FeaTures
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LTC3728L/LTC3728LX
Dual, 550kHz, 2-Phase
Synchronous Regulators
DescripTion
The LTC
®
3728L/LTC3728LX are dual high performance
step-down switching regulator controllers that drive all
N-channel synchronous power MOSFET stages. A constant-
frequency, current mode architecture allows phase-lockable
frequency of up to 550kHz. Power loss and noise due to
the ESR of the input capacitors are minimized by operating
the two controller output stages out of phase.
OPTI-LOOP compensation allows the transient response to
be optimized over a wide range of output capacitance and
ESR values. The precision 0.8V reference and power good
output indicator are compatible with future microproces-
sor generations, and a wide 4.5V to 28V (30V maximum)
input supply range encompasses all battery chemistries.
A RUN/SS pin for each controller provides both soft-
start and optional timed, short-circuit shutdown. Current
foldback limits MOSFET dissipation during short-circuit
conditions when overcurrent latchoff is disabled. Output
overvoltage protection circuitry latches on the bottom
MOSFET until V
OUT
returns to normal. The FCB mode
pin can select among Burst Mode, constant-frequency
mode and continuous inductor current mode or regulate
a secondary winding. The LTC3728L/LTC3728LX include
a power good output pin that indicates when both outputs
are within 7.5% of their designed set point.
L,
LT, LTC, LTM, Linear Technology, the Linear logo, Burst Mode, OPTI-LOOP and PolyPhase
are registered trademarks of Linear Technology Corporation. All other trademarks are the
property of their respective owners.
n
Dual, 180° Phased Controllers Reduce Required
Input Capacitance and Power Supply Induced Noise
OPTI-LOOP
®
Compensation Minimizes C
OUT
±1% Output Voltage Accuracy (LTC3728LC)
Power Good Output Voltage Indicator
Phase-Lockable Fixed Frequency 250kHz to 550kHz
Dual N-Channel MOSFET Synchronous Drive
Wide V
IN
Range: 4.5V to 28V Operation
Very Low Dropout Operation: 99% Duty Cycle
Adjustable Soft-Start Current Ramping
Foldback Output Current Limiting
Latched Short-Circuit Shutdown with Defeat Option
Output Overvoltage Protection
Low Shutdown I
Q
: 20µA
5V and 3.3V Standby Regulators
3 Selectable Operating Modes: Constant-Frequency,
Burst Mode
®
Operation and PWM
5mm × 5mm QFN and 28-Lead Narrow SSOP
Packages
Notebook and Palmtop Computers
Telecom Systems
Portable Instruments
Battery-Operated Digital Devices
DC Power Distribution Systems
applicaTions
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Typical applicaTion
4.7µF
M1
L1
3.2µH
C
B1
, 0.1µF
D1
+
D3
V
IN
PGOOD INTV
CC
TG1
BOOST1
SW1
BG1
f
IN
500kHz
PLLIN
SENSE1
+
LTC3728L/
LTC3728LX
TG2
BOOST2
SW2
BG2
PGND
SENSE2
+
1000pF
SENSE1
–
V
OSENSE1
SENSE2
–
V
OSENSE2
I
TH2
C
SS2
0.1µF
C
C2
220pF
R
C2
15k
C
B2
, 0.1µF
D4
1µF
CERAMIC
M2
V
IN
5.2V TO 28V
C
IN
22µF
50V
CERAMIC
L2
3.2µH
D2
R
SENSE1
0.01Ω
V
OUT1
5V
5A
1000pF
R
SENSE2
0.01Ω
V
OUT2
3.3V
5A
+
C
OUT1
47µF
6V
SP
R2
105k
1%
R1
20k
1%
C
C1
220pF
R
C1
15k
I
TH1
C
SS1
0.1µF
RUN/SS1 SGND RUN/SS2
R3
20k
1%
R4
63.4k
1%
C
OUT
56µF
6V
SP
+
M1, M2: FDS6982S
Figure 1. High Efficiency Dual 5V/3.3V Step-Down Converter
3728 F01
3728lxff
1
LTC3728L/LTC3728LX
absoluTe MaxiMuM raTings
(Note 1)
Input Supply Voltage (V
IN
) ........................ 30V to – 0.3V
Topside Driver Voltages
(BOOST1, BOOST2) .............................. 36V to –0.3V
Switch Voltage (SW1, SW2) ........................ 30V to –5V
INTV
CC,
EXTV
CC
, RUN/SS1, RUN/SS2,
(BOOST1-SW1), (BOOST2-SW2), PGOOD .... 7V to –0.3V
SENSE1
+
, SENSE2
+
, SENSE1
–
,
SENSE2
–
Voltages ........................ (1.1)INTV
CC
to –0.3V
PLLIN, PLLFLTR, FCB Voltages ............ INTV
CC
to –0.3V
I
TH1,
I
TH2
, V
OSENSE1
, V
OSENSE2
Voltages ... 2.7V to –0.3V
Peak Output Current <10µs (TG1, TG2, BG1, BG2) .... 3A
INTV
CC
Peak Output Current ................................. 40mA
Operating Temperature Range (Note 7)
LTC3728LC/LTC3728LXC ........................ 0°C to 85°C
LTC3728LE/LTC3728LI ........................ –40°C to 85°C
Junction Temperature (Note 2) ............................ 125°C
Storage Temperature Range.................. –65°C to 125°C
Reflow Peak Body Temperature (UH Package) ..... 260°C
Lead Temperature (Soldering, 10 sec)
(GN Package) .................................................... 300°C
pin conFiguraTion
TOP VIEW
SENSE1
–
RUN/SS1
SENSE1
+
SENSE1
–
V
OSENSE1
PLLFLTR
PLLIN
FCB
I
TH1
SGND
1
2
3
4
5
6
7
8
9
28 PGOOD
NC
27 TG1
26 SW1
25 BOOST1
24 V
IN
23 BG1
22 EXTV
CC
21 INTV
CC
20 PGND
19 BG2
18 BOOST2
17 SW2
16 TG2
15 RUN/SS2
V
OSENSE1
1
PLLFLTR 2
PLLIN 3
FCB 4
I
TH1
5
SGND 6
3.3V
OUT
7
I
TH2
8
9 10 11 12 13 14 15 16
V
OSENSE2
RUN/SS2
TG2
SW2
NC
SENSE2
–
SENSE2
+
NC
33
TOP VIEW
RUN/SS1
SENSE1
+
PGOOD
SW1
24 BOOST1
23 V
IN
22 BG1
21 EXTV
CC
20 INTV
CC
19 PGND
18 BG2
17 BOOST2
TG1
NC
32 31 30 29 28 27 26 25
3.3V
OUT
10
I
TH2
11
V
OSENSE2
12
SENSE2
–
13
SENSE2
+
14
GN PACKAGE
28-LEAD NARROW PLASTIC SSOP
T
JMAX
= 125°C,
θ
JA
= 95°C/W
UH PACKAGE
32-LEAD (5mm × 5mm) PLASTIC QFN
T
JMAX
= 125°C,
θ
JA
= 34°C/W
EXPOSED PAD IS SGND (PIN 33), MUST BE SOLDERED TO PCB
3728lxff
2
LTC3728L/LTC3728LX
orDer inForMaTion
LEAD FREE FINISH
LTC3728LCGN#PBF
LTC3728LEGN#PBF
LTC3728LIGN#PBF
LTC3728LCUH#PBF
LTC3728LEUH#PBF
LTC3728LIUH#PBF
LTC3728LXCUH#PBF
LEAD BASED FINISH
LTC3728LCGN
LTC3728LEGN
LTC3728LIGN
LTC3728LCUH
LTC3728LEUH
LTC3728LIUH
LTC3728LXCUH
TAPE AND REEL
LTC3728LCGN#TRPBF
LTC3728LEGN#TRPBF
LTC3728LIGN#TRPBF
LTC3728LCUH#TRPBF
LTC3728LEUH#TRPBF
LTC3728LIUH#TRPBF
LTC3728LXCUH#TRPBF
TAPE AND REEL
LTC3728LCGN#TR
LTC3728LEGN#TR
LTC3728LIGN#TR
LTC3728LCUH#TR
LTC3728LEUH#TR
LTC3728LIUH#TR
LTC3728LXCUH#TR
3728L
3728LE
3728LI
3728LX
3728L
3728LE
3728LI
3728LX
PART MARKING
PART MARKING
PACKAGE DESCRIPTION
28-Lead Narrow Plastic SSOP
28-Lead Narrow Plastic SSOP
28-Lead Narrow Plastic SSOP
32-Lead (5mm
×
5mm) Plastic QFN
32-Lead (5mm
×
5mm) Plastic QFN
32-Lead (5mm
×
5mm) Plastic QFN
32-Lead (5mm
×
5mm) Plastic QFN
PACKAGE DESCRIPTION
28-Lead Narrow Plastic SSOP
28-Lead Narrow Plastic SSOP
28-Lead Narrow Plastic SSOP
32-Lead (5mm
×
5mm) Plastic QFN
32-Lead (5mm
×
5mm) Plastic QFN
32-Lead (5mm
×
5mm) Plastic QFN
32-Lead (5mm
×
5mm) Plastic QFN
TEMPERATURE RANGE
0°C to 85°C
–40°C to 85°C
–40°C to 85°C
0°C to 85°C
–40°C to 85°C
–40°C to 85°C
0°C to 85°C
TEMPERATURE RANGE
0°C to 85°C
–40°C to 85°C
–40°C to 85°C
0°C to 85°C
–40°C to 85°C
–40°C to 85°C
0°C to 85°C
Consult LTC Marketing for parts specified with wider operating temperature ranges.
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/
3728lxff
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LTC3728L/LTC3728LX
elecTrical characTerisTics
SYMBOL
V
OSENSE1, 2
I
VOSENSE1, 2
V
REFLNREG
V
LOADREG
g
m1, 2
g
mGBW1, 2
I
Q
V
FCB
I
FCB
V
BINHIBIT
UVLO
V
OVL
I
SENSE
DF
MAX
I
RUN/SS1, 2
V
RUN/SS1, 2
LT
I
SCL1, 2
I
SDLHO
V
SENSE(MAX)
TG1, 2 t
r
TG1, 2 t
f
BG1, 2 t
r
BG1, 2 t
f
TG/BG t
1D
BG/TG t
2D
t
ON(MIN)
PARAMETER
Regulated Feedback Voltage
Main Control Loops
(Note 3); I
TH1, 2
Voltage = 1.2V (LTC3728LC)
(Note 3); I
TH1, 2
Voltage = 1.2V
(LTC3728LE/LTC3728LX/LTC3728LI)
(Note 3)
V
IN
= 3.6V to 30V (Note 3)
(Note 3)
Measured in Servo Loop; ΔI
TH
Voltage = 1.2V to 0.7V
Measured in Servo Loop; ΔI
TH
Voltage = 1.2V to 2.0V
I
TH1, 2
= 1.2V; Sink/Source 5µA (Note 3)
I
TH1, 2
= 1.2V (Note 8)
(Note 4)
V
IN
= 15V; EXTV
CC
Tied to V
OUT1
; V
OUT1
= 5V
V
RUN/SS1, 2
= 0V
●
●
●
●
●
The
l
denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at T
A
= 25°C. V
IN
= 15V, V
RUN/SS1, 2
= 5V unless otherwise noted.
CONDITIONS
MIN
0.792
0.788
TYP
0.800
0.800
–5
0.002
0.1
– 0.1
1.3
3
450
20
0.76
– 0.50
0.800
–0.18
4.3
●
●
MAX
0.808
0.812
–50
0.02
0.5
–0.5
UNITS
V
V
nA
%/V
%
%
mmho
MHz
µA
µA
V
µA
V
V
V
µA
%
µA
Feedback Current
Reference Voltage Line Regulation
Output Voltage Load Regulation
Transconductance Amplifier g
m
Transconductance Amplifier GBW
Input DC Supply Current
Normal Mode
Shutdown
Forced Continuous Threshold
Forced Continuous Pin Current
Burst Inhibit (Constant-Frequency)
Threshold
Undervoltage Lockout
Feedback Overvoltage Lockout
Sense Pins Total Source Current
Maximum Duty Factor
Soft-Start Charge Current
RUN/SS Pin Latchoff Arming
Threshold
RUN/SS Discharge Current
Shutdown Latch Disable Current
Maximum Current Sense Threshold
TG Transition Time:
Rise Time
Fall Time
BG Transition Time:
Rise Time
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
35
0.84
–0.1
4.8
4
0.88
V
FCB
= 0.85V
Measured at FCB Pin
V
IN
Ramping Down
Measured at V
OSENSE1, 2
(Each Channel); V
SENSE1
–
, 2
– = V
SENSE1
+
, 2
+ = 0V
In Dropout
V
RUN/SS1, 2
= 1.9V
V
RUN/SS1,
V
RUN/SS2
Rising
V
RUN/SS1,
V
RUN/SS2
Rising from 3V
Soft-Short Condition V
OSENSE1, 2
= 0.5V;
V
RUN/SS1, 2
= 4.5V
V
OSENSE1, 2
= 0.5V
V
OSENSE1, 2
= 0.7V, V
SENSE1–, 2
–
= 5V
V
OSENSE1, 2
= 0.7V, V
SENSE1–, 2–
= 5V
(Note 5)
C
LOAD
= 3300pF
C
LOAD
= 3300pF
(Note 5)
C
LOAD
= 3300pF
C
LOAD
= 3300pF
C
LOAD
= 3300pF Each Driver
C
LOAD
= 3300pF Each Driver
Tested with a Square Wave (Note 6)
●
3.5
0.84
–90
98
0.5
1.0
0.86
–60
99.4
1.2
1.5
4.1
0.5
2
1.6
65
62
75
75
55
55
45
45
80
80
100
V
RUN/SS1, 2
ON RUN/SS Pin ON Threshold
2.0
4.75
4
5
85
88
100
100
100
90
V
V
µA
µA
mV
mV
ns
ns
ns
ns
ns
ns
ns
3728lxff
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LTC3728L/LTC3728LX
elecTrical characTerisTics
SYMBOL
V
INTVCC
V
LDO
INT
V
LDO
EXT
V
EXTVCC
V
LDOHYS
f
NOM
f
LOW
f
HIGH
R
PLLIN
I
PLLFLTR
PARAMETER
Internal V
CC
Voltage
INTV
CC
Load Regulation
EXTV
CC
Voltage Drop
EXTV
CC
Switchover Voltage
EXTV
CC
Hysteresis
Nominal Frequency
Lowest Frequency
Highest Frequency
PLLIN Input Resistance
Phase Detector Output Current
Sinking Capability
Sourcing Capability
3.3V Regulator Output Voltage
3.3V Regulator Load Regulation
3.3V Regulator Line Regulation
PGOOD Voltage Low
PGOOD Leakage Current
PGOOD Trip Level, Either Controller
f
PLLIN
< f
OSC
f
PLLIN
> f
OSC
No Load
I
3.3
= 0 to 10mA
6V < V
IN
< 30V
I
PGOOD
= 2mA
V
PGOOD
= 5V
V
OSENSE
with Respect to Set Output Voltage
V
OSENSE
Ramping Negative
V
OSENSE
Ramping Positive
–6
6
–7.5
7.5
●
The
l
denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at T
A
= 25°C. V
IN
= 15V, V
RUN/SS1, 2
= 5V unless otherwise noted.
CONDITIONS
6V < V
IN
< 30V, V
EXTVCC
= 4V
I
CC
= 0 to 20mA, V
EXTVCC
= 4V
I
CC
= 20mA, V
EXTVCC
= 5V
I
CC
= 20mA, EXTV
CC
Ramping Positive
●
MIN
4.8
TYP
5.0
0.2
100
MAX
5.2
2.0
200
UNITS
V
%
mV
V
V
INTV
CC
Linear Regulator
4.5
4.7
0.2
Oscillator and Phase-Locked Loop
V
PLLFLTR
= 1.2V
V
PLLFLTR
= 0V
V
PLLFLTR
≥ 2.4V
360
230
480
400
260
550
50
–15
15
3.2
3.35
0.5
0.05
0.1
3.45
2
0.2
0.3
±1
– 9.5
9.5
440
290
590
kHz
kHz
kHz
kΩ
µA
µA
V
%
%
V
µA
%
%
3.3V Linear Regulator
V
3.3OUT
V
3.3IL
V
3.3VL
PGOOD Output
V
PGL
I
PGOOD
V
PG
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 formulas:
LTC3728LUH/LTC3728LXUH: T
J
= T
A
+ (P
D
• 34°C/W)
LTC3728LGN: T
J
= T
A
+ (P
D
• 95°C/W)
Note 3:
The IC is tested in a feedback loop that servos V
ITH1, 2
to a
specified voltage and measures the resultant V
OSENSE1, 2.
Note 4:
Dynamic supply current is higher due to the gate charge being
delivered at the switching frequency. See the Applications Information
section.
Note 5:
Rise and fall times are measured using 10% and 90% levels. Delay
times are measured using 50% levels.
Note 6:
The minimum on-time is tested under an ideal condition without
external power FETs. It can be larger when the IC is operating in an
actual circuit. See Minimum On-Time Considerations in the Applications
Information section.
Note 7:
The LTC3728LC/LTC3728LXC are guaranteed to meet performance
specifications from 0°C to 85°C. The LTC3728LE is guaranteed to meet
performance specifications over the –40°C to 85°C operating temperature
range as assured by design, characterization and correlation with statistical
process controls. The LTC3728LI is guaranteed to meet performance
specifications over the –40°C to 85°C operating temperature range.
Note 8:
Guaranteed by design.
3728lxff
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