FeaTures
n
LT8640S/LT8643S
42V, 6A Synchronous
Step-Down Silent Switcher 2 with
2.5µA Quiescent Current
DescripTion
The
LT
®
8640S/LT8643S
synchronous step-down regulator
features second generation Silent Switcher architecture
designed to minimize EMI/EMC emissions while delivering
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.
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
C
pin
to enable current sharing and fast transient response
at high switching frequencies. A CLKOUT pin enables
synchronizing other regulators to the LT8640S/LT8643S.
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.
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.
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Silent Switcher
®
2 Architecture
n
Ultralow EMI/EMC Emissions on Any PCB
n
Eliminates PCB Layout Sensitivity
n
Internal Bypass Capacitors Reduce Radiated EMI
n
Optional Spread Spectrum Modulation
High Efficiency at High Frequency
n
Up to 96% Efficiency at 1MHz, 12V to 5V
IN
OUT
n
Up to 95% Efficiency at 2MHz, 12V to 5V
IN
OUT
Wide Input Voltage Range: 3.4V to 42V
6A Maximum Continuous, 7A Peak Output
Ultralow Quiescent Current Burst Mode
®
Operation
n
2.5µA I Regulating 12V to 3.3V
Q
IN
OUT
(LT8640S)
n
Output Ripple < 10mV
P-P
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
Adjustable and Synchronizable: 200kHz to 3MHz
Output Soft-Start and Tracking
Small 24-Lead 4mm
×
4mm LQFN Package
applicaTions
n
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Automotive and Industrial Supplies
General Purpose Step-Down
Typical applicaTion
5V 6A Step-Down Converter
EFFICIENCY (%)
V
IN
5.7V TO 42V
4.7µF
3.3µH
V
IN
EN/UV
LT8640S
RT
41.2k
f
SW
= 1MHz
GND
FB
243k
8640S TA01a
100
95
90
V
OUT
5V
6A
1M
100µF
85
80
75
70
65
12V
5V
OUT
OUT
Efficiency
12V
IN
to
IN
to 5V
Efficiency
3.2
2.8
EFFICIENCY
2.4
POWER LOSS (W)
2.0
1.6
1.2
SW
BIAS
10pF
POWER LOSS
0.8
1MHz, L = 3.3µH
2MHz, L = 2.2µH 0.4
3MHz, L = 1µH
0
60
0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 5.5 6
LOAD CURRENT (A)
8640S TA01b
8640sfa
For more information
www.linear.com/LT8640S
1
LT8640S/LT8643S
absoluTe MaxiMuM raTings
(Note 1)
V
IN
, EN/UV, PG..........................................................42V
BIAS ..........................................................................25V
FB, TR/SS . .................................................................4V
SYNC Voltage .............................................................6V
Operating Junction Temperature Range (Note 2)
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
SYNC/MODE
SYNC/MODE
20
TOP VIEW
TR/SS
LT8643S
CLKOUT
TOP VIEW
TR/SS
GND
CLKOUT
19
18 RT
25
GND
26
GND
17 EN
16 GND
15 NC
14 V
IN
13 V
IN
7
BST
8
SW
9
SW
10
SW
11
SW
12
SW
PG
PG
23
24
BIAS
INTV
CC
GND
NC
V
IN
V
IN
1
2
3
4
5
6
7
BST
23
22
21
20
19
18 RT
BIAS
INTV
CC
GND
NC
V
IN
V
IN
1
2
3
4
5
6
24
V
C
22
FB
FB
21
25
GND
26
GND
17 EN
16 GND
15 NC
14 V
IN
13 V
IN
27
GND
28
GND
27
GND
28
GND
8
SW
9
SW
10
SW
11
SW
12
SW
LQFN PACKAGE
24-LEAD (4mm
×
4mm
×
0.94mm)
θ
JA
= 38°C/W,
θ
JC(PAD)
= 7°C/W (Note 3)
EXPOSED PAD (PINS 25–28) ARE GND, SHOULD BE SOLDERED TO PCB
LQFN PACKAGE
24-LEAD (4mm
×
4mm
×
0.94mm)
θ
JA
= 38°C/W,
θ
JC(PAD)
= 7°C/W (Note 3)
EXPOSED PAD (PINS 25–28) ARE GND, SHOULD BE SOLDERED TO PCB
orDer inForMaTion
PART NUMBER
LT8640SEV#PBF
LT8640SIV#PBF
LT8643SEV#PBF
LT8643SIV#PBF
PART MARKING*
8640S
e4
8643S
http://www.linear.com/product/LT8640S#orderinfo
PAD FINISH
PACKAGE
TYPE**
LQFN (Laminate Package
with QFN Footprint)
MSL
RATING
TEMPERATURE RANGE
FINISH CODE
Au (RoHS)
3
–40°C to 125°C
•
Consult Marketing for parts specified with wider operating temperature
ranges. *Device temperature grade is identified by a label on the shipping
container.
•
Pad finish code is per IPC/JEDEC J-STD-609.
•
Terminal Finish Part Marking:
www.linear.com/leadfree
•
Recommended PCB Assembly and Manufacturing Procedures:
www.linear.com/umodule/pcbassembly
•
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.
2
8640sfa
For more information
www.linear.com/LT8640S
LT8640S/LT8643S
elecTrical characTerisTics
PARAMETER
Minimum Input Voltage
V
IN
Quiescent Current in Shutdown
LT8640S V
IN
Quiescent Current in Sleep
LT8643S V
IN
Quiescent Current in Sleep
V
EN/UV
= 0V
l
The
l
denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at T
A
= 25°C.
CONDITIONS
l
MIN
TYP
3.0
0.75
0.75
1.7
1.7
230
230
19
200
MAX
3.4
3
10
4
10
290
340
25
260
60
390
0.976
0.982
0.02
20
UNITS
V
µA
µA
µA
µA
µA
µA
µA
µA
µA
µA
V
V
%/V
nA
mS
µA
µA
A/V
V
mA
V
EN/UV
= 2V, V
FB
> 0.97V, V
SYNC
= 0V
l
V
EN/UV
= 2V, V
FB
> 0.97V, V
SYNC
= 0V, V
BIAS
= 0V
l
V
EN/UV
= 2V, V
FB
> 0.97V, V
SYNC
= 0V, V
BIAS
= 5V
LT8643S BIAS Quiescent Current in Sleep
LT8640S V
IN
Current in Regulation
Feedback Reference Voltage
Feedback Voltage Line Regulation
Feedback Pin Input Current
LT8643S Error Amp Transconductance
LT8643S Error Amp Gain
LT8643S V
C
Source Current
LT8643S V
C
Sink Current
LT8643S V
C
Pin to Switch Current Gain
LT8643S V
C
Clamp Voltage
BIAS Pin Current Consumption
Minimum On-Time
Minimum Off-Time
Oscillator Frequency
R
T
= 221k
R
T
= 60.4k
R
T
= 18.2k
I
SW
= 1A
l
l
l
l
V
EN/UV
= 2V, V
FB
> 0.97V, V
SYNC
= 0V, V
BIAS
= 5V
V
OUT
= 0.97V, V
IN
= 6V, I
LOAD
= 100µA, V
SYNC
= 0
V
OUT
= 0.97V, V
IN
= 6V, I
LOAD
= 1mA, V
SYNC
= 0
V
IN
= 6V
V
IN
= 6V
V
IN
= 4.0V to 36V
V
FB
= 1V
V
C
= 1.25V
V
FB
= 0.77V, V
C
= 1.25V
V
FB
= 1.17V, V
C
= 1.25V
l
l
l
l
21
220
0.964
0.958
–20
1.7
260
350
350
5
2.6
0.970
0.970
0.004
V
BIAS
= 3.3V, f
SW
= 2MHz
I
LOAD
= 1.5A, SYNC = 0V
I
LOAD
= 1.5A, SYNC = 2V
l
l
14
30
30
80
180
665
1.8
7.5
–1.5
l
50
45
110
240
735
2.1
12.5
1.5
ns
ns
ns
kHz
kHz
MHz
mΩ
A
mΩ
µA
V
mV
nA
%
%
%
nA
Ω
V
V
V
210
700
1.95
66
10
27
Top Power NMOS On-Resistance
Top Power NMOS Current Limit
Bottom Power NMOS On-Resistance
SW Leakage Current
EN/UV Pin Threshold
EN/UV Pin Hysteresis
EN/UV Pin Current
PG Upper Threshold Offset from V
FB
PG Lower Threshold Offset from V
FB
PG Hysteresis
PG Leakage
PG Pull-Down Resistance
SYNC/MODE Threshold
V
INTVCC
= 3.4V, I
SW
= 1A
V
IN
= 42V, V
SW
= 0V, 42V
EN/UV Rising
V
EN/UV
= 2V
V
FB
Falling
V
FB
Rising
V
PG
= 3.3V
V
PG
= 0.1V
SYNC/MODE DC and Clock Low Level Voltage
SYNC/MODE Clock High Level Voltage
SYNC/MODE DC High Level Voltage
l
l
l
l
l
l
0.94
–20
5
–5.25
–40
1.0
40
1.06
20
7.5
–8
0.2
10.25
–10.75
40
700
0.7
2.2
0.9
1.2
2.55
2000
1.4
2.9
8640sfa
For more information
www.linear.com/LT8640S
3
LT8640S/LT8643S
elecTrical characTerisTics
PARAMETER
Spread Spectrum Modulation
Frequency Range
Spread Spectrum Modulation Frequency
TR/SS Source Current
TR/SS Pull-Down Resistance
Output Sink Current in Forced Continuous
Mode
V
IN
to Disable Forced Continuous Mode
Fault Condition, TR/SS = 0.1V
V
FB
= 1.01V, L = 6.8µH, R
T
= 60.4k
V
IN
Rising
0.25
35
CONDITIONS
R
T
= 60.4k, V
SYNC
= 3.3V
V
SYNC
= 3.3V
l
The
l
denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at T
A
= 25°C.
MIN
TYP
22
3
1.2
1.9
200
0.6
37
1.1
39
2.6
MAX
UNITS
%
kHz
µA
Ω
A
V
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 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
operating junction temperature range. The junction temperature (T
J
, in
°C) is calculated from the ambient temperature (T
A
in °C) and power
dissipation (PD, in Watts) according to the formula:
T
J
= T
A
+ (PD
•
θ
JA
)
where
θ
JA
(in °C/W) is the package thermal impedance.
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 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.
4
8640sfa
For more information
www.linear.com/LT8640S
LT8640S/LT8643S
Typical perForMance characTerisTics
12V
IN
to 5V
OUT
Efficiency
vs Frequency
100
95
90
EFFICIENCY (%)
85
80
75
70
POWER LOSS
EFFICIENCY
3.2
2.8
2.4
POWER LOSS (W)
EFFICIENCY (%)
2.0
1.6
1.2
100
95
90
85
80
75
70
POWER LOSS
EFFICIENCY
12V
IN
to 3.3V
OUT
Efficiency
vs Frequency
3.2
2.8
2.4
POWER LOSS (W)
2.0
1.6
1.2
EFFICIENCY (%)
100
95
90
85
80
75
70
65
60
55
50
Efficiency at 5V
OUT
3.0
2.7
EFFICIENCY
2.4
2.1
1.8
POWER LOSS (W)
POWER LOSS
1.5
1.2
L = WE-LHMI1040 0.8
1MHz, L = 3.3µH
65
2MHz, L = 2.2µH 0.4
3MHz, L = 1µH
0
60
0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 5.5 6
LOAD CURRENT (A)
8640S G01
L = WE-LHMI1040 0.8
1MHz, L = 2.2µH
0.4
65
2MHz, L = 1µH
3MHz, L = 1µH
0
60
0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 5.5 6
LOAD CURRENT (A)
8640S G02
0
1
V
IN
= 12V 0.9
V
IN
= 24V
0.6
V
IN
= 36V
f
SW
= 1MHz 0.3
L = IHLP3232DZ-01, 3.3µH
0
2
3
4
5
6
LOAD CURRENT (A)
8640S G03
100
95
90
EFFICIENCY (%)
85
80
75
70
65
60
55
50
Efficiency
Efficiency
at 3.3V
OUT
3.0
2.7
2.4
100
90
80
POWER LOSS (W)
EFFICIENCY (%)
70
60
50
40
30
LT8640S Low Load Efficiency at
5V
OUT
OUT
100
90
80
EFFICIENCY (%)
70
60
50
40
30
20
LT8643S Low Load Efficiency at
5V
OUT
EFFICIENCY
2.1
1.8
1.5
POWER LOSS
1.2
0
1
V
IN
= 12V 0.9
V
IN
= 24V
0.6
V
IN
= 36V
f
SW
= 1MHz 0.3
L = IHLP3232DZ-01, 2.2µH
0
2
3
4
5
6
LOAD CURRENT (A)
8640S G04
V
IN
= 12V
V
IN
= 24V
V
IN
= 36V
f
SW
= 1MHz
L = IHLP3232DZ-01, 4.7µH
0.1
1
10
100
LOAD CURRENT (mA)
1000
8640 G05
V
IN
= 12V
V
IN
= 24V
V
IN
= 36V
f
SW
= 1MHz
L = IHLP3232DZ–01, 4.7µH
1
10
100
LOAD CURRENT (mA)
1000
8640 G06
20
0.01
10
0.1
100
90
80
EFFICIENCY (%)
70
60
50
40
30
LT8640S Low Load Efficiency at
Efficiency
3.3V
OUT
at 3.3V
OUT
100
90
80
EFFICIENCY (%)
LT8643S Low Load Efficiency at
3.3V
OUT
Efficiency
vs Frequency
Efficiency
96
94
92
EFFICIENCY (%)
90
88
86
84
82
80
0
V
IN
= 12V
V
OUT
= 3.3V
I
LOAD
= 2A
L = IHLP3232DZ-01, 4.7µH
0.5
1
1.5
2
2.5
SWITCHING FREQUENCY (MHz)
3
8640S G09
70
60
50
40
30
20
10
0.1
1
V
IN
= 12V
V
IN
= 24V
V
IN
= 36V
f
SW
= 1MHz
L = IHLP3232DZ–01, 4.7µH
10
100
LOAD CURRENT (mA)
1000
8640S G08
V
IN
= 12V
V
IN
= 24V
V
IN
= 36V
f
SW
= 1MHz
L = IHLP3232DZ-01, 4.7µH
0.1
1
10
100
LOAD CURRENT (mA)
1000
8640S G07
20
0.01
8640sfa
For more information
www.linear.com/LT8640S
5
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