LT3514 - Triple Step-Down Switching Regulator with 100% Duty Cycle Operation

LT3514
Triple Step-Down Switching
Regulator with 100%
Duty Cycle Operation
DESCRIPTION
FEATURES
Wide Input Voltage Range: 3.2V to 36V
(40V Transient)
n Three Outputs: 2A, 1A, 1A
n 100% Duty Cycle Operation
n Resistor-Programmed Constant Frequency
n Short-Circuit Robust
n Wide SYNC Range: 350kHz to 2.2MHz
n Anti-Phase Switching Reduces Ripple
n Feedback Voltage: 800mV
n Independent Run/Soft-Start Pins
n Shutdown with UVLO
n Internal Compensation
n Thermal Shutdown
n Tiny 28-Lead (4mm × 5mm) Thermally Enhanced
QFN Package
n 24-Lead Exposed Pad TSSOP
The LT®3514 consists of three buck regulators (2A, 1A,
1A output current). The device has a wide operating input
range of 3.2V to 36V. An on-chip boost regulator allows
each channel to operate up to 100% duty cycle. The LT3514
is designed to minimize external component count and
results in a simple and small application circuit.
APPLICATIONS
The LT3514 has one fewer channel (CH2) than the LT3504,
and has one channel (CH3) that outputs 2A instead of 1A.
The LT3514 in QFN is pin compatible with the LT3504. The
LT3504 provides four 1A outputs.
n
n
n
n
n
The LT3514 operates robustly in fault conditions. Cycleby-cycle peak current limit and catch diode current limit
sensing protect the IC during overload conditions. Thermal shutdown protects the power switches at elevated
temperatures. Soft-start helps control the peak inductor
current during startup.
The LT3514 also features output voltage tracking and
sequencing, programmable frequency, programmable
undervoltage lockout, and a power good pin to indicate
when all outputs are in regulation.
Automotive Battery Regulation
Industrial Control Supplies
Wall Transformer Regulation
Distributed Supply Regulation
L, LT, LTC, LTM, Linear Technology, the Linear logo and Burst Mode are registered trademarks
of Linear Technology Corporation. All other trademarks are the property of their respective
owners.
TYPICAL APPLICATION
Triple Buck Regulator
3.3µH
SW1
SKY
SW5
1µF
VIN
5.4V TO 20V
TRANSIENT TO 40V
10µH
1µF
×2
PG
LT3514
47nF
1.8V/1A
12.7k
22µF
10.2k
4.7µH
SW3
PG
DA3
FB3
RT/SYNC
18.2k
1MHz
DA1
FB1
VIN
VIN
EN/UVLO
RUN/SS1
RUN/SS3
RUN/SS4
100pF
LT3514 Start-Up and Shutdown
Waveform. VIN (Top Trace) Is Ramped
from OV Up to 8V and Then Back Down
to 0V. The Other Three Traces Are the
Output Voltages of All Three Channels
47pF
3.3V/2A
31.6k
47µF
10.2k
5V CHANNEL BEGINS
100% DC OPERATION
3.3V CHANNEL BEGINS
100% DC OPERATION
8.2µH
SW4
DA4
FB4
22pF
UVLO = ~2.9V
PARTS SHUTS
OFF
5V/1A
53.6k
22µF
10.2k
GND
100ms/DIV
VIN 1V/DIV
CH4 1V/DIV
CH3 1V/DIV
CH1 1V/DIV
3514 TA01b
3514 TA01a
3514fa
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1
LT3514
ABSOLUTE MAXIMUM RATINGS
(Note 1)
EN/UVLO....................................................................40V
EN/UVLO Pin Above VIN...............................................5V
VIN.............................................................................40V
SKY............................................................................46V
SW5...........................................................................47V
RUN/SS........................................................................6V
FB................................................................................6V
RT/SYNC......................................................................6V
PG..............................................................................25V
Operating Junction Temperature Range (Notes 2, 7)
LT3514EUFD....................................... –40°C to 125°C
LT3514IUFD........................................ –40°C to 125°C
LT3514EFE.......................................... –40°C to 125°C
LT3514IFE........................................... –40°C to 125°C
LT3514HFE.......................................... –40°C to 150°C
Storage Temperature Range................... –65°C to 150°C
PIN CONFIGURATION
PG
TOP VIEW
SW5
SKY
VIN
GND
VIN
TOP VIEW
VIN
1
24 SKY
NC
2
23 SW5
NC 1
22 NC
DA3
3
22 GND
NC 2
21 FB3
SW3
4
21 PG
DA3 3
20 FB1
SW3
5
20 FB3
SW3 4
19 FB4
SW1
6
18 GND
DA1
7
DA1 6
17 RT/SYNC
8
17 RT/SYNC
SW4 7
16 EN/UVLO
SW4
DA4 8
15 RUN/SS3
DA4
9
16 EN/UVLO
28 27 26 25 24 23
29
GND
SW1 5
NC
RUN/SS1
RUN/SS4
VIN
VIN
GND
9 10 11 12 13 14
UFD PACKAGE
28-LEAD (4mm × 5mm) PLASTIC QFN
θJA = 43°C/W
EXPOSED PAD (PIN 29) IS GND, MUST BE SOLDERED TO PCB
25
GND
19 FB1
18 FB4
NC 10
15 RUN/SS3
VIN 11
14 RUN/SS1
VIN 12
13 RUN/SS4
FE PACKAGE
24-LEAD PLASTIC TSSOP
θJA = 33°C/W
EXPOSED PAD (PIN 25) IS GND, MUST BE SOLDERED TO PCB
ORDER INFORMATION
LEAD FREE FINISH
TAPE AND REEL
PART MARKING*
PACKAGE DESCRIPTION
TEMPERATURE RANGE
LT3514EUFD#PBF
LT3514EUFD#TRPBF
3514
28-Lead (4mm × 5mm) Plastic QFN
–40°C to 125°C
LT3514IUFD#PBF
LT3514IUFD#TRPBF
3514
28-Lead (4mm × 5mm) Plastic QFN
–40°C to 125°C
LT3514EFE#PBF
LT3514EFE#TRPBF
LT3514FE
24-Lead Plastic TSSOP
–40°C to 125°C
LT3514IFE#PBF
LT3514IFE#TRPBF
LT3514FE
24-Lead Plastic TSSOP
–40°C to 125°C
LT3514HFE#PBF
LT3514HFE#TRPBF
LT3514FE
24-Lead Plastic TSSOP
–40°C to 150°C
Consult LTC Marketing for parts specified with wider operating temperature ranges. *The temperature grade is identified by a label on the shipping container.
Consult LTC Marketing for information on non-standard lead based finish parts.
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/
3514fa
2
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LT3514
ELECTRICAL
CHARACTERISTICS
The
l denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at TA = 25°C. VIN = 12V unless otherwise noted.
SYMBOL
CONDITIONS
EN/UVLO Threshold Voltage
Rising
l
MIN
TYP
MAX
1.2
1.44
1.6
EN/UVLO Threshold Voltage Hysteresis
EN/UVLO Threshold Current Hysteresis
VEN/UVLO = Measured Rising Threshold – 50mV
(Note 3)
Internal VIN Undervoltage Lockout
2.4
UNITS
V
110
mV
1.3
µA
2.9
3.2
V
0.01
2
µA
4
10
Quiescent Current (VIN) in Shutdown
VEN/UVLO = 0V
Quiescent Current (VIN)
VEN/UVLO = 1V
Quiescent Current (VIN)
VEN/UVLO = 1.5V, VRUN/SS(1,3,4) = Open,
VFB(1,3,4) = 0.9V, VSKY = 17V (Note 4)
2.7
mA
Quiescent Current (SKY)
VEN/UVLO = 1.5V, VRUN/SS(1,3,4) = Open,
VFB(1,3,4) = 0.9V, VSKY = 17V (Note 4)
4.4
mA
RUN/SS Pin Source Current
VRUN/SS = 0V
RUN/SS Pin Threshold for Switching
VFB = 0V
Feedback Voltage
l
FB Pin Current
VFB = Measured VFB (Note 5)
Reference Line Regulation
VIN = 5V to 40V
SKY Pin Current
ISW1 = 1A or ISW4 = 1A
1.3
µA
50
100
mV
790
784
800
800
810
816
mV
mV
15
150
nA
l
–0.015
40
mA
80
mA
SKY Pin Current
ISW3 = 2A
54
VSKY – VIN
4.85
Switching Frequency
RT = 6.34k
RT = 18.2k
RT = 100k
Switching Phase
RT = 18.2k
1.8
0.85
220
150
SYNC Threshold Voltage
%/V
27
SKY Voltage above VIN Voltage
l
l
l
V
2.1
1
270
2.4
1.15
320
MHz
MHz
kHz
180
210
Deg
1.25
SYNC Input Frequency
0.35
Switch Current Limit (SW1,4)
(Note 6)
Switch VCESAT (SW1,4)
ISW1, SW4 = 1A
µA
1.45
V
2.2
1.75
2.1
400
Switch Leakage Current (SW1,4)
MHz
A
mV
0.1
2
µA
Catch Diode Current Limit (SW1,4)
FB = 0V
FB = 0.7V
0.75
1.0
1.15
1.45
1.33
1.67
A
A
Switch Current Limit (SW3)
(Note 6)
3
3.5
4.2
A
Switch VCESAT (SW3)
ISW3 = 2A
400
Switch Leakage Current (SW3)
mV
0.1
4
µA
Catch Diode Current Limit (SW3)
FB = 0V
FB = 0.7V
1.5
2.0
2
2.5
2.4
3.0
A
A
Switch Current Limit (SW5)
(Note 6)
220
320
mA
Switch VCESAT (SW5)
ISW = 200mA
230
mV
Switch Leakage Current (SW5)
0.1
Boost Diode Current Limit (SW5)
VIN = 5V
350
450
PG Threshold Offset
VFB Rising
65
90
PG Hysteresis
VFB Rising – VFB Falling
35
2
µA
mA
125
mV
mV
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LT3514
The
l denotes the specifications which apply over the full operating
ELECTRICAL
CHARACTERISTICS
temperature range, otherwise specifications are at TA = 25°C. VIN = 12V unless otherwise noted.
SYMBOL
CONDITIONS
PG Voltage Output Low
PG Pin Leakage
TYP
MAX
UNITS
IPG = 250µA
180
300
mV
VPG = 2V
0.01
1
µA
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 LT3514E 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
LT3514I is guaranteed over the full –40°C to 125°C operating junction
temperature range.
Note 3: Current flows into pin.
Note 4: The VIN pin quiescent current and the SKY pin quiescent current
are specified in the Electrical Characteristics table. However, the quiescent
MIN
current for an application circuit is higher than the sum of these two
currents because the SKY voltage is higher than VIN, and there are power
losses in the boost regulator. See the Typical Performance Characteristics
section for a plot of input quiescent current vs input voltage for a typical
application.
Note 5: Current flows out of pin.
Note 6: Current limit is guaranteed by design and/or correlation to static
test. Slope compensation reduces current limit at higher duty cycles.
Note 7: This IC includes overtemperature protection that is intended
to protect the device during momentary overload conditions. Junction
temperature will exceed 125°C when overtemperature protection is active.
Continuous operation above the specified maximum operating junction
temperature may impair device reliability.
TYPICAL PERFORMANCE CHARACTERISTICS
Efficiency, Channel 4, f = 1MHz
Efficiency, Channel 4, f = 1MHz
90
VOUT = 1.8V
Efficiency, Channel 4, f = 1MHz
90
VOUT = 2.5V
60
70
70
60
60
40
30
20
VIN = 6V
VIN = 12V
VIN = 24V
VIN = 36V
10
0
0
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9
LOAD CURRENT (A)
EFFICIENCY (%)
80
50
50
40
30
VIN = 6V
VIN = 12V
VIN = 24V
VIN = 36V
20
10
1
314 G01
0
0
VOUT = 3.3V
80
70
EFFICIENCY (%)
EFFICIENCY (%)
80
TA = 25°C, unless otherwise noted.
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9
LOAD CURRENT (A)
50
40
30
VIN = 6V
VIN = 12V
VIN = 24V
VIN = 36V
20
10
1
3514 G02
0
0
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9
LOAD CURRENT (A)
1
3514 G03
3514fa
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LT3514
TYPICAL PERFORMANCE CHARACTERISTICS
Efficiency, Channel 4, f = 1MHz
100
Efficiency, Channel 3, f = 1MHz
80
VOUT = 5V
90
60
50
40
30
70
10
0
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9
LOAD CURRENT (A)
50
40
30
20
VIN = 6V
VIN = 12V
VIN = 24V
VIN = 36V
20
VIN = 6V
VIN = 12V
VIN = 24V
VIN = 36V
10
0
1
0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8
LOAD CURRENT (A)
0
3514 G04
50
40
30
60
50
40
2
0
0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8
LOAD CURRENT (A)
–3.5
–2.0
–5.0
2
2
3514 G10
0
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9
LOAD CURRENT (A)
EN/UVLO Pin Current
1.55
1.8
1.6
1.4
RISING
1.45
1.40
FALLING
1.35
1.20
–50
1
3514 G09
2.0
–45°C
1.2
25°C
1.0
0.8
0.6
150°C
0.4
1.25
VIN = 12V
VOUT = 1.8V
VOUT = 2.5V
VOUT = 3.3V
VOUT = 5V
–4.5
1.30
1.5
1
LOAD CURRENT (A)
–2.5
–3.0
–4.0
IEN/UVLO (µA)
–1.5
0.5
–2.0
1.60
1.50
–1.0
0
–1.5
EN/UVLO Threshold
–0.5
VIN = 12V
3514 G08
THRESHOLD (V)
PERCENT ERROR (%)
3514 G06
VIN = 6V
VIN = 12V
VIN = 24V
VIN = 36V
10
0
2
–1.0
20
VOUT = 1.8V
VOUT = 2.5V
VOUT = 3.3V
VOUT = 5V
0.0
0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8
LOAD CURRENT (A)
Load Regulation Channels 1 and 4
70
Load Regulation Channel 3
0.5
0
–0.5
30
VIN = 6V
VIN = 12V
VIN = 24V
VIN = 36V
0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8
LOAD CURRENT (A)
VIN = 6V
VIN = 12V
VIN = 24V
VIN = 36V
0
3514 G07
–2.5
0
2
PERCENT ERROR (%)
EFFICIENCY (%)
EFFICIENCY (%)
60
0
30
10
80
10
40
20
VOUT = 5V
90
70
20
50
Efficiency, Channel 3, f = 1MHz
100
VOUT = 3.3V
80
60
3514 G05
Efficiency, Channel 3, f = 1MHz
90
VOUT = 2.5V
80
EFFICIENCY (%)
EFFICIENCY (%)
EFFICIENCY (%)
VOUT = 1.8V
60
70
0
Efficiency, Channel 3, f = 1MHz
90
70
80
0
TA = 25°C, unless otherwise noted.
0.2
–25
25
50
0
75
TEMPERATURE (°C)
100
125
3514 G11
0
0
0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0
VEN/UVLO (V)
3514 G12
3514fa
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LT3514
TYPICAL PERFORMANCE CHARACTERISTICS
Input Voltage Undervoltage
Lockout
10
40
3.4
9
35
8
7
3.0
IVIN (µA)
UVLO (V)
INPUT QUIESCENT CURRENT (mA)
3.6
2.8
2.6
6
5
4
3
2.4
2
2.2
1
2.0
–50
–25
25
50
0
75
TEMPERATURE (°C)
100
0
125
0
30
25
20
15
10
0
1.20
800
–0.2
1.15
700
–0.4
FREQUENCY (MHz)
IRUN/SS (µA)
–0.8
–1.0
–1.2
–1.4
200
–1.6
100
–1.8
0
200
600
800 1000
400
RUN/SS VOLTAGE (mV)
–25
25
50
0
75
TEMPERATURE (°C)
CURRENT LIMIT (A)
CHANNELS 1, 4
CHANNEL 3
200
100
0
0
100
0.95
0.80
–50 –25
125
500
1000
1500
SWITCH CURRENT (mA)
2000
3514 G19
0
25 50 75 100 125 150
TEMPERATURE (°C)
3514 G18
Switch and Diode Current Limit
4.0
2.0
3.8
1.9
3.6
1.8
3.4
1.7
CURRENT LIMIT (A)
600
300
1.00
3514 G17
700
400
1.05
Switch and Diode Current Limit,
Channel 3
500
RT = 18.2k
0.85
3514 G16
Switch Voltage Drop
45
0.90
–2.0
–50
1200
40
1.10
–0.6
300
0
15 20 25 30 35
INPUT VOLTAGE (V)
Switching Frequency
vs Temperature
900
400
10
3514 G15
Soft Start Current
500
5
0
3514 G14
FB Voltage vs RUN/SS
600
ALL SS = 2V
ALL SS = 0V
5
0
0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0
VEN/UVLO (V)
3514 G13
FB VOLTAGE (mV)
Input Quiescent Current vs Input
Voltage
VIN Pin Current
3.2
SWITCH VOLTAGE DROP (mV)
TA = 25°C, unless otherwise noted.
3.2
3.0
2.8
2.6
1.6
1.5
1.4
1.3
2.4
1.2
2.2
1.1
2.0
–50
–25
25
50
0
75
TEMPERATURE (°C)
100
125
3514 G20
CHANNELS 1, 4
1.0
–50
–25
25
50
0
75
TEMPERATURE (°C)
100
125
3514 G21
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LT3514
TYPICAL PERFORMANCE CHARACTERISTICS
3.6
56
3.4
54
3.2
52
3.0
2.0
1.9
1A
50
2A
2.8
48
2.6
46
2.4
44
2.2
42
2.0
40
–50 –25
60
40
DUTY CYCLE (%)
80
100
0
25 50 75 100 125 150
TEMPERATURE (°C)
Switch Beta, Channels 1 and 4
1.6
1.5
1.4
1.3
1.2
1.0
0
20
40
60
DUTY CYCLE (%)
80
100
3514 G24
Minimum On-Time
120
65
110
0.5A
ON-TIME (ns)
100
55
50
1.7
3514 G23
70
60
1.8
1.1
3514 G22
BETA
90
80
70
1A
45
60
40
–50
–25
25
50
0
75
TEMPERATURE (°C)
100
50
–50
125
–25
25
50
0
75
TEMPERATURE (°C)
3514 G25
Feedback Voltage
125
Power Good Threshold
805
740
804
RISING
720
803
802
801
800
799
798
797
700
680
FALLING
660
640
620
796
795
–50
100
3514 G26
THRESHOLD (mV)
20
SWITCH CURRENT LIMIT (A)
58
BETA
60
3.8
0
Switch Current Limit, Channels 1
and 4
Switch Beta, Channel 3
4.0
FEEDBACK VOLTAGE (mV)
SWITCH CURRENT LIMIT (A)
Switch Current Limit, Channel 3
TA = 25°C, unless otherwise noted.
–25
25
50
0
75
TEMPERATURE (°C)
100
125
600
–50
3514 G27
–25
25
50
0
75
TEMPERATURE (°C)
100
125
3514 G28
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LT3514
TYPICAL PERFORMANCE CHARACTERISTICS
Operating Waveforms,
Discontinuous Mode
Operating Waveforms,
Continuous Mode
SW1
10V/DIV
SW1
10V/DIV
SW3
10V/DIV
SW3
10V/DIV
SW4
10V/DIV
SW4
10V/DIV
500ns/DIV
3514 G29
IOUT1,3,4 = 40mA
VOUT1,3,4 = 5V
PIN FUNCTIONS
TA = 25°C, unless otherwise noted.
500ns/DIV
3514 G30
IOUT1,3,4 = 0.5A
VOUT1,3,4 = 5V
(QFN/TSSOP)
NC (Pins 1, 2, 14, 22/Pins 2, 10): No Connection. These
pins have no connection to internal circuitry. They can be
grounded or left floating.
DA (Pins 3, 6, 8/Pins 3, 7, 9): Return the Schottky catch
diode anode to the diode anode (DA) pin. An internal comparator senses the diode current and prevents switching
when the diode current is higher than the DA pin current
limit.
SW (Pins 4, 5, 7/Pins 4, 5, 6, 8): The SW pins are the
output of the internal power switches. Connect each SW
pin to an inductor and Schottky catch diode cathode.
VIN (Pins 9, 11, 26, 28/Pins 1, 11, 12): The VIN pins supply current to the LT3514’s internal regulator and to the
internal power switches. The VIN pins should be locally
bypassed with a capacitor to ground, preferably to pins
27 and 10. They must be tied to the same input source.
GND (Pins 10, 18, 27, Exposed Pad Pin 29/Pin 22,
Exposed Pad Pin 25): Tie the GND pins to a local ground
plane below the LT3514 and the circuit components. The
exposed pad must be soldered to the PCB and electrically
connected to ground. Use a large ground plane and thermal
vias to optimize thermal performance.
RUN/SS (Pins 12, 13, 15/Pins 13, 14, 15): The RUN/SS
pins are used to soft start each channel and to allow
each channel to track other outputs. Output tracking is
implemented by connecting a resistor divider to this pin
from the tracked output. For soft start, tie a capacitor from
this pin to ground. An internal 1.3µA soft-start current
charges the capacitor to create a voltage ramp at the pin.
Each channel can be individually shut down by pulling
RUN/SS below 0.1V.
EN/UVLO (Pin 16/Pin 16): The EN/UVLO pin is used to
start up the internal regulator to power the reference and
oscillator. It also starts up the internal boost regulator. Pull
the EN/UVLO pin below 1.44V to shut down the LT3514.
The LT3514 will draw less than 10µA of current from the
VIN pin when EN/UVLO is less than 1.44V. Pull EN/UVLO
pin below 0.7V to put the LT3514 in a state where the part
draws 0µA from the VIN pin. The threshold can function
as an accurate undervoltage lockout (UVLO), preventing
the regulator from operating until the input voltage has
reached the programmed level. Do not drive the EN/UVLO
pin more than 5V above VIN.
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LT3514
PIN FUNCTIONS
(QFN/TSSOP)
RT/SYNC (Pin 17/Pin 17): Set the switching frequency
of the LT3514 by tying an external resistor from this pin
to ground. Select the value of the programming resistor
(RT) according to Table 1 in the Applications Information
section. The RT/SYNC pin is also used to synchronize the
internal oscillator of the LT3514 to an external signal. The
synchronization (sync) signal is directly logical compatible
and can be driven by any signal with pulse width greater
than 50ns. The synchronization range is from 350kHz to
2.2MHz.
FB (Pins 19, 20, 21/Pins 18,19, 20): Each feedback pin is
regulated to 800mV. Connect the feedback resistor divider
to this pin. The output voltage is programmed according
to the following equation:
V

R1= R2 •  OUT − 1
 0.8V 
PG (Pin 23/Pin 21): The Power Good pin is the open
collector output of an internal comparator. PG remains
low until all FB pins are greater than 710mV. If not in use,
this pin can be left unconnected. The PG comparator is
disabled in shutdown.
SW5 (Pin 24/Pin 23): The SW5 pin is an open collector
of an internal boost regulator power switch. This power
switch generates the drive voltage 4.85V above the input
voltage (VIN), to drive the internal buck regulator power
switches. Connect an inductor from this pin to the VIN pin.
SKY (Pin 25/Pin 24): The SKY pin is the output of an integrated power Schottky diode and is the source of drive
voltage to the internal buck regulator power switches.
Connect a 1µF capacitor from this pin to the VIN pin. Do
not drive this pin with an external voltage source. Do not
draw current from this pin with an external component.
where R1 connects between OUT and FB, and R2 connects
between FB and GND. A good value for R2 is 10.2kΩ.
3514fa
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9
LT3514
BLOCK DIAGRAM
PRECISION UVLO
ON
1.44V
VIN
REF
EN/UVLO
SW5
BOOST ERROR AMP
S
SKY
VIN
4.5V
Q5
R NQ
5V
VIN
1µA
BOOST SWITCH AND DRIVE
SKYBAD
SKY
0.7V
Σ
0.4V
LOCK
D5
BOOST REGULATOR
1SHOT
STARTUP/SHUTDOWN
THERMAL SHUTDOWN
SLOPE
CLK1
CLK2
TO CH3, CH4
SKY
OSC
1SHOT
SLOPE
0
SYNC
DETECT
1
VIN
FREQUENCY
TO CURRENT
0.7V
2.2V
1µA
S
Σ
0.8V
Q1
R NQ
SW1
0.1V
+–
OUT1
SWITCH AND DRIVE
SKYBAD
DA1
SKYBAD
FB1
ONE OF THREE BUCK REGULATORS SHOWN
0.8V
CURRENT LIMIT FOLDBACK
PG
PGOOD
0.72V
FB1
COMPARATORS FROM OTHER CHANNELS
RT/SYNC
RUN/SS1
POWER GOOD LOGIC
GND
3514 BD
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LT3514
OPERATION
A comparator starts the reference when the EN/UVLO pin
rises above the 1.44V rising threshold. Other comparators
prevent switching when the input voltage is below 2.9V or
the die temperature is above 175°C. When the EN/UVLO
is above 1.44V, the input voltage is above 3.2V, and the
temperature is below 175°C, the boost regulator begins
switching and charges the SKY capacitor to 4.85V above
VIN. When the SKY voltage is less than 4.5V above VIN,
the RUN/SS pins and VC nodes are actively pulled low to
prevent the buck regulators from switching.
The boost regulator (Channel 5) consists of an internal
0.4A power switch (Q5), an internal power Schottky diode
(D5), and the necessary logic and other control circuitry
to drive the switch. The switch current is monitored to
enforce cycle-by-cycle current limit. The diode current
is monitored to prevent inductor current runaway during
transient conditions. An error amplifier servos the SKY
voltage to 4.85V above VIN. A comparator detects when
the SKY voltage is 4.5V above VIN and allows the buck
regulators to begin switching.
The oscillator produces two antiphase clock signals running
at 50% duty cycle. Channel 5 runs antiphase to Channels
3 and 4. The oscillator can be programmed by connecting
a single resistor from RT/SYNC to ground, or by applying
an external clock signal to RT/SYNC. A sync detect circuit
distinguishes between the type of input. Tying a resistor
to GND directly sets the bias current of the oscillator. The
sync signal is converted to a current to set the bias current of the oscillator.
The oscillator enables an RS flip-flop, turning on the power
switch Q1. 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 the VC node. A second comparator enforces
a catch diode current limit to prevent inductor current
runaway during transient conditions. An error amplifier
measures the output voltage through an external resistor
tied to the FB pin and servos the VC node. If the error
amplifier’s output increases, more current is delivered
to the output; if it decreases, less current is delivered. A
clamp on the VC pin provides switch current limit. Each
buck regulator switch driver operates by drawing current
from the SKY pin. Regulating the SKY pin to 4.85V above
the VIN pin voltage is necessary to fully saturate the bipolar
power switch for efficient operation.
Soft-start is implemented by generating a voltage ramp at
the RUN/SS pin. An internal 1.3µA current source pulls the
RUN/SS pin up to 2.1V. Connecting a capacitor from the
RUN/SS pin to ground programs the rate of the voltage
ramp on the RUN/SS pin. A voltage follower circuit with a
0.1V offset connected from the RUN/SS pin to the RAMP
node prevents switching until the voltage at the RUN/SS
pin increases above 0.1V. When the voltage at the RAMP
node is less than 0.9V, the error amplifier servos the FB
voltage to the RAMP node voltage. When the RAMP node
voltage increases above 0.9V, then the error amplifier servos the FB voltage to 0.8V. Additionally, a current amplifier
reduces the catch diode current limit when the FB voltage
is below 0.8V to limit the inductor current during startup.
Each channel can be placed in shutdown by pulling the
respective RUN/SS pin below 0.1V. The EN/UVLO pin can
be pulled low (below a VBE) to place the entire part in
shutdown, disconnecting the outputs and reducing the
input current to less than 2µA.
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11
LT3514
APPLICATIONS INFORMATION
The three step-down converters in the LT3514 are referred
to as channels 1, 3, and 4, while the boost converter is
referred to as channel 5. There is no channel 2. This naming convention is intended to maintain consistency and
limited pin compatibility with the LT3504, a four channel
step-down converter. Essentially, two 1A converters (channels 2 and 3) of the LT3504 were combined to make the
2A converter (channel 3) of the LT3514.
FB Resistor Network
The output voltage is programmed with a resistor divider
connected from the output and the FB pin. Choose the 1%
resistor according to:
V

R1= R2 •  OUT − 1
 0.8V 
A good value for R2 is 10.2kΩ, R2 should not exceed
20kΩ to avoid bias current error.
Input Voltage Range
The input voltage range for LT3514 applications depends
on the output voltage and on the absolute maximum rating of the VIN pin.
The minimum input voltage to regulate the output generally has to be at least 400mV greater than the greatest
programmed output voltage. The only exception is when
the largest programmed output voltage is less than 2.8V.
In this case the minimum input voltage is 3.2V.
The absolute maximum input voltage of the LT3514 is
40V and the part will regulate output voltages as long
as the input voltage remains less than or equal to 40V.
However for constant-frequency operation (no pulseskipping) the maximum input voltage is determined by
the minimum on-time of the LT3514 and the programmed
switching frequency. The minimum on-time is the shortest
period of time that it takes the switch to turn on and off.
Therefore the maximum input voltage to operate without
pulse-skipping is:
VIN(PS) = [ (VOUT + VD)/(fSW • tON(MIN)) ] + VSW – VD
where:
•VIN(PS) is the maximum input voltage to operate in
constant frequency operation without skipping pulses.
•VOUT is the programmed output voltage
•VSW is the switch voltage drop, at IOUT1,4 = 1A,
VSW1,4 = 0.4V, at IOUT3 = 2A, VSW3 = 0.4V.
•VD is the catch diode forward voltage drop, for an
appropriately sized diode, VD = 0.4V
•fSW is the programmed switching frequency
•tON(MIN) is the minimum on-time, worst-case over
temperature = 110ns (at T = 125°C)
At input voltages that exceed VIN(PS) the part will continue
to regulate the output voltage up to 40V. However the
part will skip pulses (see Figure 1) resulting in unwanted
harmonics, increased output voltage ripple, and increased
IL
0.5A/DIV
VSW
10V/DIV
2µs/DIV
3514 F01a
Figure 1a. The LT3514 Operating in Constant-Frequency
Operation (Below VIN(PS)), VIN = 26.5V, VOUT = 3.3V,
fSW = 2MHz, tON(MIN) = 74ns at T = 25°C
IL
0.5A/DIV
VSW
10V/DIV
2µs/DIV
3514 F01b
Figure 1b. The LT3514 Operating in Pulse-Skipping
Mode (Above VIN(PS)), VIN = 27V, VOUT = 3.3V, fSW =
2MHz, tON(MIN) = 74ns at T = 25°C
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LT3514
APPLICATIONS INFORMATION
peak inductor current. Provided that the inductor does not
saturate and that the switch current remains below 2A
(SW1, SW4) or below 4A (SW3), operation above VIN(PS)
is safe and will not damage the part. For a more detailed
discussion on minimum on-time and pulse-skipping, refer to the Applications Information section of the LT3505
data sheet.
Avoid starting up the LT3514 at input voltages greater
than 36V, as the LT3514 must simultaneously conduct
maximum currents at high VIN. The maximum operating
junction temperature of 125°C may be exceeded due to
the high instantaneous power dissipation.
Frequency Selection
The maximum frequency that the LT3514 can be programmed to is 2.5MHz. The minimum frequency is 250kHz.
The switching frequency can be programmed in two ways.
The first method is by tying a 1% resistor (RT) from the
RT/SYNC pin to ground. Table 1 can be used to select the
value of RT. The second method is to synchronize (sync)
the internal oscillator to an external clock. The external
clock must have a minimum amplitude from 0V to 1.5V
and a minimum pulse-width of 50ns.
Table 1. RT/SYNC Pin Resistance to Program Oscillator
Frequency
FREQUENCY (MHz)
RT/SYNC PIN RESISTANCE (kΩ)
0.20
140
0.3
82.5
0.4
56.2
0.5
43.2
0.6
34.8
0.7
28.0
0.8
23.7
0.9
20.5
1.0
18.2
1.1
16.9
1.2
14.7
1.3
13.0
1.4
11.5
FREQUENCY (MHz)
RT/SYNC PIN RESISTANCE (kΩ)
1.5
10.7
1.6
9.76
1.7
8.66
1.8
8.06
1.9
7.32
2.0
6.81
2.1
6.34
2.2
6.04
2.3
5.62
2.4
5.36
2.5
4.99
In certain applications, the LT3514 may be required to be
alive and switching for a period of time before it begins
to receive a sync signal. If the sync signal is in a high
impedance state when it is inactive then the solution is to
simply tie an RT resistor from the RT/SYNC pin to ground
(Figure 2). The sync signal should be capable of driving the
RT resistor. If the sync signal is in a low impedance state
or an unknown state when it is inactive, then the solution
is to tie the RT resistor from the RT/SYNC pin to ground
and then to drive the RT/SYNC pin with the sync signal
through a 1nF capacitor as shown in Figure 3.
LT3514
PORT
RT/SYNC
GND
RT
3514 F02
Figure 2. Driving the RT/SYNC Pin From a Port That
Is in a High Impedance State When it Is Inactive
LT3514
1nF
PORT
RT/SYNC
RT
GND
3514 F03
Figure 3. Driving the RT/SYNC Pin from a Port That Is
in a Low Impedance State When it Is Inactive
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13
LT3514
APPLICATIONS INFORMATION
BOOST Regulator and SKY Pin Considerations
The on-chip boost regulator generates the SKY voltage
to be 4.85V above VIN. The SKY voltage is the source of
drive current for the buck regulators which is used to fully
saturate the power switch. The boost regulator requires
two external components: an inductor and a capacitor.
A good first choice for an inductor is given by:
20.5µH
L=
f
Thus, for a 250kHz programmed switching frequency,
a good first choice for an inductor value is 82µH. For a
2.5MHz programmed switching frequency, a good first
choice for an inductor value is 8.2µH. These values will
ensure that each buck regulator will have sufficient drive
current to saturate the power switch in all applications
and under all operating conditions.
A user desiring a lower inductor current value can calculate
their optimum inductor size based on their output current requirements. Each buck regulator instantaneously
requires 20mA from the SKY pin per 1A of switch current.
The average current that each buck regulator draws from
the SKY pin is 20mA per 1A of switch current multiplied by
the duty cycle. So if all three buck regulators run at 100%
duty cycle with channels 1 and 4 supplying 1A of output
current and channel 3 supplying 2A of output current, then
the SKY pin should be able to source 80mA. However if
each channel runs at 50% duty cycle then the SKY pin only
has to source 40mA. Alternatively if each channel runs
at 100% duty cycle but the output current requirements
are reduced by half, then again the SKY pin only has to
source 40mA. To summarize, the SKY pin output current
requirement is calculated from the following equation:
(I
Once the SKY pin output current requirement is determined, the inductor value can be calculated based on
the maximum tolerable inductor current ripple from the
following equation:
L=
where f is in MHz.
ISKY =
where IOUTX is the desired output current from Channel
X, VOUTX is the programmed output voltage of Channel X,
and VIN is input voltage.
OUT1 • VOUT1 + IOUT3
• VOUT3 + IOUT4 • VOUT4
50 • VIN
)
VIN • DC5
2 • fSW • 0.3 • (1− 0.25 • DC5) − ISKY 
where fSW is the programmed switching frequency and
DC5 is the boost regulator duty cycle, given by: DC5 =
5V/(VIN + 5V).
For a 1MHz application, with VIN = 12V, VOUT1 = 5V, VOUT3
= 2.5V, VOUT4 = 1.8V, IOUT1,4 = 1A, IOUT3 = 2A, and the
required SKY pin current is 20mA and the inductor value
is 6.8µH.
Soft-Start/Tracking
The RUN/SS pin can be used to soft-start the corresponding channel, reducing the maximum input current during
start-up. The RUN/SS pin is pulled up through a 1µA current
source to about 2.1V. A capacitor can be tied to the pin to
create a voltage ramp at this pin. The buck regulator will
not switch while the RUN/SS pin voltage is less than 0.1V.
As the RUN/SS pin voltage increases above 0.1V, the channel will begin switching and the FB pin voltage will track
the RUN/SS pin voltage (offset by 0.1V), until the RUN/SS
pin voltage is greater than 0.8V + 0.1V. At this point the
output voltage will be at 100% of it’s programmed value
and the FB pin voltage will cease to track the RUN/SS
pin voltage and remain at 0.8V (the RUN/SS pin will
continue ramping up to about 2.1V with no effect on the
output voltage). The ramp rate can be tailored so that the
peak start up current can be reduced to the current that
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LT3514
APPLICATIONS INFORMATION
is required to regulate the output, with little overshoot.
Figure 4 shows the start-up waveforms with and without
a soft-start capacitor (CSS) on the RUN/SS pin.
IL
0.5A/DIV
VOUT
2V/DIV
3514 F04a
100µs/DIV
Undervoltage Lockout
The LT3514 prevents switching when the input voltage
decreases below 3.2V. Alternatively, the EN/UVLO pin
can be used to program an undervoltage lockout at input
voltages exceeding 3.2V by tapping a resistor divider from
VIN to EN/UVLO as shown in Figure 5.
The rising threshold on the EN/UVLO pin is 1.44V. The
falling threshold on the EN/UVLO pin is 1.33V. When EN/
UVLO is rising and less than 1.44V then the EN/UVLO pin
sinks 1.3µA of current. This 1.3µA current can be used to
program additional hysteresis on the EN/UVLO pin. For the
circuit in Figure 5, R1 can be determined from:
Figure 4a. Inductor Current Waveform During
Start-Up without a Soft-Start Capacitor
R1=
(
0.11
V
1.33 IN,FALLING
1.3µA
VIN,HYSTERESIS −
)
where VIN,HYSTERESIS is the desired amount of hysteresis
on the input voltage and VIN,FALLING is the desired input
voltage threshold at which the part will shut down. Notice
that for a given falling threshold (VIN,FALLING), the amount
of hysteresis (VIN,HYSTERESIS) must be at least:
IL
0.5A/DIV
VOUT
2V/DIV
3514 F04b
100µs/DIV
VIN, HYSTERESIS >
(
0.11
• VIN,FALLING
1.33
)
Figure 4b. Inductor Current Waveform During
Start-Up with a 1nF Soft-Start Capacitor (CSS)
SWITCHING
VIN
VIN
R1
133k
LT3514
EN/UVLO
R2
20.5k
GND
VIN, FALLING = 10V
VIN, RISING = 11V
NOT
SWITCHING
9
10
11
VIN (V)
12
3514 F05
Figure 5. Circuit to Prevent Switching When VIN < 10V, with 700mV of Hysteresis
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LT3514
APPLICATIONS INFORMATION
For a falling threshold of 10V, the minimum hysteresis
is 0.827V. For a falling threshold of 30V, the minimum
hysteresis is 2.48V.
R2 can be calculated once R1 is known:
R2 = R1•
1.33
VIN, FALLING − 1.33
The circuit shown in Figure 5 will start when the input
voltage rises above 11V and will shutdown when the input
voltage falls below 10V.
Inductor Selection and Maximum Output Current
A good first choice for the inductor value is:
L = 2 • (VOUT + VD)/fSW for Channels 1, 4
L = (VOUT + VD)/fSW for Channel 3
CH4 or below 2A for CH3, then you can decrease 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. Low
inductance may result in discontinuous mode operation,
which is okay, but further reduces maximum load current.
For details on maximum output current and discontinuous
mode operation, see Linear Technology Application Note 44.
Catch Diode
Use a 1A Schottky diode for channels 1 and 4 and a 2A
Schottky diode for channel 3. The diode must have a reverse voltage rating equal to or greater than the maximum
input voltage.
Input Capacitor
where VD is the voltage drop of the catch diode (~0.4V),
L is in µH and fSW is in MHz. With this value there will
be no subharmonic oscillation for applications with 50%
or greater duty cycle. The inductor’s RMS current rating
must be greater than your maximum load current and
its saturation current should be about 30% higher. For
robust operation in fault conditions, the saturation current should be above 2A for CH1, CH4 and above 4A for
CH3. To keep efficiency high, the series resistance (DCR)
should be less than 0.1 . Table 2 lists several vendors
and types that are suitable.
Of course, such a simple design guide will not always
result in the optimum inductor for your application. A
larger value provides a higher maximum load current and
reduces output voltage ripple at the expense of slower
transient response. If your load is lower than 1A for CH1,
The input of the LT3514 circuit must be bypassed with a
X7R or X5R type ceramic capacitor. Y5V types have poor
performance over temperature and amplified voltage
and should not be used. There are four VIN pins. Each
VIN pin should be bypassed to the nearest ground pin.
However it is not necessary to use a dedicated capacitor for each VIN pin. Pins 9 and 11 may be tied together
on the board layout so that both pins can share a single
bypass capacitor. Since the channels running on Pins 9
and 11 are 180 degrees out-of-phase, it is not necessary
to double the capacitor value either. Similarly, Pins 26
and 28 may be tied together on the board layout to save
a bypass capacitor. For switching frequencies greater than
750kHz, a 1µF capacitor or higher value ceramic capacitor
should be used to bypass each group of two VIN pins. For
switching frequencies less than 750kHz, a 2.2µF or higher
value ceramic capacitor should be used to bypass each
Table 2. Inductor Vendors
VENDOR
URL
PART SERIES
Sumida
www.sumida.com
CDRH4D28
CDRH5D28
CDRH5D28
INDUCTANCE (µH)
1.2 TO 4.7
2.5 TO 10
2.5 TO 33
SIZE (mm)
4.5 × 4.5
5.5 × 5.5
8.3 × 8.3
Toko
www.toko.com
A916CY
D585LC
2 TO 12
1.1 TO 39
6.3 × 6.2
8.1 × 8
Würth Elektronik
www.we-online.com
WE-TPC(M)
WE-PD2(M)
WE-PD(S)
1 TO 10
2.2 TO 22
1 TO 27
4.8 × 4.8
5.2 × 5.8
7.3 × 7.3
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LT3514
APPLICATIONS INFORMATION
group of two VIN pins. The ceramic bypass capacitors
should be located as close to the VIN pins as possible.
See the sample layout shown in the PCB Layout section.
All four VIN pins should be tied together on the board and
bypassing with a low performance electrolytic capacitor
is recommended especially if the input power source has
high impedance, or there is significant inductance due to
long wires or cables.
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 LT3514 and to force this very high frequency
switching current into a tight local loop, minimizing EMI.
To accomplish this task, the input bypass capacitor must
be placed close to the LT3514 and the catch diode; see
the PCB Layout section. A second precaution regarding
the ceramic input capacitor concerns the maximum input
voltage rating of the LT3514. A ceramic input capacitor
combined with trace or cable inductance forms a high
quality (underdamped) tank circuit. If the LT3514 circuit
is plugged into a live supply, the input voltage can ring to
twice its nominal value, possibly exceeding the LT3514’s
voltage rating. This situation can be easily avoided by adding an electrolytic capacitor in parallel with the ceramic
input capacitors. See Application Note 88.
Output Capacitor
The output capacitor has two essential functions. Along
with the inductor, it filters the square wave generated by
the LT3514 to produce the DC output. In this role it deter-
mines 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
LT3514’s control loop.
Ceramic capacitors have very low equivalent series resistance (ESR) and provide the best ripple performance.
A good value is:
COUT = 33/(VOUT • fSW) for Channels 1, 4
COUT = 132/(VOUT • fSW) for Channel 3
where COUT is in µF and fSW is in MHz. Use X5R or X7R
types and keep in mind that a ceramic capacitor biased
with VOUT will have less than its nominal capacitance. This
choice will provide low output ripple and good transient
response. Transient performance can be improved with a
high value capacitor, if the compensation network is also
adjusted to maintain the loop bandwidth.
A lower value of output capacitor can be used, but transient performance will suffer. Also, a lower value output
capacitor may result in increased sensitivity to noise which
can be alleviated by adding a 100pF phase lead capacitor
from FB to VOUT.
High performance electrolytic capacitors can be used for
the output capacitor. Low ESR is important, so choose one
that is intended for use in switching regulators. The ESR
should be specified by the supplier and should be 0.1Ω
or less. Such a capacitor will be larger than a ceramic
capacitor and will have a larger capacitance, because the
capacitor must be large to achieve low ESR. Table 3 lists
several capacitor vendors.
Table 3. Capacitor Vendors
VENDOR
PHONE
URL
PART SERIES
COMMENTS
Panasonic
(714) 373-7366
www.panasonic.com
Ceramic, Polymer, Tantalum
EEF Series
Kemet
(864) 963-6300
www.kemet.com
Ceramic, Tantalum
T494, T495
Sanyo
(408) 749-9714
www.sanyovideo.com
Ceramic, Polymer, Tantalum
POSCAP
Murata
(404) 436-1300
www.murata.com
Ceramic
www.avxcorp.com
Ceramic, Tantalum
www.taiyo-yuden.com
Ceramic
AVX
Taiyo Yuden
(864) 963-6300
TPS Series
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For more information www.linear.com/LT3514
17
LT3514
APPLICATIONS INFORMATION
Figure 6 shows the transient response of the LT3514 with
several output capacitor choices. The output is 3.3V. The
load current is stepped from 500mA to 1A and back to
500mA and the oscilloscope traces show the output voltage. The upper photo shows the recommended value. The
second photo shows the improved response (less voltage
drop) resulting from a larger output capacitor and a larger
phase lead capacitor. The last photo shows the response
to a high performance electrolytic capacitor. Transient performance is improved due to the large output capacitance.
VOUT
LT3514
31.6k
FB
Shorted and Reversed Input Protection
If the inductor is chosen so that it won’t saturate excessively, an LT3514 buck regulator will tolerate a shorted
output. There is another situation to consider in systems
where the output will be held high when the input to the
LT3514 is absent. This may occur in battery charging applications or in battery backup systems where a battery
or some other supply is diode OR-ed with the LT3514’s
output. If the VIN pin is allowed to float and the EN/UVLO
pin is held high (either by a logic signal or because it is
IOUT
1A/DIV
10µF
10k
VOUT
20mV/DIV
VOUT
LT3514
31.6k
100pF
20µs/DIV
3514 F06a
20µs/DIV
3514 F06b
20µs/DIV
3514 F06c
IOUT
1A/DIV
10µF
×2
FB
10k
VOUT
20mV/DIV
VOUT
LT3514
31.6k
FB
+
IOUT
1A/DIV
22µF
10k
VOUT
20mV/DIV
Figure 6. Transient Load Response of the LT3514 with Different Output Capacitors as the
Load Current Is Stepped from 500mA to 1A. VIN = 12V, VOUT = 3.3V, L = 10µH, RT = 19.1k
3514fa
18
For more information www.linear.com/LT3514
LT3514
APPLICATIONS INFORMATION
tied to VIN), then the LT3514’s internal circuitry will pull
its quiescent current through its SW pin. This is fine if
your system can tolerate a few mA in this state. If you
ground the EN/UVLO pin, the SW pin current will drop to
essentially zero. However, if the VIN pin is grounded while
the output is held high, then parasitic diodes inside the
LT3514 can pull large currents from the output through
the SW pin and the VIN pin. Figure 7 shows a circuit that
will run only when the input voltage is present and that
protects against a shorted or reversed input.
reduce the dependence of efficiency on input voltage. The
die temperature is calculated by multiplying the LT3514
power dissipation by the thermal resistance from junction to ambient. Power dissipation within the LT3514 can
be estimated by calculating the total power loss from an
efficiency measurement and subtracting the catch diode
losses. Thermal resistance depends on the layout of the
circuit board, but 43°C/W is typical for the QFN package and 33°C/W is typical for the FE package. Thermal
shutdown will turn off the buck regulators and the boost
regulator when the die temperature exceeds 175°C, but
this is not a warrant to allow operation at die temperatures
exceeding 125°C.
High Temperature Considerations
While the LT3514 is capable of delivering total output
current up to 4A, total power dissipation for an application circuit and the resulting temperature rise must be
considered, especially if all three channels are operating
at high duty cycle.
Outputs Greater Than 9V
For outputs greater than 9V, add a 1k resistor in series
with a 1nF capacitor across the inductor to damp the
discontinuous ringing of the SW node, preventing unintended SW current.
The die temperature of the LT3514 must be lower than the
maximum rating of 125°C. This is generally not a concern
unless the ambient temperature is above 85°C. For higher
temperatures, extra care should be taken in the layout of
the circuit to ensure good heat sinking of the LT3514. The
maximum load current should be derated as the ambient
temperature approaches 125°C. Programming the LT3514
to a lower switching frequency will improve efficiency and
VIN
Other Linear Technology Publications
Application Notes 19, 35, 44 contain more detailed descriptions and design information for step-down regulators and
other switching regulators. Design Note 318 shows how to
generate a bipolar output supply using a step-down regulator.
EN/UVLO
VOUT
SW1
SKY
SW5
LT3514
D4
DA1
VIN
VIN
RUN/SS1
BACKUP
FB1
RT/SYNC
GND
3514 F07
Figure 7. Diode D4 Prevents a Shorted Input from Discharging a Backup Battery Tied to the Output; It
Also Protects the Circuit from a Reversed Input. The LT3514 Runs Only When the Input Is Present
3514fa
For more information www.linear.com/LT3514
19
LT3514
APPLICATIONS INFORMATION
PCB Layout
For proper operation and minimum EMI, care must be taken
during printed circuit board layout. Figure 8 shows the
recommended component placement with trace, ground
plane, and via locations for the QFN package.
Note that large, switched currents flow in the LT3514’s
VIN, SW and DA pins, the catch diodes (D1, D3, D4) and
the input capacitors (C5, C6). The loop formed by these
+
C3
GND
components should be as small as possible and tied to
system ground in only one place. These components, along
with the inductors (L1, L3, L4, L5) and output capacitors
(C1, C3, C4, C7), 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, and tie this ground plane to system
ground at one location (ideally at the ground terminal
of the output capacitors). For the QFN package ground
C8
OUT3
L5
SW5
L3
GND
VIN
C7
SKY
SW3
GND
C5
D3
GND
PG
NC
NC
NC
SW1
VIN
SW4
R3
R1
R7
R9
GND
L1
R6
RT/SYNC
RUN/SS3
EN/UVLO
GND
C6
R5
NC
R2
D4
RUN/SS4
RUN/SS1
D1
FB3
FB1
FB4
GND
GND
L4
C4
OUT1
C1
GND
OUT4
GND
VIA TO LOCAL GROUND PLANE
OUTLINE OF LOCAL GROUND PLANE
VIA TO VIN
3514 F08
Figure 8
3514fa
20
For more information www.linear.com/LT3514
LT3514
APPLICATIONS INFORMATION
pins (Pins 10, 27) are provided near the VIN pins so that
the VIN pins can be bypassed to these ground pins. The
SW nodes should be kept as small as possible and kept
far away from the RT/SYNC and FB nodes. Keep the RT/
SYNC node and FB nodes small so that the ground pin
and ground traces will shield them from the SW nodes. If
the user plans on using a SYNC signal to set the oscillator
frequency then the RT/SYNC node should be kept away
from the FB nodes. Include vias near the exposed pad of
the LT3514 to help transfer heat from the LT3514 to the
ground plane. Keep the SW5 pad/trace as far away from
the FB pads as possible.
Zener diode D1 clamps Q1’s gate voltage to 36V. The source
follower configuration prevents VIN from rising any further
than about 33V (a VGS below the Zener clamp voltage ) .
Figure 10 shows the LT3514 regulating all three channels
through a 180V surge event without interruption.
VSUPPLY
50V/DIV
VIN
50V/DIV
VOUT1,3,4
2V/DIV
Overvoltage Transient Protection
Figure 9 shows the complete application circuit for a 3-output step-down regulator with 100% duty cycle operation
that withstands 180V surges. Under normal operating
conditions (VIN < 33V), the VSKY rail supplies gate drive to
MOSFET Q1, providing the LT3514 with a low resistance
path to VSUPPLY. In the event that a supply surge occurs,
3514 F10
100ms/DIV
Figure 10. Overvoltage Protection Withstands 180V Surge
VSUPPLY
3.2V TO 30V
SURGE PROTECTION TO 180V
R1
10Ω
Q1
R2
100k
D2
6.8V
R3
1k
C1
0.1µF
D3
D1
36V
VSKY
CSKY
2.2µF
L5
10µH
VIN
+
22µF
2.2µF
×2
PG
Q1: FQB34N20L
D1: BZT52C36-7-F
D2: BZT52C6V8-7-F
D3: BAT54-7-F
L1: CDRH5D28-4R2
L3: CDRH8D28-4R7
L4: CDRH5D28-8R2
L5: CBC2016100M
4.2µH
SW1
SKY
SW5
LT3514
0.1µF
18.2k
2.5V/1A
21.5k
22µF
10.2k
4.7µH
SW3
PG
DA3
FB3
RT/SYNC
82pF
DA1
FB1
VIN
VIN
EN/UVLO
RUN/SS1
RUN/SS3
RUN/SS4
L1
L3
47pF
3.3V/2A
31.6k
47µF
10.2k
8.2µH
SW4
L4
22pF
DA4
FB4
5V/1A
53.6k
22µF
10.2k
GND
3514 F09
Figure 9. Complete Triple Buck Regulator with 180V Surge Protection
3514fa
For more information www.linear.com/LT3514
21
LT3514
APPLICATIONS INFORMATION
Bear in mind that significant power dissipation occurs in
Q1 during an overvoltage event. The MOSFET junction
temperature must be kept below its absolute maximum
rating. For the overvoltage transient shown in Figure 10,
MOSFET Q1 conducts 0.55A (full load on all buck channels) while withstanding the voltage difference between
VSUPPLY (180V peak) and VIN (33V). This results in a peak
power of 81W. Since the overvoltage pulse in Figure 10
is roughly triangular, average power dissipation during
the transient event (about 400ms) is approximately half
the peak power. As such, the average power is given by:
1
• PPEAK (W) = 40.5W
2
In order to approximate the MOSFET junction temperature
rise from an overvoltage transient, one must determine
the MOSFET transient thermal response as well as the
MOSFET power dissipation. Fortunately, most MOSFET
transient thermal response curves are provided by the
manufacturer (as shown in Figure 11). For a 400ms pulse
duration, the FQB34N20L MOSFET thermal response
ZθJC(t) is 0.65°C/W. The MOSFET junction temperature
rise is given by:
PAVG (W) =
TRISE (°C) = ZθJC (t) • PAVG (W) = 26.3°C
Note that, by properly selecting MOSFET Q1, it is possible
to withstand even higher input voltage surges. Consult
manufacturer data sheets to ensure that the MOSFET
operates within its maximum safe operating area.
The application circuit start-up behavior is shown in
Figure 12. Resistor R2 pulls up on the gate of Q1, forcing
source connected VIN to follow approximately 3V below
VSUPPLY. Once VIN reaches the LT3514’s 3.2V minimum
start-up voltage, the on-chip boost converter immediately regulates the VSKY rail 4.85V above VIN. Diode D3
and resistor R3 bootstrap Q1’s gate voltage to the VSKY,
fully enhancing Q1. This connects VIN directly to VSUPPLY
through Q1’s low resistance drain-source path. It should
be noted that, prior to VSKY being present, the minimum
input voltage is about 6.2V. However, with VSKY in regulation
and Q1 enhanced, the minimum run voltage drops to 3.2V,
permitting the LT3514 to maintain regulation through deep
input voltage dips Figure 13 shows all channels operating
down to the LT3514’s 3.2V minimum input voltage.
SKY
2V/DIV
VSUPPLY
2V/DIV
VIN
2V/DIV
20ms/DIV
3514 F12
Figure 12. Figure 9’s Start-Up Behavior
ZθJC(t), THERMAL RESPONSE (°C/W)
1
VIN
1V/DIV
0.1
VOUT4
1V/DIV
0.01
10–3
10–5
SINGLE PULSE
VOUT3
1V/DIV
VOUT1
1V/DIV
D = 0.5
D = 0.2
D = 0.1
D = 0.05
D = 0.02
D = 0.01
0.1
1
10–4 10–3 0.01
10
t1, SQUARE WAVE PULSE DURATION (s)
PDM
t1
t2
ZθJC(t) = 0.7°C/W MAX
DUTY FACTOR = D = t1/t2
TJM – TC = PDM • ZθJC(t)
100ms/DIV
3514 F13
Figure 13. Figure 9’s Dropout Performance
3514 F11
Figure 11. FQB34N20L Transient Thermal Response
3514fa
22
For more information www.linear.com/LT3514
LT3514
PACKAGE DESCRIPTION
Please refer to http://www.linear.com/designtools/packaging/ for the most recent package drawings.
FE Package
24-Lead Plastic TSSOP (4.4mm)
(Reference LTC DWG # 05-08-1771 Rev B)
Exposed Pad Variation AA
7.70 – 7.90*
(.303 – .311)
3.25
(.128)
3.25
(.128)
24 23 22 21 20 19 18 17 16 15 14 13
6.60 ±0.10
2.74
(.108)
4.50 ±0.10
6.40
2.74 (.252)
(.108) BSC
SEE NOTE 4
0.45 ±0.05
1.05 ±0.10
0.65 BSC
1 2 3 4 5 6 7 8 9 10 11 12
RECOMMENDED SOLDER PAD LAYOUT
4.30 – 4.50*
(.169 – .177)
0.09 – 0.20
(.0035 – .0079)
0.25
REF
0.50 – 0.75
(.020 – .030)
NOTE:
1. CONTROLLING DIMENSION: MILLIMETERS
MILLIMETERS
2. DIMENSIONS ARE IN
(INCHES)
3. DRAWING NOT TO SCALE
1.20
(.047)
MAX
0° – 8°
0.65
(.0256)
BSC
0.195 – 0.30
(.0077 – .0118)
TYP
0.05 – 0.15
(.002 – .006)
FE24 (AA) TSSOP REV B 0910
4. RECOMMENDED MINIMUM PCB METAL SIZE
FOR EXPOSED PAD ATTACHMENT
*DIMENSIONS DO NOT INCLUDE MOLD FLASH. MOLD FLASH
SHALL NOT EXCEED 0.150mm (.006") PER SIDE
3514fa
For more information www.linear.com/LT3514
23
LT3514
PACKAGE DESCRIPTION
Please refer to http://www.linear.com/designtools/packaging/ for the most recent package drawings.
UFD Package
UFDQFN
Package
28-Lead Plastic
(4mm × 5mm)
28-Lead
Plastic
QFN (4mm × 5mm)
(Reference LTC DWG # 05-08-1712 Rev B)
(Reference LTC DWG # 05-08-1712 Rev B)
0.70 ±0.05
4.50 ±0.05
3.10 ±0.05
2.50 REF
2.65 ±0.05
3.65 ±0.05
PACKAGE OUTLINE
0.25 ±0.05
0.50 BSC
3.50 REF
4.10 ±0.05
5.50 ±0.05
RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS
APPLY SOLDER MASK TO AREAS THAT ARE NOT SOLDERED
4.00 ±0.10
(2 SIDES)
0.75 ±0.05
R = 0.05
TYP
PIN 1 NOTCH
R = 0.20 OR 0.35
× 45° CHAMFER
2.50 REF
R = 0.115
TYP
27
28
0.40 ±0.10
PIN 1
TOP MARK
(NOTE 6)
1
2
5.00 ±0.10
(2 SIDES)
3.50 REF
3.65 ±0.10
2.65 ±0.10
(UFD28) QFN 0506 REV B
0.200 REF
0.00 – 0.05
0.25 ±0.05
0.50 BSC
BOTTOM VIEW—EXPOSED PAD
NOTE:
1. DRAWING PROPOSED TO BE MADE A JEDEC PACKAGE OUTLINE MO-220 VARIATION (WXXX-X).
2. DRAWING NOT TO SCALE
3. ALL DIMENSIONS ARE IN MILLIMETERS
4. DIMENSIONS OF EXPOSED PAD ON BOTTOM OF PACKAGE DO NOT INCLUDE
MOLD FLASH. MOLD FLASH, IF PRESENT, SHALL NOT EXCEED 0.15mm ON ANY SIDE
5. EXPOSED PAD SHALL BE SOLDER PLATED
6. SHADED AREA IS ONLY A REFERENCE FOR PIN 1 LOCATION
ON THE TOP AND BOTTOM OF PACKAGE
3514fa
24
For more information www.linear.com/LT3514
LT3514
REVISION HISTORY
REV
DATE
DESCRIPTION
PAGE NUMBER
A
01/14
Added H-grade option
2
Clarified Switching Frequency parameters
3
Clarified resistor value for R2
11
3514fa
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/LT3514
tion that the interconnection
of its circuits
as described
herein will not infringe on existing patent rights.
25
LT3514
TYPICAL APPLICATION
Complete Triple Buck Regulator with 180V Surge Protection
VSUPPLY
3.2V TO 30V
SURGE PROTECTION TO 180V
R1
10Ω
Q1
R2
100k
D2
6.8V
R3
1k
C1
0.1µF
D3
D1
36V
VSKY
CSKY
2.2µF
L5
10µH
VIN
+
22µF
2.2µF
×2
PG
Q1: FQB34N20L
D1: BZT52C36-7-F
D2: BZT52C6V8-7-F
D3: BAT54-7-F
L1: CDRH5D28-4R2
L3: CDRH8D28-4R7
L4: CDRH5D28-8R2
L5: CBC2016100M
4.2µH
SW1
SKY
SW5
LT3514
0.1µF
18.2k
2.5V/1A
21.5k
22µF
10.2k
4.7µH
SW3
PG
DA3
FB3
RT/SYNC
82pF
DA1
FB1
VIN
VIN
EN/UVLO
RUN/SS1
RUN/SS3
RUN/SS4
L1
L3
47pF
3.3V/2A
31.6k
47µF
10.2k
8.2µH
SW4
DA4
FB4
L4
22pF
5V/1A
53.6k
22µF
10.2k
GND
3514 TA02
RELATED PARTS
PART
DESCRIPTION
COMMENTS
LT3504
40V, Quad 1A Step-Down 2.5MHz DC/DC Converter with 100% Duty
Cycle Operation
VIN(MIN) = 3.2V, VIN(MAX) = 40V, IQ = 7.1mA, ISD < 1µA,
4mm x 5mm QFN-28 Package
LT3507/
LT3507A
36V 2.5MHz, Triple [2.4A + 1.5A + 1.5A (IOUT)] with LDO Controller
High Efficiency Step-Down DC/DC Converter
VIN(MIN) = 4V, VIN(MAX) = 36V, VOUT(MIN) = 0.8V, IQ = 7mA,
ISD = 1µA, 5mm x 7mm QFN-38 Package
LT8610
42V 2.2MHz, Synchronous, Low IQ = 2.5µA, Step-Down DC/DC
Converter
VIN(MIN) = 3.4V, VIN(MAX) = 42V, VOUT(MIN) = 0.97V, IQ = 2.5µA,
ISD = 1µA, MSOP-16E Package
LT3988
60V with Transient Protection to 80V, 2.5MHz, Dual 1A High
Efficiency Step-Down DC/DC Converter
VIN(MIN) = 4.0V, VIN(MAX) = 60V, VOUT(MIN) = 0.75V, IQ = 2mA,
ISD = 1µA, MSOP-16E Package
LT3509
36V with Transient Protection to 60V, Dual 0.70(IOUT), 2.2MHz,
High Efficiency Step-Down DC/DC Converter
VIN(MIN) = 3.6V, VIN(MAX) = 36V, VOUT(MIN) = 0.8V, IQ = 1.9mA,
ISD = 1µA, 3mm × 4mm DFN-14, MSOP-16E Packages
LT3500
36V, 40VMAX, 2A, 2.5MHz High Efficiency Step-Down DC/DC
Converter and LDO Controller
VIN(MIN) = 3.6V, VIN(MAX) = 36V, VOUT(MIN) = 0.8V, IQ = 2.5mA,
ISD <10µA, 3mm × 3mm DFN-10 Package
LT3508
36V with Transient Protection to 40V, Dual 1.4A (IOUT), 3MHz,
High Efficiency Step-Down DC/DC Converter
VIN(MIN) = 3.7V, VIN(MAX) = 37V, VOUT(MIN) = 0.8V, IQ = 4.6mA,
ISD = 1µA, 4mm × 4mm QFN-24, TSSOP-16E Packages
LT3980
58V with Transient Protection to 80V, 2A (IOUT), 2.4MHz, High
Efficiency Step-Down DC/DC Converter with Burst Mode® Operation
VIN(MIN) = 3.6V, VIN(MAX) = 58V, Transient to 80V, VOUT(MIN) = 0.8V,
IQ = 85µA, ISD <1µA, 3mm × 4mm DFN-16 and MSOP-16E Packages
LT3480
36V with Transient Protection to 60V, 2A (IOUT), 2.4MHz, High
Efficiency Step-Down DC/DC Converter with Burst Mode Operation
VIN(MIN) = 3.6V, VIN(MAX) = 38V, VOUT(MIN) = 0.78V, IQ = 70µA,
ISD <1µA, 3mm × 3mm DFN-10, MSOP-10E Packages
LT3689
36V, 60V Transient Protection, 800mA, 2.2MHz High Efficiency
Micropower Step-Down DC/DC Converter with POR Reset and
Watchdog Timer
VIN(MIN) = 3.6V, VIN(MAX) = 36V, Transient to 60V, VOUT(MIN) = 0.8V,
IQ = 75µA, ISD <1µA. 3mm × 3mm QFN-16 Package
LT3970
40V, 350mA, 2MHz High Efficiency Micropower Step-Down
DC/DC Converter
VIN(MIN) = 4V, VIN(MAX) = 40V, Transient to 60V, VOUT(MIN) = 1.21V,
IQ = 2µA, ISD <1µA, 3mm × 2mm DFN-10 and MSOP-10 Packages
LT3682
36V, 60VMAX, 1A, 2.2MHz High Efficiency Micropower Step-Down
DC/DC Converter
VIN(MIN) = 3.6V, VIN(MAX) = 36V, VOUT(MIN) = 0.8V, IQ = 75µA,
ISD < 1µA, 3mm × 3mm DFN-12 Package
3514fa
26 Linear Technology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417
For more information www.linear.com/LT3514
(408) 432-1900 ● FAX: (408) 434-0507
●
www.linear.com/LT3514
LT 0114 REV A • PRINTED IN USA
 LINEAR TECHNOLOGY CORPORATION 2013