FEATURES
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LTC3548-2
Dual Synchronous,
Fixed /Adjustable Output, 2.25MHz
Step-Down DC/DC Regulator
DESCRIPTION
The LTC
®
3548-2 is a dual, constant-frequency, synchro-
nous step-down DC/DC converter. Intended for low power
applications, it operates from 2.5V to 5.5V input voltage
range and has a constant 2.25MHz switching frequency,
allowing the use of tiny, low cost capacitors and inductors
with a profile ≤1.2mm. For channel 1, the output voltage is
fixed at 1.8V/800mA; for channel 2, the output voltage is
adjustable from 0.6V to V
IN
Internal synchronous 0.35Ω,
1.2A/0.7A power switches provide high efficiency without
the need for external Schottky diodes.
A user selectable mode input is provided to allow the
user to trade-off noise ripple for low power efficiency.
Burst Mode
®
operation provides high efficiency at light
loads, while pulse-skipping mode provides low noise
ripple at light loads.
To further maximize battery run time, the P-channel
MOSFETs are turned on continuously in dropout (100%
duty cycle), and both channels draw a total quiescent cur-
rent of only 40μA. In shutdown, the device draws <1μA.
L,
LT, LTC, LTM, Linear Technology, the Linear logo and Burst Mode are registered trademarks
of Linear Technology Corporation. All other trademarks are the property of their respective
owners. Protected by U.S. Patents, including 5481178, 6127815, 6304066, 6498466, 6580258
6611131..
High Efficiency: Up to 95%
1.8V/800mA Fixed Output and 400mA Adjustable
Output Voltage
Only 40μA Quiescent Current (Both Channels)
2.25MHz Constant-Frequency Operation
High Switch Current Limits: 1.2A and 0.7A
No Schottky Diodes Required
Low R
DS(ON)
Internal Switches: 0.35Ω
Current Mode Operation for Excellent Line
and Load Transient Response
Short-Circuit Protected
Low Dropout Operation: 100% Duty Cycle
Ultralow Shutdown Current: I
Q
< 1μA
Power-On Reset Output
Externally Synchronizable Oscillator
Small, Thermally Enhanced 3mm
×
3mm DFN Package
APPLICATIONS
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PDAs/Palmtop PCs
Digital Cameras
Cellular Phones
Portable Media Players
PC Cards
Wireless and DSL Modems
TYPICAL APPLICATION
V
IN
2.8V TO 5.5V
10μF
RUN2
V
IN
RUN1
POR
100k
95
RESET
90
EFFICIENCY (%)
LTC3548-2
V
OUT2
2.5V
400mA
4.7μH
68pF
SW2
SW1
2.2μH
V
OUT1
1.8V
800mA
85
100
MODE/SYNC
Efficiency and Power Loss
vs Load Current
1000
EFFICIENCY
100
POWER LOSS (mW)
POWER LOSS
80
75
70
V
IN
= 3.6V
1
V
OUT
= 1.8V
Burst Mode OPERATION
CHANNEL 1
NO LOAD ON CHANNEL 2
0.1
10
100
1000
LOAD CURRENT (mA)
35482 F01b
10
887k
4.7μF
V
FB2
280k
GND
V
OUT1
10μF
35482 F01
65
60
1
Figure 1. 2.5V/1.8V at 400mA/800mA Step-Down Regulators
35482fb
1
LTC3548-2
ABSOLUTE MAXIMUM RATINGS
(Note 1)
PIN CONFIGURATION
TOP VIEW
V
OUT1
RUN1
V
IN
SW1
GND
1
2
3
4
5
11
10
V
FB2
9
RUN2
8
POR
7
SW2
6
MODE/
SYNC
V
IN
Voltages .................................................– 0.3V to 6V
V
OUT1
, V
FB2
Voltages ........................ –0.3V to V
IN
+ 0.3V
RUN1, RUN2 Voltages .................................–0.3V to V
IN
MODE/SYNC Voltage........................ –0.3V to V
IN
+ 0.3V
SW1, SW2 Voltages ......................... –0.3V to V
IN
+ 0.3V
POR
Voltage ................................................. –0.3V to 6V
Ambient Operating Temperature Range
(Note 2)....................................................– 40°C to 85°C
Junction Temperature (Note 5) ............................. 125°C
Storage Temperature Range...................– 65°C to 125°C
DD PACKAGE
10-LEAD (3mm
×
3mm) PLASTIC DFN
T
JMAX
= 125°C,
θ
JA
= 40°C/W,
θ
JC
= 3°C/W (4-LAYER BOARD)
EXPOSED PAD (PIN 11) IS GND, MUST BE SOLDERED TO PCB GND
ORDER INFORMATION
LEAD FREE FINISH
LTC3548EDD-2#PBF
TAPE AND REEL
LTC3548EDD-2#TRPBF
PART MARKING
LCDK
PACKAGE DESCRIPTION
10-Lead (3mm
×
3mm) Plastic DFN
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/
ELECTRICAL CHARACTERISTICS
SYMBOL
V
IN
I
FB2
V
FB2
V
OUT1
ΔV
LINEREG
ΔV
LOADREG
I
S
PARAMETER
Operating Voltage Range
Feedback Pin Input Current
Feedback Voltage of Channel 2 (Note 3)
Output Voltage of Channel 1 (Note 3)
Reference Voltage Line Regulation
Output Voltage Load Regulation
Input DC Supply Current (Note 4)
Active Mode
Sleep Mode
Shutdown
Oscillator Frequency
Synchronization Frequency
Peak Switch Current Limit Channel 1
Peak Switch Current Limit Channel 2
Top Switch On-Resistance
Bottom Switch On-Resistance
Switch Leakage Current
The
l
denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at T
A
= 25°C. V
IN
= 3.6V, unless otherwise specified. (Note 2)
CONDITIONS
l
l
MIN
2.5
0.588
0.585
1.764
1.755
TYP
MAX
5.5
30
UNITS
V
nA
V
V
V
V
%/V
%
0°C ≤ T
A
≤ 85°C
–40°C ≤ T
A
≤ 85°C
0°C ≤ T
A
≤ 85°C
–40°C ≤ T
A
≤ 85°C
V
IN
= 2.5V to 5.5V (Note 3)
(Note 3)
V
OUT1
= 1.5V, V
FB2
= 0.5V
V
OUT1
= 1.9V, V
FB2
= 0.63V, MODE/SYNC = 3.6V
RUN = 0V, V
IN
= 5.5V, MODE/SYNC = 0V
V
OUT1
= 1.8V, V
FB2
= 0.6V
V
IN
= 3V, Duty Cycle <35%
V
IN
= 3V, V
FB2
= 0.5V, Duty Cycle <35%
(Note 6)
(Note 6)
V
IN
= 5V, V
RUN
= 0V, V
FB2
= 0V, V
OUT1
= 0V
l
l
0.6
0.6
1.8
1.8
0.3
0.5
700
40
0.1
0.612
0.612
1.836
1.836
0.5
950
60
1
2.7
1.6
0.9
0.45
0.45
1
μA
μA
μA
MHz
MHz
A
A
Ω
Ω
μA
35482fb
f
OSC
f
SYNC
I
LIM
R
DS(ON)
I
SW(LKG)
l
1.8
0.95
0.6
2.25
2.25
1.2
0.7
0.35
0.30
0.01
2
LTC3548-2
ELECTRICAL CHARACTERISTICS
SYMBOL
V
MODE
POR
PARAMETER
Mode Threshold Low
Mode Threshold High
Power-On Reset Threshold
Power-On Reset On-Resistance
Power-On Reset Delay
V
RUN
I
RUN
RUN Threshold
RUN Leakage Current
l
l
The
l
denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at T
A
= 25°C. V
IN
= 3.6V, unless otherwise specified. (Note 2)
CONDITIONS
MIN
0
V
IN
– 0.5
V
OUT1
, V
FB2
Ramping Down, MODE/SYNC = 0V
–8.5
100
262,144
0.3
1
0.01
1.5
1
200
TYP
MAX
0 .5
V
IN
UNITS
V
V
%
Ω
Cycles
V
μA
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:
The LTC3548-2 is guaranteed to meet specified performance 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:
The LTC3548-2 is tested in a proprietary test mode that connects
the output of the error amplifier to an outside servo loop.
Note 4:
Dynamic supply current is higher due to the internal gate charge
being delivered at the switching frequency.
Note 5:
T
J
is calculated from the ambient T
A
and power dissipation P
D
according to the following formula: T
J
= T
A
+ (P
D
•
θ
JA
).
Note 6:
The DFN switch on-resistance is guaranteed by correlation to
wafer level measurements.
TYPICAL PERFORMANCE CHARACTERISTICS
Burst Mode Operation
SW
5V/DIV
SW
5V/DIV
I
L
200mA/DIV
V
OUT
10mV/DIV
T
A
= 25°C unless otherwise specified.
Pulse-Skipping Mode
V
OUT
200mV/DIV
I
L
500mA/DIV
I
LOAD
500mA/DIV
Load Step
I
L
200mA/
DIV
V
OUT
20mV/DIV
V
IN
= 3.6V
2μs/DIV
V
OUT
= 1.8V
I
LOAD
= 60mA
CHANNEL 1; CIRCUIT OF FIGURE 3
35482 G01
V
IN
= 3.6V
1μs/DIV
V
OUT
= 1.8V
I
LOAD
= 30mA
CHANNEL 1; CIRCUIT OF FIGURE 3
35482 G02
V
IN
= 3.6V
20μs/DIV
V
OUT
= 1.8V
I
LOAD
= 80mA TO 800mA
CHANNEL 1; CIRCUIT OF FIGURE 3
35482 G03
35482fb
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LTC3548-2
TYPICAL PERFORMANCE CHARACTERISTICS
Efficiency vs Input Voltage
100
95
90
EFFICIENCY (%)
10mA
85
80
75
70
65
60
2
V
OUT
= 1.8V, CHANNEL 1
Burst Mode OPERATION
CIRCUIT OF FIGURE 3
3
4
INPUT VOLTAGE (V)
35482 G04
T
A
= 25°C unless otherwise specified.
Oscillator Frequency
vs Input Voltage
10
8
FREQUENCY DEVIATION (%)
Oscillator Frequency
vs Temperature
2.5
V
IN
= 3.6V
100mA
FREQUENCY (MHz)
2.4
6
4
2
0
–2
–4
–6
–8
1mA
800mA
2.3
2.2
2.1
5
6
2.0
–50 –25
50
25
75
0
TEMPERATURE (°C)
100
125
–10
2
3
4
V
IN
(V)
5
6
35482 G06
35482 G05
Reference Voltage
vs Temperature
0.615
0.610
REFERENCE VOLTAGE (V)
0.605
0.600
0.595
0.590
0.585
–50 –25
500
V
IN
= 3.6V
450
R
DS(ON)
vs Input Voltage
550
500
450
R
DS(ON)
(mΩ)
R
DS(ON)
(mΩ)
400
350
300
250
200
SYNCHRONOUS
SWITCH
MAIN
SWITCH
400
350
300
250
200
150
1
2
3
4
V
IN
(V)
5
6
7
35482 G08
R
DS(ON)
vs Junction Temperature
V
IN
= 2.7V
V
IN
= 4.2V
V
IN
= 3.6V
50
25
75
0
TEMPERATURE (°C)
100
125
100
–50 –25
MAIN SWITCH
SYNCHRONOUS SWITCH
25 50 75 100 125 150
0
JUNCTION TEMPERATURE (°C)
3548 G09
35482 G07
Efficiency vs Load Current
100
95
90
EFFICIENCY (%)
85
80
75
70
65
60
1
V
IN
= 3.6V, V
OUT
= 1.8V
NO LOAD ON OTHER CHANNEL
CHANNEL 1; CIRCUIT OF FIGURE 3
10
100
LOAD CURRENT (mA)
1000
35482 G10
Load Regulation
2.0
1.5
0.5
0.4
Burst Mode OPERATION
V
OUT
ERROR (%)
0.3
0.2
0.1
0
–0.1
–0.2
–0.3
–0.4
–0.5
Line Regulation
V
OUT
= 1.8V
I
OUT
= 200mA
Burst Mode OPERATION
V
OUT
ERROR (%)
1.0
0.5
0
PULSE-SKIPPING MODE
–0.5
–1.0
–1.5
–2.0
1
V
IN
= 3.6V, V
OUT
= 1.8V
NO LOAD ON OTHER CHANNEL
CHANNEL 1; CIRCUIT OF FIGURE 3
10
100
LOAD CURRENT (mA)
1000
35482 G11
PULSE-SKIPPING MODE
2
3
4
V
IN
(V)
5
6
35482 G12
35482fb
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LTC3548-2
TYPICAL PERFORMANCE CHARACTERISTICS
Efficiency vs Load Current
100
95
90
EFFICIENCY (%)
85
80
75
70
65
60
1
V
OUT2
= 1.5V
Burst Mode OPERATION
NO LOAD ON OTHER CHANNEL
CIRCUIT OF FIGURE 3
10
100
LOAD CURRENT (mA)
1000
35482 G13
T
A
= 25°C unless otherwise specified.
Efficiency vs Load Current
100
V
IN
= 3.6V
95
V
IN
= 2.7V
100
95
90
EFFICIENCY (%)
V
IN
= 4.2V
85
80
75
70
65
60
Efficiency vs Load Current
V
IN
= 3.6V
V
IN
= 2.7V
EFFICIENCY (%)
90
85
80
75
70
65
60
1
V
OUT2
= 2.5V
Burst Mode OPERATION
NO LOAD ON OTHER CHANNEL
CIRCUIT OF FIGURE 3
10
100
LOAD CURRENT (mA)
1000
35482 G14
V
IN
= 5V
V
IN
= 4.2V
V
OUT2
= 3.3 V
Burst Mode OPERATION
NO LOAD ON OTHER CHANNEL
CIRCUIT OF FIGURE 3
1
10
100
LOAD CURRENT (mA)
1000
35482 G15
PIN FUNCTIONS
V
OUT1
(Pin 1):
Output Voltage Feedback for Channel 1. An
internal resistive divider divides the output voltage down
for comparison to the internal reference voltage.
RUN1 (Pin 2):
Regulator 1 Enable. Forcing this pin to
V
IN
enables regulator 1, while forcing it to GND causes
regulator 1 to shut down. This pin must be driven; do
not float.
V
IN
(Pin 3):
Input Power Supply. Must be closely decoupled
to GND.
SW1 (Pin 4):
Regulator 1 Switch Node Connection to the
Inductor. This pin swings from V
IN
to GND.
GND (Pin 5):
Ground. This pin is not connected internally.
Connect to PCB ground for optimum shielding.
MODE/SYNC (Pin 6):
Combination Mode Selection and
Oscillator Synchronization. This pin controls the opera-
tion of the device. When tied to V
IN
or GND, Burst Mode
operation or pulse-skipping mode is selected, respectively.
Do not float this pin. The oscillation frequency can be
synchronized to an external oscillator applied to this pin
and pulse-skipping mode is automatically selected.
SW2 (Pin 7):
Regulator 2 Switch Node Connection to the
Inductor. This pin swings from V
IN
to GND.
POR
(Pin 8):
Power-On Reset. This common-drain logic
output is pulled to GND when the output voltage falls below
–8.5% of regulation and goes high after 117ms when both
channels are within regulation.
RUN2 (Pin 9):
Regulator 2 Enable. Forcing this pin to
V
IN
enables regulator 2, while forcing it to GND causes
regulator 2 to shut down. This pin must be driven; do
not float.
V
FB2
(Pin 10):
Output Feedback. Receives the feedback
voltage from the external resistive divider across the output.
Normal voltage for this pin is 0.6V.
Exposed Pad (GND) (Pin 11):
Ground. Connect to the
(–) terminal of C
OUT
, and (–) terminal of C
IN
. Must be
connected to PCB ground for electrical contact and rated
thermal performance.
35482fb
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