Page 1
AX3503
1.5MHz, 1A Synchronous Step-Down
Converter
GENERAL DESCRIPTION
AX3503 is a high efficiency monolithic synchronous buck regulator using a constant
frequency, current mode architecture. The device is available in an adjustable version.
Supply current with no load is 35uA and drops to <1uA in shutdown. The 2.5V to 6V input
voltage range makes AX3503 ideally suited for single Li-Ion, two to four AA battery-powered
applications. 100% duty cycle provides low dropout operation, extending battery life in
portable systems. PWM pulse skipping mode operation provides very low output ripple
voltage for noise sensitive applications. 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 decreases need of an external Schottky diode. Low output
voltages are easily supported with the 0.6V feedback reference voltage. AX3503 is available
in small SOT-23-5L and TDFN-6L (2*2) packages.
FEATURES
- 2.5V to 6V Input Voltage Range
- High Efficiency: Up to 94%
- 1.5MHz Constant Frequency Operation
- 1A Output Current at V
=3.6V and V
IN
OUT
=1.8V
- Quiescent Current: 35uA (input < 4.2V)
- No Schottky Diode Required
- 100% Duty Cycle in Dropout
- 0.6V Reference Allows Low Output Voltages
- Current Mode Operation for Excellent Line and Load Transient Response
- Current limit, Enable function
- Short Circuit Protect (SCP)
- ≤ 1µA Shutdown Current
- SOT-23-5L and TDFN-6L (2*2) Pb-Free packages
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Page 2
AX3503
(Top View)
EN GND SW
VINFB
1
2 3
4
5
SOT-23-5L
TDFN-6L (2*2) A Type
VIN
FB
SW
1 2
3
456
EN
GND
NC
GND
Name
Description
EN
Enable pin
H：normal operation
L：Shutdown
VIN
Power Supply Input Pin
SW
Switch output pin. Connect
external inductor here. Minimize
trace area at this pin to reduce
EMI.
GND
Ground Pin
FB
Output Feedback pin
TDFN-6L (2*2) B Type
FB
VIN
EN
1 2
3
456
NC
GND
SW
GND
BLOCK DIAGRAM
PIN ASSIGNMENT
The packages of AX3503 are SOT-23-5L and TDFN-6L (2*2); the pin assignment is given by:
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Rev.2.7 May.26, 2014
Page 3
AX3503
Order Information (SOT-23-5L)
Order Information (TDFN-6L) A/B Type
Package Type
Packing
A : Taping
B: SOT-23-5L
X
Blank: Bag
AX3503
X
Package Type
Packing
A : Taping
XX X
Blank: Bag
Z6: TDFN-6L(2*2)
AX3503
X
Pin Type
B : B Type
Blank: A Type
Top Marking (SOT-23-5L)
G
AX3503
WW:01~26 (A~Z)
27~52 (a~z)
Year: 1=2011
2=2012
ID code:internal
3Y
W
X
:
:
9=2019
Top Marking (TDFN-6L)
A Type
Top Marking (TDFN-6L)
B Type
G
WW: 01~26(A~Z)
27~52(a~z)
Year: 1=2011
2=2012
ID code: internal
AX3503 (A Type)
a
Y WX
:
:
9=2019
G
WW: 01~26(A~Z)
27~52(a~z)
Year: 1=2011
2=2012
ID code: internal
AX3503 (B Type)
c
Y WX
:
:
9=2019
Characteristics
Symbol
Rating
Unit
VIN Pin Voltage
VIN
VSS - 0.3 to VSS + 7
V
Feedback Pin Voltage
VFB
VSS - 0.3 to VCC + 0.3
V
EN Pin Voltage
VEN
VSS - 0.3 to VCC + 0.3
V
Switch Pin Voltage
VSW
VSS - 0.3 to VCC + 0.3
V
Peak SW Sink & Source Current
I
PSW
1.4
A
Power Dissipation
PD
( TJ-TA ) / θJA
mW
Storage Temperature Range
TST
-40 to +150
°C
Operating Temperature Range
TOP
-40 to +85
°C
Junction Temperature
TJ
+125
°C
Thermal Resistance from Junction
to case
TDFN-6L
θJC
25
°C/W
SOT-23-5L
110
Thermal Resistance from Junction
to ambient
TDFN-6L
θJA
120
°C/W
SOT-23-5L
250
ORDER/MARKING INFORMATION
ABSOLUTE MAXIMUM RATINGS (at T
Note: θJA is measured with the PCB copper area of approximately 1 in2(Multi-layer). That need connect to GND pin
of the AX3503.
Axelite Confidential Materials, do not copy or distribute without written consent.
= 25°C )
A
3/13
Rev.2.7 May.26, 2014
Page 4
AX3503
Characteristics
Symbol
Conditions
Min
Typ
Max
Units
Input Voltage Range
VIN
2.5 - 6
V
Feedback Voltage
VFB
TA= +25°C
0.588
0.600
0.612
V
TA=0°C ≤TA≤ 85°C
0.587
0.600
0.614
TA=-40°C≤TA≤
85°C
0.582
0.600
0.618
Feedback Bias Current
IFB
VFB=0.65V
- - ±30
nA
Input DC Supply Current
ICC
VFB=0V
-
35
45
uA
Shutdown Supply Current
ISD
VEN =0V
-
0.1 1 uA
Switching Current Limit
I
LIMIT
1.2
1.4 - A
Line Regulation
△V
OUT/VOUT
VIN = 2.5V~5.5V
-
0.04
0.4
%/V
Load Regulation
△V
OUT/VOUT
I
OUT
= 0.01 to 1A
- 1 -
%
Oscillation Frequency
F
OSC
SW pin, VIN=3.6V to
V
OUT
=1.2V
1.2
1.5
2.0
MHz
R
DS(ON)
of P-CH MOSFET
R
DSON
ISW= 300mA
-
0.35
0.50
Ω
R
DS(ON)
of N-CH MOSFET
R
DSON
ISW= -300mA
-
0.30
0.45
Ω
EN pin logic input threshold
voltage
V
ENL
0.6 - -
V
V
ENH
- - 1.2
EN Pin Input Current
IEN
-
±0.1
±1
uA
Efficiency
EFFI
VIN=5V, V
OUT
=3.3V,
I
OUT
=0.5A
-
92 - %
Vout = 0.6 * (1+R1/R2)
R2
30K
C1
4.7u
C2
Vout=1.8V
R2=10K~30K
L1
2.2uH
R1
U1
VIN
EN
GND
SW
FB
C3
AX3503
100p
60K
10u
R3
C4
0.1u
100K
ELECTRICAL CHARACTERISTICS
(VIN = VEN=3.6V, T
=25°C, unless otherwise specified) (Note)
A
Note: 100% production test at +25°C. Specifications over the temperature range are guaranteed by design and
characterization.
APPLICATION CIRCUIT
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Rev.2.7 May.26, 2014
Page 5
AX3503
FUNCTION DESCRIPTIONS
Operation
AX3503 is a monolithic switching mode step-down DC-DC converter. It utilizes internal
MOSFETs to achieve high efficiency and can generate very low output voltage by using
internal reference at 0.6V. It operates at a fixed switching frequency, and uses the slope
compensated current mode architecture. This step-down DC-DC Converter supplies
1000mA output current at V
= 1.8V with input voltage range from 2.5V to 6V.
OUT
Current Mode PWM Control
Slope compensated current mode PWM control provides stable switching and
cycle-by-cycle current limit for excellent load and line responses and protection of the
internal main switch (P-Ch MOSFET) and synchronous rectifier (N-CH MOSFET). During
normal operation, the internal P-Ch MOSFET is turned on for a certain time to ramp the
inductor current at each rising edge of the internal oscillator, and switched off when the peak
inductor current is above the error voltage. The current comparator, I
limits the peak
COMP,
inductor current. When the main switch is off, the synchronous rectifier will be turned on
immediately and stay on until either the inductor current starts to reverse, as indicated by the
current reversal comparator, I
, or the beginning of the next clock cycle. The OVDET
ZERO
comparator controls output transient overshoots by turning the main switch off and keeping it
off until the fault is no longer present.
Idle Mode Operation
At very light loads, AX3503 automatically enters pulse skipping Mode. In the pulse
skipping Mode, the inductor current may reach zero or reverse on each pulse. The PWM
control loop will automatically skip pulses to maintain output regulation. The bottom
MOSFET is turned off by the current reversal comparator, I
and the switch voltage will
ZERO,
ring. This is discontinuous mode operation, and is normal behavior for the switching
regulator.
Maximum Load Current
AX3503 will operate with input supply voltage as low as 2.5V, however, the maximum
load current decreases at lower input due to large IR drop on the main switch and
synchronous rectifier. The slope compensation signal reduces the peak inductor current as a
function of the duty cycle to prevent sub-harmonic oscillations at duty cycles greater than
50%. Conversely the current limit increases as the duty cycle decreases.
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Page 6
AX3503
2
1
16.0
R
R
V
V
OUT
V
OUT
R2
R1
1.2V
30K
30K
1.5V
30K
45K
1.8V
30K
60K
2.5V
30K
95K
f
I
V
VVV
L
OSC
L
IN
OUTINOUT
V
OUT
1.2V
1.5V
1.8V
2.5V
Inductor
2.2uH
2.2uH
2.2uH
2.2uH
Part Number
WE-TPC
7440430027
7440430027
7440430027
7440430022
APPLICATION INFORMATION
Setting the Output Voltage
Application circuit item shows the basic application circuit with AX3503 adjustable
output version. The external resistor sets the output voltage according to the following
equation:
Table 1 Resistor select for output voltage setting
Inductor Selection
For most designs, the AX3503 operates with inductors of 1µH to 2.2µH. Low
inductance values are physically smaller but require faster switching, which results in some
efficiency loss. The inductor value can be derived from the following equation:
Where is inductor Ripple Current. Large value inductors lower ripple current and small
value inductors result in high ripple currents. Choose inductor ripple current approximately
35% of the maximum load current 1000mA, ΔIL=350mA.
Table 2 Inductor select for output voltage setting (VCC=3.6V)
Note: Part type MH or M (www.we-online.com)
For output voltages above 2.0V, when light-load efficiency is important, the minimum
recommended inductor is 2.2µH. For optimum voltage-positioning load transients, choose
an inductor with DC series resistance in the 50mΩ to 150mΩ range. For higher efficiency at
heavy loads (above 200mA), or minimal load regulation (but some transient overshoot), the
resistance should be kept below 100mΩ. The DC current rating of the inductor should be at
least equal to the maximum load current plus half the ripple current to prevent core
saturation (1000mA+170mA).
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Page 7
AX3503
Input Capacitor Selection
The input capacitor reduces the surge current drawn from the input and switching
noise from the device. The input capacitor impedance at the switching frequency shall be
less than input source impedance to prevent high frequency switching current passing to the
input. A low ESR input capacitor sized for maximum RMS current must be used. Ceramic
capacitors with X5R or X7R dielectrics are highly recommended because of their low ESR
and small temperature coefficients. A 4.7µF ceramic capacitor for most applications is
sufficient.
Output Capacitor Selection
The output capacitor is required to keep the output voltage ripple small and to ensure
regulation loop stability. The output capacitor must have low impedance at the switching
frequency. Ceramic capacitors with X5R or X7R dielectrics are recommended due to their
low ESR and high ripple current.
Compensation Capacitor Selection
The compensation capacitors for increasing phase margin provide additional stability.
It is required and more than 22p, Refer to Demo Board Schematic, The optimum values for
C3 is 100pF.
Layout Guidance
When laying out the PC board, the following suggestions should be taken to ensure
proper operation of the AX3503. These items are also illustrated graphically in below.
1. The power traces, including the GND trace, the SW trace and the V
trace should be
IN
kept short, direct and wide to allow large current flow. Put enough multiply-layer pads
when they need to change the trace layer.
2. Connect the input capacitor C1 to the V
pin as closely as possible to get good
IN
power filter effect.
3. Keep the switching node, SW, away from the sensitive FB node.
4. Do not trace signal line under inductor.
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Page 8
AX3503
EN GND SW
FB VIN
C1
VIN
GND
L1
VOUT
C2
R1
R2
SW should be connected to Inductor by wide
and short trace, keep sensitive components
away from this trace
C2
1 2 3
45
GND
C2's ground must be as
closer to IC's GND pin as
possible
VIN
C1's ground must be as
closer to IC's GND pin as
possible
C2, R1 and R2 must be as
closer to IC's FB pin as
possible
AX3503
GND
NC
1 2
456
EN
SWFB
EP
3
VIN
C1
VIN
GND
L1
VOUT
C2
R1
R2
SW should be connected to Inductor by wide
and short trace, keep sensitive components
away from this trace
C2
GND
C1 and C2's ground must
be as closer to IC's GND
pin as possible
C2, R1 and R2 must be as
closer to IC's FB pin as
possible and keep the
switching trace away.
TSOT-23-5L Layout Guide
TDFN-6L Layout Guide
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Rev.2.7 May.26, 2014
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Page 9
AX3503
Quiescent Current vs. Input Voltage
Line Regulation
I
OUT
=0.01A
Load Regulation
VIN=3.6V, V
OUT
=1.2V, L=2.2uH
Efficiency vs. Output Current
V
OUT
=1.2V, L=2.2uH
Efficiency vs. Output Current
V
OUT
=1.8V, L=2.2uH
Efficiency vs. Output Current
V
OUT
=3.3V, L=2.2uH
TYPICAL CHARACTERISTICS
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Rev.2.7 May.26, 2014
Page 10
AX3503
Frequency vs. Temperature
VIN=3.6V, V
OUT
=1.2V, L=2.2uH
Output Error vs. Temperature
VIN=3.6V, V
OUT
=1.2V, L=2.2uH
Quiescent Current vs. Temperature
VIN=3.6V
TYPICAL CHARACTERISTICS (CONTINUES)
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Rev.2.7 May.26, 2014
Page 11
AX3503
Output Ripple(light Load)
VIN=3.3V, V
OUT
=1.2V, L=2.2uH, I
OUT
=80mA
Output Ripple(Heavy Load)
VIN=3.3V, V
OUT
=1.2V, L=2.2uH, I
OUT
=200mA
Transient
VIN=3.3V, V
OUT
=1.2V, L=2.2uH, I
OUT
=0A~1A
TYPICAL CHARACTERISTICS (CONTINUES)
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Rev.2.7 May.26, 2014
Page 12
AX3503
L
e1
E1
E
D
A2
A1
b(5x)
A
DETAL A
L1
1(4x)
2(4x)
e
GAUGE PLANE
C
DETAL A
θ
θ
θ
Symbol
Dimensions in Millimeters
Dimensions in Inches
Min.
Nom.
Max.
Min.
Nom.
Max. A - - 1.45 - -
0.057
A1
0
0.08
0.15
0
0.003
0.006
A2
0.9
1.1
1.3
0.035
0.043
0.051 b 0.3
0.4
0.5
0.012
0.016
0.02
C
0.08
0.15
0.22
0.003
0.006
0.009 D 2.7
2.9
3.1
0.106
0.114
0.122
E1
1.4
1.6
1.8
0.055
0.063
0.071 E 2.6
2.8
3
0.102
0.11
0.118 L 0.3
0.45
0.6
0.012
0.018
0.024
L1
0.5
0.6
0.7
0.02
0.024
0.028
e1
1.9 BSC
0.075 BSC
e
0.95 BSC
0.037 BSC
θ
0o
4o
8o
0o
4o
8o
θ1
5o
10o
15o
5o
10o
15o
θ2
5o
10o
15o
5o
10o
15o
JEDEC outline: MO-178 AA
PACKAGE OUTLINES
(1) SOT-23-5L
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Rev.2.7 May.26, 2014
Page 13
AX3503
E
D
A
A 1
A3
D2
e
E 2
(Top View)
(Bottom View)
b
Pin 1
L
SEATING PLANE
(SIDE View)
Symbol
Dimensions in Millimeters
Dimensions in Inches
Min.
Nom.
Max.
Min.
Nom.
Max. A 0.7
0.75
0.8
0.028
0.03
0.031
A1 0 0.02
0.05
0
0.001
0.002
A3
0.203 REF.
0.008 REF.
b
0.2
0.28
0.35
0.009
0.011
0.013
D
1.95 2 2.05
0.077
0.079
0.081
D2
1.0
1.5
1.7
0.039
0.059
0.067
E
1.95 2 2.05
0.077
0.079
0.081
E2
0.5
0.9
1.1
0.02
0.035
0.043
e
0.65 BSC.
0.026 BSC.
L
0.2
0.3
0.4
0.008
0.012
0.016
(2) TDFN-6L (2*2 0.75mm)
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Page 14
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