LTC3408
1.5MHz, 600mA
Synchronous Step-Down
Regulator with Bypass Transistor
FEATURES
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DESCRIPTIO
Dynamically Adjustable Output from 0.3V to 3.5V
600mA Output Current
Internal 0.08
Ω
P-Channel MOSFET Bypass
Transistor
High Efficiency: Up to 96%
1.5MHz Constant Frequency Operation
No Schottky Diode Required
Low Dropout Operation: 100% Duty Cycle
2.5V to 5V Input Voltage Range
Shutdown Mode Draws < 1µA Supply Current
Current Mode Operation for Excellent Line and
Load Transient Response
Overtemperature Protected
Available in 8-Lead 3mm
×
3mm DFN Package
The LTC
®
3408 is a high efficiency monolithic synchro-
nous buck regulator optimized for WCDMA power ampli-
fier applications. The output voltage can be dynamically
programmed from 0.3V to 3.5V. At V
OUT
> 3.6V an internal
0.08Ω bypass P-channel MOSFET connects V
OUT
directly
to V
IN
, eliminating power loss through the inductor.
The input voltage range is 2.5V to 5V making the LTC3408
ideally suited for single Li-Ion battery-powered applica-
tions. 100% duty cycle provides low dropout operation,
extending battery life in portable systems.
Switching frequency is internally set at 1.5MHz, allowing
the use of small surface mount inductors and capacitors.
The internal synchronous switch increases efficiency and
eliminates the need for an external Schottky diode.
The LTC3408 is available in a low profile (0.75mm) 8-lead
3mm
×
3mm DFN package.
, LTC and LT are registered trademarks of Linear Technology Corporation.
U.S. Patent Numbers: 5481178, 6580258, 6304066, 6127815, 6498466, 6611131
APPLICATIO S
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WCDMA Cell Phone Power Amplifiers
Wireless Modems
TYPICAL APPLICATIO
WCDMA Transmitter Power Supply
V
IN
2.7V
TO 5V
4.7µH*
C
IN†
10µF
CER
V
IN
SW
V
OUT
3× V
REF
C
OUT
** 600mA
4.7µF
CER
Efficiency Power Lost vs Load Current
1
100
90
80
POWER LOST (W)
LTC3408
RUN
REF
V
OUT
0.1
OUTPUT
PROGRAMMING
DAC
GND
WCDMA
RF PA
0.01
V
OUT
= 1.2V
V
OUT
= 1.5V
V
OUT
= 1.8V
V
OUT
= 2.5V
1
10
100
LOAD CURRENT (mA)
*MURATA LQH32CN4R7M11
**TAIYO YUDEN JMK212BJ475MG
†
TAIYO YUDEN JMK212BJ106MN
3403 TA01
0.01
U
70
60
50
40
30
20
10
0
1000
U
U
EFFICIENCY (%)
3408 F04
3408f
1
LTC3408
ABSOLUTE
(Note 1)
AXI U
RATI GS
PACKAGE/ORDER I FOR ATIO
TOP VIEW
V
OUT
V
IN
GND
SW
1
2
3
4
9
8
7
6
5
V
OUT
V
IN
REF
RUN
Input Supply Voltage (< 300µs) .................. – 0.3V to 6V
Input Supply Voltage (DC) ....................... – 0.3V to 5.5V
RUN, REF, V
OUT
Voltages .......................... – 0.3V to V
IN
SW Voltage (DC) ......................... – 0.3V to (V
IN
+ 0.3V)
P-Channel Switch Source Current (DC) ............. 800mA
N-Channel Switch Sink Current (DC) ................. 800mA
Peak SW Sink and Source Current ........................ 1.3A
Bypass P-Channel FET Source Current (DC) .............. 1A
Operating Temperature Range (Note 2) .. – 40°C to 85°C
Junction Temperature (Note 3) ............................ 125°C
Storage Temperature Range ................ – 65°C to 125°C
ORDER PART
NUMBER
LTC3408EDD
DD PACKAGE
8-LEAD (3mm
×
3mm) PLASTIC DFN
EXPOSED PAD IS GND (PIN 9)
MUST BE SOLDERED TO PCB
T
JMAX
= 125°C,
θ
JA
= 43°C/ W,
θ
JC
= 3°C/ W
DD PART MARKING
LAEA
Consult LTC Marketing for parts specified with wider operating temperature ranges.
ELECTRICAL CHARACTERISTICS
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
V
OUT
∆V
OUT
I
PK
V
LOADREG
V
IN
I
S
f
OSC
V
REF
R
PFET
R
NFET
R
BYPASS
I
LSW
I
LBYP
V
RUN
I
RUN
I
REF
PARAMETER
Regulated Output Voltage
Output Voltage Line Regulation
Peak Inductor Current
Output Voltage Load Regulation
Input Voltage Range
Input Current
Shutdown Current
Oscillator Frequency
Bypass PFET Turn-Off Threshold
Bypass PFET Turn-On Threshold
R
DS(ON)
of P-Channel FET
R
DS(ON)
of N-Channel FET
R
DS(ON)
of Bypass P-Channel FET
SW Leakage
Bypass PFET Leakage
RUN Threshold
RUN Input Current
REF Input Current
V
RUN
= 0V or 2.5V
V
RUN
= 1.2V, SW = Open
V
RUN
= 0V, SW = Open
V
REF
≥
0.25V
V
REF
≤
0.1V
V
REF
=
V
REF
=
I
SW
= 160mA, Wafer Level
I
SW
= 160mA, DD Package
I
SW
= –160mA, Wafer Level
I
SW
= –160mA, DD Package
I
OUT
= 100mA, V
IN
= 3V, Wafer Level
I
OUT
= 100mA, V
IN
= 3V, DD Package (Note 4)
V
RUN
= 0V, V
SW
= 0V or 5V, V
IN
= 5V
V
OUT
= 0V, V
IN
= 5V, V
REF
= 0V
●
●
●
●
●
CONDITIONS
V
REF
= 1.1V
V
REF
= 0.1V
V
IN
= 2.5V to 5V, V
REF
= 0.6V
V
IN
= 3V, V
REF
= 0.9V
●
●
●
MIN
3.23
0.25
0.70
2.5
TYP
3.3
0.3
0.1
1
0.7
MAX
3.37
0.35
0.4
1.25
5
UNITS
V
V
%/V
A
%
V
mA
µA
MHz
kHz
V
V
Ω
Ω
Ω
Ω
Ω
Ω
µA
µA
V
µA
µA
1.5
0.1
1.2
550
1.167
1.5
700
1.2
1.21
0.3
0.4
0.3
0.4
0.05
0.08
±0.01
±0.01
0.3
1
±0.01
±0.01
2.5
1
1.8
850
1.26
0.4
0.4
0.08
±1
±1
1.5
±1
±1
Note 1:
Absolute Maximum Ratings are those values beyond which the life
of a device may be impaired.
Note 2:
The LTC3408E 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:
LTC3408: T
J
= T
A
+ (P
D
)(43°C/W)
3408f
2
U
W
U
U
W W
W
LTC3408
ELECTRICAL CHARACTERISTICS
Note 4:
When V
REF
> 1.2V and V
REF
x3 > V
IN
, the P-channel FET will be on
in parallel with the bypass PFET reducing the overall R
DS(ON)
.
Note 5:
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
Efficiency vs V
OUT
110
100
100mA
T
A
= 25°C
V
IN
= 3.6V
100
EFFICIENCY (%)
EFFICIENCY (%)
EFFICIENCY (%)
90
600mA
80
70
60
50
0
1
2
V
OUT
(V)
3
Efficiency vs Output Current
100
90
80
1.70
T
A
= 25°C
V
OUT
= 2.5V
1.65
1.60
FREQUENCY (MHz)
70
EFFICIENCY (%)
OSCILLATOR FREQUENCY (MHz)
60
50
40
30
20
10
0
0.1
V
IN
= 3.6V
V
IN
= 4.2V
10
100
1
OUTPUT CURRENT (mA)
U W
4
3408 G02
3408 G05
(From Figure 1)
Efficiency vs Output Current
100
90
80
70
T
A
= 25°C
V
OUT
= 1.5V
Efficiency vs Output Current
T
A
= 25°C
90 V
OUT
= 1.2V
80
70
60
50
40
30
20
10
0
0.1
1
10
100
OUTPUT CURRENT (mA)
1000
3408 G03
V
IN
= 3.6V
V
IN
= 4.2V
60
50
40
30
20
10
0
0.1
V
IN
= 3.6V
V
IN
= 4.2V
1
10
100
OUTPUT CURRENT (mA)
1000
3408 G04
Oscillator Frequency
vs Temperature
1.8
V
IN
= 3.6V
1.7
1.6
1.5
1.4
1.3
1.2
Oscillator Frequency
vs Supply Voltage
T
A
= 25°C
1.55
1.50
1.45
1.40
1.35
1.30
–50 –25
50
25
75
0
TEMPERATURE (°C)
100
125
1000
2
3
4
5
SUPPLY VOLTAGE (V)
6
3408 G07
3408 G06
3408f
3
LTC3408
TYPICAL PERFOR A CE CHARACTERISTICS
Frequency vs V
OUT
1600
1400
T
A
= 25°C
V
IN
= 3.6V
OUTPUT VOLTAGE (V)
FREQUENCY (kHz)
1200
1000
800
600
400
1.814
1.804
1.794
1.784
1.774
0 100 200 300 400 500 600 700 800 900 1000
LOAD CURRENT (mA)
3408 G09
R
DS(ON)
(Ω)
0
0.2
0.4
0.6
0.8
V
OUT
(V)
R
DS(ON)
vs Temperature
0.7
V
IN
= 2.7V
DYNAMIC SUPPLY CURRENT (µA)
0.6
0.5
R
DS(ON)
(Ω)
V
IN
= 3.6V
MAIN
SWITCH
V
IN
= 4.2V
3000
2500
2000
1500
1000
FORCED CONTINUOUS
MODE
SWITCH LEAKAGE (nA)
0.4
0.3
0.2
0.1
0
–50 –25
BYPASS
SWITCH
V
IN
= 3V
V
IN
= 4.2V
50
25
75
0
TEMPERATURE (°C)
100
125
SYNCHRONOUS SWITCH
Switch Leakage vs Input Voltage
120
100
SWITCH LEAKAGE (pA)
80
60
40
20
0
SYNCHRONOUS
SWITCH
T
A
= 25°C
RUN = 0V
RUN
2V/DIV
V
OUT
1V/DIV
0
1
4
U W
1.0
3408 G08
(From Figure 1)
Output Voltage vs Load Current
1.844 T = 25°C
A
V
IN
= 3.6V
1.834
1.824
R
DS(ON)
vs Input Voltage
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
0
1
4
3
5
2
INPUT VOTLAGE (V)
6
7
BYPASS
SWITCH
SYNCHRONOUS
SWITCH
MAIN
SWITCH
T
A
= 25°C
1.2
3408 G10
Dynamic Supply Current
vs Supply Voltage
T
A
= 25°C
4000 V
OUT
= 1.8V
I
LOAD
= 0A
3500
4500
300
Switch Leakage vs Temperature
V
IN
= 5.5V
RUN = 0V
250
200
150
100
50
MAIN SWITCH
SYNCHRONOUS SWITCH
500
0
2
3
4
SUPPLY VOLTAGE (V)
3408 G11
3408 G12
5
6
0
–50 –25
50
25
75
0
TEMPERATURE (°C)
100
125
3408 G13
Start-Up from Shutdown
MAIN
SWITCH
I
L
500mA/DIV
V
IN
= 3.6V
V
REF
= 0.6V
R
LOAD
= 3Ω
40µs/DIV
3408 G15
2
3
4
INPUT VOLTAGE (V)
5
6
3408 G14
3408f
LTC3408
TYPICAL PERFOR A CE CHARACTERISTICS
(From Figure 1)
Output Ripple Waveform
V
OUT
100mV/DIV
I
L
500mA/DIV
I
L
100mA/DIV
I
LOAD
500mA/DIV
V
OUT
10mV/DIV
V
IN
= 3.6V
V
REF
= 0.6V
I
LOAD
= 0A
REF Transient
4.5
V
REF
0.5V/DIV
V
OUT
(V)
V
OUT
1V/DIV
V
IN
= 4.2V
40µs/DIV
V
REF
= 0V TO 1.4V
R
LOAD
= 5Ω
PI FU CTIO S
V
OUT
(Pins 1, 8):
Output Voltage Feedback Pin. An internal
resistive divider divides the output voltage down by 3 for
comparison to the external reference voltage. The drain of
the P-channel bypass MOSFET is connected to this pin.
V
IN
(Pins 2, 7):
Main Supply Pin. Must be closely de-
coupled to GND, Pin 3, with a 10µF or greater ceramic
capacitor.
GND (Pin 3):
Ground Pin.
SW (Pin 4):
Switch Node Connection to Inductor. This pin
connects to the drains of the internal main and synchro-
nous power MOSFET switches.
RUN (Pin 5):
Run Control Input. Forcing this pin above
1.5V enables the part. Forcing this pin below 0.3V shuts
down the device. In shutdown, all functions are disabled
drawing <1µA supply current. Do not leave RUN floating.
REF (Pin 6):
External Reference Input. Controls the output
voltage to 3× the applied voltage at REF. Also turns on the
bypass MOSFET when V
REF
> 1.2V.
Exposed Pad (Pin 9):
Connect to GND, Pin 3.
U W
Load Step Response
200ns/DIV
3408 G16
V
IN
= 3.6V
20µs/DIV
V
REF
= 0.6V
I
LOAD
= 0mA TO 600mA
3408 G17
V
OUT
vs V
REF
V
IN
= 4.2V
I
L
= 100mA
I
L
= 600mA
4.0
3.5
3.0
2.5
2.0
1.5
1.0
3408 G18
0.5
0
0
0.5
V
REF
(V)
3408 G19
1.0
1.5
U
U
U
3408f
5
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