LT8643SEV#PBF [Linear]
LT8643S - 42V, 6A Synchronous Step-Down Silent Switcher 2 with 2.5µA Quiescent Current; Package: LQFN; Pins: 24; Temperature Range: -40°C to 85°C;
| 型号： | LT8643SEV#PBF |
| 厂家： | 
Linear |
| 描述： | LT8643S - 42V, 6A Synchronous Step-Down Silent Switcher 2 with 2.5µA Quiescent Current; Package: LQFN; Pins: 24; Temperature Range: -40°C to 85°C 开关 |
| 文件： | 总30页 (文件大小：2237K) |
| 中文： | 中文翻译 | 下载： | 下载PDF数据表文档文件 |
LT8640S/LT8643S
42V, 6A Synchronous
Step-Down Silent Switcher 2 with
2.5µA Quiescent Current
DescripTion
FeaTures
Silent Switcher®2 Architecture
The LT®8640S/LT8643S synchronous step-down regulator
features second generation Silent Switcher architecture
designed to minimize EMI/EMC emissions while delivering
n
n
Ultralow EMI/EMC Emissions on Any PCB
n
Eliminates PCB Layout Sensitivity
n
high efficiency at high switching frequencies. This
includes the integration of bypass capacitors to optimize
all the fast current loops inside and make it easy to achieve
advertised EMI performance by reducing layout sensitivity.
This performance makes the LT8640S/LT8643S ideal for
noise- sensitive applications and environments.
Internal Bypass Capacitors Reduce Radiated EMI
n
Optional Spread Spectrum Modulation
High Efficiency at High Frequency
n
n
Up to 96% Efficiency at 1MHz, 12V to 5V
IN
IN
OUT
OUT
n
Up to 95% Efficiency at 2MHz, 12V to 5V
n
n
n
Wide Input Voltage Range: 3.4V to 42V
6A Maximum Continuous, 7A Peak Output
The fast, clean, low overshoot switching edges enable high
efficiency operation even at high switching frequencies,
leading to a small overall solution size. Peak current
mode control with a 30ns minimum on-time allows high
step-down ratios even at high switching frequencies.
The LT8643S has external compensation via the V pin
to enable current sharing and fast transient respConse
at high switching frequencies. A CLKOUT pin enables
synchronizing other regulators to the LT8640S/LT8643S.
Ultralow Quiescent Current Burst Mode® Operation
n
2.5µA I Regulating 12V to 3.3V
(LT8640S)
Q
IN
P-P
OUT
n
Output Ripple < 10mV
n
External Compensation: Fast Transient Response
and Current Sharing (LT8643S)
Fast Minimum Switch On-Time: 30ns
Low Dropout Under All Conditions: 100mV at 1A
Forced Continuous Mode
n
n
n
n
n
n
Adjustable and Synchronizable: 200kHz to 3MHz
Output Soft-Start and Tracking
Burst Mode operation enables ultralow standby current
consumption, forced continuous mode can control
frequency harmonics across the entire output load range,
or spread spectrum operation can further reduce EMI/
EMC emissions. Soft-start and tracking functionality is
accessed via the TR/SS pin, and an accurate input voltage
UVLO threshold can be set using the EN/UV pin.
Small 24-Lead 4mm × 4mm LQFN Package
applicaTions
n
Automotive and Industrial Supplies
General Purpose Step-Down
n
L, LT, LTC, LTM, Linear Technology, the Linear logo, Silent Switcher, Burst Mode and LTspice
are registered trademarks of Analog Devices, Inc. All other trademarks are the property of their
respective owners. Protected by U.S. patents, including 8823345.
12VIN to 5VOUT Efficiency
Typical applicaTion
100
95
90
85
80
75
70
65
60
3.2
2.8
2.4
2.0
1.6
1.2
0.8
0.4
0
5V 6A Step-Down Converter
EFFICIENCY
3.3µH
V
5V
6A
OUT
V
IN
V
SW
IN
5.7V TO 42V
EN/UV
4.7µF
BIAS
10pF
1M
LT8640S
GND
POWER LOSS
100µF
RT
FB
41.2k
= 1MHz
243k
1MHz, L = 3.3µH
2MHz, L = 2.2µH
3MHz, L = 1µH
f
SW
8640S TA01a
0.5
1
1.5
2
2.5
3 3.5 4 4.5 5 5.5 6
LOAD CURRENT (A)
8640S TA01b
8640sfa
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For more information www.linear.com/LT8640S
LT8640S/LT8643S
absoluTe MaxiMuM raTings (Note 1)
V , EN/UV, PG..........................................................42V
Operating Junction Temperature Range (Note 2)
IN
BIAS..........................................................................25V
FB, TR/SS . .................................................................4V
SYNC Voltage .............................................................6V
LT8640SE/LT8643SE......................... –40°C to 125°C
LT8640SI/LT8643SI........................... –40°C to 125°C
Storage Temperature Range .................. –65°C to 150°C
Maximum Reflow (Package Body) Temperature.....260°C
pin conFiguraTion
LT8640S
LT8643S
TOP VIEW
TOP VIEW
24 23 22 21 20 19
24 23 22 21 20 19
BIAS
1
2
3
4
5
6
18 RT
17 EN
16 GND
15 NC
BIAS
1
2
3
4
5
6
18 RT
17 EN
16 GND
15 NC
25
26
25
26
INTV
INTV
CC
CC
GND
GND
GND
GND
GND
GND
NC
NC
27
GND
28
GND
27
GND
28
GND
V
IN
14
13
V
V
V
IN
14
13
V
V
IN
IN
IN
IN
V
V
IN
IN
7
8
9
10 11 12
7
8
9
10 11 12
LQFN PACKAGE
24-LEAD (4mm × 4mm × 0.94mm)
= 38°C/W, θ = 7°C/W (Note 3)
LQFN PACKAGE
24-LEAD (4mm × 4mm × 0.94mm)
= 38°C/W, θ = 7°C/W (Note 3)
θ
θ
JA
JC(PAD)
JA
JC(PAD)
EXPOSED PAD (PINS 25–28) ARE GND, SHOULD BE SOLDERED TO PCB
EXPOSED PAD (PINS 25–28) ARE GND, SHOULD BE SOLDERED TO PCB
orDer inForMaTion http://www.linear.com/product/LT8640S#orderinfo
PACKAGE
MSL
PART NUMBER
LT8640SEV#PBF
LT8640SIV#PBF
LT8643SEV#PBF
LT8643SIV#PBF
PART MARKING*
FINISH CODE
PAD FINISH
TYPE**
RATING
TEMPERATURE RANGE
8640S
LQFN (Laminate Package
with QFN Footprint)
e4
Au (RoHS)
3
–40°C to 125°C
8643S
• Consult Marketing for parts specified with wider operating temperature
ranges. *Device temperature grade is identified by a label on the shipping
container.
• Terminal Finish Part Marking: www.linear.com/leadfree
• Recommended PCB Assembly and Manufacturing Procedures:
www.linear.com/umodule/pcbassembly
• Pad finish code is per IPC/JEDEC J-STD-609.
• Package and Tray Drawings: www.linear.com/packaging
Parts ending with PBF are RoHS and WEEE compliant. **The LT8640S/LT8643S package has the same dimensions as a standard 4mm × 4mm QFN package.
8640sfa
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For more information www.linear.com/LT8640S
LT8640S/LT8643S
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
TYP
MAX
UNITS
l
l
l
l
Minimum Input Voltage
3.0
3.4
V
V
Quiescent Current in Shutdown
V
V
V
= 0V
0.75
0.75
3
10
µA
µA
IN
EN/UV
EN/UV
EN/UV
LT8640S V Quiescent Current in Sleep
= 2V, V > 0.97V, V
= 0V
1.7
1.7
4
10
µA
µA
IN
FB
SYNC
LT8643S V Quiescent Current in Sleep
= 2V, V > 0.97V, V
= 0V, V
= 0V
230
230
290
340
µA
µA
IN
FB
SYNC
BIAS
V
V
= 2V, V > 0.97V, V
= 0V, V
= 0V, V
= 5V
= 5V
19
25
µA
µA
EN/UV
FB
SYNC
BIAS
BIAS
LT8643S BIAS Quiescent Current in Sleep
= 2V, V > 0.97V, V
200
260
EN/UV
FB
SYNC
l
l
LT8640S V Current in Regulation
V
OUT
V
OUT
= 0.97V, V = 6V, I
= 100µA, V = 0
SYNC
21
220
60
390
µA
µA
IN
IN
LOAD
LOAD
= 0.97V, V = 6V, I
= 1mA, V
= 0
SYNC
IN
Feedback Reference Voltage
V
V
= 6V
0.964
0.958
0.970
0.970
0.976
0.982
V
V
IN
IN
l
l
= 6V
Feedback Voltage Line Regulation
Feedback Pin Input Current
V
V
= 4.0V to 36V
= 1V
0.004
0.02
20
%/V
nA
IN
–20
FB
LT8643S Error Amp Transconductance
LT8643S Error Amp Gain
V = 1.25V
1.7
260
350
350
5
mS
C
LT8643S V Source Current
V
V
= 0.77V, V = 1.25V
µA
µA
A/V
V
C
FB
C
LT8643S V Sink Current
= 1.17V, V = 1.25V
C
C
FB
LT8643S V Pin to Switch Current Gain
C
LT8643S V Clamp Voltage
2.6
14
C
BIAS Pin Current Consumption
Minimum On-Time
V
BIAS
= 3.3V, f = 2MHz
mA
SW
l
l
I
I
= 1.5A, SYNC = 0V
= 1.5A, SYNC = 2V
30
30
50
45
ns
ns
LOAD
LOAD
Minimum Off-Time
Oscillator Frequency
80
110
ns
l
l
l
R = 221k
180
665
1.8
210
700
1.95
240
735
2.1
kHz
kHz
MHz
T
R = 60.4k
T
R = 18.2k
T
Top Power NMOS On-Resistance
Top Power NMOS Current Limit
Bottom Power NMOS On-Resistance
SW Leakage Current
I
= 1A
66
10
27
mΩ
A
SW
l
l
7.5
12.5
V
V
= 3.4V, I = 1A
mΩ
µA
V
INTVCC
SW
= 42V, V = 0V, 42V
–1.5
0.94
1.5
IN
SW
EN/UV Pin Threshold
EN/UV Rising
1.0
40
1.06
EN/UV Pin Hysteresis
mV
nA
%
EN/UV Pin Current
V
V
V
= 2V
–20
5
20
EN/UV
l
l
PG Upper Threshold Offset from V
Falling
7.5
–8
10.25
–10.75
FB
FB
FB
PG Lower Threshold Offset from V
PG Hysteresis
Rising
–5.25
%
FB
0.2
%
PG Leakage
V
V
= 3.3V
= 0.1V
–40
40
nA
Ω
PG
l
PG Pull-Down Resistance
SYNC/MODE Threshold
700
2000
PG
l
l
l
SYNC/MODE DC and Clock Low Level Voltage
SYNC/MODE Clock High Level Voltage
SYNC/MODE DC High Level Voltage
0.7
2.2
0.9
1.2
2.55
V
V
V
1.4
2.9
8640sfa
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For more information www.linear.com/LT8640S
LT8640S/LT8643S
elecTrical characTerisTics The l denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at TA = 25°C.
PARAMETER
CONDITIONS
R = 60.4k, V
MIN
TYP
MAX
UNITS
Spread Spectrum Modulation
Frequency Range
= 3.3V
SYNC
22
%
T
Spread Spectrum Modulation Frequency
TR/SS Source Current
V
= 3.3V
3
kHz
µA
Ω
SYNC
l
1.2
1.9
200
0.6
2.6
TR/SS Pull-Down Resistance
Fault Condition, TR/SS = 0.1V
Output Sink Current in Forced Continuous
Mode
V
FB
= 1.01V, L = 6.8µH, R = 60.4k
0.25
35
1.1
39
A
T
V
IN
to Disable Forced Continuous Mode
V
Rising
37
V
IN
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 3: θ values determined per JEDEC 51-7, 51-12. See the Applications
Information section for information on improving the thermal resistance
and for actual temperature measurements of a demo board in typical
operating conditions.
Note 2: The LT8640SE/LT8643SE 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 LT8640SI/LT8643SI is guaranteed over the full –40°C to 125°C
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.
operating junction temperature range. The junction temperature (T , in
J
°C) is calculated from the ambient temperature (T in °C) and power
A
dissipation (PD, in Watts) according to the formula:
T = T + (PD • θ )
JA
J
A
where θ (in °C/W) is the package thermal impedance.
JA
8640sfa
4
For more information www.linear.com/LT8640S
LT8640S/LT8643S
Typical perForMance characTerisTics
12VIN to 5VOUT Efficiency
vs Frequency
12VIN to 3.3VOUT Efficiency
vs Frequency
Efficiency at 5VOUT
100
95
90
85
80
75
70
65
60
3.2
2.8
2.4
2.0
1.6
1.2
0.8
0.4
0
100
95
90
85
80
75
70
65
60
3.2
2.8
2.4
2.0
1.6
1.2
0.8
0.4
0
100
95
90
85
80
75
70
65
60
55
50
3.0
2.7
2.4
EFFICIENCY
EFFICIENCY
2.1
EFFICIENCY
1.8
1.5
POWER LOSS
1.2
POWER LOSS
POWER LOSS
0.9
0.6
0.3
0
V
V
V
= 12V
= 24V
= 36V
IN
IN
IN
L = WE-LHMI1040
L = WE-LHMI1040
1MHz, L = 3.3µH
2MHz, L = 2.2µH
3MHz, L = 1µH
1MHz, L = 2.2µH
2MHz, L = 1µH
3MHz, L = 1µH
f
= 1MHz
SW
L = IHLP3232DZ-01, 3.3µH
0.5
1
1.5
2
2.5
3
3.5
4
4.5
5
5.5
6
0.5
1
1.5
2
2.5
3
3.5
4
4.5
5
5.5
6
0
1
2
3
4
5
6
LOAD CURRENT (A)
LOAD CURRENT (A)
LOAD CURRENT (A)
8640S G01
8640S G02
8640S G03
LT8640S Low Load Efficiency at
5VOUT
LT8643S Low Load Efficiency at
5VOUT
Efficiency at 3.3VOUT
100
3.0
100
100
90
80
70
60
50
40
30
20
95
90
85
80
75
70
65
60
55
50
2.7
2.4
2.1
1.8
1.5
1.2
0.9
0.6
0.3
0
90
80
70
60
50
40
30
20
10
EFFICIENCY
POWER LOSS
V
V
V
= 12V
V
V
V
= 12V
IN
IN
IN
IN
IN
IN
V
= 12V
= 24V
= 36V
IN
IN
IN
= 24V
= 24V
V
= 36V
= 36V
V
f
= 1MHz
f
= 1MHz
f
= 1MHz
SW
SW
SW
L = IHLP3232DZ–01, 4.7µH
L = IHLP3232DZ-01, 2.2µH
L = IHLP3232DZ-01, 4.7µH
0
1
2
3
4
5
6
0.1
1
10
100
1000
0.01
0.1
1
10
100
1000
LOAD CURRENT (A)
LOAD CURRENT (mA)
LOAD CURRENT (mA)
8640S G04
8640 G06
8640 G05
LT8640S Low Load Efficiency at
3.3VOUT
LT8643S Low Load Efficiency at
3.3VOUT
Efficiency vs Frequency
100
90
80
70
60
50
40
30
20
96
94
92
90
88
86
84
82
80
100
90
80
70
60
50
40
30
20
10
V
IN
V
IN
V
IN
= 12V
V
IN
V
IN
V
IN
= 12V
= 24V
= 36V
= 24V
V
V
I
= 12V
IN
OUT
= 36V
= 3.3V
= 2A
f
= 1MHz
f
= 1MHz
LOAD
SW
SW
L = IHLP3232DZ-01, 4.7µH
L = IHLP3232DZ-01, 4.7µH
L = IHLP3232DZ–01, 4.7µH
0.01
0.1
1
10
100
1000
0
0.5
1
1.5
2
2.5
3
0.1
1
10
100
1000
LOAD CURRENT (mA)
SWITCHING FREQUENCY (MHz)
LOAD CURRENT (mA)
8640S G07
8640S G09
8640S G08
8640sfa
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For more information www.linear.com/LT8640S
LT8640S/LT8643S
Typical perForMance characTerisTics
Burst Mode Operation Efficiency
vs Inductor Value (LT8640S)
Reference Voltage
EN Pin Thresholds
979
977
975
973
971
969
967
965
963
961
100
95
90
85
80
75
70
65
1.03
1.02
1.01
1.00
0.99
0.98
0.97
0.96
0.95
V
= 12V
IN
EN RISING
V
= 24V
IN
EN FALLING
V
LOAD
L = IHLP3232DZ-01
= 5V
OUT
I
= 10mA
–50 –25
0
25
50
75 100 125
1
2
3
4
5
6
7
8
–50 –25
0
25
50
75 100 125
TEMPERATURE (°C)
INDUCTOR VALUE (µH)
TEMPERATURE (°C)
8640S G11
8640S G10
8640S G12
LT8640S Load Regulation
LT8643S Load Regulation
LT8640S Line Regulation
0.40
0.30
0.12
0.10
0.15
0.10
0.05
0
0.08
0.20
0.06
0.10
0.04
0.00
0.02
0.00
–0.10
–0.20
–0.30
–0.40
0.05
–0.10
–0.15
–0.02
–0.04
–0.06
–0.08
V
V
= 5V
V
LOAD
= 5V
V
V
= 5V
OUT
IN
OUT
OUT
IN
= 12V
I
= 1A
= 12V
0
1
2
3
4
5
6
5
10 15 20 25 30 35 40 45
0
1
2
3
4
5
6
LOAD CURRENT (A)
INPUT VOLTAGE (V)
LOAD CURRENT (A)
8640S G14
8640S G15
8640S G13
LT8643S Line Regulation
LT8640S No-Load Supply Current
LT8643S No-Load Supply Current
0.15
0.12
0.09
0.06
0.03
0
225
200
175
150
125
100
75
4.0
3.5
3.0
2.5
2.0
1.5
1.0
V
= 5V
OUT
L = 4.7µH
IN REGULATION
–0.03
–0.06
–0.09
–0.12
–0.15
V
= 3.3V
OUT
50
V
LOAD
= 5V
= 1A
OUT
L = 4.7µH
I
IN REGULATION
25
5
10 15 20 25 30 35 40 45
5
10 15 20 25 30 35 40 45
0
5
10 15 20 25 30 35 40 45
INPUT VOLTAGE (V)
INPUT VOLTAGE (V)
INPUT VOLTAGE (V)
8640S G16
8640S G18
8640S G17
8640sfa
6
For more information www.linear.com/LT8640S
LT8640S/LT8643S
Typical perForMance characTerisTics
Top FET Current Limit vs Duty Cycle
Top FET Current Limit
Switch Drop vs Temperature
11.0
10.5
10.0
9.5
150
12
11
10
9
SWITCH CURRENT = 1A
125
100
75
50
25
0
9.0
TOP SWITCH
8.5
5% DC
8.0
7.5
7.0
BOTTOM SWITCH
6.5
6.0
8
0.1
0.3
0.5
0.7
0.9
–50 –25
0
25
50
75 100 125
–50 –25
0
25
50
75 100 125
DUTY CYCLE
TEMPERATURE (°C)
TEMPERATURE (°C)
8640S G19
8640S G21
8640S G20
Dropout Voltage
Switch Drop vs Switch Current
Minimum On-Time
500
450
400
350
300
250
200
150
100
50
44
40
36
32
28
24
20
600
V
= 5V
OUT
IN
Burst Mode OPERATION
FORCED CONTINUOUS MODE
V
SET TO REGULATE AT 5V
500 L = IHLP3232DZ-01, 1µH
400
300
200
100
0
TOP SWITCH
I
= 2A
LOAD
OUT
V
= 0.97V
f
= 3MHz
BOTTOM SWITCH
SW
0
0
1
2
3
4
5
–50 –25
0
25
50
75 100 125
0
0.5
1
1.5
2
2.5
3
3.5
4
4.5
5
SWITCH CURRENT (A)
TEMPERATURE (°C)
LOAD CURRENT (A)
8640S G22
8640S G24
8640S G23
Switching Frequency
Burst Frequency
LT8640S Soft-Start Tracking
740
730
720
710
700
690
680
670
660
1200
1000
800
600
400
200
0
1.2
1.0
0.8
0.6
R
T
= 60.4k
0.4
0.2
0
FRONT PAGE APPLICATION
V
OUT
= 12V
= 5V
IN
V
–50 –25
0
25
50
75 100 125
0
100
200
300
400
500
600
0.8
TR/SS VOLTAGE (V)
1.2
1.4
0
0.2
0.4 0.6
1.0
TEMPERATURE (°C)
LOAD CURRENT (mA)
8640S G25
8640S G26
8640S G27
8640sfa
7
For more information www.linear.com/LT8640S
LT8640S/LT8643S
Typical perForMance characTerisTics
LT8643S Error Amp Output
Current
LT8643S Soft-Start Tracking
Soft-Start Current
1.2
1.0
0.8
0.6
0.4
0.2
0
2.2
2.1
2.0
1.9
1.8
1.7
1.6
1.5
1.4
500
375
V
= 0.5V
SS
250
125
0
–125
–250
–375
–500
V
= 1.25V
C
0
0.2 0.4 0.6 0.8
1
1.2 1.4 1.6
–50 –25
0
25
50
75 100 125
–200
–100
FB PIN ERROR VOLTAGE (mV)
8640S G30
0
100
200
TR/SS VOLTAGE (V)
TEMPERATURE (°C)
8640S G28
8640S G29
RT Programmed Switching
Frequency
PG High Thresholds
PG Low Thresholds
–6.0
–6.5
–7.0
–7.5
–8.0
–8.5
–9.0
–9.5
–10.0
10.0
9.5
9.0
8.5
8.0
7.5
7.0
6.5
6.0
250
225
200
175
150
125
100
75
FB RISING
FB RISING
FB FALLING
FB FALLING
50
25
0
–50 –25
0
25
50
75 100 125
–50 –25
0
25
50
75 100 125
0.2 0.6
1
1.4 1.8 2.2 2.6
3
TEMPERATURE (°C)
TEMPERATURE (°C)
SWITCHING FREQUENCY (MHz)
8640S G32
8640S G31
8640S G33
Bias Pin Current
Minimum Input Voltage
Bias Pin Current
8.5
8.0
7.5
7.0
6.5
6.0
5.5
25
20
15
10
5
3.6
3.4
3.2
3.0
2.8
2.6
2.4
V
V
V
= 5V
BIAS
OUT
IN
= 5V
= 12V
= 1A
I
LOAD
V
= 5V
BIAS
OUT
V
= 5V
I
f
= 1A
= 1MHz
LOAD
SW
0
5
10 15 20 25 30 35 40 45
0.2 0.6
1
1.4 1.8 2.2 2.6 3.0
–50 –25
0
25
50
75 100 125
INPUT VOLTAGE (V)
SWITCHING FREQUENCY (MHz)
TEMPERATURE (°C)
8640S G35
8640S G36
8640S G34
8640sfa
8
For more information www.linear.com/LT8640S
LT8640S/LT8643S
Typical perForMance characTerisTics
Case Temperature Rise vs 7A
Pulsed Load
Switching Rising Edge
Case Temperature Rise
80
70
60
50
40
30
20
10
0
90
80
70
60
50
40
30
20
10
0
DC2530A DEMO BOARD
DC2530A DEMO BOARD
V
= 12V
IN
OUT
SW
V
V
V
V
= 12V, f = 1MHz
IN
IN
IN
IN
SW
V
f
= 5V
= 24V, f = 1MHz
SW
= 2MHz
= 12V, f = 2MHz
SW
STANDBY LOAD = 0.25A
1kHz PULSED LOAD = 7A
= 24V, f = 2MHz
SW
V
SW
2V/DIV
8640S G39
2ns/DIV
V
LOAD
= 12V
IN
I
= 2A
0
1
2
3
4
5
6
0
0.2
0.4
0.6
0.8
1
LOAD CURRENT (A)
DUTY CYCLE OF 7A LOAD
8640S G37
8640S G38
Switching Waveforms, Full
Frequency Continuous Operation
Switching Waveforms, Burst
Mode Operation
Switching Waveforms
I
L
I
L
1A/DIV
I
L
1A/DIV
500mA/DIV
V
SW
V
SW
V
5V/DIV
SW
10V/DIV
5V/DIV
8640S G41
8640S G42
8640S G40
5µs/DIV
FRONT PAGE APPLICATION
12V TO 5V AT 10mA
500ns/DIV
500ns/DIV
FRONT PAGE APPLICATION
FRONT PAGE APPLICATION
36V TO 5V
AT 1A
IN
OUT
12V TO 5V
AT 1A
IN
SYNC
OUT
IN
OUT
V
= 0V
LT8643S Transient Response;
External Compensation
LT8640S Transient Response;
Internal Compensation
I
I
LOAD
LOAD
2A/DIV
2A/DIV
V
V
OUT
OUT
100mV/DIV
100mV/DIV
8640S G43
8640S G44
20µs/DIV
20µs/DIV
2A TO 4A TRANSIENT
2A TO 4A TRANSIENT
12V , 5V
12V , 5V
IN
OUT
IN
OUT
f
= 2MHz
f
= 2MHz
SW
SW
C
C
= 100µF, C
= 10pF
C
= 330pF, R = 8.45k
= 100µF, C
OUT
LEAD
C
C
= 4.7pF
OUT
LEAD
8640sfa
9
For more information www.linear.com/LT8640S
LT8640S/LT8643S
Typical perForMance characTerisTics
LT8640S Transient Response;
100mA to 1.1A Transient
LT8643S Transient Response;
100mA to 1.1A Transient
I
I
LOAD
LOAD
1A/DIV
1A/DIV
Burst Mode OPERATION
Burst Mode OPERATION
V
OUT
V
OUT
100mV/DIV
100mV/DIV
FCM
FCM
8640S G45
8640S G46
50µs/DIV
50µs/DIV
FRONT PAGE APPLICATION
FRONT PAGE APPLICATION
C
= 330pF, R = 6.49k, C
= 4.7pF
100mA TO 1.1A TRANSIENT
C
C
LEAD
100mA TO 1.1A TRANSIENT
12V , 5V , f = 1MHz
IN
OUT
OUT SW
= 100µF
12V , 5V , f = 1MHz
C
IN
OUT SW
= 100µF
C
OUT
Start-Up Dropout Performance
Start-Up Dropout Performance
V
IN
V
IN
V
V
IN
IN
2V/DIV
2V/DIV
V
V
OUT
OUT
V
V
OUT
2V/DIV
OUT
2V/DIV
8640S G48
8640S G47
100ms/DIV
20Ω LOAD
(250mA IN REGULATION)
100ms/DIV
2.5Ω LOAD
(2A IN REGULATION)
8640sfa
10
For more information www.linear.com/LT8640S
LT8640S/LT8643S
Typical perForMance characTerisTics
Conducted EMI Performance
60
50
40
30
20
10
0
–10
–20
SPREAD SPECTRUM MODE
FIXED FREQUENCY MODE
–30
–40
0
3
6
9
12
15
18
21 24 27
30
FREQUENCY (MHz)
8640S G49
DC2530A DEMO BOARD
(WITH EMI FILTER INSTALLED)
14V INPUT TO 5V OUTPUT AT 4A, f = 2MHz
SW
Radiated EMI Performance
(CISPR25 Radiated Emission Test with Class 5 Peak Limits)
50
VERTICAL POLARIZATION
PEAK DETECTOR
45
40
35
30
25
20
15
10
5
0
CLASS 5 PEAK LIMIT
SPREAD SPECTRUM MODE
FIXED FREQUENCY MODE
–5
0
100
200
300
400
500
600
700
800
900 1000
FREQUENCY (MHz)
8640S G50a
50
45
40
35
30
25
20
15
10
5
HORIZONTAL POLARIZATION
PEAK DETECTOR
CLASS 5 PEAK LIMIT
SPREAD SPECTRUM MODE
0
FIXED FREQUENCY MODE
–5
0
100
200
300
400
500
600
700
800
900 1000
FREQUENCY (MHz)
8640S G50b
DC2530A DEMO BOARD
(WITH EMI FILTER INSTALLED)
14V INPUT TO 5V OUTPUT AT 4A, f = 2MHz
SW
8640sfa
11
For more information www.linear.com/LT8640S
LT8640S/LT8643S
pin FuncTions
BIAS (Pin 1): The internal regulator will draw current from
SW (Pins 8–12): The SW pins are the outputs of the inter-
nal power switches. Tie these pins together and connect
them to the inductor. This node should be kept small on
the PCB for good performance and low EMI.
BIAS instead of V when BIAS is tied to a voltage higher
IN
than 3.1V. For output voltages of 3.3V to 25V this pin
should be tied to V . If this pin is tied to a supply other
OUT
than V
use a 1µF local bypass capacitor on this pin.
OUT
EN/UV (Pin 17): The LT8640S/LT8643S 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
If no supply is available, tie to GND. However, especially
for high input or high frequency applications, BIAS should
be tied to output or an external supply of 3.3V or above.
INTV (Pin 2): Internal 3.4V Regulator Bypass Pin. The
used. An external resistor divider from V can be used
CC
IN
internal power drivers and control circuits are powered
to program a V threshold below which the LT8640S/
IN
from this voltage. INTVCC maximum output current is
LT8643S will shut down.
20mA. Do not load the INTV pin with external circuitry.
CC
RT (Pin 18): A resistor is tied between RT and ground to
set the switching frequency.
INTV current will be supplied from BIAS if BIAS > 3.1V,
CC
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. This pin should be floated.
CLKOUT (Pin 19): In forced continuous mode, spread
spectrum, and synchronization modes, the CLKOUT pin
will provide a ~200ns wide pulse at the switch frequency.
The low and high levels of the CLKOUT pin are ground and
INTVCC respectively, and the drive strength of the CLKOUT
pin is several hundred ohms. In Burst Mode operation,
the CLKOUT pin will be low. Float this pin if the CLKOUT
function is not used.
GND (Pins 3, 16, Exposed Pad Pins 25–28): Ground.
Place the negative terminal of the input capacitor as close
to the GND pins as possible. The exposed pads should
be soldered to the PCB for good thermal performance. If
necessary due to manufacturing limitations Pins 25 to 28
may be left disconnected, however thermal performance
will be degraded.
SYNC/MODE (Pin 20): For the LT8640S/LT8643S,
this pin programs four different operating modes: 1)
Burst Mode operation. Tie this pin to ground for Burst
Mode operation at low output loads—this will result in
ultralow quiescent current. 2) Forced Continuous mode
(FCM). This mode offers fast transient response and
full frequency operation over a wide load range. Float
this pin for FCM. When floating, pin leakage currents
should be <1µA. 3) Spread spectrum mode. Tie this pin
NC (Pins 4, 15): No Connect. This pin is not connected
to internal circuitry and can be tied anywhere on the PCB,
typically ground.
V
(Pins 5, 6, 13, 14): The V pins supply current to
IN
IN
the LT8640S/LT8643S internal circuitry and to the internal
topside power switch. These pins must be tied together
and be locally bypassed with a capacitor of 2.2µF or more.
Be sure to place the positive terminal of the input capaci-
high to INTV (~3.4V) or an external supply of 3V to
CC
tor as close as possible to the V pins, and the negative
capacitor terminal as close as possible to the GND pins.
4V for forced continuous mode with spread-spectrum
modulation. 4) Synchronization mode. Drive this pin with
a clock source to synchronize to an external frequency.
During synchronization the part will operate in forced
continuous mode.
IN
BST (Pin 7): This pin is used to provide a drive voltage,
higher than the input voltage, to the topside power switch.
This pin should be floated.
8640sfa
12
For more information www.linear.com/LT8640S
LT8640S/LT8643S
pin FuncTions
TR/SS (Pin 21): Output Tracking and Soft-Start Pin. This
pin allows user control of output voltage ramp rate during
start-up. For the LT8640S, a TR/SS voltage below 0.97V
forces it to regulate the FB pin to equal the TR/SS pin volt-
age. When TR/SS is above 0.97V, the tracking function is
disabled and the internal reference resumes control of the
error amplifier. For the LT8643S, a TR/SS voltage below
1.6V forces it to regulate the FB pin to a function of the
TR/SS pin voltage. See plot in the Typical Performance
Characteristics section. When TR/SS is above 1.6V, the
tracking function is disabled and the internal reference
resumes control of the error amplifier. An internal 1.9µA
GND (Pin 22, LT8640S Only): Ground. Connect this pin
to system ground and to the ground plane.
VC (Pin 22, LT8643S Only): The VC pin is the output of the
internal error amplifier. The voltage on this pin controls
the peak switch current. Tie an RC network from this pin
to ground to compensate the control loop.
PG (Pin 23): The PG pin is the open-drain output of an
internal comparator. PG remains low until the FB pin is
within 8% of the final regulation voltage, and there are
no fault conditions. PG is also pulled low when EN/UV is
below 1V, INTV has fallen too low, V is too low, or
CC
IN
thermal shutdown. PG is valid when V is above 3.4V.
pull-up current from INTV on this pin allows a capacitor
IN
CC
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.
FB (Pin 24): The LT8640S/LT8643S 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 V . Typically, this capacitor is 4.7pF to 22pF.
OUT
Corner Pins: These pins are for mechanical support only
and can be tied anywhere on the PCB, typically ground.
8640sfa
13
For more information www.linear.com/LT8640S
LT8640S/LT8643S
block DiagraM
V
IN
13, 14
C
IN2
0.1µF
V
5, 6
IN
V
IN
C
IN1
C
IN3
–
+
0.1µF
INTERNAL 0.97V REF
SHDN
BIAS
3.4V
REG
1
2
R3
1V
+
–
OPT
EN/UV
INTV
CC
17
SLOPE COMP
OSCILLATOR
C
VCC
2.2µF
R4
OPT
LT8643S ONLY
V
C
22
23
200kHz TO 3MHz
R
C
C
F
ERROR
AMP
PG
BST
BST
8ꢀ
C
7
C
V
C
+
+
–
BURST
DETECT
C
0.22µF
V
OUT
M1
SW
L
SHDN
8–12
SWITCH LOGIC
AND
ANTI-SHOOT
THROUGH
LT8640S
ONLY
C1 R1
R2
THERMAL SHDN
V
OUT
INTV UVLO
CC
C
V
IN
UVLO
OUT
FB
24
M2
SHDN
THERMAL SHDN
UVLO
C
SS
OPT
1.9µA
V
IN
TR/SS
RT
GND
3, 6, 12, 25–28
21
18
R
T
INTV
CC
GND
LT8640S
ONLY
22
19
60k
CLKOUT
SYNC/MODE
20
600k
8640S BD
8640sfa
14
For more information www.linear.com/LT8640S
LT8640S/LT8643S
operaTion
The LT8640S/LT8643S is a monolithic, constant fre-
quency, 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 begin-
ning 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 ampli-
fier servos the VC node by comparing the voltage on the
The LT8640S/LT8643S can operate in forced continuous
mode (FCM) for fast transient response and full frequency
operation over a wide load range. When in FCM the oscil-
lator operates continuously and positive SW transitions
are aligned to the clock. Negative inductor current is
allowed. The LT8640S/LT8643S can sink current from
the output and return this charge to the input in this mode,
improving load step transient response.
To improve EMI/EMC, the LT8640S/LT8643S can oper-
ate in spread spectrum mode. This feature varies the
clock with a triangular frequency modulation of +20%.
For example, if the LT8640S/LT8643S’s frequency is pro-
grammed to switch at 2MHz, spread spectrum mode will
modulate the oscillator between 2MHz and 2.4MHz. The
V
pin with an internal 0.97V reference. When the load
FB
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 10A flowing
through the bottom switch, the next clock cycle will be
delayed until switch current returns to a safe level.
SYNC/MODE pin should be tied high to INTV (~3.4V) or
CC
an external supply of 3V to 4V to enable spread spectrum
modulation with forced continuous mode.
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
The “S” in LT8640S/LT8643S refers to the second genera-
tion silent switcher technology. This technology allows
fast switching edges for high efficiency at high switching
frequencies, while simultaneously achieving good EMI/
EMC performance. This includes the integration of ceramic
capacitors into the package for V , BST, and INTV (see
Block Diagram). These caps keep all the fast AC current
loops small, which improves EMI/EMC performance.
draw current from V . The BIAS pin should be connected
IN
to V
if the LT8640S/LT8643S output is programmed
OUT
at 3.3V to 25V.
The VC pin optimizes the loop compensation of the
switching regulator based on the programmed switch-
ing frequency, allowing for a fast transient response. The
VC pin also enables current sharing and a CLKOUT pin
enables synchronizing other regulators to the LT8643S.
IN
CC
If the EN/UV pin is low, the LT8640S/LT8643S is shut
down and draws 1µA from the input. When the EN/UV pin
is above 1V, the switching regulator will become active.
Comparators monitoring the FB pin voltage will pull the PG
pin low if the output voltage varies more than 8% (typi-
cal) from the set point, or if a fault condition is present.
To optimize efficiency at light loads, the LT8640S/LT8643S
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 (LT8640S) or 230µA (LT8643S with BIAS
= 0). In a typical application, 2.5µA (LT8640S) or 120µA
The oscillator reduces the LT8640S/LT8643S’s operat-
ing 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 condi-
tions. When a clock is applied to the SYNC/MODE pin, the
SYNC/MODE pin is floated, or held DC high, the frequency
foldback is disabled and the switching frequency will slow
down only during overcurrent conditions.
(LT8643S with BIAS = 5V ) will be consumed from the
OUT
input supply when regulating with no load. The SYNC/
MODE pin is tied low to use Burst Mode operation and can
be floated to use forced continuous mode (FCM). If a clock
is applied to the SYNC/MODE pin, the part will synchronize
to an external clock frequency and operate in FCM.
8640sfa
15
For more information www.linear.com/LT8640S
LT8640S/LT8643S
applicaTions inForMaTion
Low EMI PCB Layout
Note that large, switched currents flow in the LT8640S/
LT8643S V and GND pins and the input capacitors. The
IN
The LT8640S/LT8643S is specifically designed to mini-
mize EMI/EMC emissions and also to maximize efficiency
when switching at high frequencies. For optimal perfor-
mance the LT8640S/LT8643S should use multiple VIN
bypass capacitors.
loops formed by the input capacitors should be as small
as possible by placing the capacitors adjacent to the V
IN
and GND pins. Capacitors with small case size such as
0603 are optimal due to lowest parasitic inductance.
The input capacitors, along with the inductor and out-
put capacitors, 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.
Two small <1µF capacitors can be placed as close as pos-
sible to the LT8640S/LT8643S, one capacitor on each
side of the device (C
, C
). A third capacitor with a
OPT1 OPT2
larger value, 2.2µF or higher, should be placed near C
OPT1
or C
.
OPT2
See Figure 1 for a recommended PCB layouts.
For more detail and PCB design files refer to the Demo
Board guide for the LT8640S/LT8643S.
C
C
R2
R2
C
F
R
C
C
C
SS
SS
C1
C1
R1
R
R
T
T
R1
C
IN3
C
IN3
C
C
C
C
OPT2
OPT1
OPT2
OPT1
C
OUT
C
OUT
L
L
8640S F01a
8640S F01b
GROUND VIA
V
IN
VIA
V
OUT
VIA
OTHER SIGNAL VIAS
GROUND VIA
V
IN
VIA
V VIA
OUT
OTHER SIGNAL VIAS
(a) LT8640S
(b) LT8643S
Figure 1. Recommended PCB Layouts for the LT8640S and LT8643S
8640sfa
16
For more information www.linear.com/LT8640S
LT8640S/LT8643S
applicaTions inForMaTion
The exposed pads on the bottom of the package should be
soldered to the PCB to reduce thermal resistance to ambi-
ent. To keep thermal resistance low, extend the ground
plane from GND as much as possible, and add thermal
vias to additional ground planes within the circuit board
and on the bottom side.
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.
In order to achieve higher light load efficiency, more
energy must be delivered to the output during the
single small pulses in Burst Mode operation such that
the LT8640S/LT8643S can stay in sleep mode longer
between each pulse. This can be achieved by using
a larger value inductor (i.e., 4.7µH), and should be
considered independent of switching frequency when
choosing an inductor. For example, while a lower induc-
tor value would typically be used for a high switching
frequency application, if high light load efficiency is
desired, a higher inductor value should be chosen. See
curve in Typical Performance Characteristics.
Achieving Ultralow Quiescent Current (Burst Mode
Operation)
To enhance efficiency at light loads, the LT8640S/LT8643S
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 minimiz-
ing output voltage ripple. In Burst Mode operation the
LT8640S/LT8643S 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 LT8640S consumes 1.7µA and
the LT8643S consumes 230µA.
While in Burst Mode operation the current limit of the top
switch is approximately 900mA (as shown in Figure 3),
resulting in low output voltage ripple. Increasing the out-
put capacitance will decrease 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 2.
As the output load decreases, the frequency of single cur-
rent pulses decreases (see Figure 2) and the percentage
of time the LT8640S/LT8643S is in sleep mode increases,
resulting in much higher light load efficiency than for typi
-
cal converters. By maximizing the time between pulses,
the LT8640S’s quiescent current approaches 2.5µA for a
typical application when there is no output load. Therefore,
The output load at which the LT8640S/LT8643S reaches
the programmed frequency varies based on input voltage,
output voltage and inductor choice. To select low ripple
Burst Mode operation, tie the SYNC/MODE pin below 0.4V
(this can be ground or a logic low output).
Burst Frequency
1200
1000
800
600
400
I
L
500mA/DIV
V
SW
5V/DIV
FRONT PAGE APPLICATION
200
V
= 12V
IN
V
8640S F03
= 5V
OUT
5µs/DIV
0
FRONT PAGE APPLICATION
0
100
200
300
400
500
600
12V TO 5V
SYNC
AT 10mA
IN
OUT
LOAD CURRENT (mA)
V
= 0V
8640S F02
Figure 2. SW Frequency vs Load Information
in Burst Mode Operation
Figure 3. Burst Mode Operation
8640sfa
17
For more information www.linear.com/LT8640S
LT8640S/LT8643S
applicaTions inForMaTion
Forced Continuous Mode
For robust operation over a wide V and V
range, use
OUT
IN
:
an inductor value greater than L
MIN
The LT8640S/LT8643S can operate in forced continu-
ous mode (FCM) for fast transient response and full
frequency operation over a wide load range. When in
FCM, the oscillator operates continuously and positive
SW transitions are aligned to the clock. Negative induc-
tor current is allowed at light loads or under large tran-
sient conditions. The LT8640S/LT8643S can sink cur-
rent from the output and return this charge to the input
in this mode, improving load step transient response
(see Figure 4). At light loads, FCM operation is less effi-
cient than Burst Mode operation, but may be desirable
in applications where it is necessary to keep switching
harmonics out of the signal band. FCM must be used if
the output is required to sink current. To enable FCM,
float the SYNC/MODE pin. Leakage current on this pin
should be <1µA. See Block Diagram for internal pull-up
and pull-down resistance.
VOUT
2 • fSW
VOUT
40
⎛
⎝
⎞
LMIN
=
• 1–
⎜
⎟
⎠
Spread Spectrum Mode
The LT8640S/LT8643S features spread spectrum opera-
tion to further reduce EMI/EMC emissions. To enable
spread spectrum operation, the SYNC/MODE pin should
be tied high to INTV (~3.4V)or an external supply of 3V
CC
to 4V. In this mode, triangular frequency modulation is
used to vary the switching frequency between the value
programmed by RT to approximately 20% higher than
that value. The modulation frequency is approximately
3kHz. For example, when the LT8640S/LT8643S is pro-
grammed to 2MHz, the frequency will vary from 2MHz to
2.4MHz at a 3kHz rate. When spread spectrum operation
is selected, Burst Mode operation is disabled, and the part
will run in forced continuous mode.
FCM is disabled if the VIN pin is held above 37V or if
the FB pin is held greater than 8% above the feedback
reference voltage. FCM is also disabled during soft-start
until the soft-start capacitor is fully charged. When FCM
is disabled in these ways, negative inductor current is
not allowed and the LT8640S/LT8643S operates in pulse-
skipping mode.
Synchronization
To synchronize the LT8640S/LT8643S oscillator to an
external frequency, connect a square wave to the SYNC/
MODE pin. The square wave amplitude should have val-
leys that are below 0.4V and peaks above 1.5V (up to 6V)
with a minimum on-time and off-time of 50ns.
I
LOAD
The LT8640S/LT8643S will not enter Burst Mode opera-
tion at low output loads while synchronized to an external
clock, but instead will run forced continuous mode to
maintain regulation. The LT8640S/LT8643S may be syn-
chronized over a 200kHz to 3MHz range. The RT resistor
should be chosen to set the LT8640S/LT8643S 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 sub-
harmonic oscillations is established by the inductor size,
input voltage and output voltage. Since the synchroniza-
tion frequency will not change the slopes of the inductor
current waveform, if the inductor is large enough to avoid
1A/DIV
Burst Mode OPERATION
V
OUT
100mV/DIV
FCM
8640S F04
50µs/DIV
FRONT PAGE APPLICATION
100mA TO 1.1A TRANSIENT
12V , 5V , f = 1MHz
IN
OUT
OUT SW
C
= 100µF
Figure 4. LT8640S Load Step Transient Response
with and without Forced Continuous Mode
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LT8640S/LT8643S
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subharmonic oscillations at the frequency set by RT, then
the slope compensation will be sufficient for all synchro-
nization frequencies.
The R resistor required for a desired switching frequency
T
can be calculated using:
46.5
RT =
–5.2
(3)
fSW
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:
where R is in kΩ and f is the desired switching fre-
quency in MHz.
T
SW
Table 1. SW Frequency vs R Value
T
VOUT
0.970V
⎛
⎜
⎝
⎞
⎠
(1)
f
(MHz)
0.2
0.3
0.4
0.5
0.6
0.7
0.8
1.0
1.2
1.4
1.6
1.8
2.0
2.2
3.0
R (kΩ)
SW
T
R1=R2
–1
⎟
232
150
Reference designators refer to the Block Diagram. 1%
resistors are recommended to maintain output voltage
accuracy.
110
88.7
71.5
60.4
52.3
41.2
33.2
28.0
23.7
20.5
17.8
15.8
10.7
For the LT8640S, if low input quiescent current and good
light-load efficiency are desired, use large resistor val-
ues 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:
⎛
⎞
VOUT
R1+R2
VOUT
1
⎝ ⎠
n
⎛
⎜
⎝
⎞
⎟
⎠
⎛ ⎞
I =1.7µA+
(2)
⎜ ⎟
Q
⎜
⎟
V
⎝
⎠
IN
where 1.7µA is the quiescent current of the LT8640S 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
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 disad-
vantages are lower efficiency and a smaller input voltage
range.
R2 = 412k, the feedback divider draws 2.3µA. With V =
IN
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 V ; this is plotted in the Typical
Performance Characteristics section.
IN
The highest switching frequency (f
) for a given
When using large FB resistors, a 4.7pF to 22pF phase-lead
SW(MAX)
application can be calculated as follows:
capacitor should be connected from V
to FB.
OUT
V
OUT + VSW(BOT)
Setting the Switching Frequency
(4)
fSW(MAX)
=
tON(MIN) V – VSW(TOP) + VSW(BOT)
IN
The LT8640S/LT8643S uses a constant frequency PWM
architecture that can be programmed to switch from
200kHz to 3MHz by using a resistor tied from the RT pin
to ground. A table showing the necessary R value for a
desired switching frequency is in Table 1.
where V is the typical input voltage, V
drops (~0S.4WV(,TO~P0).15V, rSeWsp(BeOcTti)vely at maximum load)
is the output
OUT
voltage,INV
and V
are the internal switch
T
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A good first choice for the inductor value is:
and t
is the minimum top switch on-time (see the
ON(MIN)
Electrical Characteristics). This equation shows that a
⎛
⎜
⎞
⎟
VOUT + VSW(BOT)
slower switching frequency is necessary to accommodate
a high V /V
L =
• 0.7
⎜
⎜
⎟
⎟
(6)
fSW
⎝
⎠
ratio.
IN OUT
For transient operation, V may go as high as the abso-
where fSW is the switching frequency in MHz, VOUT is
IN
lute maximum rating of 42V regardless of the R value,
the output voltage, V
is the bottom switch drop
T
SW(BOT)
however the LT8640S/LT8643S will reduce switching
frequency as necessary to maintain control of inductor
current to assure safe operation.
(~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.
The LT8640S/LT8643S is capable of a maximum duty
cycle of approximately 99%, and the V -to-V
dropout
In addition, the saturation current (typically labeled I
)
IN
OUT
SAT
is limited by the R
of the top switch. In this mode
rating of the inductor must be higher than the load current
plus 1/2 of in inductor ripple current:
DS(ON)
the LT8640S/LT8643S skips switch cycles, resulting in a
lower switching frequency than programmed by RT.
1
2
I
L(PEAK) =ILOAD(MAX) + ΔIL
(7)
For applications that cannot allow deviation from the pro-
grammed switching frequency at low V /V
ratios use
IN OUT
where ∆I is the inductor ripple current as calculated in
L
the following formula to set switching frequency:
Equation 9 and I
for a given application.
is the maximum output load
LOAD(MAX)
V
OUT + VSW(BOT)
(5)
V
=
– VSW(BOT) + VSW(TOP)
IN(MIN)
1– fSW •tOFF(MIN)
As a quick example, an application requiring 3A output
should use an inductor with an RMS rating of greater than
where VIN(MIN) is the minimum input voltage without
skipped cycles, V
SW(BOT)
is the output voltage, V
and
3A and an I
of greater than 4A. During long duration
SW(TOP)
V
are theOinUtTernal switch drops (~0.4V, ~0.15V,
overload orSsAhTort-circuit conditions, the inductor RMS
rating requirement is greater to avoid overheating of the
inductor. To keep the efficiency high, the series resistance
(DCR) should be less than 0.02Ω, and the core material
should be intended for high frequency applications.
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 LT8640S/LT8643S limits the peak switch current in
order to protect the switches and the system from over-
Inductor Selection and Maximum Output Current
load faults. The top switch current limit (I ) is 10A at
LIM
The LT8640S/LT8643S is designed to minimize solution
size by allowing the inductor to be chosen based on the
output load requirements of the application. During over-
load or short-circuit conditions the LT8640S/LT8643S
safely tolerates operation with a saturated inductor
through the use of a high speed peak-current mode
architecture.
low duty cycles and decreases linearly to 7A at DC = 0.8.
The inductor value must then be sufficient to supply the
desired maximum output current (I
), which is a
OUT(MAX)
function of the switch current limit (I ) and the ripple
LIM
current.
ΔIL
2
IOUT(MAX) =ILIM
–
(8)
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The peak-to-peak ripple current in the inductor can be
calculated as follows:
For more information about maximum output current
and discontinuous operation, see Linear Technology’s
Application Note 44.
⎛
⎞
VOUT
L•fSW
VOUT
V
IN(MAX)
ΔIL =
• 1–
(9)
For duty cycles greater than 50% (VOUT/VIN > 0.5), a
minimum inductance is required to avoid sub-harmonic
oscillation. See Application Note 19.
⎜
⎟
⎝
⎠
where f is the switching frequency of the LT8640S/
SW
LT8643S, and L is the value of the inductor. Therefore, the
maximum output current that the LT8640S/LT8643S 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 cur-
rent does not allow sufficient maximum output current
Input Capacitors
The VIN of the LT8640S/LT8643S should be bypassed
with at least three ceramic capacitors for best perfor-
mance. Two small ceramic capacitors of <1µF can be
placed close to the part; one on each side of the device
(C
, C
). These capacitors should be 0402 or 0603
in OsPizTe1. FOoPrTa2utomotive applications requiring 2 series
input capacitors, two small 0402 or 0603 may be placed
(I
) given the switching frequency, and maximum
inOpUuTt(vMoAlXta)ge used in the desired application.
In order to achieve higher light load efficiency, more
energy must be delivered to the output during the sin-
gle small pulses in Burst Mode operation such that the
LT8640S/LT8643S can stay in sleep mode longer between
each pulse. This can be achieved by using a larger value
inductor (i.e., 4.7µH), and should be considered indepen-
dent of switching frequency when choosing an inductor.
For example, while a lower inductor value would typi-
cally be used for a high switching frequency application,
if high light load efficiency is desired, a higher inductor
value should be chosen. See curve in Typical Performance
Characteristics.
at each side of the LT8640S/LT8643S near the V and
GND pins.
IN
A third, larger ceramic capacitor of 2.2µF or larger should
be placed close to C
more detail. X7R or X5R capacitors are recommended for
best performance across temperature and input voltage
variations.
or C
. See layout section for
OPT1
OPT2
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.
The optimum inductor for a given application may dif-
fer 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 LT8640S/LT8643S may operate
with higher ripple 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.
A ceramic input capacitor combined with trace or cable
inductance forms a high quality (under damped) tank cir-
cuit. If the LT8640S/LT8643S circuit is plugged into a
live supply, the input voltage can ring to twice its nominal
value, possibly exceeding the LT8640S/LT8643S’s volt-
age rating. This situation is easily avoided (see Linear
Technology Application Note 88).
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LT8640S/LT8643S
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Output Capacitor and Output Ripple
Afinalprecautionregardingceramiccapacitorsconcernsthe
maximuminputvoltageratingofthe LT8640S/LT8643S. As
previously mentioned, a ceramic input capacitor com-
bined with trace or cable inductance forms a high quality
(underdamped) tank circuit. If the LT8640S/LT8643S
circuit is plugged into a live supply, the input voltage
can ring to twice its nominal value, possibly exceeding
the LT8640S/LT8643S’s rating. This situation is easily
avoided (see Linear Technology Application Note 88).
The output capacitor has two essential functions. Along
with the inductor, it filters the square wave generated by
the LT8640S/LT8643S 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 LT8640S/LT8643S’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.
Enable Pin
The LT8640S/LT8643S 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 V if the shutdown feature is
not used, or tied to a logicIlNevel if shutdown control is
required.
Use X5R or X7R types. This choice will provide low out-
put ripple and good transient response. Transient perfor-
mance can be improved with a higher value output capaci-
tor and the addition of a feedforward capacitor placed
between V
and FB. Increasing the output capacitance
OUT
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.
Adding a resistor divider from V to EN programs the
IN
LT8640S/LT8643S to regulate the output only when V is
IN
above a desired voltage (see the Block Diagram). Typically,
this threshold, V
, is used in situations where the
IN(EN)
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
When choosing a capacitor, special attention should be
given to the data sheet to calculate the effective capaci-
tance under the relevant operating conditions of voltage
bias and temperature. A physically larger capacitor or one
with a higher voltage rating may be required.
V
threshold prevents the regulator from operating
IN(EN)
Ceramic Capacitors
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:
Ceramic capacitors are small, robust and have very low
ESR. However, ceramic capacitors can cause problems
when used with the LT8640S/LT8643S due to their
piezoelectric nature. When in Burst Mode operation, the
LT8640S/LT8643S’s switching frequency depends on
the load current, and at very light loads the LT8640S/
LT8643S can excite the ceramic capacitor at audio fre-
quencies, generating audible noise. Since the LT8640S/
LT8643S 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.
R3
R4
⎛
⎞
⎠
(10)
V
=
+1 •1.0V
⎟
⎜
⎝
IN(EN)
where the LT8640S/LT8643S will remain off until V is
IN
above V
. Due to the comparator’s hysteresis, switch-
ing will not stop until the input falls slightly below VIN(EN)
IN(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 LT8640S/LT8643S. Therefore, the V
resis-
IN(EN)
tors should be large to minimize their effect on efficiency
at low loads.
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INTV Regulator
A zero is required and comes from a resistor R in series
CC
C
with C . This simple model works well as long as the value
of theCinductor is not too high and the loop crossover
frequency is much lower than the switching frequency. A
phase lead capacitor (C ) across the feedback divider can
be used to improve thePtrLansient response and is required
to cancel the parasitic pole caused by the feedback node
to ground capacitance.
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 cur-
rent for the LT8640S/LT8643S’s circuitry. 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 LT8640S/
LT8643S, or can be tied to an external supply of 3.3V or
LT8643S
above. If BIAS is connected to a supply other than V
,
be sure to bypass with a local ceramic capacitor. IfOtUhTe
BIAS pin is below 3.0V, the internal LDO will consume
CURRENT MODE
POWER STAGE
current from V . Applications with high input voltage and
IN
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
OUTPUT
C
PL
R1
R2
g
= 5S
m
connect an external load to the INTV pin.
CC
g
= 1.7mS
m
FB
C1
V
C
–
Frequency Compensation (LT8643S Only)
+
0.97V
R
C
150k
Loop compensation determines the stability and transient
performance, and is provided by the components tied to
C
F
C
C
the V pin. Generally, a capacitor (C ) and a resistor (R )
C
C
C
in series to ground are used. Designing the compensation
network is a bit complicated and the best values depend
on the application. A practical approach is to start with
one of the circuits in this data sheet that is similar to your
application and tune the compensation network to opti-
mize the performance. LTspice® simulations can help in
this process. Stability should then be checked across all
operating conditions, including load current, input voltage
and temperature. The LT1375 data sheet contains a more
thorough discussion of loop compensation and describes
how to test the stability using a transient load.
8640S F05
Figure 5. Model for Loop Response
Output Voltage Tracking and Soft-Start
he LT8640S/LT8643S allows the user to program its out-
put 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 soft-
start ramp the output voltage will proportionally track the
TR/SS pin voltage.
T
Figure 5 shows an equivalent circuit for the LT8643S
control loop. The error amplifier is a transconductance
amplifier with finite output impedance. The power section,
consisting of the modulator, power switches, and inductor,
is modeled as a transconductance amplifier generating an
For output tracking applications, TR/ SS can be externally
driven by another voltage source. For the LT8640S, from
0V to 0.97V, the TR/SS voltage will override the internal
0.97V reference input to the error amplifier, thus regulat-
ing 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. For
the LT8643S, from 0V to 1.6V, the TR/SS voltage will
output current proportional to the voltage at the V pin.
C
Note that the output capacitor integrates this current, and
that the capacitor on the V pin (C ) integrates the error
C
C
amplifier output current, resulting in two poles in the loop.
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override the internal 0.97V reference input to the error
amplifier, thus regulating the FB pin voltage to a func-
tion of the TR/SS pin. See plot in the Typical Performance
Characteristics section. When TR/SS is above 1.6V, track-
ing 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.
Output Power Good
When the LT8640S/LT8643S’s output voltage is within
the 8% window of the regulation point, the output volt-
age is considered good and the open-drain PG pin goes
high impedance and is typically pulled high with an exter-
nal resistor. Otherwise, the internal pull-down device will
pull the PG pin low. To prevent glitching both the upper
and lower thresholds include 0.2% of hysteresis. PG is
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
valid when V is above 3.4V.
IN
The PG pin is also actively pulled low during several fault
conditions: EN/UV pin is below 1V, INTV has fallen too
CC
capacitor are the EN/UV pin transitioning low, V voltage
IN
low, V is too low, or thermal shutdown.
IN
falling too low, or thermal shutdown.
Shorted and Reversed Input Protection
Paralleling (LT8643S Only)
The LT8640S/LT8643S 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 switch-
ing of the top switch will be delayed until such time as the
inductor current falls to safe levels.
To increase the possible output current, two LT8643s can
be connected in parallel to the same output. To do this, the
VC and FB pins are connected together, and each LT8643’s
SW node is connected to the common output through its
own inductor. The CLKOUT pin of one LT8643S should be
connected to the SYNC/MODE pin of the second LT8643S
to have both devices operate in the same mode. During
FCM, spread spectrum, and synchronization modes,
both devices will operate at the same frequency. Figure 6
shows an application where two LT8643s are paralleled
to get one output capable of up to 12A.
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 pro-
grammed level. If the SYNC pin is connected to a clock
source, floated or tied high, the LT8640S/LT8643S will stay
at the programmed frequency without foldback and only
slow switching if the inductor current exceeds safe levels.
LT8643S
L1
V
OUT
V
C
SW
12A
C1
C
OUT
R1
R2
CLKOUT
FB
There is another situation to consider in systems where
the output will be held high when the input to the
LT8640S/LT8643S 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
R
C
LT8643S
C
C
SYNC/MODE
FB
L2
V
C
SW
LT8640S/LT8643S’s output. If the V pin is allowed to
IN
8640S F06
float and the EN pin is held high (either by a logic signal
or because it is tied to V ), then the LT8640S/LT8643S’s
IN
Figure 6. Paralleling Two LT8643Ss
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 V pin
IN
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acceptable level. Figure 8 shows examples of how case
is grounded while the output is held high, regardless of
EN, parasitic body diodes inside the LT8640S/LT8643S
can pull current from the output through the SW pin and
temperature rise can be managed by reducing V , switch-
IN
ing frequency, or load.
the V pin. Figure 7 shows a connection of the V and
IN
IN
The LT8640S/LT8643S’s internal power switches are
capable of safely delivering up to 7A of peak output cur-
rent. However, due to thermal limits, the package can
only handle 7A loads for short periods of time. This
time is determined by how quickly the case temperature
approaches the maximum junction rating. Figure 9 shows
an example of how case temperature rise changes with
the duty cycle of a 1kHz pulsed 7A load.
EN/UV pins that will allow the LT8640S/LT8643S to run
only when the input voltage is present and that protects
against a shorted or reversed input.
D1
V
IN
V
IN
LT8640S/
LT8643S
EN/UV
GND
8640S F07
The LT8640S/LT8643S’s top switch current limit
decreases with higher duty cycle operation for slope
compensation. This also limits the peak output current
the LT8640S/LT8643S can deliver for a given application.
See curve in Typical Performance Characteristics.
Figure 7. Reverse VIN Protection
Thermal Considerations and Peak Output Current
80
For higher ambient temperatures, care should be taken
in the layout of the PCB to ensure good heat sinking of
the LT8640S/LT8643S. The ground pins on the bottom
of the package should 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 LT8640S/LT8643S. 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 dissi-
pation within the LT8640S/LT8643S 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 LT8640S/
LT8643S power dissipation by the thermal resistance
from junction to ambient.
DC2530A DEMO BOARD
70
V
V
V
V
= 12V, f = 1MHz
IN
IN
IN
IN
SW
= 24V, f = 1MHz
SW
60
50
40
30
20
10
0
= 12V, f = 2MHz
SW
= 24V, f = 2MHz
SW
0
1
2
3
4
5
6
LOAD CURRENT (A)
8640S F08
Figure 8. Case Temperature Rise
90
80
70
60
50
40
30
20
10
0
DC2530A DEMO BOARD
V
= 12V
OUT
= 2MHz
IN
V
= 5V
f
SW
STANDBY LOAD = 0.25A
The internal overtemperature protection monitors the
junction temperature of the LT8640S/LT8643S. If the
junction temperature reaches approximately 180°C, the
LT8640S/LT8643S will stop switching and indicate a fault
condition until the temperature drops about 10°C cooler.
1kHz PULSED LOAD = 7A
Temperature rise of the LT8640S/LT8643S is worst when
operating at high load, high V , and high switching fre-
IN
0
0.2
0.4
0.6
0.8
1
quency. If the case temperature is too high for a given
DUTY CYCLE OF 7A LOAD
8640S F09
application, then either V , switching frequency, or load
IN
Figure 9. Case Temperature Rise vs 7A Pulsed Load
current can be decreased to reduce the temperature to an
8640sfa
25
For more information www.linear.com/LT8640S
LT8640S/LT8643S
Typical applicaTions
V
IN
V
IN
3.3µH
5.7V TO 42V
V
OUT
EN/UV
SW
5V
4.7µF
6A
LT8640S/LT8643S
100k
CLKOUT
PG
PINS NOT USED IN
THIS CIRCUIT:
SYNC/MODE
6.49k
BIAS
V *
C
100µF
1210
X5R/X7R
BST, INTV
CC
4.7pF (LT8643S)
10pF (LT8640S)
1M
TR/SS
330pF
FB
RT
10nF
GND
243k
41.2k
8640S F10
f
= 1MHz
SW
L: XEL6030
Figure 10. 5V 6A Step-Down Converter with Soft-Start and Power Good
V
IN
V
IN
4V TO 42V
2.2µH
V
OUT
EN/UV
4.7µF
SW
5V
6A
LT8640S/LT8643S
100k
CLKOUT
PG
PINS NOT USED IN
THIS CIRCUIT:
SYNC/MODE
8.45k
BIAS
100µF
1210
X5R/X7R
BST, INTV
CC
V *
C
4.7pF (LT8643S)
10pF (LT8640S)
1M
TR/SS
330pF
FB
RT
GND
10nF
412k
41.2k
8640S F11
f
= 1MHz
SW
L: XEL6030
Figure 11. 3.3V, 6A Step-Down Converter with Soft-Start and Power Good
FB1
BEAD
V
IN
5.7V TO 42V
10µF
1210
10µF
1210
10µF
1210
EN/UV
V
V
IN
IN
1µF
1µF
0603
0603
GND
LT8640S/LT8643S
GND
PINS NOT USED IN
THIS CIRCUIT:
1.5µH
V
OUT
INTV
SW
5V
CC
6A
BST, CLKOUT, PG, TR/SS
SYNC/MODE
BIAS
8.45k
4.7pF (LT8643S)
10pF (LT8640S)
1M
V *
C
RT
100µF
1210
X5R/X7R
FB
330pF
GND
17.8k
243k
8640S F12
f
= 2MHz
SW
L: XEL6030
FB1 BEAD: WE-MPSB 100Ω 8A 1812
Figure 12. Ultralow EMI 5V, 6A Step-Down Converter with Spread Spectrum
* V pin and components only apply to LT8643S.
C
8640sfa
26
For more information www.linear.com/LT8640S
LT8640S/LT8643S
Typical applicaTions
LT8640S/LT8643S
V
IN
5.7V TO 42V
V
IN
EN/UV
INTV
1.5µH
V
OUT
4.7µF
SW
5V
6A
CC
SYNC/MODE
PINS NOT USED IN
BIAS
THIS CIRCUIT:
4.7pF (LT8643S)
10pF (LT8640S)
8.45k
1M
100µF
1210
X5R/X7R
V *
RT
C
BST, CLKOUT, PG, TR/SS
FB
330pF
GND
243k
17.8k
8640S F13
f
= 2MHz
SW
L: XEL6030
Figure 13. 2MHz 5V, 6A Step-Down Converter with Spread Spectrum
LT8640S/LT8643S
V
IN
V
IN
1µH
5.7V TO 42V
V
OUT
4.7µF
SW
3.3V
EN/UV
INTV
6A
CC
PINS NOT USED IN
BIAS
SYNC/MODE
THIS CIRCUIT:
4.7pF (LT8643S)
10pF (LT8640S)
16.2k
1M
100µF
1210
X5R/X7R
V *
RT
C
BST, CLKOUT, PG, TR/SS
FB
220pF
GND
412k
17.8k
8640S F14
f
= 2MHz
SW
L: XEL6030
Figure 14. 2MHz 3.3V, 6A Step-Down Converter with Spread Spectrum
4.7µH
V
12V
6A
OUT
V
IN
V
SW
BIAS
IN
12.7V TO 42V
EN/UV
4.7µF
4.7pF
1M
LT8640S
GND
47µF
1210
RT
FB
PINS NOT USED IN THIS CIRCUIT:
CC
PG, SYNC/MODE, TR/SS
X5R/X7R
41.2k
88.7k
BST, CLKOUT, INTV
,
8640S F15
f
= 1MHz
SW
L: XEL6060
Figure 15. 12V, 6A Step-Down Converter
* V pin and components only apply to LT8643S.
C
8640sfa
27
For more information www.linear.com/LT8640S
LT8640S/LT8643S
package DescripTion
Please refer to http://www.linear.com/product/LT8640S#packaging for the most recent package drawings.
Y
X
Z
M c c c
d d d
Z
Z
× 2 4
Z
/ / b b b
Z
1 . 2 5 0 0
0 . 7 5 0 0
0 . 2 5 0 0
0 . 0 0 0 0
0 . 2 5 0 0
0 . 7 5 0 0
1 . 2 5 0 0
a a a
Z
× 2
8640sfa
28
For more information www.linear.com/LT8640S
LT8640S/LT8643S
revision hisTory
REV
DATE
DESCRIPTION
PAGE NUMBER
A
06/17 Added LT8643S
All
8640sfa
Information furnished by Linear Technology Corporation is believed to be accurate and reliable.
However, no responsibility is assumed for its use. Linear Technology Corporation makes no representa-
tion that the interconnection of its circuits as described herein will not infringe on existing patent rights.
29
LT8640S/LT8643S
Typical applicaTions
2MHz 1.8V, 6A Step-Down Converter
V
1µH
V
1.8V
6A
IN
OUT
3.4V TO 22V
V
SW
IN
(42V TRANSIENT)
4.7µF
EXTERNAL
SOURCE >3.1V
OR GND
EN/UV
BIAS
1µF
10pF
866k
1M
LT8640S
GND
100µF
1210
X5R/X7R
RT
FB
PINS NOT USED IN THIS CIRCUIT:
17.8k
= 2MHz
BST, CLKOUT, INTV
,
CC
PG, SYNC/MODE, TR/SS
8640S TA02
f
SW
L: XEL6030
relaTeD parTs
PART
DESCRIPTION
42V, 5A, 96% Efficiency, 3MHz Synchronous MicroPower Step-Down
DC/DC Converter with I = 2.5μA
COMMENTS
= 3.4V, V
LT8640/
V
I
= 42V, V
= 0.99V, I = 2.5µA,
Q
IN(MIN)
IN(MAX)
OUT(MIN)
LT8640-1
< 1µA, 3mm × 4mm QFN-18
Q
SD
LT8645S
LT8641
LT8609
65V, 8A, Synchronous Step-Down Silent Switcher 2 with I = 2.5μA
V
SD
= 3.4V, V
= 65V, V
= 0.97V, I = 2.5µA,
Q
Q
IN(MIN)
IN(MAX)
OUT(MIN)
I
< 1µA, 4mm × 6mm LQFN-32
65V, 3.5A, 95% Efficiency, 3MHz Synchronous MicroPower Step-Down
V
= 3V, V
= 65V, V
= 0.81V, I = 2.5µA,
Q
IN(MIN)
IN(MAX)
OUT(MIN)
DC/DC Converter with I = 2.5μA
I
SD
< 1µA, 3mm × 4mm QFN-18
Q
42V, 2A, 94% Efficiency, 2.2MHz Synchronous MicroPower Step-Down
V
= 3V, V
= 42V, V
= 0.8V, I = 2.5µA,
Q
IN(MIN)
IN(MAX)
OUT(MIN)
DC/DC Converter with I = 2.5µA
I
SD
< 1µA, MSOP-10E
Q
LT8610A/
LT8610AB
42V, 3.5A, 96% Efficiency, 2.2MHz Synchronous MicroPower Step-
V
SD
= 3.4V, V
= 42V, V = 0.97V, I = 2.5µA,
OUT(MIN) Q
IN(MIN)
IN(MAX)
Down DC/DC Converter with I = 2.5µA
I
< 1µA, MSOP-16E
Q
LT8610AC
42V, 3.5A, 96% Efficiency, 2.2MHz Synchronous MicroPower Step-
V
SD
= 3V, V
= 42V, V = 0.8V, I = 2.5µA,
OUT(MIN) Q
IN(MIN)
IN(MAX)
Down DC/DC Converter with I = 2.5µA
I
< 1µA, MSOP-16E
Q
LT8610
42V, 2.5A, 96% Efficiency, 2.2MHz Synchronous MicroPower Step-
V
SD
= 3.4V, V
= 42V, V
= 42V, V
= 0.97V, I = 2.5µA,
Q
IN(MIN)
IN(MAX)
OUT(MIN)
Down DC/DC Converter with I = 2.5µA
I
< 1µA, MSOP-16E
Q
LT8611
42V, 2.5A, 96% Efficiency, 2.2MHz Synchronous MicroPower Step-
V
= 3.4V, V
= 0.97V, I = 2.5µA,
Q
IN(MIN)
IN(MAX)
OUT(MIN)
Down DC/DC Converter with I = 2.5µA and Input/Output Current
I
SD
< 1µA, 3mm × 5mm QFN-24
Q
Limit/Monitor
LT8616
LT8620
LT8614
LT8612
LT8613
LT8602
42V, Dual 2.5A + 1.5A, 95% Efficiency, 2.2MHz Synchronous
V
SD
= 3.4V, V
= 42V, V
= 0.8V, I = 5µA,
IN(MIN)
IN(MAX)
OUT(MIN) Q
MicroPower Step-Down DC/DC Converter with I = 5µA
I
< 1µA, TSSOP-28E, 3mm × 6mm QFN-28
Q
65V, 2.5A, 94% Efficiency, 2.2MHz Synchronous MicroPower Step-
V
= 3.4V, V
= 65V, V
= 0.97V, I = 2.5µA,
IN(MIN)
IN(MAX)
OUT(MIN) Q
Down DC/DC Converter with I = 2.5µA
I
SD
< 1µA, MSOP-16E, 3mm × 5mm QFN-24
Q
42V, 4A, 96% Efficiency, 2.2MHz Synchronous Silent Switcher Step-
V
= 3.4V, V
< 1µA, 3mm × 4mm QFN18
= 42V, V
= 0.97V, I = 2.5µA,
Q
IN(MIN)
IN(MAX)
OUT(MIN)
OUT(MIN)
OUT(MIN)
Down DC/DC Converter with I = 2.5µA
I
SD
Q
42V, 6A, 96% Efficiency, 2.2MHz Synchronous MicroPower Step-Down
V
= 3.4V, V
< 1µA, 3mm × 6mm QFN-28
= 42V, V
= 0.97V, I = 3.0µA,
Q
IN(MIN)
IN(MAX)
DC/DC Converter with I = 2.5µA
I
SD
Q
42V, 6A, 96% Efficiency, 2.2MHz Synchronous MicroPower Step-Down
DC/DC Converter with Current Limiting
V
= 3.4V, V
< 1µA, 3mm × 6mm QFN-28
= 42V, V
= 0.97V, I = 3.0µA,
Q
IN(MIN)
IN(MAX)
I
SD
42V, Quad Output (2.5A + 1.5A + 1.5A + 1.5A) 95% Efficiency, 2.2MHz
Synchronous MicroPower Step-Down DC/DC Converter with I = 25µA
V
= 3V, V
= 42V, V
= 0.8V, I = 2.5µA,
IN(MIN)
IN(MAX)
OUT(MIN) Q
I
SD
< 1µA, 6mm × 6mm QFN-40
Q
8640sfa
LT 0617 REV A • PRINTED IN USA
www.linear.com/LT8640S
30
LINEAR TECHNOLOGY CORPORATION 2017
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