LINER LTC3371 4-channel 8a configurable buck dc/dcs with watchdog and power-on reset Datasheet

LTC3371
4-Channel 8A Configurable
Buck DC/DCs with Watchdog
and Power-On Reset
Description
Features
n
n
n
n
n
n
n
n
n
n
8 × 1A Buck Power Stages Configurable as 2, 3 or 4
Output Channels
8 Unique Output Configurations (1A to 4A Per Channel)
Independent VIN Supplies for Each DC/DC (2.25V to 5.5V)
Low Total No Load Supply Current:
n 15µA In Shutdown (All Channels Off)
n 68µA One Channel Active in Burst Mode® Operation
n 18µA Per Additional Channel
Precision Enable Pin Thresholds for Autonomous
Sequencing
1MHz to 3MHz RT Programmable Frequency
(2MHz Default) or PLL Synchronization
Temp Monitor Indicates Die Temperature
CT Programmed Watchdog Timer
Independent RST Pins Indicate Buck in Regulation
Thermally Enhanced 38-Lead 5mm × 7mm QFN and
TSSOP Packages
Applications
n
General Purpose Multichannel Power Supplies:
Automotive, Industrial, Distributed Power Systems
The LTC®3371 is a highly flexible multioutput power supply
IC. The device includes four synchronous buck converters, configured to share eight 1A power stages, each of
which is powered from independent 2.25V to 5.5V inputs.
The DC/DCs are assigned to one of eight possible power
configurations via pin programmable C1-C3 pins. The
common buck switching frequency may be programmed
with an external resistor, synchronized to an external oscillator, or set to a default internal 2MHz clock. The operating
mode for all DC/DCs may be programmed for Burst Mode
or forced continuous mode operation.
The CT pin programs the timing parameters of four independent RST pins as well as the watchdog timer.
To reduce input noise, the buck converters are phased in
90° steps. Precision enable pin thresholds facilitate reliable power sequencing. The LTC3371 is available in low
profile 38-lead 5mm × 7mm QFN and TSSOP packages.
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
100
Buck Efficiency vs ILOAD
90
80
2.7V TO 5.5V
VOUT1 = 1.2V/2A
324k
645k
VOUT2 = 1.5V/2A
2.25V TO 5.5V
2.2µH
412k
475k
VINA
VINB
VCC
VINE
VINF
SWA
SWB
SWE
SWF
FB1
EN1
RST1
FB3
EN3
RST3
LTC3371
VINC
VIND
VING
VINH
SWC
SWD
SWG
SWH
FB2
EN2
RST2
FB4
EN4
RST4
TEMP
WDI
WDO
CT
C1 C2 C3 GND
70
EFFICIENCY (%)
2.25V TO 5.5V
2.2µH
2.25V TO 5.5V
VOUT3 = 1.8V/2A
806k
60
50
40
30 Burst Mode OPERATION
V = 3.3V
20 VIN = 1.8V
OUT
10 f OSC = 1MHz
L = 3.3µH
0
1
10
100
LOAD CURRENT (mA)
649k
2.5V TO 5.5V
VOUT4 = 2.5V/2A
665k
309k
PLL/MODE
RT
3371 TA01
1A BUCK
2A BUCK
3A BUCK
4A BUCK
1000
4000
3371 TA01b
C3
C2
C1
0
0
0
0
1
1
1
1
0
0
1
1
0
0
1
1
0
1
0
1
0
1
0
1
BUCK1 BUCK2 BUCK3 BUCK4
2A
3A
3A
4A
3A
4A
4A
4A
2A
1A
1A
1A
2A
–
–
–
2A
2A
1A
1A
–
2A
1A
–
2A
2A
3A
2A
3A
2A
3A
4A
3371fa
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1
LTC3371
Table of Contents
Features...................................................... 1
Applications................................................. 1
Typical Application ......................................... 1
Description.................................................. 1
Absolute Maximum Ratings............................... 3
Pin Configuration........................................... 3
Order Information........................................... 3
Electrical Characteristics.................................. 4
Typical Performance Characteristics.................... 6
Pin Functions............................................... 12
Block Diagram.............................................. 14
Operation................................................... 15
Buck Switching Regulators...................................... 15
Buck Regulators with Combined Power Stages....... 15
Power Failure Reporting Via RST Pins..................... 16
Temperature Monitoring and Overtemperature
Protection................................................................ 16
Programming the Operating Frequency................... 16
Windowed Watchdog Timer..................................... 17
Choosing the CT Capacitor....................................... 17
2
Applications Information................................. 18
Buck Switching Regulator Output Voltage and
Feedback Network................................................... 18
Buck Regulators...................................................... 18
Combined Buck Power Stages................................. 18
Input and Output Decoupling Capacitor Selection... 18
PCB Considerations.................................................20
Typical Applications....................................... 20
Package Description...................................... 23
Revision History........................................... 25
Typical Application........................................ 26
Related Parts............................................... 26
3371fa
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LTC3371
Absolute Maximum Ratings
(Note 1)
VINA-H, FB1-4, EN1-4, VCC, CT, WDI, WDO, RST1-4,
RT, PLL/MODE, C1-3.................................... –0.3V to 6V
TEMP................... –0.3V to Lesser of (VCC + 0.3V) or 6V
IRST1-4, IWDO..............................................................5mA
Operating Junction Temperature Range
(Notes 2, 3)............................................. –40°C to 150°C
Storage Temperature Range................... –65°C to 150°C
Pin Configuration
TOP VIEW
RST4
RT
PLL/MODE
VCC
TEMP
RST1
EN1
TOP VIEW
38 37 36 35 34 33 32
VCC
1
38 PLL/MODE
TEMP
2
37 RT
RST1
3
36 RST4
EN1
4
35 EN4
FB1
5
34 FB4
VINA
6
33 VINH
SWA
7
32 SWH
31 SWG
FB1 1
31 EN4
VINA 2
30 FB4
SWA 3
29 VINH
SWB 4
28 SWH
SWB
8
VINB 5
27 SWG
VINB
9
26 VING
VINC 10
25 VINF
SWC 11
SWD 8
24 SWF
SWD 12
27 SWE
VIND 9
23 SWE
VIND 13
26 VINE
FB2 10
22 VINE
FB2 14
25 FB3
EN2 11
21 FB3
EN2 15
24 EN3
RST2 12
20 EN3
RST2 16
23 RST3
VINC 6
39
GND
SWC 7
RST3
CT
WDO
WDI
C3
C2
C1
13 14 15 16 17 18 19
UHF PACKAGE
38-LEAD (5mm × 7mm) PLASTIC QFN
39
GND
30 VING
29 VINF
28 SWF
C1 17
22 CT
C2 18
21 WDO
C3 19
20 WDI
FE PACKAGE
38-LEAD PLASTIC TSSOP
TJMAX = 150°C, θJA = 34°C/W
EXPOSED PAD (PIN 39) IS GND, MUST BE SOLDERED TO PCB
TJMAX = 150°C, θJA = 25°C/W
EXPOSED PAD (PIN 39) IS GND, MUST BE SOLDERED TO PCB
Order Information
LEAD FREE FINISH
TAPE AND REEL
PART MARKING*
PACKAGE DESCRIPTION
TEMPERATURE RANGE
LTC3371EUHF#PBF
LTC3371EUHF#TRPBF
3371
38-Lead (5mm × 7mm) Plastic QFN
–40°C to 125°C
LTC3371IUHF#PBF
LTC3371IUHF#TRPBF
3371
38-Lead (5mm × 7mm) Plastic QFN
–40°C to 125°C
LTC3371HUHF#PBF
LTC3371HUHF#TRPBF
3371
38-Lead (5mm × 7mm) Plastic QFN
–40°C to 150°C
LTC3371EFE#PBF
LTC3371EFE#TRPBF
LTC3371FE
38-Lead Plastic TSSOP
–40°C to 125°C
LTC3371IFE#PBF
LTC3371IFE#TRPBF
LTC3371FE
38-Lead Plastic TSSOP
–40°C to 125°C
LTC3371HFE#PBF
LTC3371HFE#TRPBF
LTC3371FE
38-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 nonstandard 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/
3371fa
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3
LTC3371
Electrical Characteristics
The l denotes the specifications which apply over the specified operating
junction temperature range, otherwise specifications are at TA = 25°C. (Note 2) VCC = VINA-H = 3.3V, unless otherwise specified.
SYMBOL
PARAMETER
CONDITIONS
VCC
VCC Voltage Range
VCC(UVLO)
Undervoltage Threshold on VCC
VCC Voltage Falling
VCC Voltage Rising
IVCC(ALLOFF)
VCC Input Supply Current
All Switching Regulators in Shutdown
IVCC
VCC Input Supply Current
One Buck Active
PLL/MODE = 0V, RT = 400k,
VFB_BUCK = 0.85V
PLL/MODE = 2MHz
fOSC
Internal Oscillator Frequency
VRT = VCC, PLL/MODE = 0V
VRT = VCC, PLL/MODE = 0V
RT = 400k, PLL/MODE = 0V
MIN
l
2.7
l
l
2.325
2.425
l
l
1.8
1.75
1.8
fPLL/MODE
Synchronization Frequency
tLOW, tHIGH > 60ns
l
1
VPLL/MODE
PLL/MODE Level High
PLL/MODE Level Low
For Synchronization
For Synchronization
l
l
1.2
VRT
RT Servo Voltage
RT = 400k
l
780
180
TYP
MAX
UNITS
5.5
V
2.45
2.55
2.575
2.675
V
V
15
25
µA
50
75
µA
175
250
µA
2
2
2
2.2
2.25
2.2
MHz
MHz
MHz
3
MHz
0.4
V
V
800
820
mV
220
260
Temp Monitor
VTEMP(ROOM)
TEMP Voltage at 25°C
ΔVTEMP/°C
VTEMP Slope
OT
Overtemperature Shutdown
170
°C
OT Hyst
Overtemperature Hysteresis
10
°C
7
mV
mV/°C
1A Buck Regulators
VIN
Buck Input Voltage Range
l
2.25
5.5
V
VOUT
Buck Output Voltage Range
l
VFB
VIN
V
VIN(UVLO)
Undervoltage Threshold on VIN
l
l
1.95
2.05
2.05
2.15
2.15
2.25
V
V
IVIN
Burst Mode Operation Input Current
VFB = 0.85V (Note 4)
Forced Continuous Mode Operation Input Current ISW(BUCK) = 0µA, FB = 0V
Shutdown Input Current
18
400
0
30
600
2.5
µA
µA
µA
IFWD
PMOS Current Limit
1.9
2.3
2.7
A
VFB1
Feedback Regulation Voltage for Buck 1
l
792
800
808
mV
VFB
Feedback Regulation Voltage for Bucks 2-4
l
780
800
820
mV
50
nA
VIN Voltage Falling
VIN Voltage Rising
(Note 5)
IFB
Feedback Leakage Current
VFB = 0.85V
DMAX
Maximum Duty Cycle
VFB = 0V
RPMOS
PMOS On-Resistance
ISW = 100mA
300
mΩ
RNMOS
NMOS On-Resistance
ISW = –100mA
240
mΩ
ILEAKP
PMOS Leakage Current
EN = 0
–2
ILEAKN
NMOS Leakage Current
EN = 0
–2
tSS
Soft-Start Time
VPGOOD(FALL)
Falling PGOOD Threshold for Buck 1
% of Regulated VFB
96.8
98
99.2
%
Falling PGOOD Threshold for Bucks 2 to 4
% of Regulated VFB
93
95
97
%
PGOOD Hysteresis for Bucks 1 to 4
% of Regulated VFB
VPGOOD(HYS)
4
–50
l
100
%
2
2
1
0.3
µA
µA
ms
%
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LTC3371
Electrical Characteristics
The l denotes the specifications which apply over the specified operating
junction temperature range, otherwise specifications are at TA = 25°C. (Note 2) VCC = VINA-H = 3.3V, unless otherwise specified.
SYMBOL
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
Buck Regulators Combined
IFWD2
PMOS Current Limit
2 Buck Power Stages Combined (Note 5)
4.6
A
IFWD3
PMOS Current Limit
3 Buck Power Stages Combined (Note 5)
6.9
A
IFWD4
PMOS Current Limit
4 Buck Power Stages Combined (Note 5)
9.2
A
Interface Logic Pins (RST1-4, WDO, WDI, PLL/MODE, C1, C2, C3)
IOH
Output High Leakage Current
RST1-4, WDO 5.5V at Pin
VOL
Output Low Voltage
RST1-4, WDO 3mA into Pin
VIH
WDI Input High Threshold
l
VIL
WDI, C1, C2, C3 Input Low Threshold
l
tWDI(WIDTH)
WDI Pulse Width
l
40
VIH
PLL/MODE, C1, C2, C3 Input High Threshold
l
VCC – 0.4
VIL
PLL/MODE Input Low Threshold
l
0.1
1
µA
0.4
V
1.2
V
0.4
V
ns
V
VCC – 1.2
V
Interface Logic Pins (EN1, EN2, EN3, EN4)
VHI(ALLOFF)
Enable Rising Threshold
All Regulators Disabled
l
730
1200
mV
VHI
Enable Rising Threshold
At Least One Regulator Enabled
l
400
420
mV
VLO
Enable Falling Threshold
IEN
Enable Pin Leakage Current
340
390
EN = 3.3V
mV
1
µA
CT Timing Parameters; CT = 10nF
tWDI0
tWDI
tWDL
Time from WDO Low Until Next WDO Low
Time from Last WDI Until Next WDO Low
Watchdog Lower Boundary
CT = 10nF
10.3
6.2
12.9
12.9
15.5
l
Sec
Sec
1.30
0.77
1.62
1.62
1.95
l
Sec
Sec
40
50.6
50.6
60
65
ms
ms
CT = 10nF
CT = 10nF
l
tWDO
WDO Low Time Absent a Transition at WDI
CT = 10nF
160
202
280
ms
tRST
RST Assertion Delay
CT = 10nF
160
202
240
ms
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 LTC3371 is tested under pulsed load conditions such that
TJ ≈ TA. The LTC3371E is guaranteed to meet specifications from
0°C to 85°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
LTC3371I is guaranteed over the –40°C to 125°C operating junction
temperature range. The LTC3371H is guaranteed over the –40°C to 150°C
operating junction temperature range. High junction temperatures degrade
operating lifetimes; operating lifetime is derated for junction temperatures
greater than 125°C. 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. The junction temperature
(TJ in °C) is calculated from the ambient temperature (TA in °C) and power
dissipation (PD in Watts) according to the formula:
TJ = TA + (PD • θJA)
where θJA (in °C/W) is the package thermal impedance.
Note 3: The LTC3371 includes overtemperature protection which protects
the device during momentary overload conditions. Junction temperatures
will exceed 150°C when overtemperature protection is active. Continuous
operation above the specified maximum operating junction temperature
may impair device reliability.
Note 4: Static current, switches not switching. Actual current may be
higher due to gate charge losses at the switching frequency.
Note 5: The current limit features of this part are intended to protect the
IC from short term or intermittent fault conditions. Continuous operation
above the maximum specified pin current rating may result in device
degradation over time.
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5
LTC3371
Typical Performance Characteristics
100
3000
2500
80
POWER LOSS (mW)
EFFICIENCY (%)
70
60
50
40
Burst Mode OPERATION
VIN = 3.3V
VOUT = 1.8V
fOSC = 2MHz
L = 2.2µH
20
10
0
1
2.65
2.60
2000
1500
1000
1A BUCK
2A BUCK
3A BUCK
4A BUCK
100
1000
10
LOAD CURRENT (mA)
2.70
1A BUCK
2A BUCK
3A BUCK
4A BUCK
90
30
Burst Mode OPERATION
VIN = 3.3V
VOUT = 1.8V
fOSC = 2MHz
L = 2.2µH
500
0
4000
2.20
30
1.90
–55
VIN RISING
VIN FALLING
–25
25
20
15
VCC = 2.7V
VCC = 3.3V
VCC = 5.5V
5
5
35
65
95
TEMPERATURE (°C)
125
0
–55
155
125
100
10
1.95
–25
5
35
65
95
TEMPERATURE (°C)
3371 G04
2.20
AT LEAST ONE BUCK ENABLED
360 PLL/MODE = 2MHz
2.15
320
280
f OSC (MHz)
120
VCC = 2.7V
80
VCC = 3.3V
VCC = 5.5V
40
0
–55
–25
125
50
0
–55
155
5
35
65
95
TEMPERATURE (°C)
125
155
3371 G07
VCC = 2.7V
VCC = 3.3V
VCC = 5.5V
–25
5
35
65
95
TEMPERATURE (°C)
125
Default Oscillator Frequency
vs Temperature
2.20
2.15
2.10
2.10
2.05
2.05
2.00
1.95
1.85
1.80
–55
5
35
65
95
TEMPERATURE (°C)
VRT = VCC
2.00
1.95
1.90
VCC = 2.7V
VCC = 3.3V
VCC = 5.5V
–25
155
3371 G06
RT = 400k
1.90
155
AT LEAST ONE BUCK ENABLED
PLL/MODE = 0V
FB = 850mV
25
f OSC (MHz)
400
160
125
75
RT Programmed Oscillator
Frequency vs Temperature
200
5
35
65
95
TEMPERATURE (°C)
3371 G05
VCC Supply Current
vs Temperature
240
–25
VCC Supply Current
vs Temperature
IVCC (µA)
IVCC_ALLOFF (µA)
UV THRESHOLD (V)
2.25
ALL REGULATORS
35 IN SHUTDOWN
2.05
VCC RISING
VCC FALLING
3371 G03
40
2.00
IVCC (µA)
2.30
–55
4000
VCC Supply Current
vs Temperature
2.30
2.10
2.45
3371 G02
Buck VIN Undervoltage Threshold
vs Temperature
2.15
2.50
2.35
10
100
1000
LOAD CURRENT (mA)
1
2.55
2.40
3371 G01
6
VCC Undervoltage Threshold
vs Temperature
Buck Power Loss vs ILOAD
UV THRESHOLD (V)
Buck Efficiency vs ILOAD
TA = 25°C unless otherwise noted.
VCC = 2.7V
VCC = 3.3V
VCC = 5.5V
1.85
125
155
3371 G08
1.80
–55
–25
5
35
65
95
TEMPERATURE (°C)
125
155
3371 G09
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LTC3371
Typical Performance Characteristics
Oscillator Frequency vs VCC
4
VCC = 3.3V
1000
2.5
800
VRT = VCC
2
RT = 400k
1.95
VTEMP (mV)
3
2.05
2
1.5
600
1
200
1.85
0.5
0
3.9 4.3
VCC (V)
4.7
5.1
5.5
3371 G10
900
800
650
600
550
500
450
400
350
–55
–25
50
410
EN FALLING
125
EN RISING
400
395
390
EN FALLING
385
550
1.88
–25
5
35
65
95
TEMPERATURE (°C)
125
0
–55
155
VIN = 2.25V
VIN = 3.3V
VIN = 5.5V
1.84
350
1.82
VIN = 2.25V
VIN = 3.3V
VIN = 5.5V
50 FORCED CONTINUOUS MODE
FB = 0V
0
–55 –25
5
35
65
95
TEMPERATURE (°C)
VOUT (V)
400
250
155
2.6
3371 G16
155
VIN = 3.3V
2.5
1.80
1.78
2.3
2.2
1.76
1.72
–55
125
2.4
VIN = 2.25V
VIN = 3.3V
VIN = 5.5V
1.74
125
5
35
65
95
TEMPERATURE (°C)
PMOS Current Limit
vs Temperature
VOUT vs Temperature
IFWD (A)
450
300
–25
3371 G15
FORCED CONTINUOUS MODE
1.86 I LOAD = 0mA
500
100
20
3371 G14
Buck VIN Supply Current
vs Temperature
150
30
10
375
–55
155
155
BURST MODE OPERATION
FB = 850mV
380
5
35
65
95
TEMPERATURE (°C)
125
40
405
3371 G13
200
5
35
65
95
TEMPERATURE (°C)
Buck VIN Supply Current
vs Temperature
415
EN THRESHOLD (mV)
700
–25
3371 G12
Enable Pin Precision Threshold
vs Temperature
EN RISING
750
IDEAL VTEMP
3371 G11
ALL REGULATORS DISABLED
VCC = 3.3V
850
–200
–55
IVIN_BURST (µA)
3.5
0
250 300 350 400 450 500 550 600 650 700 750 800
RT (kΩ)
ACTUAL VTEMP
400
1.9
3.1
VTEMP vs Temperature
ILOAD = 0mA
1200 VCC = 3.3V
2.1
Enable Threshold vs Temperature
IVIN_FORCED_CONTINUOUS (µA)
1400
3.5
1.8
2.7
EN THRESHOLD (mV)
Oscillator Frequency vs RT
2.15
fOSC (MHz)
fOSC (MHz)
2.2
TA = 25°C unless otherwise noted.
–25
5
35
65
95
TEMPERATURE (°C)
125
155
3371 G17
2.1
2.0
–55
–25
5
35
65
95
TEMPERATURE (°C)
125
155
3371 G18
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LTC3371
Typical Performance Characteristics
500
450
400
350
300
90
80
350
300
250
250
–25
5
35
65
95
TEMPERATURE (°C)
125
150
–55
155
–25
5
35
65
95
TEMPERATURE (°C)
125
3371 G19
1000
EFFICIENCY (%)
POWER LOSS (mW)
700
600
500
400
300
VIN = 2.25V
VIN = 3.3V
VIN = 5.5V
200
100
0
1
1000
90
900
80 BURST
MODE
70
800
60
50
40
30
FORCED
CONTINUOUS MODE
10
0
1000
1
VIN = 2.25V
VIN = 3.3V
VIN = 5.5V
VIN = 2.25V
VIN = 3.3V
VIN = 5.5V
VOUT = 1.8V
fOSC = 2MHz
L = 2.2µH
10
100
LOAD CURRENT (mA)
3371 G22
1A Buck Efficiency vs ILOAD,
VOUT = 2.5V
EFFICIENCY (%)
1000
60
50
40
30
20
FORCED
CONTINUOUS MODE
10
0
1
VIN = 2.7V
VIN = 3.3V
VIN = 5.5V
VIN = 2.7V
VIN = 3.3V
VIN = 5.5V
VOUT = 2.5V
fOSC = 2MHz
L = 2.2µH
10
100
LOAD CURRENT (mA)
1000
3371 G25
8
700
600
500
400
300
0
1000
VIN = 2.25V
VIN = 3.3V
VIN = 5.5V
1
10
100
LOAD CURRENT (mA)
1000
3371 G24
100
BURST MODE
VOUT = 2.5V
fOSC = 2MHz
L = 2.2µH
800
70
BURST MODE
VOUT = 1.8V
fOSC = 2MHz
L = 2.2µH
100
1A Buck Power Loss vs ILOAD,
VOUT = 2.5V
900
BURST
MODE
80
1000
1A Buck Power Loss vs ILOAD,
VOUT = 1.8V
200
700
80
600
500
400
300
VIN = 2.7V
VIN = 3.3V
VIN = 5.5V
200
100
0
1
10
100
LOAD CURRENT (mA)
1A Buck Efficiency vs ILOAD,
VOUT = 3.3V
90
EFFICIENCY (%)
90
10
100
LOAD CURRENT (mA)
3371 G23
POWER LOSS (mW)
100
1
3371 G21
100
20
10
100
LOAD CURRENT (mA)
0
155
1A Buck Efficiency vs ILOAD,
VOUT = 1.8V
BURST MODE
VOUT = 1.2V
fOSC = 2MHz
L = 2.2µH
800
VIN = 2.25V
VIN = 3.3V
VIN = 5.5V
VIN = 2.25V
VIN = 3.3V
VIN = 5.5V
30
3371 G20
1A Buck Power Loss vs ILOAD,
VOUT = 1.2V
900
VOUT = 1.2V
fOSC = 2MHz
L = 2.2µH
60 FORCED
50 CONTINUOUS
MODE
40
10
POWER LOSS (mW)
150
–55
BURST
70 MODE
20
200
200
1A Buck Efficiency vs ILOAD,
VOUT = 1.2V
100
VIN = 2.25V
VIN = 3.3V
VIN = 5.5V
400
RDS(ON) (mΩ)
RDS(ON) (mΩ)
450
VIN = 2.25V
VIN = 3.3V
VIN = 5.5V
NMOS RDS(ON) vs Temperature
EFFICIENCY (%)
550
PMOS RDS(ON) vs Temperature
TA = 25°C unless otherwise noted.
BURST
70 MODE
60
VIN = 4.2V
VIN = 5.5V
VIN = 4.2V
VIN = 5.5V
50
40
30
20
FORCED
CONTINUOUS MODE
10
1000
3371 G26
0
1
VOUT = 3.3V
fOSC = 2MHz
L = 2.2µH
10
100
LOAD CURRENT (mA)
1000
3371 G27
3371fa
For more information www.linear.com/LTC3371
LTC3371
Typical Performance Characteristics
EFFICIENCY (%)
700
600
500
400
300
200
1
80
70 FORCED
CONTINUOUS
60 MODE
50
70
VOUT = 1.8V
fOSC = 2MHz
L = 2.2µH
40
VIN = 2.25V
VIN = 3.3V
VIN = 5.5V
VIN = 2.25V
VIN = 3.3V
VIN = 5.5V
30
10
10
100
LOAD CURRENT (mA)
0
1000
1
10
100
LOAD CURRENT (mA)
100
100
90
40
VIN = 2.25V
VIN = 3.3V
VIN = 5.5V
VIN = 2.25V
VIN = 3.3V
VIN = 5.5V
30
20
10
0
1
10
100
LOAD CURRENT (mA)
EFFICIENCY (%)
EFFICIENCY (%)
VOUT = 1.8V
fOSC = 2MHz
L = 2.2µH
60
50 FORCED
CONTINUOUS
40
MODE
30
0
1000
1
50
FORCED
CONTINUOUS
MODE
40
30
20
10
1
VIN = 2.7V
VIN = 3.3V
VIN = 5.5V
VIN = 2.7V
VIN = 3.3V
VIN = 5.5V
10
100
1000
LOAD CURRENT (mA)
3371 G34
EFFICIENCY (%)
EFFICIENCY (%)
60
1000
4A Buck Efficiency vs ILOAD,
VOUT = 1.8V
VOUT = 2.5V
fOSC = 2MHz
L = 2.2µH
VIN = 2.7V
VIN = 3.3V
VIN = 5.5V
VIN = 2.7V
VIN = 3.3V
VIN = 5.5V
10
100
LOAD CURRENT (mA)
BURST MODE
60
50 FORCED
CONTINUOUS
40 MODE
30
20
10
0
1000
1
1A Buck Efficiency vs Frequency
(Forced Continuous Mode)
100
90
90
40
20
10
0
VOUT = 1.8V
VIN = 3.3V
fOSC = 1MHz, L = 3.3µH
fOSC = 2MHz, L = 2.2µH
fOSC = 3MHz, L = 1µH
fOSC = 1MHz, L = 3.3µH
fOSC = 2MHz, L = 2.2µH
fOSC = 3MHz, L = 1µH
30
1
VIN = 2.25V
VIN = 3.3V
80
BURST MODE
FORCED
60 CONTINUOUS
50 MODE
10
100
LOAD CURRENT (mA)
VIN = 2.25V
VIN = 3.3V
VIN = 5.5V
VIN = 2.25V
VIN = 3.3V
VIN = 5.5V
3371 G33
100
70
VOUT = 1.8V
fOSC = 2MHz
L = 2.2µH
10
100
1000
LOAD CURRENT (mA)
3371 G32
80
VOUT = 2.5V
fOSC = 2MHz
L = 2.2µH
10
100
LOAD CURRENT (mA)
70
1A Buck Efficiency vs ILOAD
(Across Operating Frequency)
70
1
80
BURST MODE
10
BURST MODE
80
0
100
70
20
4A Buck Efficiency vs ILOAD,
VOUT = 2.5V
90
VIN = 2.7V
VIN = 3.3V
VIN = 5.5V
VIN = 2.7V
VIN = 3.3V
VIN = 5.5V
30
90
3371 G31
100
40
VOUT = 2.5V
fOSC = 2MHz
L = 2.2µH
3371 G30
3A Buck Efficiency vs ILOAD,
VOUT = 2.5V
80
BURST MODE
FORCED
60 CONTINUOUS
MODE
50
FORCED
CONTINUOUS
MODE
50
0
90
70
60
3371 G29
3A Buck Efficiency vs ILOAD,
VOUT = 1.8V
80
BURST MODE
10
1000
3371 G28
2A Buck Efficiency vs ILOAD,
VOUT = 2.5V
20
EFFICIENCY (%)
0
90
80
20
VIN = 4.2V
VIN = 5.5V
100
100
BURST MODE
90
EFFICIENCY (%)
800
POWER LOSS (mW)
100
BURST MODE
VOUT = 3.3V
fOSC = 2MHz
L = 2.2µH
900
2A Buck Efficiency vs ILOAD,
VOUT = 1.8V
1000
3371 G35
EFFICIENCY (%)
1000
1A Buck Power Loss vs ILOAD,
VOUT = 3.3V
TA = 25°C unless otherwise noted.
70
VIN = 5.5V
60
50
40
30
20
VOUT = 1.8V
ILOAD = 100mA
L = 3.3µH
10
0
1
1.2 1.4 1.6 1.8 2 2.2 2.4 2.6 2.8
FREQUENCY (MHz)
3
3371 G36
3371fa
For more information www.linear.com/LTC3371
9
LTC3371
Typical Performance Characteristics
100
100
VIN = 3.3V
90
60
50
40
30
20
50
40
30
VOUT = 1.8V
ILOAD = 200mA
L = 3.3µH
10
0
60
20
1
1.2 1.4 1.6 1.8 2 2.2 2.4 2.6 2.8
FREQUENCY (MHz)
0
3
1
1.815
1.812
VIN = 5.5V
1.2 1.4 1.6 1.8 2 2.2 2.4 2.6 2.8
FREQUENCY (MHz)
3
1.78
DROPOUT
1
10
100
LOAD CURRENT (mA)
1000
3371 G39
1A Buck Regulator No-Load Startup
Transient (Burst Mode Operation)
VIN = 3.3V
VOUT = 1.8V
fOSC = 2MHz
L = 2.2µH
VIN = 3.3V
1.8
VIN = 2.25V
1.796
VOUT
500mV/DIV
1.805
VOUT (V)
1.804
VOUT (V)
1.792
1.81
1.808
ILOAD = 100mA
1.8
INDUCTOR
CURRENT
500mA/DIV
ILOAD = 500mA
1.795
1.792
EN
2V/DIV
1.79
1.788
DROPOUT
1.784
1.78
VIN = 2.25V
1.796
1A Buck Regulator Line Regulation
(Forced Continuous Mode)
1.82
fOSC = 2MHz
L = 2.2µH
1.8
3371 G38
4A Buck Regulator Load Regulation
(Forced Continuous Mode)
1.82
VIN = 5.5V
VIN = 3.3V
1.804
1.784
3371 G37
1.816
1.808
1.788
VOUT = 1.8V
VIN = 3.3V
L = 3.3µH
10
fOSC = 2MHz
L = 2.2µH
1.812
ILOAD = 20mA
70
EFFICIENCY (%)
70
1.82
1.816
ILOAD = 500mA
80
VIN = 5.5V
1A Buck Regulator Load Regulation
(Forced Continuous Mode)
ILOAD = 100mA
90
VIN = 2.25V
80
EFFICIENCY (%)
1A Buck Efficiency vs Frequency
(Forced Continuous Mode)
POWER LOSS (mW)
1A Buck Efficiency vs Frequency
(Forced Continuous Mode)
TA = 25°C unless otherwise noted.
1
10
100
LOAD CURRENT (mA)
1.785
1000
1.78
200µs/DIV
2.5
3
3.5
4
VIN (V)
3371 G40
1A Buck Regulator No-Load Startup
Transient (Forced Continuous Mode)
4.5
5
5.5
3371 G41
4A Buck Regulator No-Load Startup
Transient (Burst Mode Operation)
VIN = 3.3V
VOUT = 1.8V
3371 G42
4A Buck Regulator No-Load Startup
Transient (Forced Continuous Mode)
VIN = 3.3V
VOUT = 1.8V
VIN = 3.3V
VOUT = 1.8V
VOUT
500mV/DIV
VOUT
500mV/DIV
VOUT
500mV/DIV
INDUCTOR
CURRENT
500mA/DIV
INDUCTOR
CURRENT
500mA/DIV
INDUCTOR
CURRENT
500mA/DIV
EN
2V/DIV
EN
2V/DIV
EN
2V/DIV
200µs/DIV
10
3371 G43
200µs/DIV
3371 G44
200µs/DIV
3371 G45
3371fa
For more information www.linear.com/LTC3371
LTC3371
Typical Performance Characteristics
1A Buck Regulator, Transient
Response (Burst Mode Operation)
TA = 25°C unless otherwise noted.
1A Buck Regulator, Transient
Response (Forced Continuous Mode)
VOUT
100mV/DIV
AC-COUPLED
VOUT
100mV/DIV
AC-COUPLED
INDUCTOR
CURRENT
200mA/DIV
INDUCTOR
CURRENT
200mA/DIV
0mA
0mA
50µs/DIV
3371 G46
50µs/DIV
LOAD STEP = 100mA TO 700mA
VIN = 3.3V
VOUT = 1.8V
3371 G47
LOAD STEP = 100mA TO 700mA
VIN = 3.3V
VOUT = 1.8V
4A Buck Regulator, Transient
Response (Burst Mode Operation)
4A Buck Regulator, Transient
Response (Forced Continuous Mode)
VOUT
100mV/DIV
AC-COUPLED
VOUT
100mV/DIV
AC-COUPLED
INDUCTOR
CURRENT
1A/DIV
INDUCTOR
CURRENT
1A/DIV
0mA
0mA
50µs/DIV
LOAD STEP = 400mA TO 2.8A
VIN = 3.3V
VOUT = 1.8V
3371 G48
50µs/DIV
3371 G49
LOAD STEP = 400mA TO 2.8A
VIN = 3.3V
VOUT = 1.8V
3371fa
For more information www.linear.com/LTC3371
11
LTC3371
Pin Functions
(QFN/TSSOP)
FB1 (Pin 1/Pin 5): Buck Regulator 1 Feedback Pin. Receives
feedback by a resistor divider connected across the output.
VINA (Pin 2/Pin 6): Power Stage A Input Supply. Bypass
to GND with a 10µF or larger ceramic capacitor.
SWA (Pin 3/Pin 7): Power Stage A Switch Node. External
inductor connects to this pin.
SWB (Pin 4/Pin 8): Power Stage B Switch Node. External
inductor connects to this pin.
VINB (Pin 5/Pin 9): Power Stage B Input Supply. Bypass
to GND with a 10µF or larger ceramic capacitor.
VINC (Pin 6/Pin 10): Power Stage C Input Supply. Bypass
to GND with a 10µF or larger ceramic capacitor.
SWC (Pin 7/Pin 11): Power Stage C Switch Node. External
inductor connects to this pin.
SWD (Pin 8/Pin 12): Power Stage D Switch Node. External
inductor connects to this pin.
VIND (Pin 9/Pin 13): Power Stage D Input Supply. Bypass
to GND with a 10µF or larger ceramic capacitor.
FB2 (Pin 10/Pin 14): Buck Regulator 2 Feedback Pin.
Receives feedback by a resistor divider connected across
the output. In configurations where Buck 2 is not used,
FB2 should be tied to ground.
EN2 (Pin 11/Pin 15): Buck Regulator 2 Enable Input.
Active high. In configurations where Buck 2 is not used,
tie EN2 to ground. Do not float.
C2 (Pin 14/Pin 18): Configuration Control Input Bit. With
C1 and C3, C2 configures the Buck output current power
stage combinations. C2 should either be tied to VCC or
ground. Do not float.
C3 (Pin 15/Pin 19): Configuration Control Input Bit. With
C1 and C2, C3 configures the Buck output current power
stage combinations. C3 should either be tied to VCC or
ground. Do not float.
WDI (Pin 16/Pin 20): Watchdog Timer Input. The WDI pin
must be toggled either low to high or high to low every
1.62 seconds. Failure to toggle WDI results in the WDO
pin being pulled low for 202ms. All times correspond to
a 10nF capacitor on the CT pin.
WDO (Pin 17/Pin 21): Watchdog Timer Output. Open-drain
output. WDO is pulled low for 202ms during a watchdog
timeout period. The WDO pin pulls low if the WDI input
does not transition in less than 1.62 seconds since its last
transition or 12.9 seconds after a watchdog timeout period.
A VCC UVLO event resets the watchdog timer and WDO
asserts itself low for the 202ms watchdog timeout period.
All times correspond to a 10nF capacitor on the CT pin.
CT (Pin 18/Pin 22): Timing Capacitor Pin. A capacitor
connected to GND sets a time constant which is scaled
for use by the WDI, WDO, and RST1-4 pins.
RST3 (Pin 19/Pin 23): Buck Regulator 3 Reset Pin
(Active Low). Open-drain output. When Buck 3 is disabled
or its regulated output voltage is more than 5% below its
programmed level, this pin is driven low. Assertion delay
is scaled by the CT capacitor.
RST2 (Pin 12/Pin 16): Buck Regulator 2 Reset Pin
(Active Low). Open-drain output. When Buck 2 is disabled
or its regulated output voltage is more than 5% below its
programmed level, this pin is driven low. Assertion delay
is scaled by the CT capacitor.
EN3 (Pin 20/Pin 24): Buck Regulator 3 Enable Input.
Active high. In configurations where Buck 3 is not used,
tie EN3 to ground. Do not float.
C1 (Pin 13/Pin 17): Configuration Control Input Bit. With
C2 and C3, C1 configures the Buck output current power
stage combinations. C1 should either be tied to VCC or
ground. Do not float.
FB3 (Pin 21/Pin 25): Buck Regulator 3 Feedback Pin.
Receives feedback by a resistor divider connected across
the output. In configurations where Buck 3 is not used,
FB3 should be tied to ground.
12
3371fa
For more information www.linear.com/LTC3371
LTC3371
Pin Functions
(QFN/TSSOP)
VINE (Pin 22/Pin 26): Power Stage E Input Supply. Bypass
to GND with a 10µF or larger ceramic capacitor.
VING (Pin 26/Pin 30): Power Stage G Input Supply. Bypass
to GND with a 10µF or larger ceramic capacitor.
PLL/MODE (Pin 34/Pin 38): Oscillator Synchronization
and Buck Mode Select Pin. Driving PLL/MODE with an
external clock signal synchronizes all switches to the
applied frequency, and the buck converters operate in
forced continuous mode. The slope compensation is automatically adapted to the external clock frequency. The
absence of an external clock signal enables the frequency
programmed by the RT pin. When not synchronizing to
an external clock this input determines how the LTC3371
operates at light loads. Pulling this pin to ground selects
Burst Mode operation. Tying this pin to VCC invokes forced
continuous mode operation. Do not float.
SWG (Pin 27/Pin 31): Power Stage G Switch Node. External
inductor connects to this pin.
VCC (Pin 35/Pin 1): Internal Bias Supply. Bypass to GND
with a 10µF or larger ceramic capacitor.
SWH (Pin 28/Pin 32): Power Stage H Switch Node. External
inductor connects to this pin.
TEMP (Pin 36/Pin 2): Temperature Indication Pin. TEMP
outputs a voltage of 220mV (typical) at 25°C. The TEMP
voltage increases by 7mV/°C (typical) at higher temperatures giving an external indication of the LTC3371 internal
die temperature.
SWE (Pin 23/Pin 27): Power Stage E Switch Node. External
inductor connects to this pin.
SWF (Pin 24/Pin 28): Power Stage F Switch Node. External
inductor connects to this pin.
VINF (Pin 25/Pin 29): Power Stage F Input Supply. Bypass
to GND with a 10µF or larger ceramic capacitor.
VINH (Pin 29/Pin 33/): Power Stage H Input Supply. Bypass
to GND with a 10µF or larger ceramic capacitor.
FB4 (Pin 30/Pin 34): Buck Regulator 4 Feedback Pin.
Receives feedback by a resistor divider connected across
the output.
EN4 (Pin 31/Pin 35): Buck Regulator 4 Enable Input.
Active high. Do not float.
RST4 (Pin 32/Pin 36): Buck Regulator 4 Reset Pin
(Active Low). Open-drain output. When Buck 4 is disabled
or its regulated output voltage is more than 5% below its
programmed level, this pin is driven low. Assertion delay
is scaled by the CT capacitor.
RT (Pin 33/Pin 37): Oscillator Frequency Pin. This pin
provides two modes of setting the switching frequency.
Connecting a resistor from RT to ground sets the switching
frequency based on the resistor value. If RT is tied to VCC
the internal 2MHz oscillator is used. Do not float.
RST1 (Pin 37/Pin 3): Buck Regulator 1 Reset Pin (Active
Low). Open-drain output. When Buck 1 is disabled or
its regulated output voltage is more than 2% below its
programmed level, this pin is driven low. Assertion delay
is scaled by the CT capacitor.
EN1 (Pin 38/Pin 4): Buck Regulator 1 Enable Input. Active
high. Do not float.
GND (Exposed Pad Pin 39): Ground. The exposed pad
must be connected to a continuous printed circuit board
ground plane directly under the LTC3371.
3371fa
For more information www.linear.com/LTC3371
13
LTC3371
Block Diagram
1
(Pin numbers reflect TSSOP package)
VCC
BANDGAP OT
37
38
RT
PLL/MODE
4
REF
UVLO
UV
TEMP
MONITOR
TEMP
2
CLK
OSCILLATOR
MODE
SD
22
20
21
3
CT
CT
OSCILLATOR
WDI
WDO
WATCHDOG TIMER
STATE MACHINE
CT
CLOCK
RST1
DELAY
16
RST2
RST LOGIC
DELAY
23
4 PGOOD
RST3
DELAY
36
VINA
RST4
DELAY
1A POWER
STAGE A
REF
1A POWER
STAGE B
SWA
VINB
SD
CLK
MODE
4
VINC
1A POWER
STAGE C
VINB
4
5
EN1
FB1
14
EN2
FB2
1A POWER
STAGE D
25
EN3
FB3
1A POWER
STAGE E
35
34
FB4
1A POWER
STAGE F
BUCK REGULATOR 3
CONTROL
1A POWER
STAGE G
CONFIGURATION LINES
17
14
C3
C2
18
SWF
VING
BUCK REGULATOR 4
CONTROL
C1
SWE
VINF
VING
EN4
SWD
VINE
BUCK REGULATOR 2
CONTROL
VINE
24
SWC
VIND
BUCK REGULATOR 1
CONTROL
VIND
15
SWB
19
SWG
VINH
GND
(EXPOSED PAD)
39
1A POWER
STAGE H
SWH
6
7
9
8
10
11
13
12
26
27
29
28
30
31
33
32
3371 BD
3371fa
For more information www.linear.com/LTC3371
LTC3371
Operation
Buck Switching Regulators
The LTC3371 contains eight monolithic 1A synchronous
buck switching channels. These are controlled by up to
four current mode regulator controllers. All of the switching regulators are internally compensated and need only
external feedback resistors to set the output voltage. The
switching regulators offer two operating modes: Burst
Mode operation (PLL/MODE = LOW) for higher efficiency
at light loads and forced continuous PWM mode (PLL/
MODE = HIGH or switching) for lower noise at light loads.
In Burst Mode operation at light loads, the output capacitor
is charged to a voltage slightly higher than its regulation
point. The regulator then goes into a sleep state, during
which time the output capacitor provides the load current.
In sleep most of the regulator’s circuitry is powered down,
helping conserve input power. When the output capacitor droops below its programmed value, the circuitry is
powered on and another burst cycle begins. The sleep
time decreases as load current increases. In Burst Mode
operation, the regulator bursts at light loads whereas at
higher loads it operates at constant frequency PWM mode
operation. In forced continuous mode, the oscillator runs
continuously and the buck switch currents are allowed
to reverse under very light load conditions to maintain
regulation. This mode allows the buck to run at a fixed
frequency with minimal output ripple.
Each buck switching regulator can operate at an independent VIN voltage and has its own FB and EN pin to maximize flexibility. The enable pins have two different enable
threshold voltages that depend on the operating state of
the LTC3371. With all regulators disabled, the enable pin
threshold is set to 730mV (typical). Once any regulator
is enabled, the enable pin thresholds of the remaining
regulators are set to a bandgap-based 400mV and the EN
pins are each monitored by a precision comparator. This
precision EN threshold may be used to provide eventbased sequencing via feedback from other previously
enabled regulators. All buck regulators have forward and
reverse-current limiting, soft-start to limit inrush current
during start-up and short-circuit protection.
The buck switching regulators are phased in 90° steps to
reduce noise and input ripple. The phase step determines
the fixed edge of the switching sequence, which is when
the PMOS turns on. The PMOS off (NMOS on) phase is
subject to the duty cycle demanded by the regulator. Buck 1
is set to 0°, Buck 2 is set to 90°, Buck 3 is set to 270°, and
Buck 4 is set to180°. In shutdown all SW nodes are high
impedance. The buck regulator enable pins may be tied
to VOUT voltages through a resistor divider, to program
power-up sequencing.
The buck switching regulators feature a controlled shutdown scheme where the inductor current ramps down to
zero through the NMOS switch. If any event causes the
buck regulator to shut down (EN = LOW, OT, VINA-H or VCC
UVLO) the NMOS switch turns on until the inductor current
reaches 0mA (typical). Then, the switch pin becomes Hi-Z.
Buck Regulators with Combined Power Stages
Up to four adjacent buck regulators may be combined in
a master-slave configuration by setting the configuration
via the C1, C2, and C3 pins. These pins should either be
tied to ground or pin strapped to VCC in accordance with
the desired configuration code (Table 1). Any combined
SW pins must be tied together, as must any of the combined VIN pins. EN1 and FB1 are utilized by Buck 1, EN2
and FB2 by Buck 2, EN3 and FB3 by Buck 3, and EN4 and
FB4 by Buck 4. If any buck is not used or is not available
in the desired configuration, then the associated FB and
EN pins must be tied to ground.
Any available combination of 2, 3, or 4 adjacent buck
regulators serve to provide up to either 2A, 3A, or 4A
of average output load current. For example, code 110
(C3C2C1) configures Buck 1 to operate as a 4A regulator through VIN/SW pairs A, B, C, and D, while Buck 2 is
disabled, Buck 3 operates as a 1A regulator through VIN/
SW pair E, and Buck 4 operates as a 3A regulator through
VIN/SW pairs F, G, and H.
3371fa
For more information www.linear.com/LTC3371
15
LTC3371
Operation
Table 1. Master Slave Program Combinations (Each Letter
Corresponds to a VIN and SW Pair)
PROGRAM
CODE
C3C2C1
BUCK 1
BUCK 2
BUCK 3
BUCK 4
000
AB
CD
EF
GH
001
ABC
D
EF
GH
010
ABC
D
E
FGH
011
ABCH
D
E
FG
100
ABC
DE
Not Used
FGH
101
ABCD
Not Used
EF
GH
110
ABCD
Not Used
E
FGH
111
ABCD
Not Used
Not Used
EFGH
Power Failure Reporting Via RST Pins
Power failure conditions are reported back by each buck’s
associated RST pin. Each buck switching regulator has an
internal power good (PGOOD) signal. When the regulated
output voltage of an enabled switcher falls below 98% for
Buck 1 or 95% for Bucks 2-4 of its programmed value,
the PGOOD signal is pulled low. If any PGOOD signal
stays low for greater than 100µs, then the associated
RST pin is pulled low, indicating to a microprocessor that
a power failure fault has occurred. The 100µs filter time
prevents the pin from being pulled low due to a transient.
The PGOOD signal has a 0.3% hysteresis such that when
the regulated output voltage of an enabled switcher rises
above 98.3% or 95.3%, respectively, of its programmed
value, the PGOOD signal transitions high.
Once an enabled regulator has its output PGOOD for 202ms
(typical, CT = 10nF) its associated RST output goes Hi-Z.
Any disabled or inactive switchers will assert a RST low.
Temperature Monitoring and Overtemperature
Protection
To prevent thermal damage to the LTC3371 and its surrounding components, the LTC3371 incorporates an
overtemperature (OT) function. When the LTC3371 die
temperature reaches 170°C (typical) all enabled buck
switching regulators are shut down and remain in shutdown
until the die temperature falls to 160°C (typical).
16
The temperature may be read back by the user by sampling
the TEMP pin analog voltage. The temperature, T, indicated
by the TEMP pin voltage is given by:
T=
VTEMP – 45mV
•1°C
7mV
(1)
If none of the buck switching regulators are enabled, then
the temperature monitor is also shut down to further
reduce quiescent current.
Programming the Operating Frequency
Selection of the operating frequency is a trade-off between
efficiency and component size. High frequency operation
allows the use of smaller inductor and capacitor values.
Operation at lower frequencies improves efficiency by
reducing internal gate charge losses but requires larger
inductance values and/or capacitance to maintain low
output voltage ripple.
The operating frequency for all of the LTC3371 regulators
is determined by an external resistor that is connected
between the RT pin and ground. The operating frequency
can be calculated using the following equation:
fOSC =
8 •1011 • ΩHz
RT
(2)
While the LTC3371 is designed to function with operating frequencies between 1MHz and 3MHz, it has safety
clamps that will prevent the oscillator from running faster
than 4MHz (typical) or slower than 250kHz (typical). Tying
the RT pin to VCC sets the oscillator to the default internal
operating frequency of 2MHz (typical).
The LTC3371’s internal oscillator can be synchronized
through an internal PLL circuit to an external frequency
by applying a square wave clock signal to the PLL/MODE
pin. During synchronization, the top MOSFET turn-on of
Buck regulator 1 is phase locked to the rising edge of
the external frequency source. All other buck switching
regulators are locked to the appropriate phase of the
external frequency source (see Buck Switching Regulators).
3371fa
For more information www.linear.com/LTC3371
LTC3371
Operation
The synchronization frequency range is 1MHz to 3MHz.
A synchronization signal on the PLL/MODE pin will force
all active buck switching regulators to operate in forced
continuous mode PWM.
Windowed Watchdog Timer
A standard watchdog function is used to ensure that the
system is in a valid state by continuously monitoring the
microprocessor’s activity. The microprocessor must toggle
the logic state of the WDI pin periodically in order to clear
the watchdog timer. The WDI pin reset is read only on a
WDI falling edge, such that a single reset signal may be
asserted by pulsing the WDI pin for a time greater than
the minimum pulse width. If timeout occurs, the LTC3371
asserts a WDO low for the reset timeout period, issuing a
system reset. Once the reset timeout completes, WDO is
released to go high and the watchdog timer starts again.
During power-up, the watchdog timer initiates in the
timeout state with WDO asserted low. As soon as the
reset timer times out, WDO goes high and the watchdog
timer is started.
The LTC3371 implements a windowed watchdog function
by adding a lower boundary condition to the standard
watchdog function. If the WDI input receives a falling edge
prior to the watchdog lower boundary, the part considers
this a watchdog failure, and asserts WDO low (releasing
again after the reset timeout period as described above).
This will again be followed by another lower boundary
time period.
Choosing the CT Capacitor
For example, using a standard capacitor value of 10nF
gives a 202ms watchdog timeout period. Further, the other
watchdog timing periods scale with tWDO. The watchdog
lower boundary time (tWDL) scales as precisely 1/4 of
tWDO, the watchdog upper boundary time following the
previous WDI pulse scales as eight times that of tWDO, and
the watchdog upper boundary time following a watchdog
timeout scales as 64 times that of tWDO. Finally the RST
assertion delay will scale to the same time as tWDO.
These timing periods are illustrated in Figure 1. Each WDO
low period is equal to the time period t2-t1 (202ms for a
10nF CT capacitor, typical). If a WDI falling edge occurs
before the watchdog lower boundary, indicated by t3-t2
(50.6ms for a 10nF CT capacitor, typical), then another
watchdog timeout period occurs. If a WDI falling edge
occurs after the watchdog lower boundary (t4), then the
watchdog counter resets, beginning with another watchdog lower boundary period. In the case where a WDI low
transition is not detected by the specified time another
watchdog timeout period is initiated. This time is indicated
by t5-t4 (1.62s for a 10nF CT capacitor, typical). If a WDI
low transition is not detected within the specified time following a watchdog timeout period, then another watchdog
timeout period is initiated. This time is indicated by t7-t6
(12.9s for a 10nF CT capacitor, typical).
WDO
WDI
3371 F01
t1 t2 t3
t4
t5
t6
t7
Figure 1. WDO Timing Parameters
The watchdog timeout period is adjustable and can be
optimized for software execution. The watchdog timeout
period is adjusted by connecting a capacitor between CT
and ground. Given a specified watchdog timeout period,
the capacitor is determined by:
CT = tWDO • 49.39[nF/s]
(3)
3371fa
For more information www.linear.com/LTC3371
17
LTC3371
Applications Information
Buck Switching Regulator Output Voltage and
Feedback Network
The output voltage of the buck switching regulators is
programmed by a resistor divider connected from the
switching regulator’s output to its feedback pin and is
given by VOUT = VFB(1 + R2/R1) as shown in Figure 2.
Typical values for R1 range from 40k to 1M. The buck
regulator transient response may improve with optional
capacitor, CFF, that helps cancel the pole created by the
feedback resistors and the input capacitance of the FB pin.
Experimentation with capacitor values between 2pF and
22pF may improve transient response.
VOUT
BUCK
SWITCHING
REGULATOR
CFF
R2
FB
R1
+
COUT
3371 F02
OPTIONAL
Figure 2. Feedback Components
Buck Regulators
All four buck regulators are designed to be used with
inductors ranging from 1µH to 3.3µH depending on the
lowest switching frequency at which the buck regulator
must operate. When operating at 1MHz a 3.3µH inductor
should be used, while at 3MHz a 1µH inductor may be
used, or a higher value inductor may be used if reduced
current ripple is desired. Table 2 shows some recommended inductors for the buck regulators. The bucks are
compensated to operate across the range of possible VIN
and VOUT voltages when the appropriate inductance is
used for the desired switching frequency.
The input supply should be decoupled with a 10µF capacitor
while the output should be decoupled with a 22µF capacitor. Refer to the Capacitor Selection section for details on
selecting a proper capacitor.
Combined Buck Power Stages
The LTC3371 has eight power stages that can handle average load currents of 1A each. These power stages may be
combined in any one of eight possible combinations, via
18
the C1, C2, and C3 pins (see Table 1). Tables 3, 4, and 5
show recommended inductors for the combined power
stage configurations.
The input supply should be decoupled with a 22µF capacitor
while the output should be decoupled with a 47µF capacitor for a 2A combined buck regulator. Likewise for 3A and
4A configurations the input and output capacitance must
be scaled up to account for the increased load. Refer to
the Capacitor Selection section for details on selecting a
proper capacitor.
In some cases it may be beneficial to use more power
stages than needed to achieve increased efficiency of the
active regulators. In general the efficiency will improve by
adding stages for any regulator running close to what the
rated load current would be without the additional stage.
For example, if the application requires a 1A regulator that
supplies close to 1A at a high duty cycle, a 3A regulator
that only peaks at 3A but averages a lower current, and
a 2A regulator that runs at 1.5A at a high duty cycle, better efficiency may be achieved by using the 3A, 3A, 2A
configuration.
Input and Output Decoupling Capacitor Selection
The LTC3371 has individual input supply pins for each
buck power stage and a separate VCC pin that supplies
power to all top level control and logic. Each of these
pins must be decoupled with low ESR capacitors to GND.
These capacitors must be placed as close to the pins as
possible. Ceramic dielectric capacitors are a good compromise between high dielectric constant and stability versus
temperature and DC bias. Note that the capacitance of a
capacitor deteriorates at higher DC bias. It is important
to consult manufacturer data sheets and obtain the true
capacitance of a capacitor at the DC bias voltage that it
will be operated at. For this reason, avoid the use of Y5V
dielectric capacitors. The X5R/X7R dielectric capacitors
offer good overall performance.
The input supply voltage Pins 35/1, 2/6, 5/9, 6/10, 9/13,
22/26, 25/29, 26/30, and 29/33 (QFN/TSSOP packages)
all need to be decoupled with at least 10µF capacitors. If
power stages are combined the supplies should be shorted
with as short of a trace as possible, and the decoupling
capacitor should be scaled accordingly.
3371fa
For more information www.linear.com/LTC3371
LTC3371
Applications Information
Table 2. Recommended Inductors for 1A Buck Regulators
PART NUMBER
L (µH)
MAX IDC (A)
MAX DCR (mΩ)
SIZE IN mm (L × W × H)
1.0
3
38
3 × 3.6 × 1.2
1239AS-H-1R0N
1
2.5
65
2.5 × 2.0 × 1.2
XFL4020-222ME
2.2
3.5
23.5
4 × 4 × 2.1
1277AS-H-2R2N
2.2
2.6
84
3.2 × 2.5 × 1.2
IHLP1212BZER2R2M-11
2.2
3
46
3 × 3.6 × 1.2
XFL4020-332ME
3.3
2.8
38.3
4 × 4 × 2.1
IHLP1212BZER3R3M-11
3.3
2.7
61
3 × 3.6 × 1.2
SIZE IN mm (L × W × H)
IHLP1212ABER1R0M-11
MANUFACTURER
Vishay
Toko
CoilCraft
Toko
Vishay
CoilCraft
Vishay
Table 3. Recommended Inductors for 2A Buck Regulators
PART NUMBER
L (µH)
MAX IDC (A)
MAX DCR (mΩ)
XFL4020-102ME
1.0
5.1
11.9
4 × 4 × 2.1
1
5
27
4.45 × 4.06 × 1.8
XAL4020-222ME
2.2
5.6
38.7
4 × 4 × 2.1
FDV0530-2R2M
2.2
5.3
15.5
6.2 × 5.8 × 3
IHLP2020BZER2R2M-11
2.2
5
37.7
5.49 × 5.18 × 2
XAL4030-332ME
3.3
5.5
28.6
4 × 4 × 3.1
FDV0530-3R3M
3.3
4.1
34.1
6.2 × 5.8 × 3
SIZE IN mm (L × W × H)
74437324010
MANUFACTURER
CoilCraft
Wurth Elektronik
CoilCraft
Toko
Vishay
CoilCraft
Toko
Table 4. Recommended Inductors for 3A Buck Regulators
PART NUMBER
L (µH)
MAX IDC (A)
MAX DCR (mΩ)
MANUFACTURER
XAL4020-102ME
1.0
8.7
14.6
4 × 4 × 2.1
FDV0530-1R0M
1
8.4
11.2
6.2 × 5.8 × 3
XAL5030-222ME
2.2
9.2
14.5
5.28 × 5.48 × 3.1
IHLP2525CZER2R2M-01
2.2
8
20
6.86 × 6.47 × 3
Vishay
74437346022
2.2
6.5
20
7.3 × 6.6 × 2.8
Wurth Elektonik
XAL5030-332ME
3.3
8.7
23.3
5.28 × 5.48 × 3.1
SPM6530T-3R3M
3.3
7.3
27
7.1 × 6.5 × 3
CoilCraft
Toko
CoilCraft
CoilCraft
TDK
Table 5. Recommended Inductors for 4A Buck Regulators
PART NUMBER
XAL5030-122ME
SPM6530T-1R0M120
XAL5030-222ME
L (µH)
MAX IDC (A)
MAX DCR (mΩ)
SIZE IN mm (L × W × H)
1.2
12.5
9.4
5.28 × 5.48 × 3.1
1
14.1
7.81
7.1 × 6.5 × 3
2.2
9.2
14.5
5.28 × 5.48 × 3.1
SPM6530T-2R2M
2.2
8.4
19
7.1 × 6.5 × 3
IHLP2525EZER2R2M-01
2.2
13.6
20.9
6.86 × 6.47 × 5
XAL6030-332ME
3.3
8
20.81
6.36 × 6.56 × 3.1
FDVE1040-3R3M
3.3
9.8
10.1
11.2 × 10 × 4
MANUFACTURER
CoilCraft
TDK
CoilCraft
TDK
Vishay
CoilCraft
Toko
3371fa
For more information www.linear.com/LTC3371
19
LTC3371
Applications Information
PCB Considerations
should be minimized to reduce radiated EMI and parasitic
coupling. Due to the large voltage swing of the switching
nodes, high input impedance sensitive nodes, such as the
feedback nodes, should be kept far away or shielded from
the switching nodes or poor performance could result.
When laying out the printed circuit board, the following list
should be followed to ensure proper operation of the LTC3371:
1. The exposed pad of the package (Pin 39) should connect
directly to a large ground plane to minimize thermal and
electrical impedance.
2. Each of the input supply pins should have a decoupling
capacitor.
3. The connections to the switching regulator input supply
pins and their respective decoupling capacitors should
be kept as short as possible. The GND side of these
capacitors should connect directly to the ground plane
of the part. These capacitors provide the AC current
to the internal power MOSFETs and their drivers. It is
important to minimize inductance from these capacitors
to the VIN pins of the LTC3371.
4. The switching power traces connecting SWA, SWB,
SWC, SWD, SWE, SWF, SWG, and SWH to the inductors
Typical Applications
2.2µH
47µF
6. In a multiple power stage buck regulator application
the trace length of switch nodes to the inductor must
be kept equal to ensure proper operation.
7. Care should be taken to minimize capacitance on the
TEMP pin. If the TEMP voltage must drive more than
~30pF, then the pin should be isolated with a resistor
placed close to the pin of a value between 10k and 100k.
Keep in mind that any load on the isolation resistor will
create a proportional error.
4 × 2A Quad Buck Application
2.25V TO 5.5V
22µF
1.2V
2A
5. The GND side of the switching regulator output capacitors should connect directly to the thermal ground plane
of the part. Minimize the trace length from the output
capacitor to the inductor(s)/pin(s).
232k
VINA
VINB
VING
VINH
SWA
SWB
SWG
SWH
FB1
FB4
2.25V TO 5.5V
2.2µH
806k
464k
649k
2.5V TO 5.5V
22µF
2.5V
2A
2.2µH
47µF
47µF
1.8V
2A
22µF
665k
VINC
VIND
VINE
VINF
SWC
SWD
SWE
SWF
FB2
LTC3371
3.3V TO 5.5V
22µF
2.2µH
511k
47µF
3.3V
2A
FB3
309k
162k
EN1
EN2
EN3
EN4
PLL/MODE
C1
C2
C3
MICROPROCESSOR
CONTROL
RT
402k
EXPOSED PAD
VCC
2.7V TO 5.5V
1M
RST1
RST2
RST3
RST4
WDO
WDI
TEMP
CT
10µF
MICROPROCESSOR
CONTROL
3371 TA02
20
3371fa
For more information www.linear.com/LTC3371
LTC3371
Typical Applications
Buck Regulators with Sequenced Start-Up Diven from a High Voltage Upstream Buck Converter
VIN
5.5V TO 36V
CIN
22µF
VIN
100k
INTVCC
INTVCC
2.2µF
PGOOD
PLLIN/MODE
D1
TG
ILIM
LTC2955TS8-1
VIN
EN
KILL
INT
PB
MICROPROCESSOR
CONTROL
PGND
0.1µF
LTC3891
RUN
BOOST
470pF
SENSE+
0.1µF
47µF
2.2µH
1.2V
4A
COUT: SANYO 10TPE330M
D1: DFLS1100
L1 COILCRAFT SER1360-802KL
MTOP, MBOT: Si7850DP
100µF
VINF
VING
SWH
SWA
SWB
SWC
FB1
232k
SWF
SWG
2.5V
1A
2.2µH
806k
649k
2.2µH
SWD
VINE
LTC3371
SWE
10µF
2.2µH
665k
511k
FB2
22µF
3.3V
1A
FB3
309k
MICROPROCESSOR
CONTROL
47µF
1.8V
2A
FB4
VIND
22µF
19.1k
22µF
464k
10µF
5V
6A
100k
SGND
VINH
VINA
VINB
VINC
COUT
330µF
1nF
–
TRACK/SS SENSE
EXTVCC
SGND
VFB
1M
RSENSE
7mΩ
MBOT
BG
ITH
TMR GND ON
L1
8µH
SW
FREQ
34.8k
MTOP
162k
EN1
EN2
EN3
EN4
PLL/MODE
C1
C2
C3
VCC
RT
402k
EXPOSED PAD
VCC
1M
RST1
RST2
RST3
RST4
WDO
WDI
TEMP
CT
10µF
MICROPROCESSOR
CONTROL
3371 TA03
3371fa
For more information www.linear.com/LTC3371
21
LTC3371
Typical Applications
Combined Buck Regulators with Common Input Supply
2.7V TO 5.5V
10µF
1.2V
4A
2.2µH
100µF
324k
VINA
VINH
SWA
SWB
SWC
SWD
FB1
SWH
SWG
SWF
2.2µH
511k
10µF
511k
VINB
VING
VINC
VINF
VIND
10µF
LTC3371
VINE
SWE
10µF
10µF
2.2µH
665k
FB2
EN2
C1
FB3
C2
C3
VCC
22
10µF
10µF
1M
RT
CT
1M
22µF
2.5V
1A
309k
EN1
EN3
EN4
PLL/MODE
MICROPROCESSOR
CONTROL
10µF
FB4
649k
10µF
68µF
1.6V
3A
EXPOSED PAD
RST1
RST3
RST4
WDO
WDI
TEMP
RST2
MICROPROCESSOR
CONTROL
NO CONNECT
3371 TA04
3371fa
For more information www.linear.com/LTC3371
LTC3371
Package Description
Please refer to http://www.linear.com/designtools/packaging/ for the most recent package drawings.
FE Package
38-Lead Plastic TSSOP (4.4mm)
(Reference LTC DWG # 05-08-1772 Rev C)
Exposed Pad Variation AA
4.75 REF
38
9.60 – 9.80*
(.378 – .386)
4.75 REF
(.187)
20
6.60 ±0.10
4.50 REF
2.74 REF
SEE NOTE 4
6.40
2.74
REF (.252)
(.108)
BSC
0.315 ±0.05
1.05 ±0.10
0.50 BSC
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
2. DIMENSIONS ARE IN MILLIMETERS
(INCHES)
3. DRAWING NOT TO SCALE
1
19
1.20
(.047)
MAX
0° – 8°
0.50
(.0196)
BSC
0.17 – 0.27
(.0067 – .0106)
TYP
0.05 – 0.15
(.002 – .006)
FE38 (AA) TSSOP REV C 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
3371fa
For more information www.linear.com/LTC3371
23
LTC3371
Package Description
Please refer to http://www.linear.com/designtools/packaging/ for the most recent package drawings.
FE Package
38-Lead Plastic TSSOP (4.4mm)
(Reference LTC DWG # 05-08-1772 Rev C)
Exposed Pad Variation AA
4.75 REF
38
9.60 – 9.80*
(.378 – .386)
4.75 REF
(.187)
20
6.60 ±0.10
4.50 REF
2.74 REF
SEE NOTE 4
6.40
2.74
REF (.252)
(.108)
BSC
0.315 ±0.05
1.05 ±0.10
0.50 BSC
RECOMMENDED SOLDER PAD LAYOUT
4.30 – 4.50*
(.169 – .177)
0.09 – 0.20
(.0035 – .0079)
0.50 – 0.75
(.020 – .030)
NOTE:
1. CONTROLLING DIMENSION: MILLIMETERS
2. DIMENSIONS ARE IN MILLIMETERS
(INCHES)
3. DRAWING NOT TO SCALE
24
1
0.25
REF
19
1.20
(.047)
MAX
0° – 8°
0.50
(.0196)
BSC
0.17 – 0.27
(.0067 – .0106)
TYP
0.05 – 0.15
(.002 – .006)
FE38 (AA) TSSOP REV C 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
3371fa
For more information www.linear.com/LTC3371
LTC3371
Revision History
REV
DATE
DESCRIPTION
A
05/15
Modified Buck Efficiency graphs legends
PAGE NUMBER
Changed Recommended Inductor value, Table 3
Modified Typical Application circuits
1, 6
19
21, 22, 23, 26
3371fa
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 representation that the interconnection
of its circuits
as described
herein will not infringe on existing patent rights.
For more
information
www.linear.com/LTC3371
25
LTC3371
Typical Application
Combined Bucks with 3MHz Switching Frequency and Sequenced Power Up
2.25V TO 5.5V
10µF
10µF
10µF
1.2V
3A
1µH
68µF
324k
VINA
VINH
VINB
VING
2.25V TO 5.5V
10µF
10µF
1µH
VINC
SWH
SWG
SWA
SWB
SWC
FB4
FB1
VINF
649k
VIND
LTC3371
3.3V
1A
22µF
SWD
511k
VINE
FB2
SWE
SWF
10µF
1µH
665k
309k
C1
C2
C3
TEMP
PLL/MODE
EN1
EN2
EN3
EN4
EXPOSED PAD
MICROPROCESSOR
CONTROL
2.5V
2A
47µF
FB3
162k
VCC
2.5V TO 5.5V
10µF
10µF
1µH
2V
2A
432k
649k
3.3V TO 5.5V
47µF
VCC
1M
RST1
RST2
RST3
RST4
WDO
WDI
CT
RT
2.7V TO 5.5V
10µF
MICROPROCESSOR
CONTROL
267k
3371 TA05
Related Parts
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Triple I2C Adjustable High Efficiency Step-Down DC/DC Converters: 1.6A, 1A, 1A. High Efficiency 1.2A
Buck-Boost DC/DC Converter, Triple 250mA LDO Regulators. Pushbutton On/Off Control with System
Reset, Flexible Pin-Strap Sequencing Operation. I2C and Independent Enable Control Pins, Dynamic
Voltage Scaling and Slew Rate Control. Selectable 2.25MHz or 1.12MHz Switching Frequency, 8µA
Standby Current, 40-Lead (6mm × 6mm × 0.75mm) QFN Package.
LTC3675
7-Channel Configurable High
Power PMIC
Quad Synchronous Buck Regulators (1A, 1A, 500mA, 500mA). Buck DC/DCs Can be Paralleled to
Deliver Up to 2× Current with a Single Inductor. 1A Boost, 1A Buck-Boost, 40V LED Driver. 44-Lead
(4mm × 7mm × 0.75mm) QFN Package.
LTC3676
8-Channel Power Management
Solution for Application
Processors
Quad Synchronous Buck Regulators (2.5A, 2.5A, 1.5A, 1.5A). Quad LDO Regulators
(300mA, 300mA, 300mA, 25mA). Pushbutton On/Off Control with System Reset. DDR Solution with
VTT and VTTR Reference. 40-Lead (6mm × 6mm × 0.75mm) QFN Package.
LTC3375
8-Channel Programmable
Configurable 1A DC/DC
8 × 1A Synchronous Buck Regulators. Can Connect Up to Four Power Stages in Parallel to Make a
Single Inductor, High Current Output (4A Maximum), 15 Output Configurations Possible, 48-Lead
(7mm × 7mm × 0.75mm) QFN Package.
LTC3374
8-Channel Programmable
Configurable 1A DC/DC
8 × 1A Synchronous Buck Regulators. Can Connect Up to Four Power Stages in Parallel to Make a
Single Inductor, High Current Output (4A Maximum), 15 Output Configurations Possible, 38-Lead
(5mm × 7mm × 0.75mm) QFN and TSSOP Packages.
LTC3370
4-Channel Configurable DC/DC
with 8 × 1A Power Stages
4 Synchronous Buck Regulators with 8 × 1A Power Stages. Can Connect Up to Four Power Stages
in Parallel to Make a Single Inductor, High Current Output (4A Maximum), 8 Output Configurations
Possible, Precision PGOODALL Indication, 32-Lead (5mm × 5mm × 0.75mm) QFN Package.
26 Linear Technology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417
For more information www.linear.com/LTC3371
(408) 432-1900 ● FAX: (408) 434-0507 ● www.linear.com/LTC3371
3371fa
LT 0515 REV A • PRINTED IN USA
 LINEAR TECHNOLOGY CORPORATION 2014