FEATURES
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LT8641
65V, 3.5A Synchronous
Step-Down Silent Switcher
with 2.5µA Quiescent Current
DESCRIPTION
The
LT
®
8641
step-down regulator features Silent Switcher
architecture designed to minimize EMI/EMC emissions
while delivering high efficiency at frequencies up to 3MHz.
Assembled in a 3mm
×
4mm QFN, the monolithic construc-
tion with integrated power switches and inclusion of all
necessary circuitry yields a solution with a minimal PCB
footprint. An ultralow 2.5µA quiescent current—with the
output in full regulation—enables applications requiring
highest efficiency at very small load currents. Transient
response remains excellent and output voltage ripple is
below 10mV
P-P
at any load, from zero to full current.
The LT8641 allows high V
IN
to low V
OUT
conversion at
high frequency with a fast minimum top switch on-time of
35ns. Operation is safe in overload even with a saturated
inductor.
Essential features are included and easy to use: An
open-drain PG pin signals when the output is in regula-
tion. The SYNC/MODE pin selects between Burst Mode,
pulse-skipping, or spread spectrum mode, and also al-
lows synchronization to an external clock. Soft-start and
tracking functionality is accessed via the TR/SS pin. An
accurate enable threshold can be set using the EN/UV pin
and a resistor at the RT pin programs switch frequency.
L,
LT, LTC, LTM, Linear Technology, the Linear logo, Silent Switcher and Burst Mode are
registered trademarks of Linear Technology Corporation. All other trademarks are the property
of their respective owners.
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Silent Switcher
®
Architecture
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Ultralow EMI/EMC Emissions
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Spread Spectrum Frequency Modulation
High Efficiency at High Frequency
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Up to 95% Efficiency at 1MHz
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Up to 94% Efficiency at 2MHz
Wide Input Voltage Range: 3V to 65V
3.5A Maximum Continuous Output,
5A Peak Transient Output
Ultralow Quiescent Current Burst Mode
®
Operation
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2.5μA I Regulating 12V to 3.3V
Q
IN
OUT
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Output Ripple < 10mV
P-P
Fast Minimum Switch-On Time: 35ns
Low Dropout Under All Conditions: 130mV at 1A
Safely Tolerates Inductor Saturation in Overload
Adjustable and Synchronizable: 200kHz to 3MHz
Peak Current Mode Operation
Output Soft-Start and Tracking
Small 18-Lead 3mm
×
4mm QFN
APPLICATIONS
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Automotive and Industrial Supplies
General Purpose Step-Down
GSM Power Supplies
TYPICAL APPLICATION
5V 3.5A Step-Down Converter
V
IN
5.5V TO 65V
4.7µF
1µF
EN/UV
EFFICIENCY (%)
100
95
90
12V
IN
to 5V
OUT
Efficiency
2.6
2.3
EFFICIENCY
1.9
POWER LOSS (W)
1.6
1.3
1.0
V
IN1
GND1
PG
V
IN2
GND2
1µF
85
80
75
70
65
60
0.5
1
POWER LOSS
LT8641
BST
SW
BIAS
10nF
1µF
41.2k
0.1µF 4.7µH
SYNC/MODE
TR/SS
INTV
CC
RT
V
OUT
5V
3.5A
1M
47µF
191k
0.7
4.7pF
FB
GND
1MHz, L = 3.3 H
2MHz, L = 2.2 H 0.3
3MHz, L = 1.5 H
0
1.5
2
2.5
3
3.5
LOAD CURRENT (A)
8641 TA01b
f
SW
= 1MHz
8641 TA01a
For more information
www.linear.com/LT8641
1
8641f
LT8641
ABSOLUTE MAXIMUM RATINGS
(Note 1)
PIN CONFIGURATION
SYNC/MODE
17
16 TR/SS
15 RT
22
SW
21
SW
7
GND1
8
SW
9
SW
10
GND2
14 EN/UV
13 VIN2
11 GND2
TOP VIEW
GND
V
IN
, EN/UV ................................................................65V
PG .............................................................................42V
BIAS ..........................................................................25V
FB, TR/SS . .................................................................4V
SYNC Voltage .............................................................6V
Operating Junction Temperature Range (Note 2)
LT8641E ............................................. –40°C to 125°C
LT8641I .............................................. –40°C to 125°C
Storage Temperature Range ......................–65 to 150°C
20 19 18
BIAS 1
INTV
CC
2
BST 3
V
IN1
4
GND1 6
UDC PACKAGE
18-LEAD (3mm
×
4mm) PLASTIC QFN
θ
JA
= 40°C/W,
θ
JC(PAD)
= 12°C/W
EXPOSED PAD (PINS 21, 22) ARE SW, SHOULD BE SOLDERED TO PCB
NOTE: PINS 5 AND 12 ARE REMOVED. CONFIGURATION DOES
NOT
MATCH
JEDEC 20-LEAD PACKAGE OUTLINE
ORDER INFORMATION
LEAD FREE FINISH
LT8641EUDC#PBF
LT8641IUDC#PBF
TAPE AND REEL
LT8641EUDC#TRPBF
LT8641IUDC#TRPBF
PART MARKING*
LGSN
LGSN
PACKAGE DESCRIPTION
18-Lead (3mm
×
4mm) Plastic QFN
18-Lead (3mm
×
4mm) Plastic QFN
TEMPERATURE RANGE
–40°C to 125°C
–40°C to 125°C
Consult LTC
®
Marketing for parts specified with wider operating temperature ranges. *The temperature grade is identified by a label on the shipping container.
For more information on lead free part marking, go to:
http://www.linear.com/leadfree/
For more information on tape and reel specifications, go to:
http://www.linear.com/tapeandreel/.
Some packages are available in 500 unit reels through
designated sales channels with #TRMPBF suffix.
ELECTRICAL CHARACTERISTICS
PARAMETER
Minimum Input Voltage
V
IN
Quiescent Current
V
EN/UV
= 0V
CONDITIONS
The
l
denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at T
A
= 25°C.
MIN
l
l
PG
FB
TYP
2.6
0.75
0.75
1.7
1.7
0.3
17
200
MAX
3.0
3
10
4
10
0.5
50
350
0.816
0.822
0.03
UNITS
V
µA
µA
µA
µA
mA
µA
µA
V
V
%/V
8641f
V
EN/UV
= 2V, Not Switching, V
SYNC
= 0V
l
V
EN/UV
= 2V, Not Switching, V
SYNC
= 2V
V
IN
Current in Regulation
Feedback Reference Voltage
Feedback Voltage Line Regulation
V
OUT
= 0.8V, V
IN
= 6V, Output Load = 100µA
V
OUT
= 0.8V, V
IN
= 6V, Output Load = 1mA
V
IN
= 6V, I
LOAD
= 0.5A
V
IN
= 6V, I
LOAD
= 0.5A
V
IN
= 4.0V to 42V, I
LOAD
= 0.5A
l
l
l
l
0.804
0.79
0.81
0.81
0.004
2
For more information
www.linear.com/LT8641
LT8641
ELECTRICAL CHARACTERISTICS
PARAMETER
Feedback Pin Input Current
BIAS Pin Current Consumption
Minimum On-Time
Minimum Off-Time
Oscillator Frequency
R
T
= 221k, I
LOAD
= 1A
R
T
= 60.4k, I
LOAD
= 1A
R
T
= 18.2k, I
LOAD
= 1A
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
V
FB
= 1V
V
BIAS
= 3.3V, I
LOAD
= 1A, 2MHz
I
LOAD
= 1.5A, SYNC = 0V
I
LOAD
= 1.5A, SYNC = 2V
l
l
MIN
–20
TYP
9
35
35
80
MAX
20
50
50
110
240
735
2.15
9.9
7.25
15
1.07
20
UNITS
nA
mA
ns
ns
ns
kHz
kHz
MHz
mΩ
A
mΩ
A
µA
V
mV
nA
%
%
%
nA
Ω
V
V
V
%
kHz
180
665
1.85
6.2
4.8
–15
210
700
2.00
105
8.2
55
5.8
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
PG Hysteresis
PG Leakage
PG Pull-Down Resistance
SYNC/MODE Threshold
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
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
l
l
l
l
0.95
–20
5
–5.25
–40
7.5
–8
0.4
10.25
–10.75
40
750
0.7
2.3
0.9
1.2
2.6
22
2.5
2000
1.4
2.9
Spread Spectrum Modulation
Frequency Range
TR/SS Source Current
TR/SS Pull-Down Resistance
Spread Spectrum Modulation Frequency V
SYNC
= 3.3V
l
1.2
1.9
220
2.6
µA
Ω
Fault Condition, TR/SS = 0.1V
Note 1:
Stresses beyond those listed under Absolute Maximum Ratings
may cause permanent damage to the device. Exposure to any Absolute
Maximum Rating condition for extended periods may affect device
reliability and lifetime.
Note 2:
The LT8641E is guaranteed to meet performance specifications
from 0°C to 125°C junction temperature. Specifications over the –40°C
to 125°C operating junction temperature range are assured by design,
characterization, and correlation with statistical process controls. The
LT8641I is guaranteed over the full –40°C to 125°C operating junction
temperature range.
The 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:
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.
For more information
www.linear.com/LT8641
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8641f
LT8641
TYPICAL PERFORMANCE CHARACTERISTICS
12V
IN
to 5V
OUT
Efficiency
vs Frequency
100
95
90
EFFICIENCY (%)
85
80
75
70
65
60
0.5
1
POWER LOSS
EFFICIENCY
2.6
2.3
1.9
POWER LOSS (W)
1.6
1.3
1.0
L = WE–LHMI7050 0.7
1MHz, L = 3.3 H
2MHz, L = 2.2 H 0.3
3MHz, L = 1.5 H
0
1.5
2
2.5
3
3.5
LOAD CURRENT (A)
8641 G01
12V
IN
to 3.3V
OUT
Efficiency
vs Frequency
100
95
90
EFFICIENCY (%)
85
80
75
70
65
60
0.5
1
POWER LOSS
EFFICIENCY
2.4
2.1
1.8
POWER LOSS (W)
1.5
1.2
0.9
L = WE–LHMI7050
0.6
1MHz, L = 2.2µH
2MHz, L = 1.5µH 0.3
3MHz, L = 1µH
0
1.5
2
2.5
3
3.5
LOAD CURRENT (A)
8641 G02
100
95
90
EFFICIENCY (%)
85
80
Efficiency at 5V
OUT
EFFICIENCY
2.5
2.3
2.0
1.8
1.5
1.3
1.0
POWER LOSS (W)
f
SW
= 1MHz
75 L = WE–LHMI7050, 4.7µH
70
65
60
55
50
0
0.8
V
IN
= 12V
V
IN
= 24V 0.5
V
IN
= 36V 0.3
V
IN
= 48V
POWER LOSS
0
0.5
1
1.5
2
2.5
3
3.5
LOAD CURRENT (A)
8641 G03
100
95
90
EFFICIENCY (%)
85
80
75
70
65
60
55
50
Efficiency at 3.3V
OUT
EFFICIENCY
2.5
f
SW
= 1MHz
L = WE–LHMI7050, 2.2µH 2.3
2.0
1.8
1.5
1.3
1.0
V
IN
= 12V
0.5
V
IN
= 24V
V
IN
= 36V 0.3
V
IN
= 48V
0
3
3.5
8641 G04
100
90
80
EFFICIENCY (%)
70
60
50
40
30
Efficiency at 5V
OUT
100
90
80
EFFICIENCY (%)
70
60
50
40
30
Efficiency at 3.3V
OUT
POWER LOSS (W)
0.8
POWER LOSS
0
0.5
1
1.5
2
2.5
LOAD CURRENT (A)
20
0.01
f
SW
= 1MHz
L = WE–LHMI7050, 4.7µH
0.1
1
10
100
LOAD CURRENT (mA)
V
IN
= 12V
V
IN
= 24V
V
IN
= 36V
V
IN
= 48V
1000
8641 G05
20
0.01
f
SW
= 1MHz
L = WE–LHMI7050, 4.7µH
0.1
1
10
100
LOAD CURRENT (mA)
V
IN
= 12V
V = 24V
V
IN
= 36V
V
IN
= 48V
1000
8641 G06
Efficiency vs Frequency
96
94
92
EFFICIENCY (%)
90
88
86
84
82
80
0.5
V
OUT
= 3.3V
I
LOAD
= 1.5A
L = WE–LHMI7050, 4.7µH
1.0
1.5
2.0
2.5
SWITCHING FREQUENCY (MHz)
3.0
8641 G07
Burst Mode Operation Efficiency
vs Inductor Value
100
819
V
IN
= 12V
REFERENCE VOLTAGE (mV)
817
815
813
811
809
807
805
803
95
90
EFFICIENCY (%)
85
80
75
70
65
1
2
3
V
OUT
= 5V
I
LOAD
= 10mA
L = WE–LHMI7050
4
5
6
7
8
INDUCTOR VALUE (µH)
9
10
Reference Voltage
Reference Voltage
V
IN
= 12V
V
IN
= 24V
V
IN
= 24V
801
–50 –25
0
8641 G08
25 50 75 100 125 150
TEMPERATURE (°C)
8641 G09
4
8641f
For more information
www.linear.com/LT8641
LT8641
TYPICAL PERFORMANCE CHARACTERISTICS
EN Pin Thresholds
EN Pin Thresholds
1.03
1.02
CHANGE IN V
OUT
(%)
1.01
EN THRESHOLD (V)
1.00
0.99
0.98
0.97
0.96
0.95
–50 –25
0
25 50 75 100 125 150
TEMPERATURE (°C)
8641 G10
Load Regulation
0.15
0.10
0.05
0
–0.05
–0.10
–0.15
V
OUT
= 5V
V
IN
= 12V
CHANGE IN V
OUT
(%)
0.15
0.10
0.05
0
–0.05
–0.10
–0.15
Line Regulation
V
OUT
= 5V
I
LOAD
= 1A
EN RISING
EN FALLING
0
0.5
1
1.5
2
2.5
LOAD CURRENT (A)
3
3.5
8641 G11
5
15
25
35
45
INPUT VOLTAGE (V)
55
65
8641 G12
No-Load Supply Current
4.5
4.0
INPUT CURRENT (µA)
3.5
3.0
2.5
2.0
1.5
1.0
0
10
20
30
40
50
INPUT VOLTAGE (V)
60
8641 G13
Top FET Current Limit vs Duty Cycle
8.5
8.0
CURRENT LIMIT (A)
9
7.5
7.0
6.5
6.0
5.5
0.1
CURRENT LIMIT (A)
10
Top FET Current Limit
Top FET Current Limit
V
OUT
= 3.3V
L = 4.7µH
IN-REGULATION
8
5% DC
7
0.3
0.5
DUTY CYCLE
0.7
0.9
8641 G14
6
–50 –25
0
25 50 75 100 125 150
TEMPERATURE (°C)
8641 G15
Switch Drop
250
SWITCH CURRENT = 1A
200
SWITCH DROP (mV)
SWITCH DROP (mV)
500
450
400
350
300
250
200
150
100
50
0
–50 –25
0
25 50 75 100 125 150
TEMPERATURE (°C)
8641 G16
Switch Drop
43
40
MINIMUM ON-TIME (ns)
37
34
31
28
Minimum On-Time
V
SYNC
= 0
V
SYNC
= FLOAT
150
TOP SWITCH
100
TOP SWITCH
50
BOTTOM SWITCH
BOTTOM SWITCH
0
0
0.5
1
1.5
2
2.5
SWITCH CURRENT (A)
3
3.5
8641 G17
25
–50
I
LOAD
= 2A
–25
0
25
50
75
TEMPERATURE (°C)
100
125
8641 G18
For more information
www.linear.com/LT8641
5
8641f
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