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PDF AMS4123 Data sheet ( Hoja de datos )
| Número de pieza | AMS4123 | |
| Descripción | 3A 20V Step-Down Converter + 1A LDO | |
| Fabricantes | Advanced Monolithic Systems | |
| Logotipo |  |
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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
3/5/2010
www.advanced-monolithic.com
1
Phone (925) 443-0722
Fax (925) 443-0723
1 page  
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
3/5/2010
www.advanced-monolithic.com
5
Phone (925) 443-0722
Fax (925) 443-0723
5 Page 
Device Summary
The AMS4123 is combines a high voltage 3 Amp fixed
frequency step-down converter combined with a 1
Amp low drop out (LDO) linear regulator on a single
die.
The peak current mode step-down converter has
internal compensation and is stable with a wide range
of ceramic, tantalum, and electrolytic output
capacitors. The step-down converter output voltage is
sensed through an external resistive divider that feeds
the negative input to an internal transconductance
error amplifier. The output of the error amplifier is
connected to the input to a peak current mode
comparator. The inductor current is sensed as it
passes through the power switch, amplified and is
also fed to the current mode comparator. The error
amplifier regulates the output voltage by controlling
the peak inductor current passing through the power
switch so that, in steady state, the average inductor
current equals the load current. The step-down
converter has an input voltage range of 4.5V to 20V
with an output voltage as low as 0.6V.
The LDO operates from an input voltage ranging from
1V to 20V and a typical dropout voltage of 350mV at
1A. The input to the LDO can be supplied by the
output of the Step-Down converter or some other
available power source that must be 2V less than the
input voltage (Vin). The LDO is also stable for a wide
range of ceramic output capacitors ranging from as
low as 1µF.
Enable
The enable input has two levels so that the step-down
converter can be enabled independently of the LDO.
The enable threshold for the step-down converter is
2.0V while the enable threshold for the linear regulator
output is 2.5V typical.
Under Voltage Lockout
The under-voltage lockout (UVLO) feature guarantees
sufficient input voltage (Vin) bias for proper operation
of all internal circuitry prior to activation. The input
voltage (Vin) is internally monitored and the converter
and LDO are enabled when the rising level of Vin
reaches 4.2V. To prevent UVLO chatter 400mV of
hysteresis is built in to the UVLO comparator so that
the step-down converter and LDO are disabled when
VIN drops to 3.8V.
AMS4123
3A 20V Step-Down Converter + 1A LDO
Fault Protection
Short circuit and over-temperature shutdown disable
the converter and LDO in the event of an overload
condition.
Application
Inductor
The step-down converter inductor is typically selected
to limit the ripple current to 40% of the full load output
current. Solve for this value at the maximum input
voltage where the inductor ripple current is greatest.
L=
Vin-Vo
·
Vo
Vin·Io·0.4·Fs
L=
15V-2.5V
·
2.5V
15V·2A·0.4·300kHz
=9.4µH
For most applications the duty cycle of the AMS4123
step down converter is less than 50% duty and does
not require slope compensation for stability. This
provides some flexibility in the selected inductor
value. Given the above selected value, others values
slightly greater or less may be examined to determine
the effect on efficiency without a detrimental effect on
stability.
With and inductor value selected, the ripple current
can be calculated:
Ipp=
(Vo+Vfwd)·(1-D)
L·Fs
Using the maximum input voltage values the ripple is:
Ipp=
(2.5V+0.2V)· 1-0.23
10μH·300kHz
=0.7A
Once the appropriate value is determined, the
component is selected based on the DC current and
the peak (saturation) current. Select an inductor that
has a DC current rating greater than the full load
current of the application. The DC current rating is
also reflected in the DC resistance (DCR)
specification of the inductor. The inductor DCR should
limit the inductor loss to less than 2% of the step-
down converter output power.
The peak current at full load is equal to the full load
DC current plus one half of the ripple current. As
mentioned before, the ripple current varies with input
3/5/2010
www.advanced-monolithic.com
11
Phone (925) 443-0722
Fax (925) 443-0723
11 Page | ||
| Páginas | Total 18 Páginas | |
| PDF Descargar | [ Datasheet AMS4123.PDF ] | |
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