FEATURES
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LTC3731
3-Phase, 600kHz,
Synchronous Buck Switching
Regulator Controller
DESCRIPTION
The LTC
®
3731 is a PolyPhase
®
synchronous step-down
switching regulator controller that drives all N-channel
external power MOSFET stages in a phase-lockable fixed
frequency architecture. The 3-phase controller drives its
output stages with 120° phase separation at frequencies
of up to 600kHz per phase to minimize the RMS current
losses in both the input and output filter capacitors. The
3-phase technique effectively triples the fundamental fre-
quency, improving transient response while operating each
controller at an optimal frequency for efficiency and ease of
thermal design. Light load efficiency is optimized by using a
choice of output Stage Shedding or Burst Mode operation.
A differential amplifier provides true remote sensing of
both the high and low side of the output voltage at the
point of load.
Soft-start and a defeatable, timed short-circuit shutdown
protect the MOSFETs and the load. Current foldback
provides protection for the external MOSFETs under
short-circuit or overload conditions.
L,
LT, LTC, LTM, Bust Mode, OPTI-LOOP PolyPhase, Linear Technology and the Linear logo
,
are registered and Stage Shedding is a trademark of Linear Technology Corporation. All other
trademarks are the property of their respective owners. Protected by U.S. Patents, including
5481178, 5929620, 6177787, 6144194, 6100678, 5408150, 6580258, 6462525, 630466,
5705919.
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3-Phase Current Mode Controller with Onboard
MOSFET Drivers
±5% Output Current Matching Optimizes Thermal
Performance and Size of Inductors and MOSFETs
Differential Amplifier Accurately Senses V
OUT
±1% V
REF
Accuracy Over Temperature
V
OUT
Range: 0.6V to 5.5V without Diffamp
V
OUT
Range: 0.6V to (V
CC
– 1.2V) with Diffamp
250kHz to 600kHz Per Phase, PLL, Fixed Frequency
PWM, Stage Shedding™ or Burst Mode
®
Operation
OPTI-LOOP
®
Compensation Minimizes C
OUT
Adjustable Soft-Start Current Ramping
Short-Circuit Shutdown Timer with Defeat Option
Overvoltage Soft Latch
Adjustable Undervoltage Lockout Threshold
Selectable Phase Output for Up to 12-Phase Operation
Available in 5mm
×
5mm QFN and 36-Pin Narrow
(0.209") SSOP Packages
APPLICATIONS
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Desktop Computers and Servers
High Performance Notebook Computers
High Output Current DC/DC Power Supplies
TYPICAL APPLICATION
V
CC
4.5V TO 7V
V
CC
10µF
LTC3731
TG1
SW1
BG1
SENSE1
+
SENSE1
–
TG2
SW2
BG2
PGND
UVADJ
680pF
I
TH
5k
0.01µF
100pF
RUN/SS
SGND
EAIN
DIFFOUT
IN
–
IN
+
SENSE2
+
SENSE2
–
TG3
SW3
BG3
SENSE3
+
SENSE3
–
0.8µH
0.003
+
V
IN
5V TO 28V
22µF
35V
0.1µF
SW3 SW2 SW1
POWER GOOD INDICATOR
OPTIONAL SYNC IN
36k
12k
BOOST1
BOOST2
BOOST3
V
IN
0.8µH
0.003
V
OUT
1.35V
55A
PGOOD
PLLIN
PLLFLTR
V
IN
V
IN
0.8µH
0.003
7.5k
6.04k
+
C
OUT
470µF
4V
3731 F01
Figure 1. High Current Triple Phase Step-Down Converter
3731fc
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LTC3731
ABSOLUTE MAXIMUM RATINGS
(Note 1)
Topside Driver Voltages (BOOSTn)............. 38V to –0.3V
Switch Voltage (SWn) ................................... 32V to –5V
Boosted Driver Voltage (BOOSTn – SWn) .... 7V to –0.3V
Peak Output Current < 1ms (TGn, BGn) ......................5A
Supply Voltages (V
CC
, V
DR
), PGOOD
Pin Voltage ................................................... 7V to –0.3V
RUN/SS, PLLFLTR, PLLIN, UVADJ,
FCB Voltages ..............................................V
CC
to –0.3V
SENSE+, SENSE– Voltages ....................... 5.5V to –0.3V
I
TH
Voltage ............................................... 2.4V to –0.3V
Operating Ambient Temperature Range
LTC3731C ................................................ 0°C to 70°C
LTC3731I..............................................–40°C to 85°C
Junction Temperature (Note 2) ............................. 125°C
Storage Temperature Range .................. –65°C to 125°C
Lead Temperature G Package (Soldering, 10 sec).. 300°C
Peak Body Temperature UH Package ................... 240°C
PIN CONFIGURATION
TOP VIEW
V
CC
PLLIN
PLLFLTR
FCB
IN
+
IN
–
DIFFOUT
EAIN
SGND
SENSE1
+
1
2
3
4
5
6
7
8
9
10
36 CLKOUT
35 PGOOD
33 TG1
32 SW1
31 BOOST2
30 TG2
29 SW2
28 V
DR
27 BG1
26 PGND
25 BG2
24 BG3
23 SW3
22 TG3
21 BOOST3
20 PHASMD
19 SGND2
IN
–
1
DIFFOUT 2
EAIN 3
SENSE1
+
4
SENSE1
–
TOP VIEW
PLLFLTR
CLKOUT
BOOST1
PLLIN
SW1
24 BOOST2
23 TG2
22 SW2
33
SGND
21 V
CC
20 BG1
19 PGND
18 BG2
17 BG3
9 10 11 12 13 14 15 16
UVADJ
PHASMD/PG
BOOST3
RUN/SS
SENSE3
+
SW3
TG3
I
TH
TG1
FCB
IN
+
5
6
7
8
SENSE2
+
SENSE2
–
SENSE3
–
34 BOOST1
32 31 30 29 28 27 26 25
SENSE1
–
11
SENSE2
+
12
SENSE2
–
13
SENSE3
–
14
SENSE3
+
15
RUN/SS 16
I
TH
17
UVADJ 18
G PACKAGE
36-LEAD PLASTIC SSOP
T
JMAX
= 125°C,
θ
JA
= 95°C/W,
θ
JC
= 32°C/W
UH PACKAGE
32-LEAD
(5mm
×
5mm)
PLASTIC QFN
T
JMAX
= 125°C,
θ
JA
= 34°C/W
EXPOSED PAD (PIN 33) IS SGND, MUST BE SOLDERED TO PCB
3731fc
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LTC3731
ORDER INFORMATION
LEAD FREE FINISH
LTC3731CG#PBF
LTC3731IG#PBF
LTC3731CUH#PBF
LTC3731IUH#PBF
TAPE AND REEL
LTC3731CG#TRPBF
LTC3731IG#TRPBF
LTC3731CUH#TRPBF
LTC3731IUH#TRPBF
PART MARKING
LTC3731CG
LTC3731IG
3731
3731I
PACKAGE DESCRIPTION
36-Lead Plastic SSOP
36-Lead Plastic SSOP
32-Lead (5mm
×
5mm) Plastic QFN
32-Lead (5mm
×
5mm) Plastic QFN
TEMPERATURE RANGE
0°C to 70°C
–40°C to 85°C
0°C to 70°C
–40°C to 85°C
Consult LTC Marketing for parts specified with wider operating temperature ranges.
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/
ELECTRICAL CHARACTERISTICS
SYMBOL
PARAMETER
Main Control Loop
V
REGULATED
Regulated Voltage at IN
+
V
SENSEMAX
I
MATCH
V
LOADREG
Maximum Current Sense Threshold
The
l
denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at T
A
= 25°C. V
CC
= V
RUN/SS
= 5V unless otherwise noted.
CONDITIONS
V
ITH
= 1.2V (Note 3)
LTC3731I
V
EAIN
= 0.5V, V
ITH
Open,
V
SENSE1–
, V
SENSE2–
, V
SENSE3–
= 0.6V, 1.8V
LTC3731I
Worst-Case Error at V
SENSEMAX
(Note 3)
Measured in Servo Loop, ∆I
TH
Voltage = 1.2V to 0.7V
LTC3731I
Measured in Servo Loop, ∆I
TH
Voltage = 1.2V to 2V
LTC3731I
V
CC
= 4.5V to 7V
I
TH
= 1.2V, Sink/Source 25µA (Note 3)
LTC3731I
I
TH
= 1.2V (g
m
• Z
L
, Z
L
= Series 1k-100kΩ-1nF)
LTC3731I
V
FCB
= 0.65V
Measured at FCB Pin
V
CC
Lowered Until the RUN/SS Pin is Pulled Low
At UVADJ Threshold
(Note 4)
V
CC
= 5V
V
RUN/SS
= 0V
V
RUN/SS
= 1.9V
–0.8
3.3
1.13
l
l
l
l
l
l
l
l
l
l
l
l
MIN
0.596
0.594
0.591
65
62
60
–5
TYP
0.600
0.600
75
75
MAX
0.604
0.606
0.609
85
88
90
5
UNITS
V
V
V
mV
mV
mV
%
%
%
%
%
%/V
mmho
mmho
MHz
V
V
µA
V
V
V
nA
mA
µA
µA
Maximum Current Threshold Match
Output Voltage Load Regulation
0.1
0.1
–0.1
–0.1
0.03
4
3
0.58
0.54
5
5
3
0.60
0.60
0.2
3.8
1.18
0.2
2.3
50
–1.5
0.5
0.7
–0.5
–0.7
6
7
0.62
0.66
0.7
4.5
1.23
50
3.5
100
–2.5
V
REFLNREG
g
m
g
mOL
V
FCB
I
FCB
V
BINHIBIT
UVR
UVADJ
I
UVADJ
I
Q
I
RUN/SS
Output Voltage Line Regulation
Transconductance Amplifier g
m
Transconductance Amplifier GBW
Forced Continuous Threshold
FCB Bias Current
Burst Inhibit Threshold
Undervoltage RUN/SS Reset
Undervoltage Lockout Threshold
Undervoltage Bias Current
Input DC Supply Current
Normal Mode
Shutdown
Soft-Start Charge Current
V
CC
– 1.5 V
CC
– 0.7 V
CC
– 0.3
3731fc
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LTC3731
ELECTRICAL CHARACTERISTICS
SYMBOL
V
RUN/SS
PARAMETER
RUN/SS Pin ON Threshold
V
RUN/SSARM
RUN/SS Pin Arming Threshold
V
RUN/SSLO
I
SCL
I
SDLHO
I
SENSE
DF
MAX
TG t
R
,t
F
BG t
R
, t
F
TG/BG t
1D
BG/TG t
2D
t
ON(MIN)
V
PGL
I
PGOOD
I
PGOOD
V
PGTHNEG
V
PGTHPOS
V
PGDLY
f
NOM
f
LOW
f
HIGH
V
PLLTH
R
PLLIN
I
PLLFLTR
R
RELPHS
CLKOUT
RUN/SS Pin Latch-Off Threshold
RUN/SS Discharge Current
Shutdown Latch Disable Current
SENSE Pins Source Current
Maximum Duty Factor
Top Gate Rise Time
Top Gate Fall Time
Bottom Gate Rise Time
Bottom Gate Fall Time
The
l
denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at T
A
= 25°C. V
CC
= V
RUN/SS
= 5V unless otherwise noted.
CONDITIONS
V
RUN/SS
, Ramping Positive
V
RUN/SS
, Ramping Positive Until Short-Circuit
Latch-Off is Armed
V
RUN/SS
, Ramping Negative
Soft-Short Condition V
EAIN
= 0.375V, V
RUN/SS
= 4.5V
V
EAIN
= 0.375V, V
RUN/SS
= 4.5V
SENSE1
+
, SENSE1
–
, SENSE2
+
, SENSE2
–
, SENSE3
+
,
SENSE3
–
All Equal 1.2V; Current at Each Pin
In Dropout, V
SENSEMAX
≤ 30mV
C
LOAD
= 3300pF
C
LOAD
= 3300pF
C
LOAD
= 3300pF
C
LOAD
= 3300pF
95
–5
MIN
1
TYP
1.5
3.8
3.2
–1.5
1.5
13
98.5
30
40
30
20
50
60
110
0.1
0.5
–10
–7
7
±3
–10
10
100
360
190
600
400
225
680
1
50
f
PLLIN
< f
OSC
f
PLLIN
> f
OSC
20
20
120
240
PHASMD = 0V
PHASMD = 5V
30
60
0.3
1.0
1
10
–13
13
150
440
260
750
90
90
90
90
5
20
MAX
1.9
4.5
UNITS
V
V
V
µA
µA
µA
%
ns
ns
ns
ns
ns
ns
ns
V
V
µA
µA
%
%
µs
kHz
kHz
kHz
V
kΩ
µA
µA
Deg
Deg
Deg
Deg
Top Gate Off to Bottom Gate On Delay All Controllers, C
LOAD
= 3300pF Each Driver
Synchronous Switch-On Delay Time
Bottom Gate Off to Top Gate On Delay All Controllers, C
LOAD
= 3300pF Each Driver
Top Switch-On Delay Time
Minimum On-Time
PGOOD Voltage Output Low
PGOOD Output Leakage
PGOOD/PHASMD Bias I
PGOOD Trip Thresholds
V
DIFFOUT
Ramping Negative
V
DIFFOUT
Ramping Positive
Power Good Fault Report Delay
Nominal Frequency
Lowest Frequency
Highest Frequency
PLLIN Input Threshold
PLLIN Input Resistance
Phase Detector Output Current
Sinking Capability
Sourcing Capability
Controller 2-Controller 1 Phase
Controller 3-Controller 1 Phase
Controller 1 TG to CLKOUT Phase
Tested with a Square Wave (Note 5)
I
PGOOD
= 2mA, G Package
I
PGOOD
= 1.6mA, UH Package
V
PGOOD
= 5V, G Package
0 ≤ V
PHASMD/PG
≤ V
CC
, UH Package
V
DIFFOUT
with Respect to Set Output Voltage,
HGOOD Goes Low After V
UVDLY
Delay
After V
EAIN
is Forced Outside the PGOOD Thresholds
V
PLLFLTR
= 1.2V
V
PLLFLTR
= 0V
V
PLLFLTR
= 2.4V
Minimum Pulse Width > 100ns
Power Good Output Indication
Oscillator and Phase-Locked Loop
3731fc
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LTC3731
ELECTRICAL CHARACTERISTICS
SYMBOL
A
V
V
OS
CMRR
I
CL
GBP
SR
V
O(MAX)
R
IN
PARAMETER
Differential Gain
Input Offset Voltage Magnitude
Common Mode Rejection Ratio
Output Current Sourcing
Gain-Bandwidth Product
Slew Rate
Maximum High Output Voltage
Input Resistance
I
OUT
= 1mA
R
L
= 2k
I
OUT
= 1mA
Measured at IN
+
Pin
IN
+
= IN
–
= 1.2V, I
OUT
= 1mA, Input Referred; Gain = 1
0V < IN
+
= IN
–
< 5V, I
OUT
= 1mA, Input Referred
50
10
Differential Amplifier
0.995
1.000
0.5
70
40
2
5
V
CC
– 1.2 V
CC
– 0.8
80
1.005
5
V/V
mV
dB
mA
MHz
V/µs
V
kΩ
The
l
denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at T
A
= 25°C. V
CC
= V
RUN/SS
= 5V unless otherwise noted.
CONDITIONS
MIN
TYP
MAX
UNITS
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:
T
J
is calculated from the ambient temperature T
A
and power
dissipation P
D
according to the following formula:
LTC3731CG/LTC3731IG: T
J
= T
A
+ (P
D
•
95°C/W)
LTC3731CG/LTC3731IG: T
J
= T
CASE
+ (P
D
•
32°C/W)
LTC3731CUH/LTC3731IUH: T
J
= T
A
+ (P
D
•
34°C/W)
Note 3:
The IC is tested in a feedback loop that includes the differential
amplifier loaded with 100µA to ground driving the error amplifier and
servoing the resultant voltage to the midrange point for the error amplifier
(V
ITH
= 1.2V).
Note 4:
Dynamic supply current is higher due to the gate charge being
delivered at the switching frequency. See Applications Information.
Note 5:
The minimum on-time condition corresponds to an inductor
peak-to-peak ripple current of ≥ 40% of I
MAX
(see minimum on-time
considerations in the Applications Information Section).
Note 6:
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.
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