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General Description
The AMS4123 combines a 3A Step-Down converter
with a 1A LDO in a single SO-8 exposed paddle
package. Both the LDO and Step-Down converter
are low ESR, ceramic capacitor output, stable. The
Step-Down converter is internally compensated with
internal soft-start to minimize the number of external
components. An Enable pin provides built-in
externally programmable power-up sequencing. The
Step-Down converter enable threshold is 2.0V and
the LDO enable threshold is 2.5V. It also has hiccup
current limit and thermal protection. Thermal
protection shuts down both the Step-Down converter
and LDO when the die temperature exceeds 135°C.
Both regulators are adjustable using a 0.6V
reference for low output voltage settings. The LDO
has options for fixed output voltages from 0.6V to 5V
in 100mV steps. The LDO external input can be
powered from the Step-Down converter output, for
improved efficiency, or from any voltage source that
is less than or equal to the device supply voltage
(Vin). With a dropout voltage of less than 350mV at
1A, the AMS4123 LDO makes the perfect solution
for a low noise 1.8V power source developed from
2.5V Step-Down converter output. The AMS4123 is
a complete solution for LCD TV power requirements
when combined with the AMS4122 (2A Dual
Switching Regulator in SO-8).
AMS4123
3A 20V Step-Down Converter + 1A LDO
Features
Step-Down Converter + LDO in SO-8EP
Internally Compensated
Up to 95% Efficiency
Low ESR Ceramic Output Capacitor Stable
Soft Start
Under-Voltage Lockout
Dual Threshold Enable
300 kHz Switching Frequency
Hiccup Current Limit
Over-Temperature Shutdown
Ultra-Low Dropout LDO 350mV @ 1A
Up to 3A Step-Down Output Current
Up to 1A LDO Output Current
Excellent Light Load Efficiency
Applications
Audio Power Amplifiers
Portable (Notebook) Computers
Point of Regulation for High Performance
Electronics
Consumer Electronics
DVD, Blue-ray DVD writers
LCD TVs and LCD monitors
Distributed Power Systems
Battery Chargers
Pre-Regulator for Linear Regulation
Typical Application
Vin 4.5V to 20V
U1
3 Vin
AMS4123
SW
2.5V
7 LDOin
BST
C1
10uF
1.8V at 1A
8 LDOout FB SW
C3
2.2uF
R1
20.0k
6 FB LDO
EN
R2
10.0k
Enable
C5
220nF
1
2
5
4
R3
10.0k
L1
10uH
2.5V at 2A
SW out
C2
D1 22uF
B340LB
C9
100uF
R4
31.6k
C8
4.7nF
R1 and R4 Voltage Options
1.8V 20.0k
2.5V 31.6k
3.3V 45.3k
5.0V 73.2k
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Fax (925) 443-0723

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AMS4123
Pin Description
3A 20V Step-Down Converter + 1A LDO
Pin #
1
2
3
4
5
6
7
8
9
Symbol
SW
BST
Vin
EN
FB SW
FB LDO
LDO in
LDO out
GND (PADDLE)
Description
Step-Down converter switching node that connects the internal power switch to the
output inductor.
The bootstrap capacitor tied to this pin is used as the bias source for the drive to the
internal power switch. Use a 220nF or greater capacitor from the BST to the SW
pin.
Input Power. Supplies bias to the IC and is also the power input to the step-down
converter main power switch. Bypass Vin with low impedance ceramic with
sufficient capacitance to minimize switching frequency ripple as well as high
frequency noise.
Enable. A voltage greater than 2V at this pin enables the switching regulator. 2.5V
enables the LDO section.
Step-Down Converter Feedback input. A resistor network of two resistors is used to
set-up the output voltage connected between VSW out and GND. The node between
the two resistors is connected to Feedback Switch pin.
LDO Feedback input. A resistive voltage divider is used to set the output voltage
connected between the LDO output and GND. The node between the two resistors
is connected to FB LDO pin.
LDO Input. Connect to the output of the Step-Down converter. LDO IN can also be
powered from any power supply as long as it is 2V less than Vin.
LDO Output pin.
Ground paddle to be connected to PCB ground plane. This is also the ground for
internal voltage reference.
Pin Configuration
8L SOIC
SO Package (S)
Top View
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AMS4123
3A 20V Step-Down Converter + 1A LDO
Absolute Maximum Ratings (1)
VIN Supply Voltage………………...………..….-0.3V to 23V
LDOIN Supply Voltage…………………..……...-0.3V to 20V
LDOOUT Output Voltage………………….….....-0.3V to 20V
BST Boot Strap Voltage………………....…. -0.3V to 27V
FBLDO,FBSW feedback pins………....……-0.3V to +12V
EN Enable Voltage………………………..….-0.3V to +20V
Storage Temperature Range……………...-65C to 150C
Lead Temperature…………………..……….….…… 260C
Junction Temperature………...………………..…… 150C
Recommended Operating Conditions (2)
Input Voltage………………………………….………..4.5V to 20V
Ambient Operating Temperature…… …………….-40C to 85C
Thermal Information
8L SOIC EP θJA (3) …………………………………….…...45C/W
θJC ...........................................................10C/W
Maximum Power Dissipation…………………………...….…….2W
Electrical Characteristics
Parameter
Vin
LDO Feedback Voltage
Switcher Feedback Voltage
LDO Output Voltage tolerance
Step-Down Converter Bias
Current
LDO+SW Bias Current
LDO Bias Current
Shutdown Supply Current
SW NPN Saturation Voltage
Converter Current Limit
LDO Current Limit
LDO Dropout Voltage
LDO Load Regulation
LDO Line Regulation
Oscillator Frequency
Maximum Duty Cycle
Minimum Duty Cycle
Converter Enable Threshold
Enable Hysteresis
LDO Enable Threshold
Enable Pull-up Current
Under Voltage Lockout
Under Voltage Lockout
Hysteresis
Total Power dissipation
Thermal Shutdown
TA= 25 °C and VIN=12V (unless otherwise noted).
Symbol
Conditions
Min.
Vin 4.5
VFBLDO
ILDO=0A
tbd
VFBSW
Isw=0A
tbd
VLDO Out
VLDO out=0.6V to 5V in
100mV increments
-1.5
IQSW
IQSW+LDO
IQLDO
VLDOin =VEN =5V
VFBSW= 1.5V
VLDOin =VEN =5V
VFBLDO =VFBSW= 1.5V
VEN= 5V; VFBLDO = 1.5V
IVinsd
VEN =0V
VSAT
ILIMSW
ILIMLDO
VDO
ΔVLDO Out /
VLDO Out
ΔVLDO Out /
VLDO Out
FOSC
DMAX
ISW out=1A
VSW out=5V
VLDO in=5V; Co=2.2μF
VLDOin=VLDOout-0.1V, Io=1A
ILDO = 0 to1A
VLDOin = VLDOout+0.5V to 20V,
Vin=20V
VFB=0V
260
DMIN
VFB=1.5V
VEN SW
VENHYS
VEN LDO
IEN VEN = 0V
VUVLO
Vin rising
VUVLO HYS
PD Note (4)
TSD
Typ.
12
0.586
0.596
1
1.4
1.3
400
90
0.66
4.2
1.1
350
0.5
0.1
300
95
0
2.0
100
2.5
0.7
4.2
200
2.5
145
Max.
20
tbd
tbd
1.5
1.9
2.0
340
99
2.1
2.55
Units
V
V
V
%
mA
mA
μA
nA
V
A
A
mV
%
%
kHz
%
%
V
mV
V
μA
V
mV
W
°C
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AMS4123
3A 20V Step-Down Converter + 1A LDO
Notes:
1.
2.
3.
4.
Stresses above those listed in Absolute Maximum Ratings may cause permanent damage to the device.
Operation outside of the recommended operating conditions is not guaranteed.
Measured on approximately 1” square of 1 oz. copper.
The total power dissipation for SO-8 EDP package is recommended to 2.5W rated at 25C ambient temperature. The thermal resistance Junction to Case
is 45C/W. Total power dissipation for the switching regulator and the LDO should be taken in consideration when calculating the output current capability
of each regulator.
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Typical Characteristics
100
90
80
70
60
50
40
30
20
0.01
Efficiency VSW out=5V, L=10µH,
B340LB Schottky
Vin =12V
Vin =23V
0.1 1
Output Current (A)
10
100
90
80
70
60
50
40
30
20
0.01
Efficiency VSW out=3.3V,
L=10µH, B340LB Schottky
Vin =12V
Vin =23V
0.1 1
Output Current (A)
10
Efficiency Vsw out=2.5V, L=10µH,
100 B340LB Schottky
90
80 Vin =12V
70
60
50 Vin =23V
40
30
20
0.01 0.1
1
Output Current (A)
10
AMS4123
3A 20V Step-Down Converter + 1A LDO
Load Regulation VSW out =5V, L=10µH
1.0
0.6
0.2 Vin =12V
-0.2 Vin =23V
-0.6
-1.0
0.01
0.1 1
Output Current (A)
10
Load Regulation VSW out=3.3V, L=10µH
1.0
0.6
0.2 Vin =23V
-0.2 Vin =12V
-0.6
-1.0
0.01
0.1 1
Output Current (A)
10
1.0 Load Regulation Vsw out=2.5V, L=10 µH
0.6
0.2 Vin =23V
-0.2
-0.6 Vin =12V
-1.0
0.01
0.1 1
Output Current (A)
10
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