LTC3448
1.5MHz/2.25MHz, 600mA
Synchronous Step-Down
Regulator with LDO Mode
DESCRIPTIO
The LTC
®
3448 is a high efficiency, monolithic, synchronous
buck regulator using a constant frequency, current mode
architecture. Supply current during operation is only 32µA
(linear regulator mode) and drops to <1µA in shutdown. The
2.5V to 5.5V input voltage range makes the LTC3448 ide-
ally suited for single Li-Ion battery-powered applications.
100% duty cycle provides low dropout operation, extend-
ing battery life in portable systems. At moderate output load
levels, PWM pulse skipping mode operation provides very
low output ripple voltage for noise sensitive applications.
The LTC3448 automatically switches into linear regulator
operation at very low load currents to maintain <5mV
P-P
output voltage ripple. Supply current in this mode is
typically 32µA. The switch to linear regulator mode occurs
at a threshold of 3mA. Linear regulator operation can be set
to on, off or automatic turn on/off.
Switching frequency is selectable at either 1.5MHz or
2.25MHz, allowing the use of small surface mount induc-
tors and capacitors.
The internal synchronous switch increases efficiency and
eliminates the need for an external Schottky diode. Low
output voltages are easily supported with the 0.6V feed-
back reference voltage. The LTC3448 is available in a low
profile 3mm
×
3mm DFN package or thermally enhanced
8-lead MSOP.
FEATURES
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■
■
■
■
■
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High Efficiency: Up to 96%
Very Low Quiescent Supply Current: 32
µ
A During
Linear Regulator Operation
600mA Output Current (Buck Converter)
Optionally Operates as Linear Regulator Below
3mA—External or Automatic ON/OFF
2.5V to 5.5V Input Voltage Range
1.5MHz or 2.25MHz Constant Frequency Operation
or External Synchronization
No Schottky Diode Required
Low Dropout Operation: 100% Duty Cycle
0.6V Reference Allows Low Output Voltages
Shutdown Mode Draws < 1µA Supply Current
Current Mode Operation for Excellent Line and
Load Transient Response
Overtemperature Protected
Low Profile (3mm
×
3mm) 8-Lead DFN and 8-Lead
MSOP Packages
APPLICATIO S
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Cellular Telephones
Personal Information Appliances
Wireless and DSL Modems
Digital Still Cameras
MP3 Players
Portable Instruments
, LTC and LT are registered trademarks of Linear Technology Corporation.
All other trademarks are the property of their respective owners.
Protected by U.S. Patents including 5481178, 6580258, 6304066, 6127815,
6498466, 6611131. Others pending.
TYPICAL APPLICATIO
V
IN
2.5V TO 5.5V
C
IN
4.7µF
V
IN
Efficiency and Power Loss vs Load Current
100
90
80
70
EFFICIENCY (%)
V
IN
= 3.6V
V
OUT
= 1.5V
T
A
= 25°C
1
1.5V High Efficiency Regulator with Automatic LDO Mode
2.2µH
SW
474k
C
OUT
4.7µF
V
OUT
RUN
LTC3448
MODE
V
FB
FREQ
SYNC
GND
V
OUT
1.5V
22pF
60
50
40
30
EFFICIENCY
POWER LOSS
0.01
316k
20
10
0
0.0001
3448 TA01a
U
U
U
0.1
POWER LOSS (W)
0.001
0.0001
0.001
0.01
0.1
LOAD CURRENT (A)
1
23448 TA01b
3448f
1
LTC3448
ABSOLUTE
AXI U
RATI GS
Input Supply Voltage .................................. – 0.3V to 6V
RUN, SYNC Voltages ................... – 0.3V to (V
IN
+ 0.3V)
MODE Voltage ............................. – 0.3V to (V
IN
+ 0.3V)
FREQ, V
FB
Voltages...................... – 0.3V to (V
IN
+ 0.3V)
SW Voltage .................................. – 0.3V to (V
IN
+ 0.3V)
V
OUT
Voltage ................................ – 0.3V to (V
IN
+ 0.3V)
P-Channel Switch Source Current (DC) ............. 800mA
N-Channel Switch Sink Current (DC) ................. 800mA
PACKAGE/ORDER I FOR ATIO
TOP VIEW
V
FB
1
V
OUT
2
MODE 3
V
IN
4
9
8
7
6
5
RUN
SYNC
FREQ
SW
ORDER PART
NUMBER
LTC3448EDD
V
FB
V
OUT
MODE
V
IN
1
2
3
4
DD PACKAGE
8-LEAD (3mm
×
3mm) PLASTIC DFN
T
JMAX
= 125°C,
θ
JA
= 43°C/ W
EXPOSED PAD (PIN 9) IS GND
MUST BE SOLDERED TO PCB
DD PART MARKING
LBMJ
Consult LTC Marketing for parts specified with wider operating temperature ranges.
The
●
denotes specifications which apply over the full operating
temperature range, otherwise specifications are T
A
= 25°C. V
IN
= 3.6V unless otherwise specified.
SYMBOL
I
VFB
V
FB
PARAMETER
Feedback Current
Regulated Feedback Voltage
(Note 4)
Reference Voltage Line Regulation
Output Overvoltage Lockout
Output Voltage Line Regulation
Peak Inductor Current
Output Voltage Load Regulation
Maximum Output Voltage
Input Voltage Range
T
A
= 25°C
0°C
≤
T
A
≤
85°C
–40°C
≤
T
A
≤
85°C
V
IN
= 2.5V to 5.5V (Note 4)
∆V
OVL
= V
OVL
– V
FB
∆V
OVL
= (V
OVL
– V
OUT
) • 100/V
OUT
V
IN
= 2.5V to 5.5V (LDO)
V
FB
= 0.5V or V
OUT
= 90%,
Duty Cycle < 35%
LDO, 1mA to 10mA
(Note 9)
●
ELECTRICAL CHARACTERISTICS
CONDITIONS
●
∆V
FB
∆V
OVL
∆V
OUT
I
PK
V
LOADREG
V
OUT(MAX)
V
IN
2
U
U
W
W W
U
W
(Note 1)
V
OUT
(LDO) Source Current .................................. 25mA
Peak SW Sink and Source Current ........................ 1.3A
Operating Temperature Range (Note 2) .. – 40°C to 85°C
Junction Temperature (Notes 3, 7) ...................... 125°C
Storage Temperature Range ................ – 65°C to 125°C
Lead Temperature (Soldering, 10 sec)
MSOP Only ...................................................... 300°C
TOP VIEW
8
7
6
5
RUN
SYNC
FREQ
SW
ORDER PART
NUMBER
LTC3448EMS8E
9
MS8E PACKAGE
8-LEAD PLASTIC MSOP
T
JMAX
= 125°C,
θ
JA
= 40°C/ W
EXPOSED PAD (PIN 9) IS GND
MUST BE SOLDERED TO PCB
MS8 PART MARKING
LTBMK
MIN
0.5880
0.5865
0.5850
15
2.5
0.7
TYP
0.6
0.6
0.6
0.2
35
5.8
0.1
1
0.5
MAX
±30
0.6120
0.6135
0.6150
0.4
55
9.2
0.8
1.3
UNITS
nA
V
V
V
%/V
mV
%
%/V
A
%/V
V
●
●
V
IN
– 0.7 V
IN
– 0.3
2.5
5.5
V
3448f
LTC3448
The
●
denotes specifications which apply over the full operating
temperature range, otherwise specifications are T
A
= 25°C. V
IN
= 3.6V unless otherwise specified.
SYMBOL
I
S
PARAMETER
Input DC Bias Current
Active Mode (Pulse Skip, No LRO)
Linear Regulator Operation (LRO)
Shutdown
f
OSC
f
SYNC
V
TH(SYNC)
R
PFET
R
NFET
I
LSW
V
RUNH
V
RUNL
I
RUN
V
FREQH
V
FREQL
I
FREQ
V
MODEH
V
MODEL
I
MODE
I
SYNC
I
LDO(ON)
I
LDO(OFF)
Oscillator Frequency
Synchronization Frequency
SYNC Activation Input Threshold
R
DS(ON)
of P-Channel FET
R
DS(ON)
of N-Channel FET
SW Leakage
RUN Threshold High
RUN Threshold Low
RUN Leakage Current
FREQ Threshold High
FREQ Threshold Low
FREQ Leakage Current
MODE Threshold High
MODE Threshold Low
MODE Leakage Current
SYNC Leakage Current
LRO ON Load Current Threshold
LRO OFF Load Current Threhold
2.2mH Inductor (Note 8)
8
I
SW
= 100mA
I
SW
= –150mA
V
RUN
= 0V, V
SW
= 0V or 5V, V
IN
= 5V
●
●
●
●
●
●
●
●
●
●
ELECTRICAL CHARACTERISTICS
CONDITIONS
V
IN
= 3.6V (Note 5)
V
FB
= 0.5V or V
OUT
= 90%, I
LOAD
= 0A, 1.5MHz
V
FB
= 0.5V or V
OUT
= 90%, I
LOAD
= 0A, 2.25MHz
I
LOAD
≤
I
LDO(ON)
V
RUN
= 0V, V
IN
= 5.5V
FREQ = Low, V
IN
= 3.6V
FREQ = High
(Note 6)
●
●
MIN
TYP
250
275
32
0.1
MAX
375
400
43
1
1.8
2.7
>4
1.3
UNITS
µA
µA
µA
µA
MHz
MHz
MHz
V
Ω
Ω
1.2
1.8
1.5
1.5
2.25
1
0.4
0.35
±0.01
±1
0.3
µA
V
V
µA
V
V
µA
V
V
µA
µA
mA
mA
1.5
±0.01
V
IN
– 1
1
±0.01
V
IN
– 0.15
0.12
±0.1
±0.01
3
11
±1
±1
5
17
±1
±1
Note 1:
Absolute Maximum Ratings are those values beyond which the life
of a device may be impaired.
Note 2:
The LTC3448E 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.
Note 3:
T
J
is calculated from the ambient temperature T
A
and power
dissipation P
D
according to the following formula:
T
J
= T
A
+ (P
D
)(43°C/W)
Note 4:
The LTC3448 is tested in a proprietary test mode that connects
V
FB
to the output of the error amplifier.
Note 5:
Dynamic supply current is higher due to the gate charge being
delivered at the switching frequency. LRO is “linear regulator operation.”
Note 6:
4MHz operation is guaranteed by design but is not production
tested and is subject to duty cycle limitations.
Note 7:
This IC includes overtemperature protection that is intended to
protect the device during momentary overload conditions. Junction
temperature will exceed 125°C when overtemperature is active. Continu-
ous operation above the specified maximum operating junction tempera-
ture may impair device reliability.
Note 8:
The load current below which the switching regulator turns off and
the LDO turns on is, to first order, inversely proportional to the value of
the inductor. This effect is covered in more detail in the Operation section.
This parameter is not production tested but is guaranteed by design.
Note 9:
For 2.5V < V
IN
< 2.7V the output voltage is limited to V
IN
– 0.7V
to ensure regulation in linear regulator mode. This parameter is not
production tested but is guaranteed by design.
3448f
3
LTC3448
TYPICAL PERFOR A CE CHARACTERISTICS
(From Figure1a Except for the Resistive Divider Resistor Values)
Efficiency vs Input Voltage
100
95
90
I
OUT
= 100mA
V
OUT
= 1.8V
T
A
= 25°C
EFFICIENCY (%)
EFFICIENCY (%)
80
75
70
65
60
55
50
2
I
OUT
= 600mA
60
50
40
30
20
10
V
IN
= 2.7V
V
IN
= 3.6V
V
IN
= 4.2V
0.001
0.01
0.1
LOAD CURRENT (A)
1
23448 G02
EFFICIENCY (%)
85
I
OUT
= 30mA
3
4
5
INPUT VOLTAGE (V)
Efficiency vs Load Current
(Switcher Only)
100
90
80
70
EFFICIENCY (%)
60
50
40
30
20
10
0
0.0001
0.01
0.1
0.001
LOAD CURRENT (A)
1
23448 G04
V
IN
= 2.7V
V
OUT
= 2.5V
T
A
= 25°C
REFERENCE VOLTAGE (V)
0.605
0.600
0.595
0.590
0.585
–50 –25
FREQUENCY (MHz)
Oscillator Frequency
vs Supply Voltage
1.8
1.7
OUTPUT VOLTAGE (V)
T
A
= 25°C
FREQUENCY (MHz)
1.6
1.5
1.4
1.3
1.2
2
3
4
5
SUPPLY VOLTAGE (V)
6
3448 G07
1.505
1.500
1.495
1.490
1.485
1.480
1.475
0.0001
0.01
0.001
0.1
LOAD CURRENT (A)
1
3448 G08
R
DS(ON)
(Ω)
4
U W
6
3448 G01
Efficiency vs Load Current
100
90
80
70
V
OUT
= 1.2V
T
A
= 25°C
100
90
80
70
60
50
40
30
20
10
Efficiency vs Load Current
V
OUT
= 1.5V
T
A
= 25°C
0
0.0001
0
0.0001
V
IN
= 2.7V
V
IN
= 3.6V
V
IN
= 4.2V
0.001
0.01
0.1
LOAD CURRENT (A)
1
23448 G03
Reference Voltage
vs Temperature
0.615
0.610
V
IN
= 3.6V
1.70
1.65
1.60
1.55
1.50
1.45
1.40
1.35
50
25
75
0
TEMPERATURE (°C)
100
125
Oscillator Frequency
vs Temperature
V
IN
= 3.6V
1.30
–50
–25
0
50
75
25
TEMPERATURE (°C)
100
125
3448 G05
3448 G06
Output Voltage vs Load Current
V
IN
= 3.6V
1.520 T
A
= 25°C
1.515
1.510
1.525
0.40
0.38
0.36
0.34
0.32
0.30
0.28
0.26
0.24
0.22
0.20
R
DS(ON)
vs Input Voltage
T
A
= 25°C
MAIN
SWITCH
SYNCHRONOUS
SWITCH
2
3
4
5
INPUT VOLTAGE (V)
6
3448 G09
3448f
LTC3448
TYPICAL PERFOR A CE CHARACTERISTICS
(From Figure1a Except for the Resistive Divider Resistor Values)
Dynamic Supply Current
vs Supply Voltage
340
DYNAMIC SUPPLY CURRENT (µA)
DYNAMIC SUPPLY CURRENT (µA)
R
DS(ON)
vs Temperature
0.6
0.5
0.4
R
DS(ON)
(Ω)
0.3
0.2
0.1
MAIN SWITCH SYNCH SWITCH
2.5V
2.5V
3.6V
3.6V
4.2V
4.2V
–25
50
25
75
0
TEMPERATURE (°C)
100
125
0
–50
Switch Leakage vs Temperature
350
300
V
IN
= 5.5V
RUN = 0V
SWITCH LEAKAGE (nA)
250
200
150
100
50
0
–50 –25
SWITCH LEAKAGE (nA)
MAIN
SWITCH
SYNCHRONOUS
SWITCH
50
25
75
0
TEMPERATURE (°C)
Load Step
V
OUT
200mV/DIV
AC COUPLED
V
OUT
100mV/DIV
AC COUPLED
I
LOAD
250mA/DIV
I
L
500mA/DIV
10µs/DIV
V
IN
= 3.6V
V
OUT
= 1.5V
I
LOAD
= 100µA TO 200mA
C
OUT
= 10µF
3448 G16
I
LOAD
100mA/DIV
I
L
500mA/DIV
U W
3448 G10
Dynamic Supply Current
vs Temperature
320
300
280
260
1.5MHz
240
220
200
–50 –25
V
IN
= 3.6V
I
LOAD
= 0A
320
300
280
260
240
220
200
2
I
LOAD
= 0A
T
A
= 25°C
2.25MHz
2.25MHz
1.5MHz
3
4
SUPPLY VOLTAGE (V)
5
6
3448 G11
50
25
75
0
TEMPERATURE (°C)
100
125
3448 G12
Switch Leakage vs Input Voltage
10
RUN = 0V
T
A
= 25°C
MAIN
SWITCH
RUN
5V/DIV
V
OUT
1V/DIV
Start-Up from Shutdown
1
SYNCHRONOUS
SWITCH
0.1
I
L
500mA/DIV
0.01
V
IN
= 3.6V
V
OUT
= 1.5V
I
LOAD
= 600mA
0
1
2
3
4
INPUT VOLTAGE (V)
5
6
3448 G14
40µs/DIV
3448 G15
0.001
100
125
3448 G13
Load Step
10µs/DIV
V
IN
= 3.6V
V
OUT
= 1.5V
I
LOAD
= 50mA TO 600mA
C
OUT
= 10µF
3448 G17
3448f
5
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