1
LTC3728
3728fa
Dual, 550kHz, 2-Phase
Synchronous Step-Down Switching Regulator
Figure 1. High Efficiency Dual 5V/3.3V Step-Down Converter
s
Dual, 180
°
Phased Controllers Reduce Required
Input Capacitance and Power Supply Induced Noise
s
OPTI-LOOP
®
Compensation Minimizes C
OUT
s
±
1% Output Voltage Accuracy
s
Power Good Output Voltage Indicator
s
Phase-Lockable Fixed Frequency 250kHz to 550kHz
s
Available in 5mm
×
5mm QFN and
28-Pin SSOP Packages
s
Dual N-Channel MOSFET Synchronous Drive
s
Wide V
IN
Range: 3.5V to 36V Operation
s
Very Low Dropout Operation: 99% Duty Cycle
s
Adjustable Soft-Start Current Ramping
s
Foldback Output Current Limiting
s
Latched Short-Circuit Shutdown with Defeat Option
s
Output Overvoltage Protection
s
Remote Output Voltage Sense
s
Low Shutdown I
Q
: 20
µ
A
s
5V and 3.3V Regulators
s
3 Selectable Operating Modes: Constant Frequency,
Burst Mode
®
Operation and PWM
The LTC
®
3728 is a dual high performance step-down
switching regulator controller that drives all N-channel
synchronous power MOSFET stages. A constant fre-
quency 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 3.5V to 30V (36V 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 LTC3728 includes a power good
output pin that indicates when both outputs are within
7.5% of their designed set point.
s
Notebook and Palmtop Computers
s
Telecom Systems
s
Portable Instruments
s
Battery-Operated Digital Devices
s
DC Power Distribution Systems
, LTC and LT are registered trademarks of Linear Technology Corporation.
Burst Mode and OPTI-LOOP are registered trademarks of Linear Technology Corporation.
APPLICATIO S
U
TYPICAL APPLICATIO
U
DESCRIPTIO
U
FEATURES
+
4.7
µ
F
D3
D4
M1
C
B1
, 0.1
µ
F
R2
105k
1%
1000pF
L1
3.2
µ
H
C
C1
220pF
1
µ
F
CERAMIC
C
IN
22
µ
F
50V
CERAMIC
+
C
OUT1
47
µ
F
6V
SP
R
SENSE1
0.01
R1
20k
1%
R
C1
15k
V
OUT1
5V
5A
M2
C
B2
, 0.1
µ
F
R4
63.4k
1%
L2
3.2
µ
H
C
C2
220pF
1000pF
+
C
OUT
56
µ
F
6V
SP
R
SENSE2
0.01
R3
20k
1%
R
C2
15k
V
OUT2
3.3V
5A
TG1
TG2
BOOST1
BOOST2
SW1
SW2
BG1
BG2
SGND
PGND
SENSE1
+
SENSE2
+
SENSE1
SENSE2
V
OSENSE1
V
OSENSE2
I
TH1
I
TH2
V
IN
PGOOD INTV
CC
RUN/SS1
RUN/SS2
V
IN
5.2V TO 28V
M1, M2: FDS6982S
3728 F01
C
SS1
0.1
µ
F
C
SS2
0.1
µ
F
LTC3728
PLLIN
f
IN
500kHz
2
LTC3728
3728fa
ORDER PART
NUMBER
LTC3728EG
Input Supply Voltage (V
IN
).........................36V to 0.3V
Top Side Driver Voltages
(BOOST1, BOOST2) ...................................42V to 0.3V
Switch Voltage (SW1, SW2) .........................36V 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
T
JMAX
= 125
°
C,
JA
= 95
°
C/W
1
2
3
4
5
6
7
8
9
10
11
12
13
14
TOP VIEW
G PACKAGE
28-LEAD PLASTIC SSOP
28
27
26
25
24
23
22
21
20
19
18
17
16
15
RUN/SS1
SENSE1
+
SENSE1
V
OSENSE1
PLLFLTR
PLLIN
FCB
I
TH1
SGND
3.3V
OUT
I
TH2
V
OSENSE2
SENSE2
SENSE2
+
PGOOD
TG1
SW1
BOOST1
V
IN
BG1
EXTV
CC
INTV
CC
PGND
BG2
BOOST2
SW2
TG2
RUN/SS2
ABSOLUTE AXI U RATI GS
W
W
W
U
PACKAGE/ORDER I FOR ATIO
U
U
W
(Note 1)
PLLIN, PLLFLTR, FCB, Voltage ............ INTV
CC
to 0.3V
I
TH1,
I
TH2
, V
OSENSE1
, V
OSENSE2
Voltages ... 2.7V to 0.3V
Peak Output Current <10
µ
s (TG1, TG2, BG1, BG2) ... 3A
INTV
CC
Peak Output Current ................................ 50mA
Operating Temperature Range (Note 7) ... 40
°
C to 85
°
C
Junction Temperature (Note 2) ............................. 125
°
C
Storage Temperature Range ................. 65
°
C to 150
°
C
Lead Temperature (Soldering, 10 sec)
(G Package Only) .................................................. 300
°
C
32 31 30 29 28 27 26 25
9
10 11 12 13
TOP VIEW
14 15 16
17
18
19
20
21
22
23
24
8
7
6
5
4
3
2
1
V
OSENSE1
PLLFLTR
PLLIN
FCB
I
TH1
SGND
3.3V
OUT
I
TH2
BOOST1
V
IN
BG1
EXTV
CC
INTV
CC
PGND
BG2
BOOST2
NC
SENSE1
SENSE1
+
NC
RUN/SS1
PGOOD
TG1
SW1
V
OSENSE2
NC
SENSE2
SENSE2
+
RUN/SS2
TG2
SW2
NC
ORDER PART
NUMBER
LTC3728EUH
T
JMAX
= 125
°
C,
JA
= 34
°
C/W
EXPOSED PAD IS SGND
(MUST BE SOLDERED TO PCB)
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.
ELECTRICAL CHARACTERISTICS
SYMBOL
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
Main Control Loops
V
OSENSE1, 2
Regulated Feedback Voltage
(Note 3); I
TH1, 2
Voltage = 1.2V
q
0.792
0.800
0.808
V
I
VOSENSE1, 2
Feedback Current
(Note 3)
5
50
nA
V
REFLNREG
Reference Voltage Line Regulation
V
IN
= 3.6V to 30V (Note 3)
0.002
0.02
%/V
V
LOADREG
Output Voltage Load Regulation
(Note 3)
Measured in Servo Loop;
I
TH
Voltage = 1.2V to 0.7V
q
0.1
0.5
%
Measured in Servo Loop;
I
TH
Voltage = 1.2V to 2.0V
q
0.1
0.5
%
g
m1, 2
Transconductance Amplifier g
m
I
TH1, 2
= 1.2V; Sink/Source 5uA; (Note 3)
1.3
mmho
g
mGBW1, 2
Transconductance Amplifier GBW
I
TH1, 2
= 1.2V; (Note 3)
3
MHz
Consult LTC Marketing for parts specified with wider operating temperature ranges.
UH PART
MARKING
3728
3
LTC3728
3728fa
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.
ELECTRICAL CHARACTERISTICS
SYMBOL
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
I
Q
Input DC Supply Current
(Note 4)
Normal Mode
V
IN
= 15V; EXTV
CC
Tied to V
OUT1
; V
OUT1
= 5V
450
µ
A
Shutdown
V
RUN/SS1, 2
= 0V
20
35
µ
A
V
FCB
Forced Continuous Threshold
q
0.76
0.800
0.84
V
I
FCB
Forced Continuous Pin Current
V
FCB
= 0.85V
0.50
0.18
0.1
µ
A
V
BINHIBIT
Burst Inhibit (Constant Frequency)
Measured at FCB pin
4.3
4.8
V
Threshold
UVLO
Undervoltage Lockout
V
IN
Ramping Down
q
3.5
4
V
V
OVL
Feedback Overvoltage Lockout
Measured at V
OSENSE1, 2
q
0.84
0.86
0.88
V
I
SENSE
Sense Pins Total Source Current
(Each Channel); V
SENSE1
, 2
= V
SENSE1
+
, 2
+
= 0V
85
60
µ
A
DF
MAX
Maximum Duty Factor
In Dropout
98
99.4
%
I
RUN/SS1, 2
Soft-Start Charge Current
V
RUN/SS1, 2
= 1.9V
0.5
1.2
µ
A
V
RUN/SS1, 2
ON RUN/SS Pin ON Threshold
V
RUN/SS1,
V
RUN/SS2
Rising
1.0
1.5
1.9
V
V
RUN/SS1, 2
LT RUN/SS Pin Latchoff Arming Threshold V
RUN/SS1,
V
RUN/SS2
Rising from 3V
3.8
4.5
V
I
SCL1, 2
RUN/SS Discharge Current
Soft Short Condition V
OSENSE1, 2
= 0.5V;
0.5
2
4
µ
A
V
RUN/SS1, 2
= 4.5V
I
SDLHO
Shutdown Latch Disable Current
V
OSENSE1, 2
= 0.5V
1.6
5
µ
A
V
SENSE(MAX)
Maximum Current Sense Threshold
V
OSENSE1, 2
= 0.7V,V
SENSE1, 2
= 5V
65
75
85
mV
V
OSENSE1, 2
= 0.7V,V
SENSE1, 2
= 5V
q
62
75
88
mV
TG Transition Time:
(Note 5)
TG1, 2 t
r
Rise Time
C
LOAD
= 3300pF
50
90
ns
TG1, 2 t
f
Fall Time
C
LOAD
= 3300pF
50
90
ns
BG Transition Time:
(Note 5)
BG1, 2 t
r
Rise Time
C
LOAD
= 3300pF
40
90
ns
BG1, 2 t
f
Fall Time
C
LOAD
= 3300pF
40
80
ns
TG/BG t
1D
Top Gate Off to Bottom Gate On Delay
Synchronous Switch-On Delay Time
C
LOAD
= 3300pF Each Driver
90
ns
BG/TG t
2D
Bottom Gate Off to Top Gate On Delay
Top Switch-On Delay Time
C
LOAD
= 3300pF Each Driver
90
ns
t
ON(MIN)
Minimum On-Time
Tested with a Square Wave (Note 6)
100
ns
INTV
CC
Linear Regulator
V
INTVCC
Internal V
CC
Voltage
6V < V
IN
< 30V, V
EXTVCC
= 4V
4.8
5.0
5.2
V
V
LDO
INT
INTV
CC
Load Regulation
I
CC
= 0 to 20mA, V
EXTVCC
= 4V
0.2
1.0
%
V
LDO
EXT
EXTV
CC
Voltage Drop
I
CC
= 20mA, V
EXTVCC
= 5V
80
160
mV
V
EXTVCC
EXTV
CC
Switchover Voltage
I
CC
= 20mA, EXTV
CC
Ramping Positive
q
4.5
4.7
V
V
LDOHYS
EXTV
CC
Hysteresis
0.2
V
Oscillator and Phase-Locked Loop
f
NOM
Nominal Frequency
V
PLLFLTR
= 1.2V
360
400
440
kHz
f
LOW
Lowest Frequency
V
PLLFLTR
= 0V
230
260
290
kHz
f
HIGH
Highest Frequency
V
PLLFLTR
2.4V
480
550
590
kHz
R
PLLIN
PLLIN Input Resistance
50
k
I
PLLFLTR
Phase Detector Output Current
Sinking Capability
f
PLLIN
< f
OSC
15
µ
A
Sourcing Capability
f
PLLIN
> f
OSC
15
µ
A
4
LTC3728
3728fa
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:
LTC3728: T
J
= T
A
+ (P
D
· 95
°
C/W)
Note 3: The LTC3728 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 LTC3728E is guaranteed to meet performance specifications
from 0
°
C to 70
°
C. Specifications over the 40
°
C to 85
°
C operating
temperature range are assured by design, characterization and correlation
with statistical process controls.
Efficiency vs Output Current
and Mode (Figure 13)
OUTPUT CURRENT (A)
0.001
0
EFFICIENCY (%)
10
30
40
50
100
70
0.01
0.1
1
3728 G01
20
80
90
60
10
FORCED
CONTINUOUS
MODE (PWM)
CONSTANT
FREQUENCY
(BURST DISABLE)
Burst Mode
OPERATION
V
IN
= 15V
V
OUT
= 5V
f = 250kHz
OUTPUT CURRENT (A)
0.001
EFFICIENCY (%)
70
80
10
3728 G02
60
50
0.01
0.1
1
100
90
V
IN
= 10V
V
IN
= 15V
V
IN
= 7V
V
IN
= 20V
V
OUT
= 5V
f = 250kHz
INPUT VOLTAGE (V)
5
EFFICIENCY (%)
70
80
3728 G03
60
50
15
25
35
100
V
OUT
= 5V
I
OUT
= 3A
f = 250kHz
90
Efficiency vs Output Current
(Figure 13)
Efficiency vs Input Voltage
(Figure 13)
TYPICAL PERFOR A CE CHARACTERISTICS
U
W
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.
ELECTRICAL CHARACTERISTICS
SYMBOL
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
3.3V Linear Regulator
V
3.3OUT
3.3V Regulator Output Voltage
No Load
q
3.25
3.35
3.45
V
V
3.3IL
3.3V Regulator Load Regulation
I
3.3
= 0 to 10mA
0.5
2
%
V
3.3VL
3.3V Regulator Line Regulation
6V < V
IN
< 30V
0.05
0.2
%
PGOOD Output
V
PGL
PGOOD Voltage Low
I
PGOOD
= 2mA
0.1
0.3
V
I
PGOOD
PGOOD Leakage Current
V
PGOOD
= 5V
±
1
µ
A
V
PG
PGOOD Trip Level, Either Controller
V
OSENSE
with Respect to Set Output Voltage
V
OSENSE
Ramping Negative
6
7.5
9.5
%
V
OSENSE
Ramping Positive
6
7.5
9.5
%
5
LTC3728
3728fa
Internal 5V LDO Line Regulation
Maximum Current Sense Threshold
vs Duty Factor
Maximum Current Sense Threshold
vs Percent of Nominal Output
Voltage (Foldback)
INPUT VOLTAGE (V)
0
4.8
4.9
5.1
15
25
3728
G07
4.7
4.6
5
10
20
30
35
4.5
4.4
5.0
INTV
CC
VOLTAGE (V)
I
LOAD
= 1mA
DUTY FACTOR (%)
0
0
V
SENSE
(mV)
25
50
75
20
40
60
80
3728 G08
100
PERCENT ON NOMINAL OUTPUT VOLTAGE (%)
0
V
SENSE
(mV)
40
50
60
100
3728
G09
30
20
0
25
50
75
10
80
70
Maximum Current Sense Threshold
vs V
RUN/SS
(Soft-Start)
V
RUN/SS
(V)
0
0
V
SENSE
(mV)
20
40
60
80
1
2
3
4
3728 G10
5
6
V
SENSE(CM)
= 1.6V
Maximum Current Sense Threshold
vs Sense Common Mode Voltage
COMMON MODE VOLTAGE (V)
0
V
SENSE
(mV)
72
76
80
4
3728 G11
68
64
60
1
2
3
5
Current Sense Threshold
vs I
TH
Voltage
V
ITH
(V)
0
V
SENSE
(mV)
30
50
70
90
2
3728 G12
10
10
20
40
60
80
0
20
30
0.5
1
1.5
2.5
TYPICAL PERFOR A CE CHARACTERISTICS
U
W
INTV
CC
and EXTV
CC
Switch
Voltage vs Temperature
Supply Current vs Input Voltage
and Mode (Figure 13)
INPUT VOLTAGE (V)
0
5
0
SUPPLY CURRENT (
µ
A)
400
1000
10
20
25
3728 G04
200
800
600
15
30
35
BOTH
CONTROLLERS ON
SHUTDOWN
EXTV
CC
Voltage Drop
CURRENT (mA)
0
EXTV
CC
VOLTAGE DROP (mV)
150
200
250
40
3728
G05
100
50
0
10
20
30
50
TEMPERATURE (
°
C)
50
INTV
CC
AND EXTV
CC
SWITCH VOLTAGE (V)
4.95
5.00
5.05
25
75
3728 G06
4.90
4.85
25
0
50
100
125
4.80
4.70
4.75
INTV
CC
VOLTAGE
EXTV
CC
SWITCHOVER THRESHOLD
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