FEATURES
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LTC3731H
3-Phase, 600kHz,
Synchronous Buck Switching
Regulator Controller
DESCRIPTIO
The LTC
®
3731H 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 LTC3731H is rated for opera-
tion up to 140°C junction temperature. The 3-phase con-
troller drives its output stages with 120° phase separation
at frequencies 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 frequency, 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.
The precision reference supports output voltages from 0.6V
to 6V. Current foldback provides protection for the exter-
nal MOSFETs under short-circuit or overload conditions.
Please refer to the LTC3731 data sheet for 0°C to 70°C and
–40°C to 85°C rated versions.
, LTC and LT are registered trademarks of Linear Technology Corporation.
All other trademarks are the property of their respective owners.
Burst Mode, OPTI-LOOP and PolyPhase are registered trademarks of Linear Technology
Corporation. Stage Shedding is a trademark of Linear Technology Corporation.
Protected by U.S. Patents including 5481178, 5929620, 6177787, 6144194, 6100678,
5408150, 6580258, 6462525, 6304066, 5705919.
3-Phase Current Mode Controller with Onboard
MOSFET Drivers
±5%
Output Current Matching Optimizes Thermal
Performance and Size of Inductors and MOSFETs
±2%
V
REF
Accuracy Over Temperature up to 140°C
Reduced Power Supply Induced Noise
±10%
Power Good Output Indicator
250kHz to 600kHz Per Phase, PLL, Fixed Frequency
PWM, Stage Shedding
TM
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 36-Pin Narrow (0.209") SSOP Package
APPLICATIO S
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Automotive and Industrial Power Supplies
High Output Current DC/DC Power Supplies
TYPICAL APPLICATIO
V
CC
4.5V TO 7V
V
CC
10µF
LTC3731H
BOOST1
BOOST2
BOOST3
0.1µF
SW3 SW2 SW1
TG1
SW1
BG1
SENSE1
+
SENSE1
–
TG2
SW2
BG2
PGND
SENSE2
+
SENSE2
–
TG3
SW3
BG3
SENSE3
+
SENSE3
–
V
IN
V
IN
0.8µH
0.003Ω
POWER GOOD INDICATOR
OPTIONAL SYNC IN
PGOOD
PLLIN
PLLFLTR
0.8µH
0.003Ω
36k
V
IN
12k
680pF
I
TH
5k
0.01µF
100pF
7.5k
6.04k
RUN/SS
SGND
EAIN
UVADJ
0.8µH
0.003Ω
Figure 1. High Current Triple Phase Step-Down Converter
3731hf
U
U
U
+
V
IN
5V TO 28V
22µF
35V
V
OUT
1.35V
55A
+
C
OUT
470µF
4V
3731H F01
1
LTC3731H
ABSOLUTE
(Note 1)
AXI U
RATI GS
PACKAGE/ORDER I FOR ATIO
TOP VIEW
V
CC
PLLIN
PLLFLTR
FCB
NC
NC
NC
EAIN
SGND
SENSE1
–
SENSE2
–
SENSE3
+
1
2
3
4
5
6
7
8
9
36 CLKOUT
35 PGOOD
34 BOOST1
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 SGND
Topside Driver Voltages (BOOST
N
) ............ 38V to –0.3V
Switch Voltage (SW
N
)................................... 32V to –5V
Boosted Driver Voltage (BOOST
N
– SW
N
) .... 7V to –0.3V
Peak Output Current <1ms (TG
N
, BG
N
) ..................... 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
I
TH
Voltage ................................................ 2.4V to –0.3V
Operating Junction
Temperature Range (Note 2) .................–40°C to 140°C
Storage Temperature Range ..................–65°C to 150°C
Lead Temperature G Package (Soldering, 10sec).. 300°C
ORDER PART
NUMBER
LTC3731HG
SENSE1
+
10
11
SENSE2
+
12
13
SENSE3
–
14
15
RUN/SS 16
I
TH
17
UVADJ 18
G PACKAGE
36-LEAD PLASTIC SSOP
T
JMAX
= 140°C,
θ
JA
= 95°C/W,
θ
JC
= 32°C/W
PINS 5 AND 6 CAN BE GROUNDED OR NC
PIN 7 MUST BE LEFT AS NC
ELECTRICAL CHARACTERISTICS
SYMBOL
V
REGULATED
V
SENSEMAX
I
MATCH
V
LOADREG
PARAMETER
Regulated Voltage at EAIN
Maximum Current Sense Threshold
Maximum Current Threshold Match
Output Voltage Load Regulation
Main Control Loop
The
●
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)
●
MIN
0.594
0.590
65
60
–5
●
●
●
●
TYP
0.600
0.600
75
75
MAX
0.606
0.614
85
90
5
UNITS
V
V
mV
mV
%
%
%
%/V
mmho
MHz
V
V
EAIN
= 0.5V, V
ITH
Open,
V
SENSE1
–
,
V
SENSE2
–
,
V
SENSE3
– = 0.6V, 1.8V
Worst-Case Error at V
SENSEMAX
(Note 3)
Measured in Servo Loop,
∆I
TH
Voltage = 1.2V to 0.7V
Measured in Servo Loop,
∆I
TH
Voltage = 1.2V to 2V
V
CC
= 4.5V to 7V
I
TH
= 1.2V, Sink/Source 25µA (Note 3)
I
TH
= 1.2V (g
m
• Z
L
, Z
L
= Series 1k-100kΩ-1nF)
●
0.1
–0.1
0.03
3
0.54
5
3
0.60
0.7
–0.7
7
0.66
V
REFLNREG
g
m
g
mOL
V
FCB
Output Voltage Line Regulation
Transconductance Amplifier g
m
Transconductance Amplifier GBW
Forced Continuous Threshold
2
U
3731hf
W
U
U
W W
W
LTC3731H
ELECTRICAL CHARACTERISTICS
SYMBOL
I
FCB
V
BINHIBIT
UVR
UVADJ
I
UVADJ
I
Q
PARAMETER
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
RUN/SS Pin ON Threshold
At UVADJ Threshold
(Note 4)
V
CC
= 5V
V
RUN/SS
= 0V
V
RUN/SS
= 1.9V
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
All Controllers, C
LOAD
= 3300pF Each Driver
All Controllers, C
LOAD
= 3300pF Each Driver
Tested with a Square Wave (Note 5)
I
PGOOD
= 2mA
V
PGOOD
= 5V
V
EAIN
with Respect to Set Output Voltage,
PGOOD 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
360
190
600
–7
7
–10
10
100
400
225
680
95
–5
–0.8
1
The
●
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
FCB
= 0.65V
Measured at FCB Pin
V
CC
Lowered Until the RUN/SS Pin is Pulled Low
MIN
TYP
0.2
MAX
0.7
UNITS
µA
V
V
V
nA
mA
µA
µA
V
V
V
µA
5
20
µA
µA
%
90
90
90
90
ns
ns
ns
ns
ns
ns
ns
0.3
1
–13
13
150
440
260
750
V
µA
%
%
µs
kHz
kHz
kHz
V
CC
– 1.5 V
CC
– 0.7 V
CC
– 0.3
3.3
3.8
4.5
1.13
1.18
0.2
2.3
50
–1.5
1.5
3.8
3.2
–1.5
1.5
13
98.5
30
40
30
20
50
60
110
0.1
1.23
50
3.5
100
– 2.5
1.9
4.5
I
RUN/SS
V
RUN/SS
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
V
PGTHNEG
V
PGTHPOS
V
PGDLY
f
NOM
f
LOW
f
HIGH
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
Top Gate Off to Bottom Gate On Delay
Synchronous Switch-On Delay Time
Bottom Gate Off to Top Gate On Delay
Top Switch-On Delay Time
Minimum On-Time
PGOOD Voltage Output Low
PGOOD Output Leakage
PGOOD Trip Thesholds
V
EAIN
Ramping Negative
V
EAIN
Ramping Positive
Power Good Fault Report Delay
Nominal Frequency
Lowest Frequency
Highest Frequency
Power Good Output Indication
Oscillator and Phase-Locked Loop
3731hf
3
LTC3731H
ELECTRICAL CHARACTERISTICS
SYMBOL
V
PLLTH
R
PLLIN
I
PLLFLTR
PARAMETER
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
PHASMD = 0V
PHASMD = 5V
f
PLLIN
< f
OSC
f
PLLIN
> f
OSC
The
●
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
Minimum Pulse Width > 100ns
MIN
TYP
1
50
20
20
120
240
30
60
MAX
UNITS
V
kΩ
µA
µA
Deg
Deg
Deg
Deg
R
RELPHS
CLKOUT
Note 1:
Absolute Maximum Ratings are those values beyond which the life
of a device may be impaired.
Note 2:
The LTC3731H is guaranteed to meet specifications from –40°C to
140°C. Junction temperature (T
J
) is calculated from the ambient
temperature T
A
and power dissipation P
D
according to the following
formula:
LTC3731H: T
J
= T
A
+ (P
D
×
95°C/W)
LTC3731H: T
J
= T
CASE
+ (P
D
×
32°C/W)
High junction temperatures degrade operating lifetimes. Operating lifetime
at junction temperatures greater than 125°C is derated to 1000 hours.
Note 3:
The IC is tested in a feedback loop that servoes the error amplifier
output voltage to its midrange point (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.
Overtemperature protection becomes active at a junction temperature
greater than the maximum operating temperature. Continuous operation
above the specified maximum operating junction temperature may impair
device reliability.
3731hf
4
LTC3731H
TYPICAL PERFOR A CE CHARACTERISTICS
Efficiency vs I
OUT
(Figure 14)
100
90 V
FCB
= OPEN
80 V
FCB
= 5V
V
FCB
= 0V
70
60
50
40
30
20
10
0
0.1
1
10
V
IN
= 8V
V
OUT
= 1.5V
100
3731H G01
EFFICIENCY (%)
EFFICIENCY (%)
80
75
70
65
60
55
50
0
5
I
L
= 45A
EFFICIENCY (%)
INDUCTOR CURRENT (A)
Reference Voltage vs
Temperature
610
6.0
605
MAXIMUM I
SENSE
THRESHOLD (mV)
ERROR AMPLIFIER g
m
(mmho)
REFERENCE VOLTAGE (mV)
600
595
590
–50 –25
0
25 50 75 100 125 150
TEMPERATURE (°C)
3731H
G04
Oscillator Frequency vs
Temperature
700
600
500
400
V
PLLFLTR
= 1.2V
300
200
100
0
–50 –25
200
0
25
50
75
100 125 150
3731H
G07
V
PLLFLTR
= 2.4V
OPERATING FREQUENCY (kHz)
500
UNDER VOLTAGE RESET (V)
V
PLLFLTR
= 5V
FREQUENCY (kHz)
V
PLLFLTR
= 0V
TEMPERATURE (°C)
U W
Efficiency vs V
IN
(Figure 14)
100
95
90
85
I
L
= 15A
V
OUT
= 1.5V
f = 225kHz
100
Efficiency vs Frequency (Figure 14)
I
LOAD
= 20A
V
OUT
= 1.5V
95
V
IN
= 5V
90
V
IN
= 12V
85
V
IN
= 20V
80
V
IN
= 8V
10
15
V
IN
(V)
20
25
3731H G02
75
200
300
400
FREQUENCY (kHz)
500
600
3731H G03
Error Amplifier gm vs
Temperature
85
Maximum I
SENSE
Threshold vs
Temperature
5.5
80
V
O
= 1.75V
75
V
O
= 0.6V
70
5.0
4.5
4.0
–50 –25
0
25 50 75 100 125 150
TEMPERATURE (°C)
3731H
G05
65
–50 –25
0
25 50 75 100 125 150
TEMPERATURE (°C)
3731H
G06
Operating Frequency vs V
PLLFLTR
700
5
Undervoltage Reset Voltage vs
Temperature
600
4
3
400
2
300
1
0
0.5
1
1.5
2
PLLFLTR PIN VOLTAGE (V)
2.5
3731H
G08
0
–50 –25
0
25
50
75
100 125 150
3731H
G09
TEMPERATURE (°C)
3731hf
5
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