FEATURES
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LT8645S/LT8646S
65V, 8A Synchronous
Step-Down Silent Switcher 2
with 2.5µA Quiescent Current
DESCRIPTION
The
LT
®
8645S/LT8646S
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 eliminating layout sensitivity. This
performance makes the LT8645S/LT8646S ideal for noise
sensitive applications and environments.
The fast, clean, low-overshoot switching edges enable
high efficiency operation even at high switching frequen-
cies, leading to a small overall solution size. Peak current
mode control with a 40ns minimum on-time allows high
step-down ratios even at high switching frequencies. The
LT8646S has external compensation via the V
C
pin to en-
able current sharing and fast transient response at high
switching frequencies. A CLKOUT pin enables synchroniz-
ing other regulators to the LT8645S/LT8646S.
Burst Mode
®
operation enables ultralow standby current
consumption, pulse-skipping mode allows full switching
frequency at lower output loads, or spread spectrum
operation can further reduce EMI/EMC emissions.
All registered trademarks and trademarks are the property of their respective owners. Protected
by U.S. patents, including 8823345.
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Silent Switcher
®
2 Architecture
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Ultralow EMI/EMC Emissions on Any PCB
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Eliminates PCB Layout Sensitivity
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Internal Bypass Capacitors Reduce Radiated EMI
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Optional Spread Spectrum Modulation
High Efficiency at High Frequency
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Up to 95% Efficiency at 1MHz, 12V to 5V
IN
OUT
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Up to 94% Efficiency at 2MHz, 12V to 5V
IN
OUT
Wide Input Voltage Range: 3.4V to 65V
Ultralow Quiescent Current Burst Mode Operation
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2.5 A I Regulating 12V to 3.3V
Q
IN
OUT
(LT8645S)
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Output Ripple < 10mV
P-P
External Compensation: Fast Transient Response
and Current Sharing (LT8646S)
Fast Minimum Switch On-Time: 40ns
Low Dropout Under All Conditions: 60mV at 1A
Adjustable and Synchronizable: 200kHz to 2.2MHz
Peak Current Mode Operation
Output Soft-Start and Tracking
Small 32-Lead 6mm × 4mm LQFN Package
APPLICATIONS
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Automotive and Industrial Supplies
General Purpose Step-Down
GSM Power Supplies
TYPICAL APPLICATION
5V 8A Step-Down Converter
V
IN
5.5V TO 65V
2.2µH
V
IN
4.7µF
EN/UV
LT8645S
SW
BIAS
2.2pF
FB
GND
f
SW
= 1MHz
243k
8645S TA01a
12V
IN
to 5V
OUT
Efficiency
100
V
OUT
5V
8A
EFFICIENCY (%)
95
90
85
80
75
70
65
60
1
2
POWER LOSS
EFFICIENCY
4.0
3.5
3.0
POWER LOSS (W)
2.5
2.0
1.5
1.0
1M
100µF
RT
41.2k
1MHz, L = 2.2µH 0.5
2MHz, L = 1µH
0
3
4
5
6
7
8
LOAD CURRENT (A)
8645S TA01b
8645Sfa
For more information
www.linear.com/LT8645S
1
LT8645S/LT8646S
ABSOLUTE MAXIMUM RATINGS
(Note 1)
V
IN
, EN/UV ................................................................65V
PG .............................................................................42V
BIAS ..........................................................................25V
FB, TR/SS . .................................................................4V
SYNC/MODE Voltage . ................................................6V
Operating Junction Temperature Range (Note 2)
LT8645SE/LT8646SE......................... –40°C to 125°C
LT8645SI/LT8646SI........................... –40°C to 125°C
Storage Temperature Range .................. –65°C to 150°C
Maximum Reflow (Package Body) Temperature ... 260°C
PIN CONFIGURATION
LT8645S
SYNC/MODE
TOP VIEW
TR/SS
CLKOUT
LT8646S
SYNC/MODE
28
TOP VIEW
TR/SS
GND
PG
PG
32
31
30
29
28
27
26
RT
32
31
V
C
FB
FB
30
29
BIAS
INTVCC
NC
V
IN
V
IN
V
IN
NC
GND
GND
GND
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
37
GND
38
GND
35
GND
36
GND
33
GND
34
GND
BIAS
INTVCC
NC
V
IN
V
IN
V
IN
NC
GND
GND
GND
CLKOUT
27
26
RT
25
EN/UV
24
NC
23
V
IN
22
V
IN
21
V
IN
20
NC
19
GND
18
GND
17
GND
16
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
37
GND
38
GND
35
GND
36
GND
33
GND
34
GND
25
EN/UV
24
NC
23
V
IN
22
V
IN
21
V
IN
20
NC
19
GND
18
GND
17
GND
BST
SW
SW
SW
SW
SW
BST
SW
SW
SW
SW
LQFN PACKAGE
32-LEAD (6mm
×
4mm
×
0.94mm)
LQFN PACKAGE
32-LEAD (6mm
×
4mm
×
0.94mm)
JEDEC BOARD:
θ
JA
= 31°C/W,
θ
JC(PAD)
= 6°C/W (NOTE 3)
DEMO BOARD:
θ
JA
= 21°C/W
EXPOSED PADS (PINS 33-38) ARE GND, SHOULD BE SOLDERED TO PCB
JEDEC BOARD:
θ
JA
= 31°C/W,
θ
JC(PAD)
= 6°C/W (NOTE 3)
DEMO BOARD:
θ
JA
= 21°C/W
EXPOSED PADS (PINS 33-38) ARE GND, SHOULD BE SOLDERED TO PCB
ORDER INFORMATION
PART NUMBER
LT8645SEV#PBF
LT8645SIV#PBF
LT8646SEV#PBF
LT8646SIV#PBF
PART MARKING*
8645SV
http://www.linear.com/product/LT8645S#orderinfo
PAD FINISH
PACKAGE TYPE**
LQFN (Laminate Package
with QFN Footprint)
MSL
RATING
TEMPERATURE RANGE
FINISH CODE
e4
8646SV
Au (RoHS)
3
• Consult Marketing for parts specified with wider operating temperature
ranges. *Device temperature grade is indicated 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
8645Sfa
Parts ending with PBF are RoHS and WEEE compliant. **The LT8645S/LT8646S package has the same footprint as a standard 6mm × 4mm QFN Package.
2
For more information
www.linear.com/LT8645S
SW
–40°C to 125°C
LT8645S/LT8646S
ELECTRICAL CHARACTERISTICS
PARAMETER
Minimum Input Voltage
V
IN
Quiescent Current in Shutdown
LT8645S V
IN
Quiescent Current in Sleep
(Internal Compensation)
LT8646S V
IN
Quiescent Current in Sleep
(External Compensation)
LT8646S BIAS Quiescent Current in Sleep
LT8645S V
IN
Quiescent Current when Active
LT8646S V
IN
Quiescent Current when Active
LT8645S V
IN
Current in Regulation
Feedback Reference Voltage
Feedback Voltage Line Regulation
Feedback Pin Input Current
LT8646S Error Amp Transconductance
LT8646S Error Amp Gain
LT8646S V
C
Source Current
LT8646S V
C
Sink Current
LT8646S V
C
Pin to Switch Current Gain
LT8646S 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
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
0.9
0.9
1.7
1.7
230
230
16
200
0.4
0.6
MAX
3.4
3
10
4
10
290
340
25
260
0.6
0.8
60
400
0.976
0.982
0.025
20
UNITS
V
µA
µA
µA
µA
µA
µA
µA
µA
mA
mA
µA
µA
V
V
%/V
nA
mS
V/V
µA
µA
A/V
V
mA
V
EN/UV
= 0V
l
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
V
EN/UV
= 2V, V
FB
> 0.97V, V
SYNC
= 0V, V
BIAS
= 5V
V
EN/UV
= 2V, V
FB
> 0.97V, V
SYNC
= 2V, R
T
= 60.4k, V
BIAS
= 0V
V
EN/UV
= 2V, V
FB
> 0.97V, V
SYNC
= 2V, R
T
= 60.4k, V
BIAS
= 0V
V
OUT
= 0.97V, V
IN
= 6V, I
LOAD
= 100µA, V
SYNC
= 0V
V
OUT
= 0.97V, V
IN
= 6V, I
LOAD
= 1mA, V
SYNC
= 0V
V
IN
= 6V
V
IN
= 6V
V
IN
= 4.0V to 42V
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
17
200
0.964
0.958
–20
1.7
350
350
350
8
2.6
0.970
0.970
0.004
V
BIAS
= 3.3V, f
SW
= 2MHz
I
LOAD
= 2A, SYNC = 0V
I
LOAD
= 2A, SYNC = 2V
l
l
22
40
35
80
180
665
1.8
10.5
8.5
–1.5
l
65
60
110
240
735
2.1
17.5
ns
ns
ns
kHz
kHz
MHz
m
A
m
A
µA
V
mV
nA
%
%
210
700
1.95
36
14
25
11
1.01
45
Top Power NMOS On-Resistance
Top Power NMOS Current Limit
Bottom Power NMOS On-Resistance
Bottom Power NMOS Current Limit
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
V
INTVCC
= 3.4V, I
SW
= 1A
V
INTVCC
= 3.4V
V
IN
= 42V, V
SW
= 0V, 42V
EN/UV Rising
V
EN/UV
= 2V
V
FB
Falling
V
FB
Rising
l
l
13.5
1.5
1.07
20
0.95
–20
5
–10.5
7.5
–8
10
–5.5
8645Sfa
For more information
www.linear.com/LT8645S
3
LT8645S/LT8646S
ELECTRICAL CHARACTERISTICS
PARAMETER
PG Hysteresis
PG Leakage
PG Pull-Down Resistance
SYNC/MODE Threshold
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
R
T
= 60.4k, V
SYNC
= 3.3V
V
SYNC
= 3.3V
l
l
l
l
l
The
l
denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at T
A
= 25°C.
CONDITIONS
MIN
–40
750
0.7
2.2
0.9
1.2
2.55
24
2.5
1.2
1.9
220
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.
2.6
TYP
0.4
40
2000
1.4
2.9
V
V
V
%
kHz
µA
MAX
UNITS
%
nA
Spread Spectrum Modulation
Frequency Range
Spread Spectrum Modulation Frequency
TR/SS Source Current
TR/SS Pull-Down Resistance
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 LT8645SE/LT8646SE 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 LT8645SI/LT8646SI 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.
4
8645Sfa
For more information
www.linear.com/LT8645S
LT8645S/LT8646S
TYPICAL PERFORMANCE CHARACTERISTICS
12V
IN
to 5V
OUT
Efficiency
vs Frequency
100
95
90
EFFICIENCY (%)
85
80
75
70
65
60
1
2
POWER LOSS
EFFICIENCY
4.0
3.5
3.0
POWER LOSS (W)
EFFICIENCY (%)
2.5
2.0
1.5
L = XEL6060 1.0
500kHz, L = 2.7µH
1MHz, L = 2.2µH
2MHz, L = 1µH
3
4
5
6
LOAD CURRENT (A)
7
8
0.5
0
100
95
90
85
80
75
70
65
60
1
2
POWER LOSS
EFFICIENCY
12V
IN
to 3.3V
OUT
Efficiency
vs Frequency
4.0
3.5
3.0
POWER LOSS (W)
2.5
2.0
1.5
EFFICIENCY (%)
100
95
90
85
80
75
70
65
60
55
50
Efficiency at 5V
OUT
EFFICIENCY
4.0
3.6
3.2
f
SW
= 500kHz
L = XEL6060, 2.7µH
2.8
2.4
2.0
1.6
V
IN
= 12V
V
IN
= 24V
V
IN
= 36V
V
IN
= 48V
7
8
1.2
0.8
0.4
0
POWER LOSS (W)
L = XEL6060 1.0
500kHz, L = 2.7µH
1MHz, L = 1.5µH 0.5
2MHz, L = 0.82µH
0
3
4
5
6
7
8
LOAD CURRENT (A)
8645S G02
POWER LOSS
0
1
2
3
4
5
6
LOAD CURRENT (A)
8645S G01
8645S G03
Efficiency at 3.3V
OUT
100
95
90
EFFICIENCY (%)
85
80
75
70
65
60
55
50
0
1
POWER LOSS
2
3
4
5
LOAD CURRENT (A)
f
SW
= 500kHz
L = XEL6060, 2.7µH
EFFICIENCY
4.0
3.6
3.2
EFFICIENCY (%)
2.8
2.4
2.0
1.6
1.2
V
IN
= 12V
0.8
V
IN
= 24V
V
IN
= 36V 0.4
V
IN
= 48V
0
6
7
8
8645S G04
LT8645S Low Load Efficiency
at 5V
OUT
100
90
80
POWER LOSS (W)
70
60
50
40
30
20
0.01
0.1
V
IN
= 12V
V
IN
= 24V
V
IN
= 36V
V
IN
= 48V
f
SW
= 500kHz
L = WE–LHMI7050, 4.7µH
1
10
100
LOAD CURRENT (mA)
1000
8645S G05
LT8646S Low Load Efficiency
at 5V
OUT
100
90
80
EFFICIENCY (%)
70
60
50
40
30
20
10
0.1
1
V
IN
= 12V
V
IN
= 24V
V
IN
= 36V
V
IN
= 48V
f
SW
= 500kHz
L = WE–LHMI7050, 4.7µH
10
100
1000
LOAD CURRENT (mA)
8645S G06
OUT
100
90
80
EFFICIENCY (%)
LT8645S Low Load Efficiency at
3.3V
OUT
100
90
80
EFFICIENCY (%)
LT8646S Low Load Efficiency at
3.3V
OUT
Low Load Efficiency at 3.3V
OUT
LT8646S
Efficiency vs Frequency
96
94
92
EFFICIENCY (%)
90
88
86
84
82
80
0.4
V
OUT
= 3.3V
I
LOAD
= 2A
L = XEL6060, 4.7µH
0.7
1
1.3
1.6
1.9
SWITCHING FREQUENCY (MHz)
2.2
8645S G09
V
IN
= 12V
70
60
50
40
30
20
0.01
0.1
V
IN
= 12V
V
IN
= 24V
V
IN
= 36V
V
IN
= 48V
f
SW
= 500kHz
L = WE–LHMI7050, 4.7µH
1
10
100
LOAD CURRENT (mA)
1000
8645S G07
70
60
50
40
30
20
10
0.1
1
V
IN
= 12V
V
IN
= 24V
V
IN
= 36V
V
IN
= 48V
f
SW
= 500kHz
L = WE–LHMI7050, 4.7µH
10
100
1000
LOAD CURRENT (mA)
8645S G08
V
IN
= 24V
8645Sfa
For more information
www.linear.com/LT8645S
5
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