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AIC2351
Low-Noise Synchronous PWM/PFM Step-Down
DC/DC Converter
 FEATURES










2.5V to 5.5V Input Voltage Range
600mA Guaranteed Output Current
95% Efficiency
No Schottky Diode Required
30µA Low Quiescent Current
100% Duty Cycle in Low Dropout Operation
1.5MHz Fixed-Frequency PWM Operation
Accurate Reference 0.6V Provides Low
Output Voltages
Small 5-Pin SOT23 Package
Internal Soft-Start Function(100μs)
 DESCRIPTION
The AIC2351 is a low-noise, pulse-widthmodulated (PWM), DC-DC step-down converter.
The device is available in an adjustable version
and fixed output voltages of 1.2V, 1.5V, 1.8V,
2.5V, and 3.3V. AIC2351 is capable of delivering
continuous 600mA output current over a wide
input voltage range from 2.5V to 5.5V. However,
it can provide higher peak current. For example,
over 1A peak current in the condition of 5V input
and 1.2V output. See Fig.13 for detail.
The device features an internal synchronous
rectifier for high efficiency; it requires no external
Schottky diode. Internally fixed-frequency
1.5MHz operation provides easy post-filtering
and allows the use of small inductors and
capacitors. The AIC2351 is ideally suited for LiIon battery applications. PWM/PFM mode
extends battery life by switching to a pulsefrequency-modulated mode during light loads.
Shutdown mode places the device in standby,
reducing quiescent supply current to under
0.1µA.
Other features of the AIC2351 include high
efficiency, low dropout voltage, and an accurate
reference 0.6V provides low output voltages. It is
available in a space-saving 5-pin SOT23
package.
 APPLICATIONS






Cellular Phones
CPU I/O Supplies
Cordless Phones
Notebook Chipset Supplies
PDAs and Handy-Terminals
Battery-Operated Devices (1 Li-Ion or 3
NiMH/ NiCd).
 APPLICATIONS CIRCUIT
Vin=2.5~5.5V
U1
L1
VIN
LX
Vo=1.8V
ON
OFF
EN
VIN
Cout
10uF
GND
OFF
EN
Vo=2.4V
L1
LX
2.2uH
R1 273k
VFB
Cin
4.7uF
ON
AIC2351-18
R2
91k
GND
Cout
10uF
AIC2351
Fig. 1 Fixed Step-Down DC/DC Converter
Analog Integrations Corporation
U1
2.2uH
VOUT
Cin
4.7uF
Vin=3.0~5.5V
Fig. 2 Adjustable Step-Down DC/DC Converter
Si-Soft Research Center
DS-2351G-03 20120302
3A1, No.1, Li-Hsin Rd. I , Science Park , Hsinchu 300, Taiwan , R.O.C.
TEL: 886-3-5772500
FAX: 886-3-5772510
www.analog.com.tw
1
AIC2351
 ORDERING INFORMATION
AIC2351-XXXXX XX
PIN CONFIGURATION
PACKING TYPE
TR: TAPE & REEL
BG: BAG
FRONT VIEW
5
PACKAGE TYPE
V5: SOT-23-5
4
GNxxG
G: Green Package
OUTPUT VOLTAGE
DEFAULT: Adj.
12: 1.2V
15: 1.5V
18: 1.8V
25: 2.5V
33: 3.3V
VIN
VOUT
1
2
3
EN GND LX
Fixed version
FRONT VIEW
VIN
VFB
5
4
2351G
1
2
3
EN GND LX
Adjustable version
Example:
AIC2351-33GV5TR
3.3V Output Version, in SOT-23-5 Green Package & Tape & Reel
Packing Type
AIC2351GV5TR
Adjustable Version, in SOT-23-5 Green Package & Tape & Reel
Packing Type

Marking (Fixed Version)
Part No.
AIC2351-12GV5
AIC2351-15GV5
AIC2351-18GV5
AIC2351-25GV5
AIC2351-33GV5

Marking
GN12G
GN15G
GN18G
GN25G
GN33G
Marking (Adjustable Version)
Part No.
AIC2351GV5
Marking
2351G
2
AIC2351

ABSOLUTE MAXIMUM RATINGS
VIN, LX to GND
-0.3 V to 6.0V
VFB, VOUT, EN to GND
-0.3 V to VIN
Operating Ambient Temperature Range TA
-40C to 85C
Operating Maximum Junction Temperature TJ
Storage Temperature Range TSTG
150C
-65C to 150C
260C
Lead Temperature (Soldering 10 Sec.)
Thermal Resistance Junction to Case
SOT-23-5
115C/W
Thermal Resistance Junction to Ambient
SOT-23-5
250C/W
(Assume no Ambient Airflow, no Heatsink)
Absolute Maximum Ratings are those values beyond which the life of a device may be impaired.
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AIC2351

ELECTRICAL CHARACTERISTICS
(TA=25C, VIN=3.6V, unless otherwise specified.) (Note 1)
PARAMETER
CONDITIONS
SYMBOL MIN
Input Voltage Range
VIN
2.5
Output Adjustment Range
Line Regulation
Load Regulation
VFB = VOUT, VIN = 3.6V, IOUT = 0A
TA=25C
AIC2351-12, IOUT = 0.1A
AIC2351-15, IOUT = 0.1A
AIC2351-18, IOUT = 0.1A
AIC2351-25, IOUT = 0.1A
AIC2351-33, IOUT = 0.1A
Duty cycle = 100% to 23%
IOUT = 0 to 600mA
FB Input Current
VFB = 1.4V
Feedback Voltage
Fixed Output Voltage
VOUT
VFB
VFB
0.588
VOUT
-2
-2
-2
-2
-2
TYP
MAX UNITS
5.5
V
VIN V
0.2
0.6
0.612
V
+2
+2
+2
+2
+2
%
%
%
%
%
1
1.3
%
%
IFB
0.01
nA
P-Channel On-Resistance ILX = 100mA, VIN=3.6V
PRDS(ON)
0.4
0.65

N-Channel On-Resistance ILX = -100mA, VIN=3.6V
NRDS(ON)
0.35
0.8

IPK
1.4
Peak Inductor Current
Quiescent Current
VFB=0.66V, IOUT=0A
VFB=0.50V, IOUT=0A
Shutdown Supply Current
Oscillator Frequency
Maximum Duty Cycle
EN Pin Current
VEN = VIN
100
fOSC
DMAX
IEN
1.2
100
A
30
50
A
150
0.1
1.5
220
1
1.8
0.1
1
A
A
MHz
%
A
Output ON
VENH
1.6
V
Output OFF
VENL
0.25
V
Note 1: Specifications are production tested at TA=25C. Specifications over the -40C to 85C operating
temperature range are assured by design, characterization and correlation with Statistical Quality
Controls (SQC).
EN Input Threshold
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AIC2351
 TYPICAL PERFORMANCE CHARACTERISTICS
100
90
VOUT = 1.2V
VOUT=1.8V
95
85
100mA
90
80
10mA
80
EFFICIENCY(%)
EFFICIENCY(%)
85
600mA
75
70
65
1mA
60
VIN = 2.7V
75
70
VIN = 3.6V
65
VIN = 4.2V
60
55
50
55
50
2.5
3.0
3.5
4.0
4.5
5.0
45
5.5
1
10
INPUT VOLTAGE (V)
Fig. 3 Efficiency vs. Input Voltage
100
VOUT = 1.8V
VIN = 2.7V
85
VOUT = 2.5V
95
90
VIN = 2.7V
VIN = 3.6V
90
85
80
EFFICIENCY(%)
EFFICIENCY(%)
1000
Fig. 4 Efficiency vs. Output Current
95
75
VIN = 3.6V
70
VIN = 4.2V
65
75
70
65
60
55
55
50
1
10
100
1000
VIN = 3.6V
VIN = 4.2V
VIN = 4.2V
80
60
50
100
OUTPUT CURRENT (mA)
VIN = 2.7V
1
10
100
1000
OUTPUT CURRENT (mA)
OUTPUT CURRENT (mA)
Fig. 5 Efficiency vs. Output Current
Fig. 6 Efficiency vs. Output Current
100
VOUT = 3.3V
95
90
VIN = 5V
EFFICIENCY(%)
85
80
75
70
65
60
55
50
1
10
100
1000
OUTPUT CURRENT(mA)
Fig. 7 Efficiency vs. Output Current
5
AIC2351
TYPICAL PERFORMANCE CHARACTERISTICS (Continuous)
VIN=5.5V, IOUT=600mA
3.390
OSCILLATOR FREQUENCY (Mhz)
1.6
OUTPUTE VOLTAGE (V)
3.385
3.380
3.375
3.370
3.365
3.360
-40
-20
0
20
40
60
VIN =4.2V
1.5
1.4
1.3
1.2
1.1
-40
80
-20
O
20
40
60
80
TEMPERATURE ( C)
Fig. 8 Output Voltage vs. Temperature
Fig. 9 Oscillator Frequency vs. Temperature
1.6
0.7
0.6
1.5
0.5
MAIN
SWITCH
0.4
1.4
RDS(ON)
OSCILLATOR FREQUENCY( Mhz)
0
O
TEMPERATURE ( C)
0.3
SYNCHRONOUS
SWITCH
0.2
1.3
0.1
1.2
2.5
3.0
3.5
4.0
4.5
5.0
0.0
2.5
5.5
3.0
INPUT VOLTAGE(V)
4.5
5.0
5.5
2.5
VOUT= 1.2V
IOUT=0A
Current Limit (A)
48
4.0
Fig. 11 RDS(ON) vs. Input Voltage
50
49
3.5
INPUT VOLTAGE (V)
Fig. 10 Oscillator Frequency vs. Input Voltage
47
SUPPLY CURRENT (A)

46
45
44
43
42
41
40
39
VOUT=1.2V
2
1.5
1
0.5
0
38
2.5
37
3
3.5
4
4.5
5
5.5
36
35
3.0
3.5
4.0
4.5
5.0
5.5
Input Voltage (V)
INPUT VOLTAGE (V)
Fig. 12 Supply Current vs. Input Voltage
Fig. 13 Current Limit vs. Input Voltage
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AIC2351

TYPICAL PERFORMANCE CHARACTERISTICS
(Continuous)
Output Ripple
Output Ripple
ILX
ILX
IOUT
IOUT
VIN=3.6V; VOUT=1.2V; L=2.2F; COUT=10F; IOUT=0mA to 600mA
Fig. 14 Load Transient Response
VIN=3.6V; VOUT=1.2V; L=2.2F; COUT=10F; IOUT=200mA to 600mA
Fig. 15 Load Transient Response
Output Ripple
Output Ripple
ILX
ILX
IOUT
IOUT
VIN=3.6V; VOUT=1.8V; L=2.2F; COUT=10F; IOUT=50mA to 600mA
Fig. 16 Load Transient Response
Output Ripple
VIN=3.6V; VOUT=1.8V; L=2.2F; COUT=10F; IOUT=100mA to 600mA
Fig. 17 Load Transient Response
VLX
Output Ripple
ILX
IOUT
VIN=3.6V; VOUT=1.8V; L=2.2F; COUT=10F; IOUT=200mA to 600mA
Fig. 18 Load Transient Response
ILX
VIN=3.6V; VOUT=1.8V; L=2.2F; COUT=10F; IOUT=50mA
Fig. 19 PFM Operation
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AIC2351

BLOCK DIAGRAM

PIN DESCRIPTIONS
PIN 1: EN
- ON/OFF Control Pin. The Device
will turn off when EN is Low. This
pin isn’t allowed to float.
PIN 2: GND - Ground.
PIN 3: LX
- LX Pin. The LX node connects to
the inductor.
PIN 4: VIN
- Main Power Supply Pin.
PIN 5: VOUT - Output Voltage Feedback Pin.
(For fixed version)
PIN 5: VFB
- Feedback Pin. (For adjustable
version)
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AIC2351

APPLICATION INFORMATION
Operation
frequency of AIC2351 will be reduced to third of the
The AIC2351 is a low-noise and fixed-frequency step-
normal switching frequency. This lower switching
down DC/DC converter with current-mode PWM
frequency ensures the inductor current has more time
control
internal
to discharge, thereby preventing inductor current
synchronous rectifier, which eliminates the external
runaway. The switching frequency will automatically
Schottky diode and increases efficiency. During normal
return to its designed value while short circuit condition
operation, the AIC2351 can regulate its output voltage
is released.
architecture.
It
features
an
through a feedback control circuit, which is composed
of an error amplifier; a current comparator and several
control signal generators. By comparing the feedback
voltage to the reference voltage of 0.6V, the error
Shutdown
By connecting the EN pin to GND, the AIC2351 can be
shut down to reduce the supply current to 0.1A
amplifier varies its output voltage. The output voltage
(typical). At this operation mode, the output voltage of
of the error amplifier is compared with the summing
step-down converter is equal to 0V.
signal
100% Duty Cycle Operation
of
current
sensing
signal
and
slope
compensation signal to determine the duty cycle of
When the input voltage approaches the output voltage,
internal main power switch (P-channel MOSFET).
the AIC2351 smoothly transits to 100% duty cycle
While the main power switch is turned on, the
operation. This allows AIC2351 to regulate the output
synchronous power switch (N-channel MOSFET) will
voltage until AIC2351 completely enters 100% duty
be turned off through anti-short-through block. Similarly,
cycle operation. In 100% duty cycle mode, the output
when the main power switch is turned off, the
voltage is equal to the input voltage minus the voltage,
synchronous power switch will be turned on until the
which is the drop across the main power switch.
inductor current starts to reverse or the beginning of
The AIC2351 achieves 100% duty cycle operation by
the next switching cycle. In order to achieve better
extending the turn-on time of the main power switch. If
efficiency
output
the summing signal of current sensing signal and slope
pulse-frequency-
compensation signal does not reach the output voltage
modulated mode (PFM) operation while working at
level of the error amplifier at the end of 90% switching
light load conditions.
period, the main power switch is continuously turned
Current Limitation
on and the oscillator remains off until the summing
The AIC2351 provides current limit function by using
signal
an internal sensing resistor. When the main power
compensation signal reaches the output voltage level
switch turns on, current follows through the internal
of the error amplifier. After the summing signal of
sensing resistor. And current amplifier senses the
current sensing signal and slope compensation signal
voltage, which crosses the resistor, and amplifies it.
reaches the output voltage level of the error amplifier,
While the sensed voltage gets higher than reference
the main power switch is turned off and the
voltage, the current limitation function is activated.
synchronous power switch is turned on for a constant
While the current limitation function is activated, the
off time. At the end of the constant off time, the next
duty cycle will be reduced to limit the output power to
switching cycle is begun. While the input voltage
protect the internal power switches.
approaches
capacitor,
and
prevent
AIC2351
will
overcharging
enter
the
Short Circuit Protection
While the output is shorted to ground, the switching
of
current
the
sensing
output
signal
voltage,
and
the
slope
switching
frequency decreases gradually to smoothly transit to
100% duty cycle operation.
9
AIC2351
If input voltage is very close to output voltage, the
For lower output voltage ripple, the use of low ESR
switching mode goes from pure PWM mode to 100%
ceramic capacitor is recommended. The tantalum
duty cycle operation. During this transient state
capacitor can also be used well, but its ESR is larger
mentioned above, large output ripple voltage may
than that of ceramic capacitor.
appear on output terminal.
When
Components Selection
capacitors, X5R and X7R types are recommended
Inductor
The inductor selection depends on the current ripple of
inductor, the input voltage and the output voltage.
VOUT
L
fOSC  IL


V
1  OUT 
V
IN 

choosing
the
input
and
output
ceramic
because they retain their capacitance over wider
ranges of voltage and temperature than other types.
When using the ceramic capacitor as the input
capacitor, the high input voltage transient may be
generated at some start-up conditions, such as
connecting the input to a live power source. By adding
Accepting a large current ripple of inductor allows the
a small resistor in series with the input ceramic
use of a smaller inductance. However, higher current
capacitor, the high input voltage transient can be
ripple of inductor can cause higher output ripple
improved.
voltage and large core loss. By setting an acceptable
Output Voltage Programming (AIC2351 Adjustable
Version Only)
By connecting a resistive divider R1 and R2, the output
current ripple of inductor, a suitable inductance can be
obtained from above equation.
In addition, it is important to ensure the inductor
saturation current exceeds the peak value of inductor
voltage of AIC2351 step-down converter can be set.
VOUT can be calculated as:
peak value of inductor current can be calculated

R 
VOUT  0.6  1  1 
 R2 
according to the following equation.
The resistive divider should sit as close to VFB pin as
current in application to prevent core saturation. The
IPEAK  IOUT max  
possible.

VOUT 
V
1  OUT 
2  fOSC  L 
VIN 
Layout Consideration
Input Capacitor and Output Capacitor
To prevent the high input voltage ripple and noise
resulted from high frequency switching, the use of low
ESR ceramic capacitor for the maximum RMS current
is recommended. The approximated RMS current of
the input capacitor can be calculated according to the
following equation.
2
ICINRMS  IOUT
(MAX ) 
following points should be managed comprehensively.
1. The input capacitor and VIN should be placed as
close as possible to each other to reduce the input
voltage ripple and noise.
2. The output loop, which is consisted of the inductor,
the internal main power switch, the internal synchronous power switch and the output capacitor,
VOUT VIN  VOUT 
2
VIN

IL2
12
The selection of output capacitor depends on the
required output voltage ripple. The output voltage
ripple can be expressed as:
VOUT
In order to ensure a proper operation of AIC2351, the
IL

 ESR  IL
8  fOSC  COUT
should be kept as small as possible.
3. The routes with large current should be kept short
and wide.
4. Logically the large current on the converter should
flow at the same direction.
5. The VFB pin should be connected to the feedback
resistors directly and the route should be away
from the noise sources.
10
AIC2351
 PHYSICAL DIMENSIONS

SOT-23-5 (unit: mm)
A
A
E
E1
D
e
e1
SEE VIEW B
WITH PLATING
c
A
A2
b
SECTION A-A
A1
BASE METAL
0.25
S
Y
M
B
O
L
GAUGE PLANE
SEATING PLANE
θ
L
L1
A
VIEW B
Note : 1. Refer to JEDEC MO-178AA.
2. Dimension "D" does not include mold flash, protrusions
or gate burrs. Mold flash, protrusion or gate burrs shall not
exceed 10 mil per side.
3. Dimension "E1" does not include inter-lead flash or protrusions.
4. Controlling dimension is millimeter, converted inch
dimensions are not necessarily exact.
SOT-23-5
MILLIMETERS
MIN.
MAX.
0.95
1.45
A1
0.00
0.15
A2
0.90
1.30
b
0.30
0.50
c
0.08
0.22
D
2.80
3.00
E
2.60
3.00
E1
1.50
1.70
e
0.95 BSC
e1
1.90 BSC
L
0.30
L1
θ
0.60
0.60 REF
0°
8°
Note:
Information provided by AIC is believed to be accurate and reliable. However, we cannot assume responsibility for use of any circuitry other than
circuitry entirely embodied in an AIC product; nor for any infringement of patents or other rights of third parties that may result from its use. We
reserve the right to change the circuitry and specifications without notice.
Life Support Policy: AIC does not authorize any AIC product for use in life support devices and/or systems. Life support devices or systems are
devices or systems which, (I) are intended for surgical implant into the body or (ii) support or sustain life, and whose failure to perform, when
properly used in accordance with instructions for use provided in the labeling, can be reasonably expected to result in a significant injury to the
user.
11
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