LTC3561
1A, 4MHz, Synchronous
Step-Down DC/DC Converter
FEATURES
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DESCRIPTIO
Uses Tiny Capacitors and Inductor
High Frequency Operation: Up to 4MHz
High Switch Current: 1.4A
Low R
DS(ON)
Internal Switches: 0.110Ω
High Efficiency: Up to 95%
V
IN
: 2.63V to 5.5V
Stable with Ceramic Capacitors
Current Mode Operation for Excellent Line
and Load Transient Response
Short-Circuit Protected
Low Dropout Operation: 100% Duty Cycle
Low Shutdown Current: I
Q
≤
1µA
Low Quiescent Current: 240µA
Output Voltages from 0.8V to 5V
Low Noise Pulse-Skipping Operation
Small 8-Pin DFN Package
The LTC
®
3561 is a constant-frequency, synchronous,
step-down DC/DC converter. Intended for medium power
applications, it operates from a 2.63V to 5.5V input voltage
range and has a user configurable operating frequency up
to 4MHz, allowing the use of tiny, low cost capacitors and
inductors 2mm or less in height. The output voltage is
adjustable from 0.8V to 5V. Internal synchronous 0.11Ω
power switches with 1.4A peak current ratings provide
high efficiency. The LTC3561’s current mode architecture
and external compensation allow the transient response
to be optimized over a wide range of loads and output
capacitors.
To further maximize battery life, the P-channel MOSFET is
turned on continuously in dropout (100% duty cycle). The
no-load quiescent current is only 240µA. In shutdown, the
device draws <1µA.
, LT, LTC and LTM 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.
APPLICATIO S
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Wireless LAN Power
Notebook Computers
Digital Cameras
Cellular Phones
Board Mounted Power Supplies
TYPICAL APPLICATIO
Step-Down 2.5V/1A Regulator
V
IN
2.63V TO 5.5V
22µF
PV
IN
EFFICIENCY (%)
Efficiency and Power Loss vs Load Current
100
95
90
85
80
75
70
65
10
V
IN
= 3.3V
V
OUT
= 2.5V
f
O
= 1MHz
100
LOAD CURRENT (mA)
POWER LOSS
EFFICIENCY
POWER LOSS (mW)
SV
IN
I
TH
13k
1000pF
324k
LTC3561
SW
2.2µH
887k
V
OUT
2.5V/1A
22µF
SHDN/R
T
SGND
PGND
V
FB
412k
60
55
NOTE: IN DROPOUT, THE OUTPUT TRACKS
THE INPUT VOLTAGE.
3561 F01
50
10
U
1000
100
1
1000
3561 TA01
U
U
3561f
1
LTC3561
ABSOLUTE
(Note 1)
AXI U RATI GS
PACKAGE/ORDER I FOR ATIO
TOP VIEW
SHDN/R
T
1
SGND 2
SW 3
PGND 4
9
8
7
6
5
I
TH
V
FB
SV
IN
PV
IN
PV
IN
, SV
IN
Voltages .....................................– 0.3V to 6V
V
FB
, SHDN/R
T
Voltages ................ – 0.3V to (V
IN
+ 0.3V)
I
TH
Voltage ................................................– 0.3V to 1.4V
SW Voltage ................................... – 0.3V to (V
IN
+ 0.3V)
Operating Ambient Temperature Range
(Note 2) .................................................. – 40°C to 85°C
Junction Temperature (Notes 5, 8) ....................... 125°C
Storage Temperature Range ................. – 65°C to 125°C
DD PACKAGE
8-LEAD (3mm
×
3mm) PLASTIC DFN
T
JMAX
= 125°C,
θ
JA
= 43°C/W,
θ
JC
= 3°C/W
EXPOSED PAD (PIN 9) MUST BE SOLDERED TO GROUND
ORDER PART NUMBER
LTC3561EDD
DD PART MARKING
LCJJ
Order Options
Tape and Reel: Add #TR
Lead Free: Add #PBF Lead Free Tape and Reel: Add #TRPBF
Lead Free Part Marking:
http://www.linear.com/leadfree/
Consult LTC Marketing for parts specified with wider operating temperature ranges.
ELECTRICAL CHARACTERISTICS
SYMBOL
V
IN
I
FB
V
FB
∆V
LINEREG
∆V
LOADREG
g
m(EA)
I
S
V
SHDN/RT
f
OSC
I
LIM
R
DS(ON)
I
SW(LKG)
V
UVLO
PARAMETER
Operating Voltage Range
Feedback Pin Input Current
Feedback Voltage
Reference Voltage Line Regulation
Output Voltage Load Regulation
Error Amplifier Transconductance
Input DC Supply Current (Note 4)
Active Mode
Shutdown
Shutdown Threshold High
Active Oscillator Resistor
Oscillator Frequency
Peak Switch Current Limit
Top Switch On-Resistance (Note 6)
Bottom Switch On-Resistance (Note 6)
Switch Leakage Current
Undervoltage Lockout Threshold
The
●
denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at T
A
= 25°C. V
IN
= 3.3V, R
T
= 324k unless otherwise specified. (Note 2)
CONDITIONS
(Note 3)
(Note 3)
V
IN
= 2.7V to 5V
I
TH
= 0.36, (Note 3)
I
TH
= 0.84, (Note 3)
I
TH
Pin Load =
±5µA
(Note 3)
V
FB
= 0.75V
V
SHDN/RT
= 3.3V
MIN
2.625
●
●
●
TYP
0.784
0.8
0.04
0.02
– 0.02
800
MAX
5.5
±0.1
0.816
0.2
0.2
– 0.2
UNITS
V
µA
V
%/V
%
%
µS
µA
µA
V
Ω
MHz
MHz
A
Ω
Ω
µA
V
R
T
= 324k
(Note 7)
I
TH
= 1.3
V
IN
= 3.3V
V
IN
= 3.3V
V
IN
= 6V, V
ITH/RUN
= 0V, V
FB
= 0V
V
IN
Ramping Down
0.85
1.4
2.375
240
350
0.1
1
V
IN
– 0.6 V
IN
– 0.4
324k
1M
1
1.15
4
1.7
0.11
0.15
0.11
0.15
0.01
1
2.5
2.625
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. No pin shall exceed 6V.
Note 2:
The LTC3561E is guaranteed to meet specified performance from
0°C to 85°C. Specifications over the – 40°C to 85°C operating ambient
temperature range are assured by design, characterization and correlation
with statistical process controls.
Note 3:
The LTC3561 is tested in a feedback loop which servos V
FB
to the
midpoint for the error amplifier (V
ITH
= 0.6V).
Note 4:
Dynamic supply current is higher due to the internal gate charge
being delivered at the switching frequency.
3561f
2
U
W
U
U
W W
W
LTC3561
ELECTRICAL CHARACTERISTICS
Note 5:
T
J
is calculated from the ambient T
A
and power dissipation P
D
according to the following formula:
LTC3561EDD: T
J
= T
A
+ (P
D
• 43°C/W)
Note 6:
Switch on-resistance is guaranteed by correlation to wafer level
measurements.
Note 7:
4MHz operation is guaranteed by design but not production tested
and is subject to duty cycle limitations (see Applications Information).
Note 8:
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.
TYPICAL PERFOR A CE CHARACTERISTICS
Switching Waveforms
100
I
OUT
= 0.4A
V
OUT
10mV/
DIV
EFFICIENCY (%)
I
L1
100mA/
DIV
V
IN
= 3.3V
2µs/DIV
V
OUT
= 2.5V
I
LOAD
= 50mA
CIRCUIT OF FIGURE 6
3561 G02.eps
Load Regulation
0.4
0.3
0.2
V
IN
= 3.3V
V
OUT
= 2.5V
0.50
0.45
0.40
V
OUT
ERROR (%)
V
OUT
ERROR (%)
0.1
0
–0.1
–0.2
–0.3
–0.4
–0.5
1
10
100
1000
LOAD CURRENT (mA)
10000
3561 G07
0.35
0.30
0.25
0.20
0.15
0.10
0.05
0
2
3
4
V
IN
(V)
5
6
3561 G08
FREQUENCY VARIATION (%)
U W
Efficiency vs V
IN
95
90
I
OUT
= 1.0A
85
80
75
70
65
V
OUT
= 2.5V
CIRCUIT OF FIGURE 6
3.0
3.5
4.0 4.5
V
IN
(V)
5.0
5.5
6.0
I
L1
0.4A/
DIV
V
OUT
100mV/
DIV
Load Step
60
2.5
V
IN
= 3.3V
40µs/DIV
V
OUT
= 2.5V
I
LOAD
= 0.20A TO 1A
CIRCUIT OF FIGURE 6
3561 G06.eps
3561 G05
Line Regulation
10
V
OUT
= 1.8V
T
A
= 25°C
8
6
4
2
0
–2
–4
–6
–8
–10
Frequency vs V
IN
V
OUT
= 1.8V
I
OUT
= 1A
T
A
= 25°C
I
OUT
= 1.0A
I
OUT
= 400mA
2
3
4
V
IN
(V)
5
6
3561 G09
3561f
3
LTC3561
TYPICAL PERFOR A CE CHARACTERISTICS
Frequency Variation
vs Temperature
10
8
100
REFERENCE VARIATION (%)
6
4
EFFICIENCY (%)
R
DS(ON)
(mΩ)
2
0
–2
–4
–6
–8
–10
–50
–25
0
25
50
75
TEMPERATURE (°C)
PI FU CTIO S
SHDN/R
T
(Pin 1):
Combination Shutdown and Timing
Resistor Pin. The oscillator frequency is programmed by
connecting a resistor from this pin to ground. Forcing this
pin to SV
IN
causes the device to be shut down. In
shutdown all functions are disabled.
SGND (Pin 2):
The Signal Ground Pin. All small signal
components and compensation components should be
connected to this ground (see Board Layout
Considerations).
SW (Pin 3):
The Switch Node Connection to the Inductor.
This pin swings from PV
IN
to PGND.
PGND (Pin 4):
Main Power Ground Pin. Connect to the
(–) terminal of C
OUT
, and (–) terminal of C
IN
.
PV
IN
(Pin 5):
Main Supply Pin. Must be closely decoupled
to PGND.
SV
IN
(Pin 6):
The Signal Power Pin. All active circuitry is
powered from this pin. Must be closely decoupled to
SGND. SV
IN
must be greater than or equal to PV
IN
.
V
FB
(Pin 7):
Receives the feedback voltage from the
external resistive divider across the output. Nominal
voltage for this pin is 0.8V.
I
TH
(Pin 8):
Error Amplifier Compensation Point. The
current comparator threshold increases with this control
voltage. Nominal voltage range for this pin is 0V to 1.5V.
Exposed Pad (Pin 9):
Thermal Connection to PCB. This
pin should be soldered to ground to achieve rated
thermal performance.
4
U W
100
Efficiency vs Frequency
V
IN
= 3.3V
V
OUT
= 2.5V
I
OUT
= 500mA
T
A
= 25°C
R
DS(ON)
vs V
IN
120
T
A
= 25°C
115
110
SYNCHRONOUS SWITCH
105
MAIN SWITCH
100
95
95
90
85
125
0
1
2
3
FREQUENCY (MHz)
4
3561 G11
90
2.5
3
3.5
4
4.5
V
IN
(V)
5
5.5
6
3561 G10
3561 G12
U
U
U
3561f
LTC3561
BLOCK DIAGRA
ERROR
AMPLIFIER
V
B
OSCILLATOR
OPERATIO
The LTC3561 uses a constant frequency, current mode
architecture. The operating frequency is determined by
the value of the R
T
resistor.
The output voltage is set by an external divider returned to
the V
FB
pin. An error amplifier compares the divided
output voltage with a reference voltage of 0.8V and adjusts
the peak inductor current accordingly.
Main Control Loop
During normal operation, the top power switch (P-channel
MOSFET) is turned on at the beginning of a clock cycle
when the V
FB
voltage is below the reference voltage. The
current into the inductor and the load increases until the
current limit is reached. The switch turns off and energy
stored in the inductor flows through the bottom switch (N-
channel MOSFET) into the load until the next clock cycle.
The peak inductor current is controlled by the voltage on
the I
TH
pin, which is the output of the error amplifier. This
amplifier compares the V
FB
pin to the 0.8V reference.
When the load current increases, the V
FB
voltage de-
creases slightly below the reference. This decrease causes
the error amplifier to increase the I
TH
voltage until the
average inductor current matches the new load current. At
+
V
FB
7
–
–
W
SV
IN
6
SGND
2
I
TH
8
PV
IN
5
0.8V
VOLTAGE
REFERENCE
PMOS CURRENT
COMPARATOR
I
TH
LIMIT
+
+
–
SLOPE
COMPENSATION
3 SW
+
LOGIC
NMOS
COMPARATOR
–
+
REVERSE
COMPARATOR
1
SHDN/R
T
–
4 PGND
3561 BD
U
low load currents, the inductor current becomes discontinu-
ous, and pulses may be skipped to maintain regulation.
The main control loop is shut down by pulling the SHDN/R
T
pin to SV
IN
. A digital soft-start is enabled after shutdown,
which will slowly ramp the peak inductor current up over
1024 clock cycles or until the output reaches regulation,
whichever is first. Soft-start can be lengthened by ramping
the voltage on the I
TH
pin (see Applications Information
section).
Dropout Operation
When the input supply voltage decreases toward the
output voltage, the duty cycle increases to 100% which is
the dropout condition. In dropout, the PMOS switch is
turned on continuously with the output voltage being
equal to the input voltage minus the voltage drops across
the internal P-channel MOSFET and the inductor.
Low Supply Operation
The LTC3561 incorporates an undervoltage lockout circuit
which shuts down the part when the input voltage drops
below about 2.5V to prevent unstable operation.
3561f
5
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