LTC3605
15V, 5A Synchronous
Step-Down Regulator
FEATURES
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DESCRIPTION
The LTC
®
3605 is a high efficiency, monolithic synchronous
buck regulator using a phase lockable controlled on-time
constant frequency, current mode architecture. PolyPhase
operation allows multiple LTC3605 regulators to run out of
phase while using minimal input and output capacitance.
The operating supply voltage range is from 15V down to
4V, making it suitable for dual lithium-ion battery inputs
as well as point of load power supply applications from
a 12V or 5V rail.
The operating frequency is programmable from 800kHz to
4MHz with an external resistor. The high frequency capabil-
ity allows the use of small surface mount inductors. For
switching noise sensitive applications, it can be externally
synchronized from 800kHz to 4MHz. The PHMODE pin
allows user control of the phase of the outgoing clock
signal. The unique constant frequency/controlled on-time
architecture is ideal for high step-down ratio applications
that are operating at high frequency while demanding
fast transient response. Two internal phase-lock loops
synchronize the internal oscillator to the external clock
and also servos the regulator on-time to lock on to either
the internal clock or the external clock if it’s present.
L,
LT, LTC, LTM and Polyphase are registered trademarks of Linear Technology Corporation.
All other trademarks are the property of their respective owners. Protected by U.S. Patents,
including 5481178, 5847554, 6580258, 6304066, 6476589, 6774611.
High Efficiency: Up to 96%
5A Output Current
4V to 15V V
IN
Range
Integrated Power N-Channel MOSFETs
(70mΩ Top and 35mΩ Bottom)
Adjustable Frequency 800kHz to 4MHz
PolyPhase
®
Operation (Up to 12 Phases)
Output Tracking
0.6V ±1% Reference Accuracy
Current Mode Operation for Excellent Line and Load
Transient Response
Shutdown Mode Draws Less than 15μA Supply
Current
Available in 24-Pin (4mm
×
4mm) QFN Package
APPLICATIONS
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Point of Load Power Supply
Portable Instruments
Distributed Power Systems
Battery-Powered Equipment
TYPICAL APPLICATION
High Efficiency 1MHz, 5A Step-Down Regulator
V
IN
4V TO 15V
22μF
2
Efficiency and Power Loss
100
V
IN
= 12V
90 f = 1MHz
80
EFFICIENCY (%)
70
60
50
40
30
20
10
PGND
0
10
100
1000
OUTPUT CURRENT (mA)
0.1
10000
3605 TA01b
V
OUT
= 3.3V
10
PV
IN
CLKOUT
CLKIN
SV
IN
INTV
CC
POWER LOSS (W)
2.2μF
0.1μF
V
OUT
= 1.2V
1
BOOST
PGOOD
LTC3605
SW
V
ON
V
IN
RUN
RT
FB
ITH
L
11.5k
2.55k
47μF
2
V
OUT
3.3V
V
OUT
= 3.3V
V
OUT
= 1.2V
0
16k
162k
220pF
3605 TA01a
3605fa
1
LTC3605
ABSOLUTE MAXIMUM RATINGS
(Note 1)
PIN CONFIGURATION
TOP VIEW
CLKOUT
INTV
CC
BOOST
CLKIN
SGND
SV
IN
18 PV
IN
17 PV
IN
25
16 SW
15 SW
14 SW
13 SW
7
RUN
8
PGOOD
9 10 11 12
V
ON
PGND
SW
SW
PV
IN
, SV
IN
, SW Voltage .............................. –0.3V to 15V
PV
IN
, SW Transient Voltage ........................ –2V to 17.5V
BOOST Voltage .......................... –0.3V to PV
IN
+ INTV
CC
RUN Voltage ............................................. –0.3V to SV
IN
V
ON
Voltage ............................................... –0.3V to SV
IN
INTV
CC
Voltage.......................................... –0.3V to 3.6V
ITH, RT, CLKOUT, PGOOD Voltage ......... –0.3V to INTV
CC
CLKIN, PHMODE, MODE Voltage .......... –0.3V to INTV
CC
TRACK/SS, FB Voltage .......................... –0.3V to INTV
CC
Operating Temperature Range (Note 2)....–40°C to 85°C
Junction Temperature (Note 5) ............................. 125°C
Storage Temperature Range...................–65°C to 125°C
24 23 22 21 20 19
RT 1
PHMODE 2
MODE 3
FB 4
TRACK/SS 5
ITH 6
UF PACKAGE
24-LEAD (4mm 4mm) PLASTIC QFN
T
JMAX
= 125°C,
θ
JA
= 37°C/W
EXPOSED PAD (PIN 25) IS PGND, MUST BE SOLDERED TO PCB
ORDER INFORMATION
LEAD FREE FINISH
LTC3605EUF#PBF
TAPE AND REEL
LTC3605EUF#TRPBF
PART MARKING
3605
PACKAGE DESCRIPTION
24-Lead (4mm
×
4mm) Plastic QFN
TEMPERATURE RANGE
–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/
3605fa
2
LTC3605
ELECTRICAL CHARACTERISTICS
SYMBOL
PV
IN
I
Q
PARAMETER
V
IN
Supply Range
Input DC Supply Current
Active
Shutdown
Feedback Reference Voltage
Feedback Voltage Line
Regulation
Feedback Voltage Load
Regulation
Feedback Pin Input Current
Error Amplifier Transconductance
Minimum On-Time
Minimum Off-Time
Positive Inductor Valley Current Limit
Top Power NMOS On-Resistance
Bottom Power NMOS On-Resistance
INTV
CC
Undervoltage Lockout
Threshold
Run Threshold 2 (I
Q
= 2mA)
Run Threshold 1 (I
Q
= 400μA)
Internal V
CC
Voltage
INTV
CC
Load Regulation
Output Overvoltage
PGOOD Upper Threshold
Output Undervoltage
PGOOD Lower Threshold
PGOOD Hysteresis
PGOOD Pull-Down Resistance
PGOOD Leakage
TRACK Pull-Up Current
Oscillator Frequency
CLKIN Threshold
R
T
= 162k
l
The
l
denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at T
A
= 25°C.
CONDITIONS
(Note 3)
Mode = 0, R
T
= 162k
V
IN
=12V, RUN = 0
ITH =1.2V (Note 4) –40°C to 85°C
–40°C to 125°C
V
IN
= 4V to 15V
ITH = 1.2V
ITH = 0.8V to 1.6V
l
l
l
l
MIN
4
TYP
MAX
15
UNITS
V
mA
μA
V
V
%/V
%
nA
mS
ns
ns
2
11
0.594
0.592
0.600
0.600
0.001
0.1
5
40
0.606
0.608
0.02
0.2
±30
V
FB
ΔV
FB(LINE)
ΔV
FB(LOAD)
I
FB
g
m
(EA)
t
ON(MIN)
t
OFF(MIN)
I
LIM
R
TOP
R
BOTTOM
V
UVLO
V
RUN
V
INTVCC
ΔV
INTVCC
OV
UV
ΔV
FB(HYS)
R
PGOOD
I
PGOOD
I
TRACK/SS
f
OSC
CLKIN
ITH = 1.2V
1.15
1.35
40
70
1.6
V
FB
= 0.57V
INTV
CC
= 3.3V
INTV
CC
= 3.3V
INTV
CC
Falling
INTV
CC
Hysteresis (Rising)
RUN Rising
RUN Rising
4V < V
IN
< 15V
I
LOAD
= 0mA to 20mA
V
FB
Rising
V
FB
Falling
V
FB
Returning
1mA Load
0.54V < V
FB
< 0.66V
5
6
70
35
7.5
150
55
2.8
1.3
0.75
3.4
13
–7
A
mΩ
mΩ
V
V
V
V
V
%
%
%
%
2.4
1.2
0.45
3.2
7
–13
2.6
0.25
1.25
0.6
3.3
0.5
10
–10
1.5
12
2.5
25
2
4
1.2
Ω
μA
μA
MHz
V
0.85
1
0.7
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. Absolute Maximum Ratings are those values beyond
which the life of a device may be impaired.
Note 2:
The LTC3605E is guaranteed to meet performance specifications
from 0°C to 85°C. Specifications over the –40°C to 85°C operating
temperature range are assured by design, characterization and correlation
with statistical process controls.
Note 3:
Dynamic supply current is higher due to the internal gate charge
being delivered at the switching frequency.
Note 4:
The LTC3605 is tested in a feedback loop that adjusts V
FB
to
achieve a specified error amplifier output voltage (ITH).
Note 5:
T
J
is calculated from the ambient temperature T
A
and
power dissipation as follows: T
J
= T
A
+ P
D
• (37°C/W). See Thermal
Considerations 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.
3605fa
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LTC3605
TYPICAL PERFORMANCE CHARACTERISTICS
T
A
= 25°C unless otherwise specified.
Quiescent Current vs V
IN
2.18
2.16
QUIESCENT CURRENT (mA)
2.14
2.12
2.10
2.08
2.06
2.04
2.02
2.00
0
3
6
9
V
IN
(V)
12
15
3605 G01
Shutdown Current vs V
IN
14
13
SHUTDOWN CURRENT (μA)
20
12
I
INTVCC
(mA)
11
10
9
8
7
6
0
0
3
6
9
V
IN
(V)
12
15
3605 G02
I
INTVCC
Current vs Frequency
25
MODE = 3.3V
NO LOAD
MODE = 0V
NO LOAD
15
10
5
0
0.5
1
1.5 2 2.5 3 3.5
FREQUENCY (MHz)
4
4.5
3605 G03
R
DS(ON)
vs Temperature
120
100
TOP FET
R
DS(ON)
(mΩ)
80
60
BOTTOM FET
40
20
0
–45 –20
NORMALIZED (%)
0.5
0
–0.5
–1.0
–1.5
1.5
1.0
Load Regulation
INTERNAL ITH
COMPENSATION
(ITH = 3.3V)
V
IN
= 12V
V
OUT
= 1.2V
f = 1MHz
MODE = INTV
CC
V
OUT
50mV/DIV
AC COUPLED
I
OUT
5A/DIV
Load Step
(External ITH Compensation)
EXTERNAL ITH
COMPENSATION
I
L
5A/DIV
V
IN
= 12V
V
OUT
= 1.2V
I
OUT
= 0A TO 5A
20μs/DIV
3605 G06
5
30 55 80 105 130
TEMPERATURE (°C)
3605 G04
0
1
2
4
3
I
OUT
(A)
5
6
7
3605 G05
Load Step
(Internal ITH Compensation)
Output Tracking
4.5
4.0
Switching Frequency vs R
T
FREQUENCY (MHz)
V
OUT
50mV/DIV
AC COUPLED
3.5
V
TRACK
V
FB
3.0
2.5
2.0
1.5
1.0
V
IN
= 12V
V
OUT
= 1.2V
500μs/DIV
3605 G08
I
OUT
5A/DIV
V
IN
= 12V
V
OUT
= 3.3V
ITH = 3.3V
I
OUT
= 0A TO 5A
20μs/DIV
3605 G07
0.5
0
0
50 100 150 200 250 300 350 400 450 500
R
T
(kΩ)
3605 G09
3605fa
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LTC3605
TYPICAL PERFORMANCE CHARACTERISTICS
T
A
= 25°C unless otherwise specified.
Switch Leakage vs V
IN
350
300
SWITCH LEAKAGE (nA)
250
200
150
100
50
TOP SWITCH
0
0
4
8
12
V
IN
(V)
3605 G10
Efficiency vs V
IN
98
96
I
LOAD
= 1A
FREQUENCY (MHz)
EFFICIENCY (%)
V
OUT
= 3.3V
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0
3
6
9
V
IN
(V)
12
15
3605 G11
Frequency vs V
ON
Voltage
V
IN
= 14V
R
T
= 162k
BOTTOM SWITCH
94
92
90
88
86
84
16
20
I
LOAD
= 5A
0
0
2
4
8
6
V
ON
(V)
10
12
14
3605 G12
Current Limit Foldback
NORMALIZED MAXIMUM OUTPUT CURRENT (%)
120
100
NORMALIZED INTV
CC
(%)
80
60
40
20
0
0
0.1
0.2
0.3 0.4
V
FB
(V)
0.5
0.6
0.7
101
100
99
98
97
96
95
94
93
92
91
90
INTV
CC
Load Regulation
T
A
= 25°C
1.30
1.25
RUN THRESHOLD (V)
1.20
1.15
1.10
1.05
RUN Pin Threshold vs Temperature
0
80 100 120
60
40
20
INTV
CC
OUTPUT CURRENT (mA)
140
1.00
–40
–15
60
35
85
10
TEMPERATURE (°C)
110
3605 G15
3605 G13
3605 G14
DCM Operation
CLKOUT
2V/DIV
CLKOUT
2V/DIV
CCM Operation
V
SW
5V/DIV
I
L
2A/DIV
V
SW
5V/DIV
I
L
2A/DIV
V
IN
= 12V
V
OUT
= 1.2V
MODE = 0
I
OUT
= 0
L1 = 0.33μH
400ns/DIV
3605 G16
V
IN
= 12V
400ns/DIV
V
OUT
= 1.2V
MODE = 3.3V
PHMODE = 3.3V
I
OUT
= 0
L1 = 0.33μH
3605 G17
3605fa
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