LT3697
USB 5V, 2.5A Output,
35V Input Buck with
Cable Drop Compensation
Description
Features
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Accurate 5V Output
Programmable Cable Drop Compensation
Programmable Output Current Limit
Adjustable Output from 5.0V to 6.1V
Dual Input Feedback Permits Regulation on Output
of USB Switch
Active Load Reduces Output Overshoot
FLT Flag Indicates Overcurrent on the USB Output
1.5ms FLT Flag Delay Filters Hot Plug Events
USB Output Current Monitor
Wide Input Rage: Operation from 5V to 35V
Withstands Input Transient to 60V
2.5A Maximum Output Current
Survives Output Short to GND and Car Battery
Adjustable Switching Frequency: 300kHz to 2.2MHz
Synchronizable from 300kHz to 2.2MHz
Small, Thermally Enhanced 16-Lead MSOP Package
Applications
Automotive USB
Industrial USB
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The LT®3697 is a 35V, 2.5A step-down switching regulator
designed to power 5V USB applications. A precise output
voltage and programmable cable drop compensation
maintain accurate 5V regulation at the USB socket connected
to the end of a long cable. The accurate, programmable
current limit can eliminate the need for a USB power switch
and improve system reliability. The provided 180mA active
load reduces output overshoot during load transients. Dual
feedback allows regulation on the output of a USB switch
and limits cable drop compensation to a maximum of 6.1V
output, protecting USB devices during fault conditions. A
separate 5V output can be taken from the SYS terminal
to power auxiliary circuitry such as a USB hub controller.
The LT3697 also provides a load current monitor output
and an overcurrent fault indicator.
The LT3697 operates from 300kHz to 2.2MHz and withstands input voltage transients up to 60V. The device's
output survives shorts to ground and to the battery. A current
mode topology is used for fast transient response and good
loop stability. Shutdown reduces input supply current to
less than 1µA. The LT3697 is available in a 16-lead MSOP
package with an exposed pad for low thermal resistance.
L, LT, LTC, LTM, Burst Mode, Linear Technology and the Linear logo are registered trademarks
of Linear Technology Corporation. All other trademarks are the property of their respective
owners.
Typical Application
5V Step-Down Converter with Cable Drop Compensation and Output Current Limit
EN
VIN
BST
SW
255k
7.5k
330pF
1nF
10µH
GND
47µF
0.13Ω
10nF
16.5k
+
10k
VLOAD
5V, 2.5A
–
100µF
VOUT
5.50
+
SYS
ISP
ISN
USB5V
RCBL
FLT
RT
VC
RLIM
SYNC
0.022Ω
6.0
5.25
VLOAD
5.0
4.0
3.0
5.00
ILOAD
25mA/µs
4.75
CURRENT (A)
LT3697
1µF
VOLTAGE (V)
10µF
5.75
4 METERS AWG 20
TWISTED PAIR CABLE
VOUT
LOAD
VIN 6V TO 35V
TRANSIENT TO 60V
Transient Response
2.0
0.13Ω
3697 TA01
1.0
4.50
250µs/DIV
3697 TA01a
3697f
For more information www.linear.com/LT3697
1
LT3697
Absolute Maximum Ratings
Pin Configuration
(Note 1)
VIN, EN Voltage (Note 2) ...........................................60V
BST Voltage...............................................................55V
BST Above SW Voltage..............................................25V
SYNC Voltage...............................................................6V
RT, VC, RLIM, RCBL Voltage........................................3V
FLT, ISN, ISP, USB5V, SYS Voltage.............................30V
Operating Junction Temperature Range
LT3697E............................................. –40°C to 125°C
LT3697I.............................................. –40°C to 125°C
Storage Temperature Range................... –65°C to 150°C
Lead Temperature (Soldering, 10sec).................... 300°C
TOP VIEW
VIN
VIN
EN
FLT
SYNC
RT
VC
RLIM
1
2
3
4
5
6
7
8
17
GND
16
15
14
13
12
11
10
9
SW
SW
BST
SYS
USB5V
ISP
ISN
RCBL
MSE PACKAGE
16-LEAD PLASTIC MSOP
θJA = 40°C/W
EXPOSED PAD (PIN 17) IS GND, MUST BE SOLDERED TO PCB
Order Information
LEAD FREE FINISH
TAPE AND REEL
PART MARKING*
PACKAGE DESCRIPTION
TEMPERATURE RANGE
LT3697EMSE#PBF
LT3697EMSE#TRPBF
3697
16-Lead Plastic MSOP
–40°C to 125°C
LT3697IMSE#PBF
LT3697IMSE#TRPBF
3697
16-Lead Plastic MSOP
–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/
2
3697f
For more information www.linear.com/LT3697
LT3697
Electrical Characteristics
The l denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at TA = 25°C. VIN = 12V, VEN = 12V unless otherwise noted. (Notes 3, 6)
PARAMETER
CONDITIONS
MIN
VIN Undervoltage Lockout
l
VIN Overvoltage Lockout
l
Dropout Comparator Threshold
(VIN – VSYS) Falling, VIN = 5V
TYP
MAX
4.2
4.8
V
35.5
37
39.5
V
550
650
750
mV
Dropout Comparator Hysteresis
200
VIN Current
VSYS = 5V, VEN = 0.3V
VSYS = 5V, Not Switching
VSYS = 0V, Not Switching
0.5
0.8
SYS Current
VEN = 0.3V
VSYS = 5V, Not Switching
VSYS = 0V, Not Switching
USB5V Voltage
l
USB5V Line Regulation
6V < VIN < 35V
l
USB5V Current
VSENSE = 50mV, RCBL = 16.5k
VSENSE = 10mV, RCBL = 16.5k
VSENSE = 0V, RCBL = 16.5k
l
l
l
RCBL Voltage
VSENSE = 50mV, RCBL = 16.5k
VSENSE = 10mV, RCBL = 16.5k
VSENSE = 0V, RCBL = 16.5k
RCBL Current Limit
VRCBL = 0V, VSENSE = 50mV
SYS Voltage
VUSB5V = 0V
SYS Voltage to Disable Switching
VUSB5V = 0V
SENSE Voltage (Note 7)
VISP = 5V, RLIM = Open
VISP = 0V, RLIM = Open
VISP = 5V, RLIM = 56.2k
VISP = 5V, RLIM = 29.4k
l
l
l
l
l
UNITS
mV
0.01
0.75
1.1
2
1.0
1.4
μA
mA
mA
200
9
300
–75
13
500
–120
μA
μA
μA
4.95
4.99
5.03
V
1
5
mV
58
11
60
13
0
62
15
3
μA
μA
μA
960
180
1000
210
0
1030
240
50
mV
mV
mV
200
300
400
μA
6.0
6.1
6.2
V
6.5
6.8
7.1
V
56.5
20
33
18
60.5
35.2
20.5
64.5
105
37.5
23
mV
mV
mV
mV
20
–1.1
30
–1.6
μA
mA
ISP and ISN Bias Current
VISP, VISN = 5V
VISP, VISN = 0V
RLIM Current
VRLIM = 1.2V
–9
–11
–13
μA
Active Load Current from SYS
VSYS = 5V
120
180
240
mA
USB5V Voltage Offset to Enable Active Load
VUSB5V Rising
0.5
1.5
3
%
SYS Voltage Threshold to Disable Active Load
VSYS Rising
6.6
7.2
7.8
Error Amp gm
Error Amp Gain
V
400
mS
500
V/V
VC Source Current
VVC = 1.3V
–80
μA
VC Sink Current
VVC = 1.3V
80
μA
VC to Switch gm
5
A/V
VC Clamp Voltage
1.8
V
3697f
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3
LT3697
Electrical Characteristics
The l denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at TA = 25°C. VIN = 12V, VEN = 12V unless otherwise noted. (Notes 3, 6)
PARAMETER
CONDITIONS
Switching Frequency
RT = 22.1k
RT = 63.4k
RT = 453k
Foldback Frequency
RT = 63.4k, VSYS = 0V
240
Minimum Switch On-Time
ISW = 0.9A
100
160
ns
Minimum Switch Off-Time
ISW = 0.9A
140
210
ns
5.3
6.7
l
l
l
Switch Current Limit (Note 8)
l
MIN
TYP
MAX
UNITS
1.8
0.9
250
2
1
300
2.25
1.12
350
MHz
MHz
KHZ
4.3
kHz
A
Switch VCESAT
ISW = 2A
220
SW Leakage Current
VEN = 0.3V, VBST = 5V, VSW = 0V
0.1
1
Minimum BST Voltage (Note 9)
ISW = 2A
1.6
2.2
V
BST Current
ISW = 2A
35
65
mA
2.5
V
EN Input Voltage High
l
EN Input Voltage Low
l
EN Current
VEN = 2.5V
SENSE Voltage to Trigger FLT
Percentage of Nominal Sense Voltage
FLT Blanking
FLT Leakage
VFLT = 5V
FLT Sink Current
VFLT = 0.3V
SYNC Threshold
SYNC Current
VSYNC = 5V
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: Absolute maximum voltage at VIN and EN is 60V for nonrepetitive
1 second transients, and 35V for continuous operation.
Note 3: The LT3697E 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
LT3697I is guaranteed over the full –40°C to 125°C operating junction
temperature range. High junction temperatures degrade operating
lifetimes. Operating lifetime is derated at junction temperatures greater
than 125°C.
Note 4: Note that the maximum ambient temperature consistent with
these specifications is determined by specific operating conditions in
conjunction with board layout, the rated package thermal impedance and
other environmental factors.
Note 5: This IC includes overtemperature protection that is intended
4
mV
0.3
µA
V
1
2
97
99.5
100
%
0.5
1.5
4
ms
0.1
1
μA
l
100
180
l
0.4
0.7
0.1
μA
μA
1
V
μA
to protect the device during momentary overload conditions. Junction
temperature will exceed 150°C when overtemperature protection is active.
Continuous operation above the specified maximum operating junction
temperature may impair device reliability.
Note 6: Polarity specification for current into a pin is positive and out of
a pin is negative. All voltages are referenced to GND unless otherwise
specified. MAX and MIN refer to absolute values.
Note 7: SENSE Voltage is defined as the differential voltage applied across
the sense amplifier inputs, or VISP – VISN. SENSE voltage and VSENSE are
synonymous.
Note 8: Switch current limit is guaranteed by design and/or correlation to
static test. Slope compensation reduces switch current limit at higher duty
cycles.
Note 9: Boost voltage is the minimum voltage across the boost capacitor
needed to guarantee full saturation of the switch.
3697f
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LT3697
Typical Performance Characteristics
USB5V Voltage
USB5V Voltage
5.04
VIN = 12V
ILOAD = 1A
5.01
ILOAD = 1A
5.03
USB5V VOLTAGE (V)
5.00
VUSB5V (V)
USB5V Current
60
4.99
4.98
4.97
50
5.00
4.99
4.96
4.97
4.95
–50 –25
4.96
25 50 75 100 125 150
TEMPERATURE (°C)
10
VSENSE = 0mV
6
10
14
18 22 26 30 34
INPUT VOLTAGE (V)
38
300
6
RCBL = 2k
100
RCBL = 16.5k
70
20
60
3
2
Sense Voltage (VISP - VISN)
at 0.5V Output
VISP - VISN (mV)
60
RLIM = 75k
50
40 VIN = 12V
VOUT = 0.5V
30
RLIM = 29.4k
20
0.5
2.5
3.0
25 50 75 100 125 150
TEMPERATURE (°C)
3697 G07
RLIM = 29.4k
50
0.0
fSW = 2MHz
RSENSE = 22mΩ
L = 1.8µH
DCRL = 10mΩ
RCBL = ∞
VIN = 8V
VIN = 12V
VIN = 16V
0.5
25 50 75 100 125 150
TEMPERATURE (°C)
Efficiency at 400kHz
90
70
0
3697 G06
90
80
RLIM = 0Ω
3697 G05
100
RLIM = 0Ω
0
1.0
1.5
2.0
OUTPUT CURRENT (A)
100
60
10
0
–50 –25
EFFICIENCY (%)
RLIM = ∞
VIN = 12V
VOUT = 4.5V
0
–50 –25
Efficiency at 2MHz
80
70
30
10
VIN = 12V
RSENSE = 25mΩ
RCBL = ∞
3697 G04
40
20
0
0.0
70
RLIM = 75k
50
4
1
30
40
50
VSENSE (mV)
RLIM = ∞
60
1.0
1.5
2.0
LOAD CURRENT (A)
2.5
3697 G08
EFFICIENCY (%)
10
25 50 75 100 125 150
TEMPERATURE (°C)
Sense Voltage (VISP - VISN)
at 4.5V Output
RCBL = 130k
0
0
3697 G03
VISP - VISN (mV)
OUTPUT VOLTAGE (V)
IUSB5V (µA)
42
RLIM = 29.4k
RLIM = 75k
RLIM = 49.9k
RLIM = ∞
5
150
50
0
–50 –25
Output Current Limit Load Line
200
VSENSE = 10mV
3697 G02
USB5V Current
0
30
20
3697 G01
250
RCBL = 16.5k
40
5.01
4.98
0
VSENSE = 40mV
5.02
IUSB5V (µA)
5.02
TA = 25°C, unless otherwise noted.
80
fSW = 400kHz
RSENSE = 22mΩ
L = 8.2µH
DCRL = 24mΩ
RCBL = ∞
70
60
50
0.0
VIN = 8V
VIN = 12V
VIN = 24V
0.5
1.0
1.5
2.0
LOAD CURRENT (A)
2.5
3697 G09
3697f
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5
LT3697
Typical Performance Characteristics
LT3697 Temperature Rise
200
10
1
1.5
2
OUTPUT CURRENT (A)
120
80
40
VIN = 8V
VIN = 12V
VIN = 16V
0
2.5
1
0
2
3
4
5
6
7
0.0
8
5
10
VBST - VSW = 4.5V
VIN = 12V
5 CATCH DIODE:
CENTRAL SEMI
CMSH3-40MA
200
150
100
VBST - VSW = 4.5V
30
20
10
50
0
0.0
25 50 75 100 125 150
TEMPERATURE (°C)
0.5
1.5
1.0
2.0
SWITCH CURRENT (A)
0
0.0
2.5
0.5
1.0
2.0
1.5
SWITCH CURRENT (A)
3697 G14
3697 G13
Switch Current Limit
vs Temperature
2.5
3697 G15
Switch Current Limit
vs Duty Cycle
7.0
40
40
BST CURRENT (mA)
10
35
BST Pin Current
50
250
20
INCREASED SUPPLY CURRENT
DUE TO CATCH DIODE
LEAKAGE AT HIGH
TEMPERATURE
20
15
25
30
INPUT VOLTAGE (V)
3697 G12
Switch VCESAT
300
SWITCH VCESAT (mV)
NO LOAD INPUT CURRENT (mA)
1.0
3697 G11
25
0
1.5
VSYS (V)
No Load Input Current
vs Temperature
0
–50 –25
2.0
0.5
3697 G10
15
IN REGULATION
2.5
INPUT CURRENT (mA)
20
0.5
3.0
VUSB5V = 5.5V
160
ISYS (mA)
IC TEMPERATURE RISE (°C)
DC1893A DEMO BOARD
24°C AMBIENT TEMPERATURE
NO AIR FLOW
30 fSW = 500kHz
0
No Load Input Current
vs Input Voltage
SYS Pin Current
40
0
TA = 25°C, unless otherwise noted.
Switching Frequency
7
500
RT = 255k
6
30% DC
5.0
70% DC
4.0
3.0
SWITCHING FREQUENCY (kHz)
6.0
CURRENT LIMIT (A)
SWITCH CURRENT LIMIT (A)
475
5
4
3
450
425
400
375
350
325
2.0
–50 –25
0
25 50 75 100 125 150
TEMPERATURE (°C)
3697 G16
6
2
0
20
40
60
DUTY CYCLE (%)
80
100
3697 G17
300
–50 –25
0
25 50 75 100 125 150
TEMPERATURE (°C)
3697 G18
3697f
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LT3697
Typical Performance Characteristics
Frequency Foldback
Burst Frequency
Error Amp Output Current
500
100
RT = 255k
450 L = 8.2µH
400
350
300
250
200
75
400
VC PIN CURRENT (µA)
RT = 255k
SWITCHING FREQUENCY (kHz)
SWITCHING FREQUENCY (kHz)
450
350
300
250
200
150
100
VIN = 12V
VIN = 24V
VIN = 36V
50
150
0
TA = 25°C, unless otherwise noted.
1
2
3
4
5
0
6
0
20
VSYS (V)
25
0
–25
–50
–75
–100
–100
0
200
–200
100
OFFSET FROM NOMINAL USB5V VOLTAGE (mV)
40 60 80 100 120 140 160
LOAD CURRENT (mA)
3697 G19
50
3697 G20
Minimum On-Time
3697 G21
Minimum Off-Time
180
Minimum Off-Time in SYNC
220
170
210
0.9A LOAD
140
120
2.1A LOAD
100
160
MINIMUM OFF-TIME (ns)
MINIMUM OFF-TIME (ns)
MINIMUM ON-TIME (ns)
160
2.1A LOAD
150
140
0.9A LOAD
80
–50 –25
0
25 50 75 100 125 150
TEMPERATURE (°C)
130
–50 –25
0
25 50 75 100 125 150
TEMPERATURE (°C)
3697 G22
EN PIN VOLTAGE (V)
190
180
170
0.9A LOAD
160
150
FSYNC = 2.2MHz
140
25 50 75 100 125 150
–50 –25 0
TEMPERATURE (°C)
3697 G24
Start-Up/Dropout Performance
RLOAD = 2Ω
(2.5A IN REGULATION)
RCBL = ∞
EN RISING
EN FALLING
1.80
2.1A LOAD
3697 G23
Enable Threshold
2.00
200
Start-Up/Dropout Performance
RLOAD = 100Ω
(50mA IN REGULATION)
RCBL = ∞
VIN
VIN
1.60
1.40
2V/DIV
1.20
VOUT
2V/DIV
VOUT
1.00
0.80
–50 –25
0
25 50 75 100 125 150
TEMPERATURE (°C)
100ms/DIV
100ms/DIV
3697 G26
3697 G27
3697 G25
3697f
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7
LT3697
Typical Performance Characteristics
TA = 25°C, unless otherwise noted.
Load Transient Response
Through Cable
Minimum Input Voltage
5.75
VOLTAGE TO START
VOLTAGE TO RUN
6.5
5.50
5.25
VOLTAGE (V)
6.0
5.5
5.0
RSENSE = 22mΩ
4.5 RCBL = ∞
RESISTOR LOAD
DCRL = 10mΩ
4.0
1000
10
100
1
LOAD CURRENT (mA)
7.0
5.00
6.0
VOUT
5.0
VLOAD
4.0
FRONT PAGE CIRCUIT
VIN = 12V
CABLE = 4m AWG20 3.0
CLOAD = 10µF
2.0
4.75
4.50
ILOAD
4.25
1.0
25mA/µs
4.00
10000
0.0
200µs/DIV
3697 G29
3697 G28
Load Transient Response
Through Cable
Output Current Limit Transient
Response
5.75
4.25
4.0
VLOAD
FRONT PAGE CIRCUIT 3.0
VIN = 12V
CABLE = 4m AWG20 2.0
CLOAD = 10µF
ILOAD
25mA/µs
4.00
VOLTAGE (V)
5.0
VOUT
12
FRONT PAGE CIRCUIT
VIN = 12V
VLOAD
4
VFLT
6
2
4
1.0
1
0.0
0
ILOAD
1ms/DIV
3697 G31
Feedback Shorted to Ground
FLT Deglitching
8
VOUT
7
6.0
FLT VOLTAGE (V)
VOLTAGE (V)
3.5
ILOAD
VUSB5V
SHORTED
TO GROUND
5
2.5
4
2.0
3
1.5
2
1.0
1
0
0.0
3.0
LOAD CURRENT (A)
VUSB5V
3.0
1.0
0.5
VFLT
0.0
2ms/DIV
200µs/DIV
3697 G32
8
4.0
RSENSE = 22mΩ
RLIM = OPEN
6
5.0
2.0
2
0
3697 G30
4.0
8
3
200µs/DIV
7.0
10
CURRENT (A)
4.50
5
CURRENT (A)
VOLTAGE (V)
6.0
5.00
4.75
6
7.0
5.50
5.25
CURRENT (A)
MINIMUM INPUT VOLTAGE (V)
7.0
3697 G33
3697f
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LT3697
Pin Functions
VIN (Pins 1, 2): The VIN pins supply current to the LT3697’s
internal regulator and to the power switch. These pins
must be locally bypassed.
EN (Pin 3): The EN pin is used to put the LT3697 into
shutdown mode. Tie to ground to shut down the LT3697.
Tie to 2.5V or higher for normal operation. If the shutdown
feature is not used, tie this pin to the VIN pin.
FLT (Pin 4): The FLT pin is the open drain output of the
LT3697 fault comparator and timer. In normal operation
the FLT pin is high impedance. An overcurrent fault that
is sustained for at least 1.5ms causes the LT3697 to pull
the FLT pin low. The FLT pin then remains low until the
USB output current stays below the overcurrent threshold
for at least 1.5ms. The overcurrent fault threshold is 0.5%
below the current limit. The FLT output is valid when VIN
is above 4V and EN is high.
SYNC (Pin 5): The SYNC pin is the external clock synchronization input. Tie to a clock source with on and off times
greater than 50ns for synchronization. Tie pin to ground if
not used. See the Synchronization section in Applications
Information for more details.
RT (Pin 6): The RT pin is the oscillator resistor input.
Connect a resistor from this pin to ground to set the
switching frequency.
VC (Pin 7): The VC pin is the output of the internal error
amplifier. The voltage on this pin controls the peak switch
current. Tie an R-C network from this pin to ground to
compensate the control loop.
RLIM (Pin 8): The RLIM pin provides an additional reference
to the third feedback amplifier of the LT3697 to allow the
output current limit to be programmed easily. The RLIM
pin has an accurate 11μA pull-up current. When the voltage
of the output current sense amplifier exceeds the lower
of the RLIM voltage or 1.22V, the LT3697 error amplifier
will switch to current limit mode and will regulate the USB
output current. In current limit mode, the output voltage
drops. Tie a resistor from RLIM to ground to program the
LT3697 current limit. If the USB output current exceeds
99.5% of the current limit for at least 1.5ms, the LT3697
will pull down on the FLT pin. Float the RLIM pin if not used.
RCBL (Pin 9): The RCBL pin is used to program the USB5V
current as a function of sense voltage (VISP – VISN) for
cable drop compensation. Tie a resistor from RCBL to
ground to set the USB5V input current. Float the RCBL
pin if cable drop compensation is not desired. The RCBL
pin may also be used as an USB output current monitor.
Excessive capacitive loading on the RCBL pin can cause
USB output voltage overshoot during load steps when
cable drop compensation is used. Keep the capacitive
loading on the RCBL pin below 100pF.
ISN (PIN 10): The ISN pin is the inverting input of the
LT3697’s onboard USB output current sense amplifier.
Tie a resistor RSENSE from ISP to ISN to sense the USB
output current. Connect ISN to ISP if the current monitor,
USB output current limit, and cable drop compensation
functions are not desired.
ISP (Pin 11): The ISP pin is the noninverting input of the
LT3697’s onboard USB output current sense amplifier. Tie
a resistor RSENSE from ISP to ISN to sense the USB output
current. When a USB switch is used in series between
the LT3697 and the 5V USB output, tie the ISP pin to the
USB switch output.
USB5V (Pin 12): The USB5V pin is the primary feedback
input of the internal error amplifier. In normal operation,
the LT3697 regulates the voltage on this pin to 5V. The
USB5V pin also allows the output voltage to increase as a
function of output current to compensate for voltage drop
at the point of load due to cable impedance. The USB5V
pin input current is proportional to USB output current
and is programmed by the RCBL resistor. Tie a resistor
from USB5V to the 5V USB output to set this cable drop
compensation.
Tie USB5V directly to the USB output if no cable drop
compensation is desired. If a USB switch is used in series between the LT3697 and the 5V USB output, tie the
USB5V pin through the compensation resistor to the USB
switch output.
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9
LT3697
Pin Functions
SYS (Pin 13): The SYS pin is the second feedback input
of the internal error amplifier. The SYS pin allows the
LT3697 to regulate the output voltage at the output of a
USB switch. If the USB switch goes open and the USB5V pin
is no longer part of the control loop, the LT3697 regulates
the SYS pin to 6.1V to protect the input of a USB switch
from an overvoltage condition. The SYS pin also supplies
current to the internal regulator of the LT3697 and may
be used to supply power to auxiliary circuitry. The active
load also draws current from this pin to reduce output
overshoot. This pin must be locally bypassed and must
be tied to the switching regulator output.
10
BST (Pin 14): The BST pin is used to provide a drive
voltage, higher than the input voltage, to the internal NPN
power switch.
SW (Pin 15, 16): The SW pins are the output of the internal
power switch. Connect these pins to the inductor, catch
diode, and boost capacitor.
GND (Pin 17): Ground. The exposed pad must be soldered
to the PCB.
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SYNC
EN
RT
RLIM
OSC
11µA
INTERNAL
REGULATOR
AND
REFERENCE
OSCILLATOR
SYNC
RT
RLIM
EN
R5
CFF2
GND
R3
R4
REF
R1
REF
REF
R2
CFF1
OSC
REF
REF
REF
ACTIVE
LOAD
ENABLE
+
–
CIN
VC
+
–
ACTIVE
LOAD
CURRENT
LIMIT
SWITCH
CONTROL
+
–
+
–
+
+
–
For more information www.linear.com/LT3697
+
+
–
+
–
VIN
ACTIVE
LOAD
ENABLE
CURRENT
LIMIT
DELAY
+
–
VIN
+
–
3697 BD
VC
FLT
RCBL
USB5V
ISN
ISP
SYS
SW
BST
CC
RC
FLT
RCBL
DCATCH
L
CBST
+
RSENSE
CF
ILOAD MONITOR
COUT
DBST
CBUS
CCDC
RCDC
OUT
RCABLE
–
+
VLOAD
LT3697
Block Diagram
3697f
11
LOAD
LT3697
Operation
The LT3697 is a constant frequency, current mode stepdown regulator. The oscillator sets an RS flip-flop, turning
on the internal power switch. The RT resistor sets the oscillator frequency. An amplifier and comparator monitor the
current flowing between the VIN and SW pins, turning the
switch off when this current reaches a level determined by
the voltage at VC. The error amplifier measures the output
voltage on the USB5V pin through an internal resistor divider and servos the VC node to regulate the USB5V pin to
5V. If the error amplifier’s output increases, more current
is delivered to the output; if it decreases, less current is
delivered. An active clamp on the VC pin provides switch
peak current limit. The LT3697 can provide up to 2.5A of
output current.
A second error amp input on the SYS pin allows a switch to
be placed in the output path before the USB5V connection.
SYS is regulated to 6.1V if this switch is open. A third error
amp input is connected to the ISP and ISN pins through
the internal current sense amplifier. The LT3697 regulates
VSENSE voltage (VISP – VISN) to the lower of V(RLIM)/19.8
or 1.22V/19.8 to provide accurate output current limit.
To implement cable drop compensation, the LT3697 drives
the RCBL pin to 19.8 (VISP – VISN). Current sourced from
the RCBL pin is derived from the USB5V pin, creating
an output offset above the 5V USB5V pin voltage that
is proportional to the load current and the RCDC/RCBL
resistor ratio.
The LT3697 includes a 180mA active load that sinks
current from the SYS pin to ground. The purpose of this
active load is to improve load step transient response and
to charge the boost cap during startup. If USB5V is 1.5%
above its nominal 5V output or if the boost drive voltage
(VBST – VSW) is insufficient to fully saturate the internal
NPN power switch, the active load is enabled.
An internal regulator provides power to the control circuitry.
The bias regulator normally draws power from the VIN
pin, but if the SYS pin is connected to an external voltage
higher than 4V, some bias power will be drawn from the
output voltage improving efficiency.
If the EN pin is low, the LT3697 is shut down and draws
1.5ms, the FLT pin
pulls low, indicating an overcurrent fault. Once the output
current drops below the current limit for >1.5ms, the fault
logic resets and the FLT pin becomes high impedance. FLT
is valid when VIN is above 4V and when EN if high. If VIN
is below 4V or if EN is low, the fault latch state is reset
and FLT becomes high impedance.
3697f
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LT3697
Applications Information
The LT3697 includes the necessary circuitry to implement
cable drop compensation. Cable drop compensation allows
the regulator to maintain 5V regulation on the USB VLOAD
despite high cable resistance. The LT3697 increases its
local output voltage (VOUT) above 5V as the load increases
to keep the VLOAD regulated to 5V. This compensation
does not require running an additional pair of Kelvin sense
wires from the regulator to the load, but does require the
system designer to know the cable resistance RCABLE as
the LT3697 does not sense this value.
Program the cable drop compensation using the following ratio:
RCBL = 19.8 •
RSENSE •RCDC
RCABLE
where RCDC is a resistor tied between the local regulator
output and the USB5V pin, RCBL is a resistor tied between
the RCBL pin and GND, RSENSE is the sense resistor tied
between the ISP and ISN pins in series between the regulator output and the load, and RCABLE is the cable resistance.
RSENSE is typically chosen based on the desired current
limit and is typically 25mΩ for 2.1A systems and 50mΩ
for 1A systems. See the Setting the Current Limit section
for more information.
The current flowing into the USB5V pin through RCDC is
identical to the current flowing through RCBL. While the ratio
of these two resistors should be chosen per the equation
above, choose the absolute values of these resistors to
keep this current through these resistors between 30µA
and 200µA at the full load current.
If IUSB5V is too low, capacitive loading on the USB5V and
RCBL pins will degrade the load step transient performance
of the regulator. If IUSB5V is too high, the RCBL pin will
go into current limit and the cable drop compensation
feature will not work.
Capacitance across the remote load to ground downstream
of RSENSE forms a zero in the LT3697’s feedback loop due
to cable drop compensation. CBUS and the input capacitance
of a portable device tied to the USB socket typically form
this zero. CCDC reduces the cable drop compensation gain
at high frequency. The 10nF CCDC capacitor tied across
the 10k RCDC is required for stability of the LT3697’s
output. If RCDC is changed, CCDC should also be changed
to maintain roughly the same 100μs RC time constant. If
the capacitance across the remote load is large compared
to the LT3697 output capacitors COUT and CBUS, a longer
RCDC • CCDC time constant may be necessary for stability
depending on the amount of cable drop compensation
used. Output stability should always be verified in the end
application circuit.
The LT3697 limits the maximum voltage of VOUT by
limiting the voltage on the SYS pin VSYS to 6.1V. If the
cable drop compensation is programmed to compensate
for more than 1V of cable drop at the maximum ILOAD,
this VSYS maximum will prevent VOUT from rising higher
and the voltage at the point of load will drop below 5V.
The following equation shows how to derive the LT3697
output voltage VOUT:
VOUT = 4.99V +
19.8 •ILOAD•RSENSE •RCDC
RCBL
As stated earlier, LT3697’s cable drop compensation feature
does not allow VOUT to exceed the VSYS regulation point
of 6.1V. If additional resistance is placed between the SYS
pin and the OUT node such as RSENSE or a USB Switch, the
voltage drop through these resistances at the maximum
ILOAD must also be factored in to this maximum allowable
VOUT value. Please refer to Figure 1 for load lines of VOUT
and VLOAD to see how cable drop compensation works.
6.5
RCABLE = 0.26Ω
RSENSE = 22mΩ
RCBL = 16.5k
6.0 RCDC = 10k
VOLTAGE (V)
Cable Drop Compensation
VOUT
5.5
5.0
4.5
0.0
VLOAD
0.5
1.0
1.5
2.0
LOAD CURRENT (A)
2.5
3697 F01
Figure 1. Cable Drop Compensation Load Line
3697f
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13
LT3697
Applications Information
Cable Drop Compensation Over a Wide Temperature
Range
Cable drop compensation with zero temperature variation
may be used in many applications. However, matching
the cable drop compensation temperature variation to the
cable resistance temperature variation may result in better overall output voltage accuracy over a wide operating
temperature range. For example, in an application with
0.2Ω of wire resistance and a maximum output current of
2.1A, cable drop compensation adds 0.42V at 25°C to the
output at max load for a fully compensated wire resistance.
If the wire in this example is copper, the copper resistance
temperature coefficient of about 4000ppm/°C results in
an output voltage error of –170mV at 125°C and 110mV
at –40°C. Figure 2a shows this behavior.
5.8
VOLTAGE (V)
ILOAD = 2.1A
RCABLE = 0.2Ω
= 25mΩ
R
5.6 RSENSE
CBL = 24.9k
RCDC = 10k
5.4
VOUT
5.2
VLOAD
5.0
4.8
–25
5
65
35
TEMPERATURE (°C)
95
125
3697 F02a
Figure 2a. Cable Drop Compensation Through 3m of AWG 20
Twisted-Pair Cable (260mΩ) without Temperature Compensation
RCBL
17.8k
1%
12.1k
1%
MURATA
NCP21XV103J03RA
10k THERMISTOR
3697 F02b
Figure 2b. RCBL Resistor Network for Matching
Copper Wire Temperature Coefficient
14
See Table 1 for a list of copper wire resistances vs gauge.
Table 1. Copper Wire Resistance vs Wire Gauge
AWG
RESISTANCE OF Cu WIRE AT 20°C (mΩ/m)
15
10.4
16
13.2
17
16.6
18
21.0
19
26.4
20
33.3
21
42.0
22
53.0
23
66.8
24
84.2
25
106
26
134
27
169
28
213
29
268
30
339
31
427
32
538
33
679
34
856
35
1080
36
1360
37
1720
38
2160
39
2730
40
3440
Cable drop compensation can be made to vary positively
versus temperature with the addition of a negative temperature coefficient (NTC) resistor as a part of the RCBL
resistance. This circuit idea assumes the NTC resistor is
at the same temperature as the cable. Figure 2b shows an
example resistor network for RCBL that matches copper
resistance variation over a wide –40°C to 125°C temperature range. Figure 2c shows the resultant cable drop
compensation output at several temperatures using RCBL
with negative temperature variation.
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LT3697
Applications Information
5.8
6.5
ILOAD = 2.1A
RCABLE = 0.2Ω
RSENSE = 15.7mΩ Cu
= 15k
R
6.0 CBL
RCDC = 10k
VOUT
VOLTAGE (V)
VOLTAGE (V)
ILOAD = 2.1A
RCABLE = 0.2Ω
RSENSE = 25mΩ
5.6 R
CBL = FIG 2b
RCDC = 10k
5.4
5.2
VLOAD
5
65
35
TEMPERATURE (°C)
95
VLOAD
5.0
5.0
4.8
–25
VOUT
5.5
4.5
–25
125
5
65
35
TEMPERATURE (°C)
3697 F02c
Figure 2c. Cable Drop Compensation Through 3m of AWG 20
Twisted-Pair Cable (200mΩ) with Temperature Compensation
Using NTC RCBL
The NTC resistor does not give a perfectly linear transfer
function versus temperature. Here, for typical component
values, the worse case error is VDROPOUT(MIN)
where VDROPOUT(MIN) is 600mV. The 600mV dropout voltage limits the duty cycle and forces the switch to turn off
regularly to charge the boost capacitor. Since sufficient
voltage across the boost capacitor is maintained, the switch
is allowed to fully saturate and the internal switch drop
stays low for good dropout performance. Figure 8 shows
the overall VIN to VOUT performances during start-up and
dropout conditions.
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21
LT3697
Applications Information
RLOAD = 100Ω
(50mA IN REGULATION)
RCBL = ∞
The inductor value must be sufficient to supply the desired
maximum output current (IOUT(MAX)), which is a function
of the switch current limit (ILIM) and the ripple current.
VIN
VOUT
100ms/DIV
3697 F08
Figure 8. VIN to VOUT Performance
Inductor Selection and Maximum Output Current
For a given input and output voltage, the inductor value
and switching frequency will determine the ripple current.
The ripple current increases with higher VIN or VOUT and
decreases with higher inductance and faster switching
frequency. A good first choice for the inductor value is:
L=
VSYS + VD
1.5 • fSW
where fSW is the switching frequency in MHz, VSYS is the
SYS pin voltage, VD is the catch diode drop (~0.5V) and
L is the inductor value is μH.
The inductor’s RMS current rating must be greater than the
maximum load current and its saturation current should be
about 30% higher. For robust operation in fault conditions
(start-up or short circuit) and high input voltage (>30V), the
saturation current should be above 7A. To keep the efficiency
high, the series resistance (DCR) should be less than 0.1Ω,
and the core material should be intended for high frequency
applications. Table 4 lists several inductor vendors.
Table 4. Inductor Vendors
VENDOR
URL
Coilcraft
www.coilcraft.com
Sumida
www.sumida.com
Toko
www.tokoam.com
Würth Electronik
www.we-online.com
Coiltronics
www.cooperet.com
Murata
www.murata.com
22
ΔIL
2
The LT3697 limits its peak switch current in order to protect
itself and the system from overload faults. The LT3697’s
switch current limit (ILIM) is 5.3A at low duty cycles and
decreases linearly to 4A at DC = 0.8.
2V/DIV
IOUT(MAX) =ILIM –
When the switch is off, the potential across the inductor
is the output voltage plus the catch diode drop. This gives
the peak-to-peak ripple current in the inductor:
ΔIL =
(1–DC)•(VSYS + VD )
L • fSW
where fSW is the switching frequency of the LT3697, DC is
the duty cycle and L is the value of the inductor. Therefore,
the maximum output current that the LT3697 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 current does
not allow sufficient maximum output current (IOUT(MAX))
given the switching frequency and maximum input voltage
used in the desired application.
The optimum inductor for a given application may differ
from the one indicated by this simple design guide. A larger
value inductor provides a higher maximum load current
and reduces the output voltage ripple. If your load is lower
than the maximum load current, than you can relax the
value of the inductor and operate with higher ripple current. This allows you to use a physically smaller inductor,
or one with a lower DCR resulting in higher efficiency. Be
aware that if the inductance differs from the simple rule
above, then the maximum load current will depend on
the input voltage. In addition, low inductance may result
in discontinuous mode operation, which further reduces
maximum load current. For discussion regarding maximum
output current and discontinuous operation, see Linear
Technology’s Application Note 44. Additionally, for duty
cycles greater than 50% (VOUT/VIN > 0.5), a minimum
inductance is required to avoid subharmonic oscillations,
see Application Note 19.
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LT3697
Applications Information
One approach to choosing the inductor is to start with the
simple rule given above, look at the available inductors,
and choose one to meet cost or space goals. Then use the
equations above to check that the LT3697 will be able to
deliver the required output current. Note again that these
equations assume that the inductor current is continuous.
Discontinuous operation occurs when IOUT is less than:
ΔIL
2
Input Capacitor
Bypass the input of the LT3697 circuit with a ceramic capacitor of X7R or X5R type. Y5V types have poor performance
over temperature and applied voltage, and should not be
used. A 4.7μF to 10μF ceramic capacitor is adequate to
bypass the LT3697 and will easily handle the ripple current. Note that larger input capacitance is required when
a lower switching frequency is used (due to longer on
times). 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.
Step-down regulators draw current from the input supply in pulses with very fast rise and fall times. The input
capacitor is required to reduce the resulting voltage ripple
at the LT3697 input and to force this very high frequency
switching current into a tight local loop, minimizing EMI.
A 4.7μF capacitor is capable of this task, but only if it is
placed close to the LT3697 (see the PCB Layout section).
A second precaution regarding the ceramic input capacitor
concerns the maximum input voltage rating of the LT3697.
A ceramic input capacitor combined with trace or cable
inductance forms a high quality (under damped) tank
circuit. If the LT3697 circuit is plugged into a live supply,
the input voltage can ring to twice its nominal value, possibly exceeding the LT3697’s voltage rating. If the input
supply is poorly controlled or the user will be plugging
the LT3697 into an energized supply, the input network
should be designed to prevent this overshoot. See Linear
Technology Application Note 88 for a complete discussion.
Output Capacitor and Output Ripple
The LT3697 output capacitors include COUT tied to the
inductor and to the ISP side of RSENSE and CBUS tied to
the regulator output and the ISN side of RSENSE. These
output capacitors have two essential functions. Along
with the inductor, they filter the square wave generated by
the LT3697 to produce the DC output. In particular, COUT
determines the output ripple, so low impedance (at the
switching frequency) is important. The second function
is to store energy in order to satisfy transient loads and
stabilize the LT3697’s control loop.
CBUS serves some additional purposes. It helps to stabilize
the output current limit loop. To this end, CBUS must satisfy
the following relationship:
CBUS ≥ COUT
CBUS also helps provide the minimum 120µF bypassing
required for the VBUS rail as specified by the USB 2.0
standard document.
Ceramic capacitors have very low equivalent series resistance (ESR) and provide the best ripple performance.
A good starting value for COUT is 47µF in 1206 or 1210
case size. Use X5R or X7R types. A good starting value
for CBUS is 100µF. Since CBUS is only tied to the inductor
through RSENSE, the ESR rating of CBUS is less critical
and high density tantalum or electrolytic capacitor types
may be used.
When choosing a capacitor, look carefully through the
data sheet to find out what the actual capacitance is under
operating conditions (applied voltage and temperature).
A physically larger capacitor or one with a higher voltage
rating may be required. Table 5 lists several capacitor
vendors.
Table 5. Recommended Ceramic Capacitor Vendors
MANUFACTURER
URL
AVX
www.avxcorp.com
Murata
www.murata.com
Taiyo Yuden
www.t-yuden.com
Vishay Siliconix
www.vishay.com
TDK
www.tdk.com
3697f
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23
LT3697
Applications Information
Catch Diode Selection
The catch diode (DCATCH from the Block Diagram) conducts
current only during the switch off time. Average forward
current in normal operation can be calculated from:
ID(AVG)=IOUT •
( VIN – VSYS )
VIN
where IOUT is the output load current. The current rating of
the diode should be selected to be greater than or equal to
the application’s output load current, so that the diode is
robust for a wide input voltage range. The voltage rating of
the diode is equal to the maximum regulator input voltage
while switching, 37V or less. Use a 3A, 40V Schottky diode.
Do not use a 60V diode due to the high resistive voltage drop.
BST and SYS Pin Considerations
Capacitor CBST and Schottky diode DBST (see the Block
Diagram) are used to generate a boost voltage that is
higher than the input voltage to drive the internal NPN
power switch. In most cases a 0.47μF capacitor will work
well for CBST. For switching frequency below 500kHz, use
1µF. The BST pin must be more than 1.8V above the SW
pin for best efficiency and more than 2.6V above the SW
pin to allow the LT3697 to skip off times to achieve very
high duty cycles.
With the SYS pin connected to the output, a 180mA active load will charge the boost capacitor during light load
start-up and an enforced 600mV minimum dropout voltage
will keep the boost capacitor charged across operating
conditions (see Minimum Dropout Voltage section).
Synchronizing the LT3697 oscillator to an external frequency can be done by connecting a square wave (with
on and off time greater than 50ns) to the SYNC pin. The
square wave amplitude should have valleys that are below
0.4V and peaks above 1V (up to 6V).
The LT3697 will skip pulses at low output loads while
synchronized to an external clock to maintain regulation. At very light loads, the part will go to sleep between
groups of pulses, reducing the quiescent current of the
part. Holding the SYNC pin DC high yields no advantages
so it is not recommended.
The LT3697 may be synchronized over a 300kHz to
2.2MHz range. The RT resistor should be chosen to set
the LT3697 switching frequency 10% below the lowest
synchronization input. For example, if the synchronization signal will be 300kHz and higher, the RT should be
selected for 270kHz. To ensure reliable and safe operation
the LT3697 will only synchronize when the output voltage
is near regulation. It is therefore necessary to choose a
large enough inductor value to supply the required output
current at the frequency set by the RT resistor (see Inductor Selection section). The slope compensation is set by
the RT value, while the minimum slope compensation
required to avoid subharmonic oscillations is established
by the inductor size, input voltage and output voltage.
Since the synchronization frequency will not change the
slopes of the inductor current waveform, if the inductor
is large enough to avoid subharmonic oscillations at the
frequency set by RT, than the slope compensation will be
sufficient for all synchronization frequencies.
Enable
Shorted and Reversed Input Protection
The LT3697 is in shutdown with IVIN < 1µA when the EN
pin is low and active when the pin is high. The enable
threshold is about 1.5V. The EN pin can be tied to VIN
if the shutdown feature is not used. The EN pin current
depends on the EN pin voltage for VEN < 12V and reaches
about 30µA at 12V.
If the inductor is chosen so that it won’t saturate excessively,
the LT3697 will tolerate a shorted output and the power
dissipation will be limited by the current limit set by RLIM
and RSENSE (see the Setting the Current Limit section).
Synchronization
To select low ripple Burst Mode operation, tie the SYNC
pin below 0.3V (this can be ground or a logic output).
24
There is another situation to consider in systems where
the output will be held high when the input to the LT3697
is absent. This may occur in automotive systems where
the LT3697 output may be connected to the 12V VBATT
during a fault condition or if a USB peripheral with a
large, charged cap is plugged into the LT3697 output. If
the VIN pin is allowed to float and the EN pin is held high
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LT3697
Applications Information
(either by a logic signal or because it is tied to VIN), then
the LT3697’s internal circuitry will pull its quiescent current through its SW pin. This is fine if your system can
tolerate a 1mA in this state. If you ground the EN pin, the
SW pin current will drop to zero. However, if the VIN pin
is grounded while the output is held high, regardless of
EN, parasitic diodes inside the LT3697 can pull current
from the output through the SW pin and out of VIN pin,
possibly causing high power dissipation in and damage
to the LT3697 depending on the magnitude of the current.
Figure 9 shows a circuit that is robust to output shorts
high and reversed input.
DOUT
DIN
INPUT
+
CBULK
VIN
CIN
SW
LT3697
L
OUTPUT
COUT
GND
3697 F09
Figure 9. Diodes DIN and DOUT Prevent High Current
Flow in the LT3697 if the Input Is Grounded or
Floating and the Output Is Pulled High.
PCB Layout
For proper operation and minimum EMI, care must be
taken during printed circuit board layout. Figure 10 shows
a good PCB layout example with component, trace, ground
plane and via locations. Note that large currents with high
dI/dt flow in the LT3697’s VIN and SW pins, the catch diode
(DCATCH), and the input capacitor (CIN). The loop formed by
these components should be as small and low inductance
as possible. These components, along with the inductor
and output capacitor, 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 below
these components. The SW and BST nodes should be as
small as possible to minimize the capacitive coupling on
these nodes to any fixed voltage like GND or VIN. Finally,
keep the VC, RT, RLIM and RCBL nodes small so that the
ground traces will shield them from the SW and BST nodes.
The exposed pad on the bottom of the package must be
soldered to ground so that the pad acts as a heat sink. To
keep thermal resistance low, extend the ground plane as
much as possible, and add thermal vias under and near
the LT3697 to additional ground planes within the circuit
board and on the bottom side.
High Temperature Considerations and Thermal
Shutdown
For higher ambient temperatures, care should be taken in
the layout of the PCB to ensure good heat sinking of the
LT3697. The exposed pad on the bottom of the package
must be soldered to a ground plane. This ground should be
tied to large copper layers below with thermal vias; these
layers will dissipate the heat generated by the LT3697.
Placing additional vias can reduce the thermal resistance
further. When operating at high ambient temperatures, the
maximum load current should be derated as the ambient
temperature approaches the maximum junction rating.
Power dissipation within the LT3697 can be estimated by
calculating the total power loss from an efficiency measurement and subtracting the catch diode loss and inductor
loss. The die temperature is calculated by multiplying the
LT3697 power dissipation by the thermal resistance from
junction to ambient.
The LT3697 has thermal shutdown to protect the part
during periods of high power dissipation, particularly in
high ambient temperature environments. The thermal
shutdown feature detects when the LT3697 is too hot
and shuts the part down, preventing switching. When the
thermal event passes and the LT3697 cools, the part will
restart and resume switching.
Other Linear Technology Publications
Application Notes 19, 35 and 44 contain more detailed
descriptions and design information for buck regulators
and other switching regulators. The LT1376 data sheet
has a more extensive discussion of output ripple, loop
compensation and stability testing.
3697f
For more information www.linear.com/LT3697
25
LT3697
Applications Information
GND
DCATCH
L
CIN
VIN
1
CBST
DBST
CCDC
RSENSE
RT
RC
CF
RLIM
VOUT
RCDC
RCBL
COUT
CC
3697 F10
VIAS TO GROUND PLANE
VIAS TO SYNC
VIAS TO VOUT
VIAS TO FLT
VIAS TO ISP
VIAS TO EN
VIAS TO VIN
VIAS TO ISN
Figure 10. Recommended PCB Layout
26
3697f
For more information www.linear.com/LT3697
LT3697
Typical Applications
5V Step Down Converter with Cable Drop Compensation and Output Current Limit
OFF ON
DBST
CIN
10µF
EN
VIN
BST
SW
LT3697
RT
255k
CF
330pF
RC
7.5k
CC
1nF
GND
f = 400kHz FOR
VIN = 8V TO 35V
ILOAD = 0.5A TO 2.5A
4 METERS AWG 20
TWISTED PAIR CABLE
VOUT
RSENSE
0.022Ω
DCATCH
SYS
ISP
ISN
USB5V
RCBL
FLT
RT
VC
RLIM
SYNC
CBST
1µF
L
10µH
0.13Ω
COUT
47µF
CCDC
10nF
+
RCBL
16.5k
+
VLOAD
5V, 2.5A
–
CBUS
100µF
0.13Ω
3697 TA02
CBST: X7R OR X5R
CIN: X7R OR X5R
COUT: 1210 CASE SIZE, X7R OR X5R
DCATCH: CENTRAL SEMI CMSH3-40MA
DBST: CENTRAL SEMI CMDSH2-3
VIN(MIN) = 6V AT 0.5A ILOAD
7V AT 2.5A ILOAD
5.75
6.0
VOUT
5.50
5.25
VLOAD
5.0
4.0
3.0
5.00
ILOAD
25mA/µs
4.75
CURRENT (A)
VOLTAGE (V)
RCDC
10k
LOAD
VIN
6V TO 35V
TRANSIENT TO 60V
2.0
1.0
4.50
250µs/DIV
3697 TA01a
3697f
For more information www.linear.com/LT3697
27
LT3697
Typical Applications
1.2MHz, 5.2V Step Down Converter with Output Current Limit
DBST
CIN
4.7µF
OFF ON
CBST
1µF
EN
VIN
BST
SW
LT3697
RT
48.7k
CF
100pF
RSENSE
0.022Ω
COUT
47µF
DCATCH
RCDC
10k
SYS
ISP
ISN
USB5V
RCBL
FLT
RT
VC
RLIM
SYNC
RC
9.09k
L
3.3µH
+
GND
CC
330pF
f = 1.2MHz FOR
VIN = 7.5V TO 18V
ILOAD = 0.5A TO 2.5A
CBUS
100µF
–
7.0
VIN = 12V
5.22
6.0
VLOAD
5.20
5.0
5.18
4.0
5.16
3.0
5.14
2.0
ILOAD
5.12
CURRENT (A)
VOLTAGE (V)
R5V2
249k
VLOAD
5.2V, 2.5A
3697 TA03
CBST: X7R OR X5R
CIN: X7R OR X5R
COUT: 1210 CASE SIZE, X7R OR X5R
DCATCH: CENTRAL SEMI CMSH3-40MA
DBST: CENTRAL SEMI CMDSH2-3
VIN(MIN) = 6V AT 0.5A ILOAD
6.5V AT 2.5A ILOAD
5.24
+
LOAD
VIN
6V TO 18V
TRANSIENT TO 35V
1.0
1A/µs
0.0
5.10
50µs/DIV
3697 TA03a
28
3697f
For more information www.linear.com/LT3697
LT3697
Typical Applications
5V Step Down Converter with Cable Drop Compensation,
Programmable Output Current Limit and Overcurrent Fault Reporting
VLOGIC
DBST
CIN
10µF
OFF ON
B
A
RLIM2
29.4k
M2
NMOS
RLIM1
75k
M1
NMOS
RT
453k
CF
470pF
BST
SW
COUT
47µF
GND
CC
1.5nF
CCDC
10nF
+
–
+
RCBL
16.5k
CBST: X7R OR X5R
CIN: X7R OR X5R
COUT: 1210 CASE SIZE, X7R OR X5R
DCATCH: CENTRAL SEMI CMSH3-40MA
DBST: CENTRAL SEMI CMDSH2-3
VIN(MIN) = 6V AT 0.5A ILOAD
7V AT 2.1A ILOAD
5
RCDC
10k
VLOAD
5V
2.1A - > RLIM = OPEN
1.5A - > RLIM = 75k
0.5A - > RLIM = 29.4k
CBUS
100µF
0.15Ω
f = 300kHz FOR
VIN = 8V TO 35V
ILOAD = 0.5A TO 2.1A
6
0.15Ω
SYS
ISP
ISN
USB5V
RCBL
FLT
RT
VC
RLIM
SYNC
RC
7.5k
5 METERS AWG 20
TWISTED PAIR CABLE
VOUT
RSENSE
0.025Ω
DCATCH
LT3697
3697 TA04
12
VIN = 12V
RLIM = 75k
VLOAD
4
10
8
VFLT
6
3
4
2
ILOAD
1
CURRENT (A)
VOLTAGE (V)
0 0
1 0
0 1
OUTPUT
CURRENT
LIMIT
2.4A
1.9A
0.8A
VIN
RFLT
100k
FAULT
A B
EN
CBST
1µF L
15µH
LOAD
VIN
6V TO 35V
TRANSIENT TO 60V
2
0
0
1ms/DIV
3697 TA04a
3697f
For more information www.linear.com/LT3697
29
LT3697
Typical Applications
5V Step Down Converter with Cable Drop Compensation
for Copper Cabling Over a Wide Temperature Range
DBST
CIN
10µF
OFF ON
EN
VIN
BST
SW
SYS
ISP
ISN
USB5V
RCBL
FSYNC =
300kHz TO 500kHz
CF
470pF
GND
RC
7.5k
RSENSE
0.025Ω
COUT
47µF
5.8
CCDC
10nF
RCDC
10k
VLOAD
5V, 2.1A
–
RCBL1
17.8k
+
RNTC
10k
CBUS
100µF
0.1Ω
CBST: X7R OR X5R
CIN: X7R OR X5R
COUT: 1210 CASE SIZE, X7R OR X5R
DCATCH: CENTRAL SEMI CMSH3-40MA
DBST: CENTRAL SEMI CMDSH2-3
RNTC: MURATA NCP21XV103J03RA
VIN(MIN) = 6V AT 0.5A ILOAD
7V AT 2.1A ILOAD
f = FSYNC FOR
VIN = 8V TO 35V
ILOAD = 0.5A TO 2.1A
0.1Ω
+
RCBL2
12.1k
CC
1.5nF
RT
590k
3 METERS AWG 20
TWISTED PAIR CABLE
VOUT
DCATCH
LT3697
FLT
RT
VC
RLIM
SYNC
CBST
1µF L
15µH
LOAD
VIN
6V TO 35V
TRANSIENT TO 60V
3697 TA05
ILOAD = 2.1A
5.6
VOLTAGE (V)
VOUT
5.4
5.2
VLOAD
5.0
4.8
–25
5
65
35
TEMPERATURE (°C)
95
125
3697 TA05a
30
3697f
For more information www.linear.com/LT3697
LT3697
Package Description
Please refer to http://www.linear.com/designtools/packaging/ for the most recent package drawings.
MSE Package
16-Lead Plastic MSOP, Exposed Die Pad
(Reference LTC DWG # 05-08-1667 Rev F)
BOTTOM VIEW OF
EXPOSED PAD OPTION
2.845 ±0.102
(.112 ±.004)
5.10
(.201)
MIN
2.845 ±0.102
(.112 ±.004)
0.889 ±0.127
(.035 ±.005)
8
1
1.651 ±0.102
(.065 ±.004)
1.651 ±0.102 3.20 – 3.45
(.065 ±.004) (.126 – .136)
0.305 ±0.038
(.0120 ±.0015)
TYP
16
0.50
(.0197)
BSC
4.039 ±0.102
(.159 ±.004)
(NOTE 3)
RECOMMENDED SOLDER PAD LAYOUT
0.254
(.010)
0.35
REF
0.12 REF
DETAIL “B”
CORNER TAIL IS PART OF
DETAIL “B” THE LEADFRAME FEATURE.
FOR REFERENCE ONLY
9
NO MEASUREMENT PURPOSE
0.280 ±0.076
(.011 ±.003)
REF
16151413121110 9
DETAIL “A”
0° – 6° TYP
3.00 ±0.102
(.118 ±.004)
(NOTE 4)
4.90 ±0.152
(.193 ±.006)
GAUGE PLANE
0.53 ±0.152
(.021 ±.006)
DETAIL “A”
1.10
(.043)
MAX
0.18
(.007)
SEATING
PLANE
0.17 – 0.27
(.007 – .011)
TYP
1234567 8
0.50
(.0197)
BSC
NOTE:
1. DIMENSIONS IN MILLIMETER/(INCH)
2. DRAWING NOT TO SCALE
3. DIMENSION DOES NOT INCLUDE MOLD FLASH, PROTRUSIONS OR GATE BURRS.
MOLD FLASH, PROTRUSIONS OR GATE BURRS SHALL NOT EXCEED 0.152mm (.006") PER SIDE
4. DIMENSION DOES NOT INCLUDE INTERLEAD FLASH OR PROTRUSIONS.
INTERLEAD FLASH OR PROTRUSIONS SHALL NOT EXCEED 0.152mm (.006") PER SIDE
5. LEAD COPLANARITY (BOTTOM OF LEADS AFTER FORMING) SHALL BE 0.102mm (.004") MAX
6. EXPOSED PAD DIMENSION DOES INCLUDE MOLD FLASH. MOLD FLASH ON E-PAD SHALL
NOT EXCEED 0.254mm (.010") PER SIDE.
0.86
(.034)
REF
0.1016 ±0.0508
(.004 ±.002)
MSOP (MSE16) 0213 REV F
3697f
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 representaFor more
information
www.linear.com/LT3697
tion that the interconnection
of its circuits
as described
herein will not infringe on existing patent rights.
31
LT3697
Typical Application
5.1V Step Down Converter with Cable Drop Compensation and Output Current Monitor
DBST
OFF ON
CBST
1µF
EN
VIN
BST
SW
DCATCH
LT3697
RT
63.4k
CF
120pF
RC
7.5k
SYS
ISP
ISN
USB5V
RCBL
FLT
RT
VC
RLIM
SYNC
CC
470pF
0.1Ω
COUT
47µF
CCDC
10nF
RCDC
1k
+
RUSB
9.09k
RMON
100k
GND
VMON = 0.43V/A
+
RCBL
22.1k
VLOAD
5.1V, 2.5A
–
CBUS
100µF
5.50
R5V1
49.9k
VIN = 12V
5.25
0.1Ω
VLOAD
3697 TA06
CBST: X7R OR X5R
CIN: X7R OR X5R
COUT: 1210 CASE SIZE, X7R OR X5R
DCATCH: CENTRAL SEMI CMSH3-40MA
DBST: CENTRAL SEMI CMDSH2-3
VIN(MIN) = 6V AT 0.5A ILOAD
7V AT 2.5A ILOAD
5.00
4.75
ILOAD
25mA/µs
2.0
VMON
0.43V/A
1.0
2.0
CURRENT (A)
f = 1MHz FOR
VIN = 8V TO 18V
ILOAD = 0.5A TO 2.5A
3 METERS AWG 20
TWISTED PAIR CABLE
VOUT
RSENSE
0.022Ω
L
4.7µH
VOLTAGE (V)
CIN
4.7µF
LOAD
VIN
6V TO 18V
TRANSIENT TO 35V
1.0
0.0
0.0
250µs/DIV
3697 TA06a
Related Parts
PART NUMBER DESCRIPTION
COMMENTS
LT8697
5V USB,42V Input, 2.5A, 95% Efficiency, 2.2MHz Synchronous
Step-Down DC/DC Converter with Cable Drop Compensation
VIN(MIN) = 5V, VIN(MAX) = 42V, VOUT(MIN) = 5.0V to 5.25V, ISD
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