LT8641
65V, 3.5A Synchronous Step-Down
Silent Switcher with 2.5µA Quiescent Current
FEATURES
DESCRIPTION
Silent Switcher® Architecture
n Ultralow EMI Emissions
n Spread Spectrum Frequency Modulation
n High Efficiency at High Frequency
n Up to 95% Efficiency at 1MHz, 12V to 5V
IN
OUT
n Up to 94% Efficiency at 2MHz, 12V to 5V
IN
OUT
n Wide Input Voltage Range: 3V to 65V
n 3.5A Maximum Continuous Output,
5A Peak Transient Output
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 Fast Minimum Switch On-Time: 35ns
n Low Dropout Under All Conditions: 130mV at 1A
n Safely Tolerates Inductor Saturation in Overload
n Adjustable and Synchronizable: 200kHz to 3MHz
n Peak Current Mode Operation
n Output Soft-Start and Tracking
n Small 18-Lead 3mm × 4mm QFN
n AEC-Q100 Qualified for Automotive Applications
The LT®8641 step-down regulator features Silent Switcher
architecture designed to minimize EMI emissions while
delivering high efficiency at frequencies up to 3MHz.
Assembled in a 3mm × 4mm QFN, the monolithic construction with integrated power switches and inclusion of
all necessary circuitry yields a solution with a minimal PCB
footprint. An ultralow 2.5µA quiescent current—with the
output in full regulation—enables applications requiring
highest efficiency at very small load currents. Transient
response remains excellent and output voltage ripple is
below 10mVP-P at any load, from zero to full current.
n
APPLICATIONS
Automotive and Industrial Supplies
General Purpose Step-Down
n GSM Power Supplies
n
The LT8641 allows high VIN to low VOUT conversion
at high frequency with a fast minimum top switch ontime of 35ns. Operation is safe in overload even with a
saturated inductor.
Essential features are included and easy to use: An opendrain PG pin signals when the output is in regulation.
The SYNC/MODE pin selects between Burst Mode, pulseskipping, or spread spectrum mode, and also allows synchronization to an external clock. Soft-start and tracking
functionality is accessed via the TR/SS pin. An accurate
enable threshold can be set using the EN/UV pin and a
resistor at the RT pin programs switch frequency.
All registered trademarks and trademarks are the property of their respective owners. Protected
by U.S. patents, including 8823345.
n
TYPICAL APPLICATION
12VIN to 5VOUT Efficiency
5V 3.5A Step-Down Converter
4.7µF
1µF
EN/UV
GND1
PG
10nF
SYNC/MODE
TR/SS
BST
fSW = 1MHz
1µF
0.1µF 4.7µH
4.7pF
INTVCC
RT
VOUT
5V
3.5A
SW
BIAS
1µF
41.2k
95
2.275
90
VIN2
GND2
LT8641
2.600
1M
47µF
FB
GND
191k
85
1.625
80
1.300
0.975
75
70
POWER LOSS
65
60
0.5
8641 TA01a
1.950
EFFICIENCY
1
POWER LOSS (W)
VIN1
EFFICIENCY (%)
VIN
5.5V TO 65V
100
0.650
1MHz, L = 3.3μH
2MHz, L = 2.2μH 0.325
3MHz, L = 1.5μH
0
1.5
2
2.5
3
3.5
LOAD CURRENT (A)
8641 TA01b
Rev. C
Document Feedback
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1
LT8641
PIN CONFIGURATION
VIN, EN/UV.................................................................65V
PG..............................................................................42V
BIAS...........................................................................25V
FB, TR/SS ...................................................................4V
SYNC/MODE Voltage ..................................................6V
Operating Junction Temperature Range (Note 2)
LT8641E.............................................. –40°C to 125°C
LT8641I............................................... –40°C to 125°C
LT8641J.............................................. –40°C to 150°C
LT8641H............................................. –40°C to 150°C
Storage Temperature Range................... –65°C to 150°C
SYNC/MODE
17
PG
20 19 18
FB
GND
TOP VIEW
16 TR/SS
BIAS 1
INTVCC 2
15 RT
BST 3
GND1 6
21
SW
14 EN/UV
13 VIN2
11 GND2
7
8
9
10
GND2
22
SW
SW
VIN1 4
SW
(Note 1)
GND1
ABSOLUTE MAXIMUM RATINGS
UDC PACKAGE
18-LEAD (3mm × 4mm) PLASTIC QFN
θJA = 40°C/W, θJC(PAD) = 12°C/W (NOTE 3)
EXPOSED PAD (PINS 21, 22) ARE SW, SHOULD BE SOLDERED TO PCB
NOTE: PINS 5 AND 12 ARE REMOVED. CONFIGURATION DOES NOT MATCH
JEDEC 20-LEAD PACKAGE OUTLINE
ORDER INFORMATION
LEAD FREE FINISH
TAPE AND REEL
PART MARKING*
PACKAGE DESCRIPTION
TEMPERATURE RANGE
LT8641EUDC#PBF
LT8641EUDC#TRPBF
LGSN
18-Lead (3mm × 4mm) Plastic QFN
–40°C to 125°C
LT8641IUDC#PBF
LT8641IUDC#TRPBF
LGSN
18-Lead (3mm × 4mm) Plastic QFN
–40°C to 125°C
LT8641HUDC#PBF
LT8641HUDC#TRPBF
LGSN
18-Lead (3mm × 4mm) Plastic QFN
–40°C to 150°C
LT8641IUDC#WPBF
LT8641IUDC#WTRPBF
LGSN
18-Lead (3mm × 4mm) Plastic QFN
–40°C to 125°C
LT8641JUDC#WPBF
LT8641JUDC#WTRPBF
LGSN
18-Lead (3mm × 4mm) Plastic QFN
–40°C to 150°C
LT8641HUDC#WPBF
LT8641HUDC#WTRPBF
LGSN
18-Lead (3mm × 4mm) Plastic QFN
–40°C to 150°C
AUTOMOTIVE PRODUCTS**
Contact the factory for parts specified with wider operating temperature ranges. *The temperature grade is identified by a label on the shipping container.
Tape and reel specifications. Some packages are available in 500 unit reels through designated sales channels with #TRMPBF suffix.
**Versions of this part are available with controlled manufacturing to support the quality and reliability requirements of automotive applications. These
models are designated with a #W suffix. Only the automotive grade products shown are available for use in automotive applications. Contact your
local Analog Devices account representative for specific product ordering information and to obtain the specific Automotive Reliability reports for
these models.
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
2.6
3.0
V
l
0.75
0.75
3
10
µA
µA
l
1.7
1.7
4
10
µA
µA
0.3
0.5
mA
17
200
50
350
µA
µA
VEN/UV = 0V
VEN/UV = 2V, Not Switching, VSYNC = 0V
VEN/UV = 2V, Not Switching, VSYNC = 2V
VIN Current in Regulation
2
VOUT = 0.8V, VIN = 6V, Output Load = 100µA
VOUT = 0.8V, VIN = 6V, Output Load = 1mA
l
l
UNITS
Rev. C
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LT8641
ELECTRICAL
CHARACTERISTICS
The
l denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at TA = 25°C.
PARAMETER
CONDITIONS
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
BIAS Pin Current Consumption
VBIAS = 3.3V, fSW = 2MHz
Minimum On-Time
ILOAD = 1.5A, SYNC = 0V
ILOAD = 1.5A, SYNC = 2V
l
l
Oscillator Frequency
RT = 221k
RT = 60.4k
RT = 18.2k
l
l
l
Top Power NMOS On-Resistance
ISW = 1A
MIN
TYP
MAX
UNITS
0.804
0.79
0.81
0.81
0.816
0.822
V
V
0.004
0.03
%/V
–20
Minimum Off-Time
180
665
1.85
l
Bottom Power NMOS On-Resistance
VINTVCC = 3.4V, ISW = 1A
Bottom Power NMOS Current Limit
VINTVCC = 3.4V
SW Leakage Current
VIN = 42V, VSW = 0V, 42V
EN/UV Pin Threshold
EN/UV Rising
6.2
50
50
ns
ns
80
110
ns
210
700
2.00
240
735
2.15
kHz
kHz
MHz
8.2
mΩ
9.9
0.95
7.25
A
15
µA
1.01
1.07
V
45
EN/UV Pin Current
VEN/UV = 2V
PG Upper Threshold Offset from VFB
VFB Falling
l
5
PG Lower Threshold Offset from VFB
–20
VFB Rising
l
–5.25
PG Hysteresis
mV
20
nA
7.5
10.25
%
–8
–10.75
%
0.4
PG Leakage
VPG = 3.3V
–40
PG Pull-Down Resistance
VPG = 0.1V
SYNC/MODE Threshold
SYNC/MODE DC and Clock Low Level Voltage
SYNC/MODE Clock High Level Voltage
SYNC/MODE DC High Level Voltage
l
0.7
2.3
RT = 60.4k, VSYNC = 3.3V
Spread Spectrum Modulation Frequency VSYNC = 3.3V
TR/SS Source Current
l
Fault Condition, TR/SS = 0.1V
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 LT8641E 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
LT8641I is guaranteed over the full –40°C to 125°C operating junction
temperature range. The LT8641H is guaranteed over the full –40°C to
150°C operating junction temperature range. High junction temperatures
degrade operating lifetimes. Operating lifetime is derated at junction
temperatures greater than 125°C.
1.2
A
mΩ
5.8
–15
l
EN/UV Pin Hysteresis
TR/SS Pull-Down Resistance
35
35
55
4.8
nA
mA
105
Top Power NMOS Current Limit
Spread Spectrum Modulation
Frequency Range
20
9
%
40
nA
750
2000
Ω
0.9
1.2
2.6
1.4
2.9
V
V
V
22
%
2.5
kHz
1.9
220
2.6
µA
Ω
The junction temperature (TJ, in °C) is calculated from the ambient
temperature (TA in °C) and power dissipation (PD, in Watts) according to
the formula:
TJ = TA + (PD • θJA)
where θJA (in °C/W) is the package thermal impedance.
Note 3: θ values determined per JEDEC 51-7, 51-12. See Applications
Information section for information on improving the thermal resistance
and for actual temperature measurements of a demo board in typical
operating conditions.
Note 4: 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. C
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3
LT8641
TYPICAL PERFORMANCE CHARACTERISTICS
12VIN to 5VOUT Efficiency
vs Frequency
12VIN to 3.3VOUT Efficiency
vs Frequency
2.3
95
1.9
80
1.3
1.0
75
POWER LOSS
65
60
0.5
1
L = WE–LHMI7050 0.7
1MHz, L = 3.3μH
2MHz, L = 2.2μH 0.3
3MHz, L = 1.5μH
0
1.5
2
2.5
3
3.5
LOAD CURRENT (A)
1.2
80
0.9
75
POWER LOSS
70
L = WE–LHMI7050 0.6
1MHz, L = 2.2µH
2MHz, L = 1.5µH 0.3
3MHz, L = 1µH
0
1.5
2
2.5
3
3.5
LOAD CURRENT (A)
65
60
0.5
1
8641 G01
Efficiency at 3.3V OUT
EFFICIENCY
fSW = 1MHz
L = WE–LHMI7050, 2.2µH 2.3
90
2.0
85
1.8
80
1.5
75
1.3
70
1.0
65
0.8
60
55
50
POWER LOSS
0
0.5
100
2.5
1
1.5
2
2.5
LOAD CURRENT (A)
POWER LOSS (W)
EFFICIENCY (%)
95
2.0
85
1.8
80
1.5
fSW = 1MHz
75 L = WE–LHMI7050, 4.7µH
1.0
65
0.8
VIN = 12V
VIN = 24V 0.5
VIN = 36V 0.3
VIN = 48V
POWER LOSS
0
0.5
1
1.5
2
2.5
3
3.5
LOAD CURRENT (A)
60
55
50
0
8641 G03
Efficiency at 5V OUT
100
90
90
80
80
70
60
50
VIN = 12V
0.5
VIN = 24V
VIN = 36V 0.3
VIN = 48V
0
3
3.5
40
fSW = 1MHz
L = WE–LHMI7050, 4.7µH
30
20
0.01
0.1
8641 G04
VIN = 12V
VIN = 24V
VIN = 36V
VIN = 48V
1
10
100
LOAD CURRENT (mA)
Efficiency at 3.3V OUT
70
60
50
40
30
20
0.01
1000
fSW = 1MHz
L = WE–LHMI7050, 4.7µH
0.1
1
10
100
LOAD CURRENT (mA)
8641 G05
96
88
VIN = 24V
86
82
80
0.5
VIN = 24V
85
80
75
VOUT = 3.3V
ILOAD = 1.5A
L = WE–LHMI7050, 4.7µH
1.0
1.5
2.0
2.5
SWITCHING FREQUENCY (MHz)
VOUT = 5V
ILOAD = 10mA
L = WE–LHMI7050
70
3.0
8641 G07
4
817
65
1
2
3
4
5
6
7
8
INDUCTOR VALUE (µH)
9
10
8641 G08
REFERENCE VOLTAGE (mV)
90
84
VIN = 12V
90
EFFICIENCY (%)
EFFICIENCY (%)
92
819
95
VIN = 12V
1000
Reference
Reference Voltage
Voltage
100
94
VIN = 12V
V = 24V
VIN = 36V
VIN = 48V
8641 G06
Burst Mode Operation Efficiency
vs Inductor Value
Efficiency vs Frequency
1.3
70
8641 G02
EFFICIENCY (%)
100
1.5
2.3
90
EFFICIENCY (%)
70
EFFICIENCY
85
2.5
EFFICIENCY
POWER LOSS (W)
1.6
1.8
POWER LOSS (W)
85
90
EFFICIENCY (%)
EFFICIENCY
Efficiency at 5V OUT
95
2.1
95
POWER LOSS (W)
EFFICIENCY (%)
90
100
2.4
100
EFFICIENCY (%)
2.6
100
815
813
811
809
807
805
803
801
–50 –25
0
25 50 75 100 125 150
TEMPERATURE (°C)
8641 G09
Rev. C
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LT8641
TYPICAL PERFORMANCE CHARACTERISTICS
EN
EN Pin
Pin Thresholds
Thresholds
1.02
0.99
0.98
EN FALLING
0
0.05
0
–0.05
0
0.5
1
1.5
2
2.5
LOAD CURRENT (A)
3
–0.15
2.0
15
25
35
45
INPUT VOLTAGE (V)
55
65
8641 G12
Top
Top FET
FET Current
Current Limit
Limit
10
9
CURRENT LIMIT (A)
2.5
5
8.0
CURRENT LIMIT (A)
7.5
7.0
6.5
8
5% DC
7
6.0
0
10
20
30
40
50
INPUT VOLTAGE (V)
60
5.5
0.1
0.3
0.5
DUTY CYCLE
0.7
6
–50 –25
0.9
8641 G14
8641 G13
Switch Drop
500
SWITCH CURRENT = 1A
43
40
400
SWITCH DROP (mV)
150
TOP SWITCH
100
50
350
300
250
TOP SWITCH
200
150
100
BOTTOM SWITCH
0
25 50 75 100 125 150
TEMPERATURE (°C)
0
0
0.5
1
1.5
2
2.5
SWITCH CURRENT (A)
3
3.5
8641 G17
8641 G16
37
34
31
28
BOTTOM SWITCH
50
0
–50 –25
VSYNC = 0
VSYNC = FLOAT
450
200
25 50 75 100 125 150
TEMPERATURE (°C)
Minimum On-Time
Switch Drop
250
0
8641 G15
MINIMUM ON-TIME (ns)
INPUT CURRENT (µA)
3.0
1.5
SWITCH DROP (mV)
3.5
8.5
VOUT = 3.3V
L = 4.7µH
IN-REGULATION
3.5
1.0
–0.05
Top FET Current Limit vs Duty Cycle
No-Load Supply Current
4.0
0
8641 G11
8641 G10
4.5
0.05
–0.10
–0.15
25 50 75 100 125 150
TEMPERATURE (°C)
VOUT = 5V
ILOAD = 1A
0.10
–0.10
0.96
0.95
–50 –25
VOUT = 5V
VIN = 12V
CHANGE IN VOUT (%)
CHANGE IN VOUT (%)
EN THRESHOLD (V)
1.00
0.97
0.15
0.10
EN RISING
1.01
Line Regulation
Load Regulation
0.15
1.03
25
–50
ILOAD = 2A
–25
0
25
50
75
TEMPERATURE (°C)
100
125
8641 G18
Rev. C
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5
LT8641
TYPICAL PERFORMANCE CHARACTERISTICS
Dropout Voltage
VIN = 5V
VOUT SET TO REGULATE AT 5V
L = WE–LHMI7050, 1µH
400
300
200
100
0
720
710
700
690
680
0
0.5
1
1.5
2
2.5
LOAD CURRENT (A)
3
660
–50 –25
3.5
0
400
200
0
25 50 75 100 125 150
TEMPERATURE (°C)
60
40
20
200
400
600
LOAD CURRENT (mA)
600
800
Soft-Start Tracking
1.0
0.8
FB VOLTAGE (V)
SWITCHING FREQUENCY (kHz)
80
0
8641 G21
VOUT = 3.3V
VIN = 12V
VSYNC = 0V
RT = 60.4k
700
100
0
600
Frequency Foldback
800
FRONT PAGE APPLICATION
VOUT = 5V
fSW = 1MHz
120
800
8641 G20
Minimum Load to Full Frequency
(Pulse-Skipping Mode)
140
FRONT PAGE APPLICATION
VIN = 12V
VOUT = 5V
1000
670
8641 G19
LOAD CURRENT (mA)
Burst Frequency
1200
RT = 60.4k
730
SWITCHING FREQUENCY (kHz)
500
DROPOUT VOLTAGE (mV)
Switching
SwitchingFrequency
Frequency
740
SWITCHING FREQUENCY (kHz)
600
500
400
300
200
0.6
0.4
0.2
100
5
15
25
35
45
INPUT VOLTAGE (V)
55
0
65
0
0.2
8641 G22
0.4
0.6
FB VOLTAGE (V)
0.8
0
1
0
0.2
0.4
0.6
0.8
TR/SS VOLTAGE (V)
1.0
1.2
8641 G24
8641 G23
Soft-StartCurrent
Current
Soft–Start
VSS = 0.5V
2.0
1.9
1.8
1.7
1.6
1.5
1.4
–50 –25
0
–6.0
25 50 75 100 125 150
TEMPERATURE (°C)
9.5
9.0
8.5
FB RISING
8.0
7.5
FB FALLING
7.0
6.5
6.0
–50 –25
0
25 50 75 100 125 150
TEMPERATURE (°C)
8641 G25
6
PG THRESHOLD OFFSET FROM VREF (%)
SS PIN CURRENT (µA)
2.1
PG
PG Low
Low Thresholds
Thresholds
PG High Thresholds
10.0
PG THRESHOLD OFFSET FROM VREF (%)
2.2
8641 G26
–6.5
–7.0
–7.5
FB RISING
–8.0
–8.5
FB FALLING
–9.0
–9.5
–10.0
–50 –25
0
25 50 75 100 125 150
TEMPERATURE (°C)
8641 G27
Rev. C
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LT8641
TYPICAL PERFORMANCE CHARACTERISTICS
RT Programmed Switching
Frequency
VMinimum
IN UVLO Input Voltage
250
Bias Pin Current
3.2
7.5
3.0
7.0
VBIAS = 5V
VOUT = 5V
ILOAD = 1A
fSW = 1MHz
175
INPUT VOLTAGE (V)
RT PIN RESISTOR (kΩ)
200
150
125
100
75
50
BIAS PIN CURRENT (mA)
225
2.8
2.6
2.4
0
0.2
0.6
1.4 1.8 2.2 2.6
1
SWITCHING FREQUENCY (MHz)
2.0
–50 –25
3
6.0
5.5
5.0
2.2
25
6.5
0
4.5
25 50 75 100 125 150
TEMPERATURE (°C)
5
15
25
35
45
INPUT VOLTAGE (V)
55
65
8641 G30
8641 G29
8641 G28
10
5
0
0.2
0.6 1.0 1.4 1.8 2.2 2.6
SWITCHING FREQUENCY (MHz)
3.0
50
40
20
10
0
500ns/DIV
FRONT PAGE APPLICATION
12VIN TO 5VOUT AT 1A
8641 G34
70
60
50
40
30
20
10
0
0.5
1
1.5
2
2.5
LOAD CURRENT (A)
3
3.5
0
0
0.2
0.4
0.6
DUTY CYCLE OF 5A LOAD
Switching Waveforms, Burst
Mode Operation
Switching Waveforms
IL
500mA/DIV
IL
1A/DIV
VSW
5V/DIV
VSW
20V/DIV
10µs/DIV
FRONT PAGE APPLICATION
12VIN TO 5VOUT AT 10mA
VSYNC = 0V
0.8
8641 G33
8641 G32
Switching Waveforms, Full
Frequency Continuous Operation
VSW
5V/DIV
DC2373A DEMO BOARD
VIN = 12V
VOUT = 5V
fSW = 2MHz
STANDBY LOAD = 0.25A
1kHz PULSED LOAD = 5A
80
30
8641 G31
IL
1A/DIV
DC2373A DEMO BOARD
VIN = 12V, fSW = 1MHz
VIN = 24V, fSW = 1MHz
VIN = 12V, fSW = 2MHz
VIN = 24V, fSW = 2MHz
CASE TEMPERATURE RISE (°C)
15
90
60
VBIAS = 5V
VOUT = 5V
VIN = 12V
ILOAD = 1A
CASE TEMPERATURE RISE (°C)
BIAS PIN CURRENT (mA)
20
Case Temperature Rise vs 5A
Pulsed Load
Case
Case Temperature
Temperature Rise
Rise
Bias Pin Current
8641 G35
500ns/DIV
FRONT PAGE APPLICATION
48VIN TO 5VOUT AT 1A
8641 G36
Rev. C
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7
LT8641
TYPICAL PERFORMANCE CHARACTERISTICS
Transient Response; Load Current
Stepped from 300mA (Burst Mode
Operation) to 1.3A
Transient Response; Load Current
Stepped from 1A to 2A
ILOAD
1A/DIV
ILOAD
1A/DIV
VOUT
100mA/DIV
VOUT
200mA/DIV
8641 G37
50µs/DIV
50µs/DIV
FRONT PAGE APPLICATION
300mA (Burst Mode OPERATION) TO
1.3A TRANSIENT
12VIN, 5VOUT
COUT = 47µF
FRONT PAGE APPLICATION
1A TO 2A TRANSIENT
12VIN, 5VOUT
COUT = 47µF
Start-Up Dropout Performance
Start-Up Dropout Performance
VIN
VIN
2V/DIV
VIN
VIN
2V/DIV
VOUT
VOUT
2V/DIV
8641 G38
VOUT
VOUT
2V/DIV
100ms/DIV
2.5Ω LOAD
(2A IN REGULATION)
100ms/DIV
20Ω LOAD
(250mA IN REGULATION)
8641 G39
8641 G40
Conducted EMI Performance
60
50
AMPLITUDE (dBµV)
40
30
20
10
0
–10
–20
SPREAD SPECTRUM MODE
FIXED FREQUENCY MODE
–30
–40
0
3
6
9
12
15
18
21
FREQUENCY (MHz)
24
27
30
8641 G41
DC2373A DEMO BOARD
(WITH EMI FILTER INSTALLED)
14V INPUT TO 5V OUTPUT AT 3.5A, fSW = 2MHz
8
Rev. C
For more information www.analog.com
LT8641
TYPICAL PERFORMANCE CHARACTERISTICS
Radiated EMI Performance
(CISPR25 Radiated Emission Test with Class 5 Peak Limits)
50
VERTICAL POLARIZATION
PEAK DETECTOR
45
AMPLITUDE (dBµV/m)
40
35
30
25
20
15
10
5
CLASS 5 PEAK LIMIT
FIXED FREQUENCY MODE
SPREAD SPECTRUM MODE
0
–5
0
100
200
300
400
500
600
700
800
900
1000
FREQUENCY (MHz)
50
HORIZONTAL POLARIZATION
PEAK DETECTOR
45
AMPLITUDE (dBµV/m)
40
35
30
25
20
15
10
5
CLASS 5 PEAK LIMIT
FIXED FREQUENCY MODE
SPREAD SPECTRUM MODE
0
-5
0
100
200
300
400
500
600
700
800
900
1000
FREQUENCY (MHz)
DC2373A DEMO BOARD
(WITH EMI FILTER INSTALLED)
14V INPUT TO 5V OUTPUT AT 3.5A, fSW = 2MHz
8641 G42
Rev. C
For more information www.analog.com
9
LT8641
PIN FUNCTIONS
BIAS (Pin 1): 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 to 25V 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. If
no supply is available, tie to GND.
GND2 (10, 11): 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 GND2 pins as possible.
Also be sure to tie GND2 to the ground plane. See the
Applications Information section for sample layout.
INTVCC (Pin 2): 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 BIAS >
3.1V, otherwise current will be drawn from VIN. Voltage
on INTVCC will vary between 2.8V and 3.4V when BIAS
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.
VIN2 (Pin 13): The LT8641 requires two 1µF small input
bypass capacitors. One 1µF capacitor should be placed
between VIN1 and GND1. A second 1µF capacitor should
be placed between VIN2 and GND2. These capacitors
must be placed as close as possible to the LT8641. A
third larger capacitor of 2.2µF or more should be placed
close to the LT8641 with the positive terminal connected
to VIN1 and VIN2, and the negative terminal connected
to ground. See the Applications Information section for
sample layout.
BST (Pin 3): 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.
EN/UV (Pin 14): The LT8641 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 LT8641 will shut down.
VIN1 (Pin 4): The LT8641 requires two 1µF small input
bypass capacitors. One 1µF capacitor should be placed
between VIN1 and GND1. A second 1µF capacitor should
be placed between VIN2 and GND2. These capacitors must
be placed as close as possible to the LT8641. A third
larger capacitor of 2.2µF or more should be placed close
to the LT8641 with the positive terminal connected to VIN1
and VIN2, and the negative terminal connected to ground.
See applications section for sample layout.
GND1 (6, 7): 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 GND1 pins as possible.
Also be sure to tie GND1 to the ground plane. See the
Applications Information section for sample layout.
SW (Pins 8, 9): 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 and low EMI.
10
RT (Pin 15): A resistor is tied between RT and ground to
set the switching frequency.
TR/SS (Pin 16): Output Tracking and Soft-Start Pin. This
pin allows user control of output voltage ramp rate during
start-up. A TR/SS voltage below 0.8V forces the LT8641
to regulate the FB pin to equal the TR/SS pin voltage.
When TR/SS is above 0.8V, the tracking function is disabled and the internal reference resumes control of the
error amplifier. An internal 1.9μA 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 200Ω 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.
Rev. C
For more information www.analog.com
LT8641
PIN FUNCTIONS
SYNC/MODE (Pin 17): This pin programs four different
operating modes: 1) Burst Mode. Tie this pin to ground
for Burst Mode operation at low output loads—this will
result in ultralow quiescent current. 2) Pulse-skipping
mode. Float this pin for pulse-skipping mode. This mode
offers full frequency operation down to low output loads
before pulse skipping occurs. When floating, pin leakage
currents should be 0.5), a minimum inductance is required to avoid subharmonic oscillation (See Equation 10). See Analog Devices Application
Note 19 for more details.
LMIN =
VIN •(2•DC– 1)
2.5• fSW
(10)
where DC is the duty cycle ratio (VOUT/VIN) and fSW is the
switching frequency.
Input Capacitors
The VIN of the LT8641 should be bypassed with at least
three ceramic capacitors for best performance. Two small
ceramic capacitors of 1µF should be placed close to the
part; one at the VIN1/GND1 pins and a second at VIN2/
GND2 pins. These capacitors should be 0402 or 0603 in
size. For automotive applications requiring 2 series input
capacitors, two small 0402 or 0603 may be placed at
each side of the LT8641 near the VIN1/GND1 and VIN2/
GND2 pins.
A third, larger ceramic capacitor of 2.2µF or larger should
be placed close to VIN1 or VIN2. See Low EMI PCB Layout
section for more detail. X7R or X5R capacitors are recommended for best performance across temperature and
input voltage variations.
Note that larger input capacitance is required when a lower
switching frequency is used. If the input power source has
Rev. C
For more information www.analog.com
17
LT8641
APPLICATIONS INFORMATION
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.
A ceramic input capacitor combined with trace or cable
inductance forms a high quality (under damped) tank circuit. If the LT8641 circuit is plugged into a live supply, the
input voltage can ring to twice its nominal value, possibly
exceeding the LT8641’s voltage rating. This situation is
easily avoided (see Analog Devices 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 LT8641 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 LT8641’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.
18
Ceramic Capacitors
Ceramic capacitors are small, robust and have very low
ESR. However, ceramic capacitors can cause problems
when used with the LT8641 due to their piezoelectric
nature. When in Burst Mode operation, the LT8641’s
switching frequency depends on the load current, and
at very light loads the LT8641 can excite the ceramic
capacitor at audio frequencies, generating audible noise.
Since the LT8641 operates at a lower current limit during
Burst Mode operation, the noise is typically very quiet to a
casual ear. If this is unacceptable, use a high performance
tantalum or electrolytic capacitor at the output. Low noise
ceramic capacitors are also available.
A final precaution regarding ceramic capacitors concerns
the maximum input voltage rating of the LT8641. As
previously mentioned, a ceramic input capacitor combined with trace or cable inductance forms a high quality (underdamped) tank circuit. If the LT8641 circuit is
plugged into a live supply, the input voltage can ring to
twice its nominal value, possibly exceeding the LT8641’s
rating. This situation is easily avoided (see Analog Devices
Application Note 88).
Enable Pin
The LT8641 is in shutdown when the EN pin is low and
active when the pin is high. The rising threshold of the EN
comparator is 1.01V, with 45mV 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
LT8641 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
Rev. C
For more information www.analog.com
LT8641
APPLICATIONS INFORMATION
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 Equation 11:
⎛ R3 ⎞
VIN(EN) = ⎜
+ 1⎟ • 1.01V
⎝
⎠
R4
(11)
where the LT8641 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 LT8641. Therefore, the VIN(EN) resistors should be
large to minimize their effect on efficiency at low loads.
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 LT8641’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 LT8641, 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 LT8641 allows the user to program its output voltage
ramp rate by means of the TR/SS pin. An internal 1.9μA
pulls up the TR/SS pin to INTVCC. Putting an external
capacitor on TR/SS enables soft starting the output to
prevent current surge on the input supply. During the softstart 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 0.8V, the TR/SS voltage will override the internal 0.8V reference input to the error amplifier, thus regulating the FB pin voltage to that of TR/SS pin. When TR/SS
is above 0.8V, 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.
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 LT8641’s output voltage is within the ±8% window of the regulation point, 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 0.4% 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 and Spread Spectrum
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 LT8641 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 1.5V
(up to 6V).
Rev. C
For more information www.analog.com
19
LT8641
APPLICATIONS INFORMATION
The LT8641 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 LT8641
may be synchronized over a 200kHz to 3MHz range. The
RT resistor should be chosen to set the LT8641 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 LT8641 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 floated. Leakage current on this pin
should be 3.1V
OR GND
VOUT
1.8V
3.5A
10pF
1M
825k
GND
fSW = 2MHz
L: VISHAY IHLP2525CZ-01
100µF
1210
X5R/X7R
8641 TA07
RELATED PARTS
PART
DESCRIPTION
COMMENTS
LT8640/
LT8640-1
42V, 5A, 96% Efficiency, 3MHz Synchronous MicroPower Step-Down
DC/DC Converter with IQ = 2.5µA
VIN(MIN) = 3.4V, VIN(MAX) = 42V, VOUT(MIN) = 0.97V, IQ = 2.5μA,
ISD < 1μA, 3mm × 4mm QFN-18
LT8609/
LT8609A
42V, 2A, 94% Efficiency, 2.2MHz Synchronous MicroPower Step-Down
DC/DC Converter with IQ = 2.5µA
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ISD < 1μA, MSOP-10E
LT8610A/
LT8610AB
42V, 3.5A, 96% Efficiency, 2.2MHz Synchronous MicroPower Step-Down
DC/DC Converter with IQ = 2.5µA
VIN(MIN) = 3.4V, VIN(MAX) = 42V, VOUT(MIN) = 0.97V, IQ = 2.5μA,
ISD < 1μA, MSOP-16E
LT8610AC
42V, 3.5A, 96% Efficiency, 2.2MHz Synchronous MicroPower Step-Down
DC/DC Converter with IQ = 2.5µA
VIN(MIN) = 3V, VIN(MAX) = 42V, VOUT(MIN) = 0.8V, IQ = 2.5μA,
ISD < 1μA, MSOP-16E
LT8610
42V, 2.5A, 96% Efficiency, 2.2MHz Synchronous MicroPower Step-Down
DC/DC Converter with IQ = 2.5µA
VIN(MIN) = 3.4V, VIN(MAX) = 42V, VOUT(MIN) = 0.97V, IQ = 2.5μA,
ISD < 1μA, MSOP-16E
LT8611
42V, 2.5A, 96% Efficiency, 2.2MHz Synchronous MicroPower Step-Down
DC/DC Converter with IQ = 2.5µA and Input/Output Current
Limit/Monitor
VIN(MIN) = 3.4V, VIN(MAX) = 42V, VOUT(MIN) = 0.97V, IQ = 2.5μA,
ISD < 1μA, 3mm × 5mm QFN-24
LT8616
42V, Dual 2.5A + 1.5A, 95% Efficiency, 2.2MHz Synchronous MicroPower VIN(MIN) = 3.4V, VIN(MAX) = 42V, VOUT(MIN) = 0.8V, IQ = 5μA,
Step-Down DC/DC Converter with IQ = 5µA
ISD < 1μA, TSSOP-28E, 3mm × 6mm QFN-28
LT8620
65V, 2.5A, 94% Efficiency, 2.2MHz Synchronous MicroPower Step-Down
DC/DC Converter with IQ = 2.5µA
LT8614
42V, 4A, 96% Efficiency, 2.2MHz Synchronous Silent Switcher Step-Down VIN(MIN) = 3.4V, VIN(MAX) = 42V, VOUT(MIN) = 0.97V, IQ = 2.5μA,
DC/DC Converter with IQ = 2.5µA
ISD < 1μA, 3mm × 4mm QFN18
LT8612
42V, 6A, 96% Efficiency, 2.2MHz Synchronous MicroPower Step-Down
DC/DC Converter with IQ = 2.5µA
VIN(MIN) = 3.4V, VIN(MAX) = 42V, VOUT(MIN) = 0.97V, IQ = 3.0μA,
ISD < 1μA, 3mm × 6mm QFN-28
LT8613
42V, 6A, 96% Efficiency, 2.2MHz Synchronous MicroPower Step-Down
DC/DC Converter with Current Limiting
VIN(MIN) = 3.4V, VIN(MAX) = 42V, VOUT(MIN) = 0.97V, IQ = 3.0μA,
ISD < 1μA, 3mm × 6mm QFN-28
LT8602
42V, Quad Output (2.5A + 1.5A + 1.5A + 1.5A) 95% Efficiency, 2.2MHz
Synchronous MicroPower Step-Down DC/DC Converter with IQ = 25µA
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ISD < 1μA, 6mm × 6mm QFN-40
26
VIN(MIN) = 3.4V, VIN(MAX) = 65V, VOUT(MIN) = 0.97V, IQ = 2.5μA,
ISD < 1μA, MSOP-16E, 3mm × 5mm QFN-24
Rev. C
D16866-0-12/20
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