FEATURES
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LTC3728L/LTC3728LX
Dual, 550kHz, 2-Phase
Synchronous Regulators
DESCRIPTIO
The LTC
®
3728L/LTC3728LX are dual high performance
step-down switching regulator controllers that drive all
N-channel synchronous power MOSFET stages. A con-
stant 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.
, LTC and LT are registered trademarks of Linear Technology Corporation.
Burst Mode and OPTI-LOOP are registered trademarks of Linear Technology Corporation.
s
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
APPLICATIO S
s
s
s
s
s
Notebook and Palmtop Computers
Telecom Systems
Portable Instruments
Battery-Operated Digital Devices
DC Power Distribution Systems
TYPICAL APPLICATIO
+
4.7µF
M1
L1
3.2µH
D3
V
IN
PGOOD INTV
CC
TG1
C
B1
, 0.1µF
BOOST1
SW1
BG1
f
IN
500kHz
PLLIN
SENSE1
+
R
SENSE1
0.01Ω
V
OUT1
5V
5A
1000pF
SENSE1
–
V
OSENSE1
C
C1
220pF
R
C1
15k
I
TH1
C
SS1
0.1µF
SENSE2
–
V
OSENSE2
I
TH2
C
SS2
0.1µF
C
C2
220pF
R
C2
15k
LTC3728L/
LTC3728LX
TG2
BOOST2
SW2
BG2
PGND
SENSE2
+
1000pF
C
B2
, 0.1µF
D4
1µF
CERAMIC
M2
+
R2
105k
C
OUT1
1%
47µF
6V
SP
M1, M2: FDS6982S
R1
20k
1%
RUN/SS1 SGND RUN/SS2
Figure 1. High Efficiency Dual 5V/3.3V Step-Down Converter
U
U
U
V
IN
5.2V TO 28V
C
IN
22µF
50V
CERAMIC
L2
3.2µH
R
SENSE2
0.01Ω
V
OUT2
3.3V
5A
R3
20k
1%
R4
63.4k
1%
C
OUT
56µF
6V
SP
+
3728 F01
3728lxfa
1
LTC3728L/LTC3728LX
ABSOLUTE
AXI U
RATI GS
Input Supply Voltage (V
IN
)........................ 30V to – 0.3V
Top Side 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, Voltage ........... INTV
CC
to – 0.3V
PACKAGE/ORDER I FOR ATIO
TOP VIEW
RUN/SS1
SENSE1
+
SENSE
–
RUN/SS1
SENSE1
–
SENSE1
+
PGOOD
1
2
3
4
5
6
7
8
9
28 PGOOD
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
PLLFLTR
PLLIN
FCB
I
TH1
SGND
LTC3728LCGN
LTC3728LEGN
32 31 30 29 28 27 26 25
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
–
SW1
TG1
NC
NC
ORDER PART
NUMBER
3.3V
OUT
10
I
TH2
11
V
OSENSE2
12
SENSE2
–
13
SENSE2
+
14
V
OSENSE2
NC
+
TG2
RUN/SS2
SW2
SENSE2
SENSE2
NC
GN PACKAGE
28-LEAD NARROW PLASTIC SSOP
T
JMAX
= 125°C,
θ
JA
= 95°C/W
Consult LTC Marketing for parts specified with wider operating temperature ranges.
The
q
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.
SYMBOL
V
OSENSE1, 2
I
VOSENSE1, 2
V
REFLNREG
V
LOADREG
PARAMETER
Regulated Feedback Voltage
Feedback Current
Output Voltage Load Regulation
CONDITIONS
(Note 3); I
TH1, 2
Voltage = 1.2V (LTC3728LC)
q
(Note 3); I
TH1, 2
Voltage = 1.2V (LTC3728LE/LTC3728LX)
q
(Note 3)
(Note 3)
Measured in Servo Loop;
∆I
TH
Voltage = 1.2V to 0.7V
Measured in Servo Loop;
∆I
TH
Voltage = 1.2V to 2.0V
I
TH1, 2
= 1.2V; Sink/Source 5uA; (Note 3)
MIN
0.792
0.788
TYP
0.800
0.800
–5
0.002
q
q
ELECTRICAL CHARACTERISTICS
Main Control Loops
0.808
0.812
– 50
0.02
0.5
– 0.5
V
V
nA
%/V
%
%
mmho
3728lxfa
Reference Voltage Line Regulation V
IN
= 3.6V to 30V (Note 3)
g
m1, 2
Transconductance Amplifier g
m
2
U
U
W
W W
U
W
(Note 1)
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 ........................................ – 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
TOP VIEW
ORDER PART
NUMBER
24 BOOST1
23 V
IN
22 BG1
21 EXTV
CC
20 INTV
CC
19 PGND
18 BG2
17 BOOST2
LTC3728LCUH
LTC3728LEUH
LTC3728LXCUH
33
UH PART
MARKING
3728L
3728LE
3728LX
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
MAX
UNITS
0.1
– 0.1
1.3
LTC3728L/LTC3728LX
The
q
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.
SYMBOL
g
mGBW1, 2
I
Q
PARAMETER
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
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
q
q
ELECTRICAL CHARACTERISTICS
CONDITIONS
I
TH1, 2
= 1.2V; (Note 3)
(Note 4)
V
IN
= 15V; EXTV
CC
Tied to V
OUT1
; V
OUT1
= 5V
V
RUN/SS1, 2
= 0V
q
MIN
TYP
3
450
20
MAX
UNITS
MHz
µA
µA
V
µA
V
V
V
µA
%
µA
35
0.84
– 0.1
4.8
4
0.88
V
FCB
I
FCB
V
BINHIBIT
UVLO
V
OVL
I
SENSE
DF
MAX
I
RUN/SS1, 2
0.76
– 0.50
0.800
– 0.18
4.3
3.5
0.84
– 90
98
0.5
1.0
0.5
0.86
– 60
99.4
1.2
1.5
4.1
2
1.6
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
V
RUN/SS1, 2
LT RUN/SS Pin Latchoff Arming Threshold V
RUN/SS1,
V
RUN/SS2
Rising from 3V
I
SCL1, 2
RUN/SS Discharge Current
Soft Short Condition V
OSENSE1, 2
= 0.5V;
V
RUN/SS1, 2
= 4.5V
I
SDLHO
V
SENSE(MAX)
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
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
f
PLLIN
< f
OSC
f
PLLIN
> f
OSC
V
PLLFLTR
= 1.2V
V
PLLFLTR
= 0V
V
PLLFLTR
≥
2.4V
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)
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
q
q
65
62
75
75
55
55
45
45
80
80
100
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)
V
INTVCC
V
LDO
INT
V
LDO
EXT
V
EXTVCC
V
LDOHYS
f
NOM
f
LOW
f
HIGH
R
PLLIN
I
PLLFLTR
INTV
CC
Linear Regulator
4.8
5.0
0.2
100
4.5
4.7
0.2
360
230
480
400
260
550
50
–15
15
440
290
590
5.2
2.0
200
V
%
mV
V
V
kHz
kHz
kHz
kΩ
µA
µA
Oscillator and Phase-Locked Loop
3728lxfa
3
LTC3728L/LTC3728LX
The
q
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.
SYMBOL
V
3.3OUT
V
3.3IL
V
3.3VL
PGOOD Output
V
PGL
I
PGOOD
V
PG
PGOOD Voltage Low
PGOOD Leakage Current
PGOOD Trip Level, Either Controller
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
0.1
0.3
±1
– 9.5
9.5
V
µA
%
%
PARAMETER
3.3V Regulator Output Voltage
3.3V Regulator Load Regulation
3.3V Regulator Line Regulation
CONDITIONS
No Load
I
3.3
= 0 to 10mA
6V < V
IN
< 30V
q
ELECTRICAL CHARACTERISTICS
3.3V Linear Regulator
MIN
3.2
TYP
3.35
0.5
0.05
MAX
3.45
2
0.2
UNITS
V
%
%
Note 1:
Absolute Maximum Ratings are those values beyond which the life
of a device may be impaired.
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 Applications Information.
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 condition is specified for an inductor
peak-to-peak ripple current
≥
40% of I
MAX
(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.
TYPICAL PERFOR A CE CHARACTERISTICS
Efficiency vs Output Current
and Mode (Figure 13)
100
90
80
Burst Mode
OPERATION
100
V
IN
= 7V
90
90
EFFICIENCY (%)
EFFICIENCY (%)
60
50
40
30
20
10
0
0.001
80
EFFICIENCY (%)
70
FORCED
CONTINUOUS
MODE (PWM)
CONSTANT
FREQUENCY
(BURST DISABLE)
V
IN
= 15V
V
OUT
= 5V
f = 250kHz
0.1
0.01
1
OUTPUT CURRENT (A)
10
3728L G01
4
U W
Efficiency vs Output Current
(Figure 13)
100
Efficiency vs Input Voltage
(Figure 13)
V
IN
= 10V
V
IN
= 15V
V
IN
= 20V
80
70
70
60
V
OUT
= 5V
f = 250kHz
50
0.001
0.1
0.01
1
OUTPUT CURRENT (A)
60
50
10
3728L G02
V
OUT
= 5V
I
OUT
= 3A
f = 250kHz
5
25
15
INPUT VOLTAGE (V)
35
3728L G03
3728lxfa
LTC3728L/LTC3728LX
TYPICAL PERFOR A CE CHARACTERISTICS
Supply Current vs Input Voltage
and Mode (Figure 13)
1000
200
INTV
CC
AND EXTV
CC
SWITCH VOLTAGE (V)
800
SUPPLY CURRENT (µA)
EXTV
CC
VOLTAGE DROP (mV)
600
BOTH
CONTROLLERS ON
400
200
SHUTDOWN
0
0
5
20
15
10
INPUT VOLTAGE (V)
25
30
3728L G04
Internal 5V LDO Line Regulation
5.1
5.0
I
LOAD
= 1mA
INTV
CC
VOLTAGE (V)
4.9
4.8
4.7
4.6
4.5
4.4
0
5
20
15
10
INPUT VOLTAGE (V)
25
30
3728L
G07
V
SENSE
(mV)
V
SENSE
(mV)
Maximum Current Sense Threshold
vs V
RUN/SS
(Soft-Start)
80
V
SENSE(CM)
= 1.6V
80
60
V
SENSE
(mV)
V
SENSE
(mV)
72
V
SENSE
(mV)
40
20
64
0
0
1
2
3
V
RUN/SS
(V)
3728L G10
4
U W
5
6
EXTV
CC
Voltage Drop
5.05
5.00
4.95
4.90
4.85
4.80
4.75
INTV
CC
and EXTV
CC
Switch
Voltage vs Temperature
INTV
CC
VOLTAGE
150
100
50
EXTV
CC
SWITCHOVER THRESHOLD
0
0
10
20
CURRENT (mA)
30
40
3728L
G05
4.70
– 50 – 25
50
25
75
0
TEMPERATURE (°C)
100
125
3728L G06
Maximum Current Sense Threshold
vs Duty Factor
75
Maximum Current Sense Threshold
vs Percent of Nominal Output
Voltage (Foldback)
80
70
60
50
50
40
30
20
10
25
0
0
20
40
60
DUTY FACTOR (%)
80
100
3728L G08
0
50
100
0
25
75
PERCENT ON NOMINAL OUTPUT VOLTAGE (%)
3728L
G09
Maximum Current Sense Threshold
vs Sense Common Mode Voltage
90
80
76
70
60
50
40
30
20
10
0
–10
–20
60
0
1
3
4
2
COMMON MODE VOLTAGE (V)
5
3728L G11
Current Sense Threshold
vs I
TH
Voltage
68
–30
0
0.5
1
1.5
V
ITH
(V)
2
2.5
3728L G12
3728lxfa
5
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