISN
LINE TO ISP
VIAS TO GROUND PLANE
8611 F06
OUTLINE OF LOCAL
GROUND PLANE
Figure 6. Recommended PCB Layout for the LT8611
High Temperature Considerations
For higher ambient temperatures, care should be taken in
the layout of the PCB to ensure good heat sinking of the
LT8611. The exposed pad on the bottom of the package
must be soldered to a ground plane. This ground should be
tied to large copper layers below with thermal vias; these
layers will spread heat dissipated by the LT8611. Placing
additional vias can reduce thermal resistance further. The
maximum load current should be derated as the ambient
temperature approaches the maximum junction rating.
Power dissipation within the LT8611 can be estimated
by calculating the total power loss from an efficiency
measurement and subtracting the inductor loss. The
die temperature is calculated by multiplying the LT8611
power dissipation by the thermal resistance from junction
to ambient. The LT8611 will stop switching and indicate
a fault condition if safe junction temperature is exceeded.
Rev. B
For more information www.analog.com
21
LT8611
TYPICAL APPLICATIONS
5V Step-Down with 1A Output Current Limit
VIN
5.5V TO 42V
4.7µF ON OFF
VIN
BST
0.1µF
EN/UV
SYNC
IMON
4.7µH
LT8611
INTVCC
0.1µF
1µF
1µF
ISP
ISN
BIAS
PG
ICTRL
0.050Ω
SW
10pF
TR/SS
RT
1M
FB
PGND
52.3k
GND
VOUT
5V
1A
243k
47µF
8611 TA02
fSW = 800kHz
3.3V Step-Down with 1A Input Current Limit
1µF
VIN
3.8V TO 42V
0.050Ω
4.7µF ON OFF
VIN
ISN
ISP
BST
0.10µF
EN/UV
SYNC
IMON
4.7µH
LT8611
ICTRL
SW
BIAS
PG
INTVCC
0.1µF
1µF
4.7pF
TR/SS
RT
41.2k
PGND GND
VOUT
3.3V
1M
FB
412k
47µF
8611 TA03
fSW = 1MHz
3.3V Step-Down with 1A Input Current Limit and 7V VIN Undervoltage Lockout
VIN
3.8V TO 42V
0.050Ω
VIN
1µF
604k
4.7µF
ISN
ISP
BST
0.1µF
EN/UV
SYNC
100k
IMON
4.7µH
LT8611
ICTRL
SW
BIAS
PG
INTVCC
0.1µF
1µF
60.4k
4.7pF
TR/SS
RT
PGND GND
VOUT
3.3V
1M
FB
412k
fSW = 700kHz
47µF
8611 TA04
Rev. B
22
For more information www.analog.com
LT8611
TYPICAL APPLICATIONS
Digitally Controlled Current/Voltage Source
VIN
3.8V TO 42V
VIN
4.7µF ON OFF
BST
0.1µF
EN/UV
4.7µH
SYNC
µC
ADC
IMON
DAC
ICTRL
SW
LT8611
0.025Ω
1µF
VOUT
3.3V
2A
ISP
ISN
BIAS
INTVCC
4.7pF
PG
TR/SS
RT
1µF
PGND
60.4k
1M
FB
GND
412k
47µF
8611 TA05
fSW = 700kHz
CCCV Battery Charger
VIN
3.8V TO 42V
D1
4.7µF ON OFF
VIN
BST
0.1µF
EN/UV
SYNC
IMON
4.7µH
SW
LT8611
1µF
10pF
PG
INTVCC
0.1µF
1µF
ISP
ISN
BIAS
ICTRL
TR/SS
RT
60.4k
+
Li-Ion
BATTERY
324k
FB
PGND GND
VOUT
4.1V
1A
0.050Ω
100k
47µF
8611 TA06
fSW = 700kHz
–3.3V Negative Converter with 1A Output Current Limit
VIN
3.8V TO 38V
4.7µF
VIN
0.1µF
BST
EN/UV
SW
SYNC
ISP
LT8611
60.4k
4.7µF
IMON
ICTRL
ISN
BIAS
INTVCC
0.1µF
1µF
60.4k
0.1µF
4.7µH
PG
1µF
10pF
TR/SS
RT
PGND GND
1M
FB
412k
f = 700kHz
47µF
0.05Ω
8611 TA07
VOUT
–3.3V
1A
Rev. B
For more information www.analog.com
23
LT8611
TYPICAL APPLICATIONS
2MHz, 3.3V Step-Down with Power Good without Current Sense
VIN
3.8V TO 42V
4.7µF
VIN
ON OFF
BST
EN/UV
0.1µF
2.2µH
SW
ISP
ISN
BIAS
PG
SYNC
IMON LT8611
ICTRL
150k
1µF
TR/SS
RT
18.2k
PGND GND
PGOOD
4.7pF
INTVCC
0.1µF
VOUT
3.3V
2.5A
1M
FB
412k
f = 2MHz
47µF
8611 TA08
1V Step-Down with 2A Output Current Limit
VIN
3.8V TO 42V
10µF
VIN
ON OFF
BST
EN/UV
SW
ISP
SYNC
IMON LT8611
ICTRL
INTVCC
0.1µF
1µF
0.1µF
10µH
0.025Ω
VOUT
0.97V
2A
1µF
ISN
BIAS
PG
FB
TR/SS
RT
150k
100µF
PGND GND
f = 300kHz
8611 TA09
Rev. B
24
For more information www.analog.com
LT8611
TYPICAL APPLICATIONS
12V Step-Down with 1A Output Current Limit
VIN
12.5V TO 42V
10µF
VIN
BST
0.1µF
10µH
EN/UV
ON OFF
SW
ISP
SYNC
IMON LT8611
ICTRL
VOUT
12V
1A
0.05Ω
1µF
ISN
BIAS
PG
10pF
INTVCC
TR/SS
0.1µF
1µF
RT
60.4k
PGND GND
1M
FB
88.7k
f = 700kHz
22µF
8611 TA10
2A LED Driver
VIN
3.8V TO 42V
4.7µF
VIN
ON OFF
BST
EN/UV
SYNC
IMON
ICTRL
SW
ISP
LT8611
0.1µF
4.7µH
0.025Ω
1µF
ISN
BIAS
PG
2A
D1
10pF
INTVCC
0.1µF
1µF
60.4k
TR/SS
RT
PGND GND
FB
f = 700kHz
D1: LUMINUS CBT-40
420k
100k
4.7µF
8611 TA11
Rev. B
For more information www.analog.com
25
LT8611
PACKAGE DESCRIPTION
UDD Package
24-Lead Plastic QFN (3mm × 5mm)
(Reference LTC DWG # 05-08-1833 Rev Ø)
0.70 ±0.05
3.50 ±0.05
2.10 ±0.05
3.65 ±0.05
1.50 REF
1.65 ±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
3.00 ±0.10
0.75 ±0.05
1.50 REF
23
R = 0.05 TYP
PIN 1 NOTCH
R = 0.20 OR 0.25
× 45° CHAMFER
24
0.40 ±0.10
PIN 1
TOP MARK
(NOTE 6)
5.00 ±0.10
1
2
3.65 ±0.10
3.50 REF
1.65 ±0.10
(UDD24) QFN 0808 REV Ø
0.200 REF
0.00 – 0.05
R = 0.115
TYP
0.25 ±0.05
0.50 BSC
BOTTOM VIEW—EXPOSED PAD
NOTE:
1. DRAWING IS NOT A JEDEC PACKAGE OUTLINE
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. B
26
For more information www.analog.com
LT8611
REVISION HISTORY
REV
DATE
DESCRIPTION
A
01/17
Clarified Current Sense Voltage and IMON Monitor Pin Voltage Parameters
PAGE NUMBER
3
B
04/21
AECQ100 Statement
1
#W Models in Order Table
2
Rev. B
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.
more by
information
www.analog.com
27
LT8611
TYPICAL APPLICATION
Coincident Tracking Step-Downs Each with 2A Output Current Limit
VIN
3.8V TO 42V
10µF
VIN
ON OFF
BST
EN/UV
SYNC
IMON
SW
ISP
LT8611
ICTRL
0.1µF
5.6µH
VOUT
3.3V
2A
0.025Ω
1µF
ISN
BIAS
PG
16.5k
10pF
20k
INTVCC
0.1µF
1µF
TR/SS
RT
88.7k
PGND GND
232k
FB
97.6k
47µF
f = 500kHz
VIN
10µF
ON OFF
BST
EN/UV
SYNC
IMON
ICTRL
SW
ISP
LT8611
0.1µF
5.6µH
VOUT
1.8V
2A
0.025Ω
1µF
ISN
BIAS
PG
4.7pF
INTVCC
TR/SS
RT
1µF
88.7k
PGND GND
80.6k
FB
93.1k
68µF
8611 TA12
f = 500kHz
RELATED PARTS
PART NUMBER
DESCRIPTION
COMMENTS
LT8610
42V, 2.5A, 96% Efficiency, 2.2MHz Synchronous Micropower Step-Down
DC/DC Converter with IQ = 2.5µA
VIN: 3.4V to 42V, VOUT(MIN) = 0.97V, IQ = 2.5µA,
ISD < 1µA, MSOP-16E Package
LT3690
36V with 60V Transient Protection, 4A, 92% Efficiency, 1.5MHz
Synchronous Micropower Step-Down DC/DC Converter with IQ = 70µA
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LT3971
38V, 1.2A, 2.2MHz High Efficiency Micropower Step-Down DC/DC
Converter with IQ = 2.8µA
VIN: 4.2V to 38V, VOUT(MIN) = 1.21V, IQ = 2.8µA,
ISD < 1µA, 3mm × 3mm DFN-10 and MSOP-10E Packages
LT3991
55V, 1.2A, 2.2MHz High Efficiency Micropower Step-Down DC/DC
Converter with IQ = 2.8µA
VIN: 4.2V to 55V, VOUT(MIN) = 1.21V, IQ = 2.8µA,
ISD < 1µA, 3mm × 3mm DFN-10 and MSOP-10E Packages
LT3970
40V, 350mA, 2.2MHz High Efficiency Micropower Step-Down DC/DC
Converter with IQ = 2.5µA
VIN: 4.2V to 40V, VOUT(MIN) = 1.21V, IQ = 2.5µA,
ISD < 1µA, 3mm × 2mm DFN-10 and MSOP-10 Packages
LT3990
62V, 350mA, 2.2MHz High Efficiency MicroPower Step-Down DC/DC
Converter with IQ = 2.5µA
VIN: 4.2V to 62V, VOUT(MIN) = 1.21V, IQ = 2.5µA,
ISD < 1µA, 3mm × 3mm DFN-10 and MSOP-6E Packages
LT3480
36V with Transient Protection to 60V, 2A (IOUT), 2.4MHz, High Efficiency
Step-Down DC/DC Converter with Burst Mode Operation
VIN: 3.6V to 36V, Transient to 60V, VOUT(MIN) = 0.78V,
IQ = 70µA, ISD < 1µA, 3mm × 3mm DFN-10 and
MSOP-10E Packages
LT3980
58V with Transient Protection to 80V, 2A (IOUT), 2.4MHz, High Efficiency
Step-Down DC/DC Converter with Burst Mode Operation
VIN: 3.6V to 58V, Transient to 80V, VOUT(MIN) = 0.78V,
IQ = 85µA, ISD < 1µA, 3mm × 4mm DFN-16 and
MSOP-16E Packages
Rev. B
28
04/21
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