AMS4123
3A 20V Step-Down Converter + 1A LDO
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).
Features
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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
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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
2.5V
7
C1
10uF
1.8V at 1A
8
R1
20.0k
C3
2.2uF
R2
10.0k
6
LDOout FB SW
FB LDO
Enable
EN
5
4
R3
10.0k
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
LDOin
BST
2
D1
B340LB
C2
22uF
C9
100uF
Vin
AMS4123
SW
1
C5
220nF
L1
10uH
2.5V at 2A
SW out
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AMS4123
3A 20V Step-Down Converter + 1A LDO
Pin Description
Pin #
1
2
Symbol
SW
BST
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 V
SW 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.
3
4
5
6
7
8
9
Vin
EN
FB SW
FB LDO
LDO in
LDO out
GND (PADDLE)
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)
V
IN
Supply Voltage………………...………..….-0.3V to 23V
LDO
IN
Supply Voltage…………………..……...-0.3V to 20V
LDO
OUT
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……………...-65⁰C to 150⁰C
Lead Temperature…………………..……….….…… 260⁰C
Junction Temperature………...………………..…… 150⁰C
Recommended Operating Conditions
(2)
Input Voltage………………………………….………..4.5V to 20V
Ambient Operating Temperature…… …………….-40⁰C to 85⁰C
Thermal Information
8L SOIC EP
θ
JA
…………………………………….…...45⁰C/W
θ
JC
...........................................................10⁰C/W
Maximum Power Dissipation…………………………...….…….2W
(3)
Electrical Characteristics
Parameter
V
in
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
T
A
= 25
°C
and V
IN
=12V (unless otherwise noted).
Symbol
V
in
V
FBLDO
V
FBSW
V
LDO Out
I
QSW
I
QSW+LDO
I
QLDO
I
Vinsd
V
SAT
I
LIMSW
I
LIMLDO
V
DO
ΔV
LDO Out
/
V
LDO Out
ΔV
LDO Out
/
V
LDO Out
F
OSC
D
MAX
D
MIN
V
EN SW
V
ENHYS
V
EN LDO
I
EN
V
UVLO
V
UVLO HYS
P
D
T
SD
Note (4)
V
EN
= 0V
V
in
rising
I
LDO
=0A
I
sw=
0A
V
LDO out
=0.6V to 5V in
100mV increments
V
LDOin
=V
EN
=5V
V
FBSW
= 1.5V
V
LDOin
=V
EN
=5V
V
FBLDO
=V
FBSW
= 1.5V
V
EN
= 5V; V
FBLDO
= 1.5V
V
EN
=0V
I
SW out
=1A
V
SW out
=5V
V
LDO in
=5V; C
o
=2.2μF
V
LDOin
=V
LDOout
-0.1V, Io=1A
I
LDO
= 0 to1A
V
LDOin
= V
LDOout
+0.5V to 20V,
V
in
=20V
260
V
FB
=0V
V
FB
=1.5V
Conditions
Min.
4.5
tbd
tbd
-1.5
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
2.55
2.1
340
99
Max.
20
tbd
tbd
1.5
1.9
2.0
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 25⁰C ambient temperature. The thermal resistance Junction to Case
is 45⁰C/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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AMS4123
3A 20V Step-Down Converter + 1A LDO
Typical Characteristics
V
SW out
Regulation (%)
100
90
Efficiency V
SW out
=5V, L=10µH,
B340LB Schottky
V
in
=12V
V
in
=23V
Load Regulation V
SW out
=5V, L=10µH
1.0
0.6
0.2
-0.2
-0.6
-1.0
Efficiency (%)
80
70
60
50
40
30
20
0.01
V
in
=12V
V
in
=23V
0.1
1
10
0.01
0.1
1
10
Output Current (A)
Output Current (A)
100
90
Efficiency (%)
80
70
60
50
40
30
20
0.01
V
in
=12V
V
SW out
Regulation (%)
Efficiency V
SW out
=3.3V,
L=10µH, B340LB Schottky
Load Regulation V
SW out
=3.3V, L=10µH
1.0
0.6
0.2
-0.2
-0.6
-1.0
V
in
=23V
V
in
=12V
V
in
=23V
0.1
1
10
0.01
0.1
1
10
Output Current (A)
Output Current (A)
90
Efficiency (%)
80
70
60
50
40
30
20
V
in
=12V
V
SW out
Regulation (%)
100
Efficiency V
sw out
=2.5V, L=10µH,
B340LB Schottky
1.0
0.6
0.2
-0.2
-0.6
-1.0
Load Regulation V
sw out
=2.5V, L=10 µH
V
in
=23V
V
in
=12V
V
in
=23V
0.01
0.1
1
Output Current (A)
10
0.01
0.1
1
10
Output Current (A)
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