ADMOS AMS4123S Dual threshold enable Datasheet

AMS4123
3A 20V Step-Down Converter + 1A LDO
General Description
Features
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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).
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
C5
220nF
AMS4123
Vin
SW
LDOin
BST
1
L1
10uH
2.5V at 2A
SW out
2.5V
7
C1
10uF
2
1.8V at 1A
8
R1
20.0k
6
C3
2.2uF
R2
10.0k
LDOout FB SW
FB LDO
Enable
EN
D1
B340LB
5
C2
22uF
4
R3
10.0k
R4
31.6k
C8
4.7nF
3/5/2010
C9
100uF
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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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AMS4123
3A 20V Step-Down Converter + 1A LDO
Pin Description
Pin #
Symbol
1
SW
2
BST
3
Vin
4
EN
5
FB SW
6
FB LDO
7
LDO in
8
LDO out
9
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
3/5/2010
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AMS4123
3A 20V Step-Down Converter + 1A LDO
Absolute Maximum Ratings (1)
Recommended Operating Conditions (2)
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……………...-65⁰C to 150⁰C
Lead Temperature…………………..……….….…… 260⁰C
Junction Temperature………...………………..…… 150⁰C
Input Voltage………………………………….………..4.5V to 20V
Ambient Operating Temperature…… …………….-40⁰C to 85⁰C
Electrical Characteristics
Parameter
Thermal Information
(3)
8L SOIC EP θJA
…………………………………….…...45⁰C/W
θJC ...........................................................10⁰C/W
Maximum Power Dissipation…………………………...….…….2W
TA= 25 °C and VIN=12V (unless otherwise noted).
Symbol
Conditions
Min.
Typ.
Max.
Units
4.5
12
20
V
Vin
Vin
LDO Feedback Voltage
VFBLDO
ILDO=0A
tbd
0.586
tbd
V
Switcher Feedback Voltage
VFBSW
Isw=0A
tbd
0.596
tbd
V
LDO Output Voltage tolerance
VLDO Out
VLDO out=0.6V to 5V in
100mV increments
-1.5
1
1.5
%
Step-Down Converter Bias
Current
IQSW
1.4
1.9
mA
LDO+SW Bias Current
IQSW+LDO
1.3
2.0
mA
LDO Bias Current
IQLDO
VEN= 5V; VFBLDO = 1.5V
400
μA
Shutdown Supply Current
IVinsd
VEN =0V
90
nA
SW NPN Saturation Voltage
Converter Current Limit
VSAT
ISW out=1A
0.66
V
ILIMSW
VSW out=5V
4.2
A
LDO Current Limit
ILIMLDO
VLDO in=5V; Co=2.2μF
1.1
A
VDO
VLDOin=VLDOout-0.1V, Io=1A
350
mV
ILDO = 0 to1A
0.5
%
VLDOin = VLDOout+0.5V to 20V,
Vin=20V
0.1
%
LDO Dropout Voltage
VLDOin =VEN =5V
VFBSW= 1.5V
VLDOin =VEN =5V
VFBLDO =VFBSW= 1.5V
Oscillator Frequency
ΔVLDO Out /
VLDO Out
ΔVLDO Out /
VLDO Out
FOSC
300
340
kHz
Maximum Duty Cycle
DMAX
VFB=0V
95
99
%
Minimum Duty Cycle
DMIN
VFB=1.5V
0
Converter Enable Threshold
VEN SW
2.0
Enable Hysteresis
VENHYS
100
LDO Enable Threshold
VEN LDO
2.5
Enable Pull-up Current
IEN
VEN = 0V
0.7
μA
Under Voltage Lockout
VUVLO
Vin rising
4.2
V
Under Voltage Lockout
Hysteresis
VUVLO HYS
200
mV
Total Power dissipation
PD
2.5
W
Thermal Shutdown
TSD
145
°C
LDO Load Regulation
LDO Line Regulation
3/5/2010
260
Note (4)
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%
2.1
V
mV
2.55
V
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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.
3/5/2010
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AMS4123
3A 20V Step-Down Converter + 1A LDO
Typical Characteristics
Efficiency (%)
90
80
VSW out Regulation (%)
100
Efficiency V SW out=5V, L=10µH,
B340LB Schottky
Vin =12V
70
Vin =23V
60
50
40
30
0.6
Vin =12V
0.2
-0.2
Vin =23V
-0.6
-1.0
20
0.01
Load Regulation VSW out =5V, L=10µH
1.0
0.1
1
0.01
10
80
Efficiency VSW out=3.3V,
L=10µH, B340LB Schottky
Vin =12V
70
60
Vin =23V
50
40
30
1.0
0.6
Vin =23V
0.2
-0.2
Vin =12V
-0.6
-1.0
20
0.01
0.1
1
0.01
10
80
VSW out Regulation (%)
Efficiency (%)
90
Efficiency Vsw out=2.5V, L=10µH,
B340LB Schottky
Vin =12V
70
60
50
Vin =23V
40
30
20
0.01
3/5/2010
0.1
1
10
Output Current (A)
Output Current (A)
100
10
Load Regulation VSW out=3.3V, L=10µH
VSW out Regulation (%)
Efficiency (%)
90
1
Output Current (A)
Output Current (A)
100
0.1
1.0
Load Regulation Vsw out=2.5V, L=10 µH
0.6
V in =23V
0.2
-0.2
Vin =12V
-0.6
-1.0
0.1
1
Output Current (A)
10
0.01
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0.1
1
10
Output Current (A)
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AMS4123
3A 20V Step-Down Converter + 1A LDO
Typical Characteristics
No Load Input Current vs. Input Voltage
Vsw out = 2.5V, VLDO out = 1.8V
0.50
Output Error (%)
Input Current (mA)
3.2
2.4
1.6
0.8
0.0
Output Voltage Error vs. Input Voltage
VSW out = VLDO in = 2.5V, VLDO out =1.8V
0.25
V LDO out
0.00
Vsw out
-0.25
ILDO=0.6A
Isw =1.6A
-0.50
0
5
10
15
20
25
0
5
Switching Frequency vs. Input Voltage
Vsw out = 2.5V, VLDO out = 1.8V
Dropout Voltage (V)
308
304
300
296
292
0
VLDO Out Voltage (V)
2
5
10
15
20
20
0.3
0.2
0.1
VLDO out programmed f or 1.8V
Vin = 12V
0
0
0.2
0.4
0.6
0.8
LDO Output Current (V)
VLDO Out Load Regulation
VSW out = VLDO in =2.5V, VLDO Out=1.8V
Feedback Voltage Temperature
Variation
Vin=15V
1
0.5
Vin=20V
0.61
0.4
0.6
0.8
1
1
FBSW
0.60
0.59
FBLDO
0.58
ILDO=Isw=0
V LDO out=1.8V, V SW out=2.5V
0.57
0
0.2
25
LDO Dropout Voltage vs. Load Current
VLDO in = VLDO out - 0.1V
0.4
25
Vin=12V
-50
Output Current (A)
3/5/2010
15
Input Voltage (V)
1.5
0
10
Input Voltage Vin (V)
Feedback Voltage (V)
Switching Frequency (kHz)
Input Voltage (V)
-10
30
70
110
150
Ambient Temperature (ºC)
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AMS4123
3A 20V Step-Down Converter + 1A LDO
Typical Characteristics
Step-Down Converter Output Ripple
VSW out =2.5V, ISW out=1.8A, Vin=12V
LDO 200mA to 800mA Transient Response,
VLDO in=3.3V, Co=2.2µF, VLDO out = 1.8V, Vin=12V
VSW out
20mVac
/div
VLDO out
100mVac
/div
IL
1A/div
ILDO out
500mA
/div
VSW
5V/div
2 µsec/div
1 µsec/div
Step-Down Converter Load Transient
No Load to 2A,VSW out=2.5V, Vin=12V
Step-Down Converter Load Transient
200mA to 2A, Vsw out = 2.5V, Vin=12V
VSW out
100mVac
/div
VSW out
100mVac
/div
ISW out
500mA
/div
ISW out
1A/div
2 msec/div
40 µsec/div
LDO Transient Response
No Load to 1A, VLDO in=Vsw out =2.5V,
Step-Down Converter Load Transient
200mA to 1.2A, Vsw out = 2.5V,Vin=12V
VSW out
200mVac/
div
VSW out
100mVac
/div
VLDO out
100mVac
/div
ISW out
500mA
/div
ILDO out
1A/div
20 µsec/div
3/5/2010
20 µsec/div
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AMS4123
3A 20V Step-Down Converter + 1A LDO
Typical Characteristics
VSW out
1V /div
VLDO out
1V /div
IL
2A/div
Ven
5V /div
Switching Frequency (kHz)
Start-Up Response Vin=12V
Switching Frequency Temperature
Variation VSW out=2.5V, Vin =12V
300
280
260
240
220
-45
-10
25
60
95
130
Ambient Temperature (ºC)
400 µsec/div
•
VLDO out
1V /div
IL
2A/div
Vin
10V /div
Feedback Voltage Error (%)
Start-Up Response
Enable=Vin=12V
Feedback Voltage Temperature
Variation
0.8
FBSW
0.0
FBLDO
-0.8
ILDO=Isw=0
V LDO out=1.8V, V SW out=2.5V
-1.6
-2.4
-50
-10
30
70
110
150
Ambient Temperature (ºC)
2 msec/div
Start-Up Response Vin=20V
1.2
1V /div
IL
2A/div
Ven
5V /div
Vcesat (V)
VSW out
1V /div
VLDO out
Step-Down Converter Power Switch
Saturation Voltage Vin =12V
0.9
0.6
0.3
Tamb = 25⁰ C
Mounted on Eval. Board
0
0
0.7
1.4
2.1
2.8
3.5
Current (A)
1 msec/div
3/5/2010
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3A 20V Step-Down Converter + 1A LDO
Typical Characteristic
LDO Current Limit
VLDO in = 3.3V, V LDO out=1.8V
50
1.3
Ground Current (mA)
Current Limit (A)
1.4
1.2
1.1
1
0.9
0.8
Voltage Mode Load V LDO out = 1.68V
0.7
0.6
5
9
13
17
21
LDO Ground Current
VLDO in = 3.3V, VLDO out=1.8V
25
40
30
20
10
0
0
400
600
800
1000
Load Current (mA)
V in Input Voltage (V)
3/5/2010
200
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3A 20V Step-Down Converter + 1A LDO
Functional Block Diagram
Vin
3
UVLO
4.2V / 3.8V
BST
Reg.
Internal
Vcc
Vcc
3.3V Regulator
Isense
Vref
0.6V
EAout
Σ
2
R
300kHz
Oscillator
FB SW
S
SET
CLR
BST
Level
Shift
Q
Q
1
SW
SW out
EAout
5
Vref
0.6V
Switching
Regulator
Shutdown
2.0V
En
4
7
8
Pgnd
3/5/2010
P
Paddle
LDO In
Vref
Shutdown
Comparators
2.5V
PVin
LDO Out
6
FB LDO
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3A 20V Step-Down Converter + 1A LDO
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.
3/5/2010
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 ·
L= 15V-2.5V ·
Vo
Vin·Io·0.4·Fs
2.5V
=9.4µH
15V·2A·0.4·300kHz
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
=0.7A
10μH·300kHz
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 stepdown 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
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AMS4123
3A 20V Step-Down Converter + 1A LDO
voltage and is a maximum at the maximum input
voltage.
Ipkmax=Io+
(Vo+Vfwd)·(1-Dmin)
2·L·Fs
High Frequency Ripple
The following equation determines the required low
ESR ceramic output capacitance for a given inductor
current ripple (Ipp).
Vo
Dmin=
Vinmax
The duty cycle can be more accurately estimated by
including the drops of the external Schottky diode and
the internal power switch:
Dmin=
Dmin=
Vo+Vfwd
Vinmax-Vo+Vfwd
2.5V+0.2V
=0.23
15V-0.3V+0.2V
Vfwd is the diode freewheeling diode drop and Vsw is
the collector to emitter drop of the internal power
switch.
With a good estimate of the duty cycle (D) the
inductor peak current can be determined:
Ipkmax=2A+
(2.5V+0.2V)·(1-0.23)
=2.35A
2·10µH·300kHz
There are a wide range 2 and 3 Amp, shielded and
non-shielded inductors available. Table 1 lists a few.
Table 1. Inductor Selection Guide
Dimensions (mm)
Series
Type
W
L
H
Coilcraft
NonDO3316P
9.4
13
5.2
Shielded
Non9.4
13
3.0
DO3308
Shielded
Sumida
CDRH6D26
Shielded
7
7
2.8
Non7.3
7.3
5.2
CDH74
Shielded
Coiltronics
SD8328
Shielded
8.3
9.5
3.0
3/5/2010
Step-Down Converter Output Capacitor
The optimum solution for the switching regulator is to
use a large bulk capacitor for large load transients in
parallel with a smaller, low ESR, X5R or X7R ceramic
capacitor to minimize the switching frequency ripple.
C=
Ipp
0.7A
=
=15μF
Fs·8·dV 300kHz·8·20mV
Large Signal Transient
For applications with large load transients an
additional capacitor may be required to keep the
output voltage within the limits required during large
load transients.
In this case the required capacitance can be
examined for the load application and load removal.
For full load to no load transient the required
capacitance is
2
L·Io2
10μH·(2A)
Cbulk=
=
=36μF
Vos2 -Vo2 (2.7V)2 -(2.5V)2
For the application of a load pulse the capacitance
required form hold up depends on the time it takes for
the power supply loop to build up the inductor current
to match the load current. For the AMS4123 this can
be estimated to be less than 10 µsec or about three
clock cycles.
Cbulk=
Io·t 2A·10μsec
=
=100μF
dV
0.2V
For applications that do not have any significant load
transient requirements a ceramic capacitor alone is
typically sufficient.
Boot Strap Capacitor
An external capacitor is required for the high side
switch drive. The capacitor is biased during the off
time while the switch node is at ground by way of the
freewheeling diode. During the on time portion of the
switching cycle the switch node is tied to the input
voltage by way of the internal power switch. The boot
strap capacitor is always referenced to the switch
node so the charge stored in the capacitor during the
off time is then used to drive the internal power switch
during the on time.
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3A 20V Step-Down Converter + 1A LDO
Typical bootstrap capacitor values are in the 220nF to
470nF range. Insufficient values will not be able to
provide sufficient base drive current to the power
switch during the on time. Values less than 220nF are
not recommended. This will result in excessive losses
and reduced efficiency.
Optional Snubber
To reduce high frequency ringing at the switching
node a snubber network is suggested. The values
typically selected are 470pF ceramic in series with a
10Ω resistor. The power dissipation of the 10Ω
resistor is about 32mW for a 15V input with a 300kHz
switching frequency.
PR1=C3·Vin2 ·Fsw
Vin is the maximum input voltage and Fsw is the
switching frequency. The snubber capacitor must be
rated to withstand the input voltage.
Step-Down Converter Input Capacitor
The low esr ceramic capacitor required at the input to
filter out high frequency noise as well as switching
frequency ripple. Placement of the capacitor is critical
for good high frequency noise rejection. See the PCB
layout guidelines section for details. Switching
frequency ripple is also filtered by the ceramic bypass
input capacitor. Given a desired input voltage ripple
(Vripple) limit, the required input capacitor can be
estimated with:
Dmax=
C=
Linear Regulator Output Capacitor
The Linear regulator is stable with a wide range of
ceramic capacitors. The ceramic output capacitor can
range from 1uF to 100uF with either X5R or X7R
temperature coefficient. The actual values selected
within the range will depend on the expected load
transients and the output voltage tolerance
requirements during the load transient.
Linear Regulator Input Capacitor
Place a 2.2uF X5R or X7R or equivalent ceramic
bypass capacitor at the LDO input.
Feedback Resistor Selection
The step down converter and LDO both use a 0.6V
reference voltage at the positive terminal of the error
amplifier. To set the output voltage a programming
resistor form the feedback node to ground must first
be selected (R2,R3 of figure 4). A 10kΩ resistor is a
good selection for a programming resistor. A higher
value could result in an excessively sensitive
feedback node while a lower value will draw more
current and degrade the light load efficiency. The
equation for selecting the voltage specific resistor is:
R4=
Vout
Vref
-1 ·R3 =
Dmax·Io·(1-Dmax)
Fs·Vripple
0.6V
-1V ·10kΩ=31.67kΩ
Table 2. Feedback Resistor values
Vout (V)
1.8
2.5
3.3
5.0
Vo+Vfwd
Vinmin-Vo+Vfwd
2.5V
R1,R4 (kΩ)
(R2,R3=10kΩ)
20.0
31.6
45.3
73.2
2.5V 0.2V
2.5V +0.2V
·2A· 19V-0.3V+0.2V
9V-0.3V+0.2V
=
=7μF
300kHz·0.2V
.
For high voltage input converters the duty cycle is
always less than 50% so the maximum ripple is at the
minimum input voltage. The ripple will increase as the
duty cycle approaches 50% where it is a maximum.
Step-Down Converter Feedforward Capacitor
For optimum start-up and improved transient
response place a feed-forward capacitor (C6) across
the feedback resistor R2. Typical values range from
220pF to 10nF.
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AMS4123
3A 20V Step-Down Converter + 1A LDO
PCB Layout
The following guidelines should be followed to insure
proper layout.
1. Vin Capacitor. A low ESR ceramic bypass
capacitor must be placed as close to the IC as
possible.
2. Schottky Diode. During the off portion of the
switching cycle the inductor current flows through
the Schottky diode to the output cap and returns
to the inductor through the output capacitor. The
trace that connects the output diode to the output
capacitor sees a current signal with a very high
di/dt. To minimize the associated spiking and
ringing, the inductance and resistance of this
trace should be minimized by connecting the
diode anode to the output capacitor return with a
short wide trace.
3. Feedback Resistors. The feedback resistors
should be placed as close as possible the IC.
Minimize the length of the trace from the feedback
pin to the resistors. This is a high impedance
node susceptible to interference from external RF
noise sources.
4. Inductor. Minimize the length of the SW node
trace. This minimizes the radiated EMI associated
with the SW node.
5. Ground. The most quiet ground or return potential
available is the output capacitor return. The
inductor current flows through the output
capacitor during both the on time and off time,
hence it never sees a high di/dt. The only di/dt
seen by the output capacitor is the inductor ripple
current which is much less than the di/dt of an
edge to a square wave current pulse. This is the
best place to make a solid connection to the IC
ground and input capacitor. This node is used as
the star ground shown in Figure 1. This method of
grounding helps to reduce high di/dt traces, and
the detrimental effect associated with them, in a
step-down converter. The inductance of these
traces should always be minimized by using wide
traces, ground planes, and proper component
placement.
6. For good thermal performance vias are required
to couple the exposed tab of the SO-8 package to
the PCB ground plane. The via diameter should
be 0.3mm to 0.33mm positioned on a 1.2mm grid.
3/5/2010
Ion+ Ioff
Ion
Ioff
PCB Inductance
High
di/dt
Ion
Ioff
Ion+Ioff
Ion
Ioff
High di/dt trace reduction
“Star Ground”
Figure 1. Step Down Converter Layout
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AMS4123
3A 20V Step-Down Converter + 1A LDO
Output Power and Thermal Limits
The AMS4123 junction temperature, Step-Down
converter and LDO current capability depends on the
internal dissipation and the junction to case thermal
resistance of the SO8 exposed paddle package. This
gives the junction temperature rise above the device
paddle and PCB temperature.
The temperature of the paddle and PCB will be
elevated above the ambient temperature due to the
total losses of the step down converter and losses of
other circuits and or converters mounted to the PCB.
Tjmax=Pd·θjc+Tpcb+Tamb
The losses associated with the AMS4123 overall
efficiency are;
1. Output Diode Conduction Losses
2. Inductor DCR Losses
3. AMS4123 Internal losses
a. Power Switch Forward Conduction
and Switching Losses
b. Quiescent Current Losses
The internal losses contribute to the junction
temperature rise above the case and PCB
temperature.
The junction temperature depends on many factors
and should always be verified in the final application
at the maximum ambient temperature. This will assure
that the device does not enter over-temperature
shutdown when fully loaded at the maximum ambient
temperature.
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AMS4123
3A 20V Step-Down Converter + 1A LDO
Figure 2. AMS4123 Evaluation Board Top Side
Figure 3. AMS4123 Evaluation Board Bottom Side
1
2
JP1
LDO Input
R5
10
VLX
L1
10uH
Vout
C7
470pF
1
C5
C2
J6
gnd
Vin
2
220nF
J3
C9
22uF
100uF 16V
C4
22uF 35V
C1
10uF 50V
U1
3
4
SW LDO Out
BST
LDO In
Vin
FB LDO
EN
FB SW
D1
B340LB
J2
AMS4123_1
R4
gnd
45.3k
J5
Enable
J1
VLDOIn
J4
8
7
R1
31.6k
VLDOOut
6
5
C6
2.2uF
R3
10.0k
C8
R2
10.0k
C3
2.2uF
J7
gnd
4.7nF
Figure 4. AMS4123 Evaluation Board Schematic
Table 3. Evaluation Board Bill of Materials
Component Value
L1
C9
C2
C1
C3,C6
C3,C6
option
C7
C5
C8
C4
R5
3/5/2010
Manufacturer
Manufacturer Part Number
Coilcraft
DO3316P
Kemet
T491X107M016AS
10µF, 50V, X5R, 1210, Ceramic
2.2µF, 10V, X5R, 0805
2.2µF, 10V, X5R, 0603
Taiyo Yuden
TDK
Taiyo Yuden
Murata
Murata
LMK212BJ226MG-T
C3225X5R1A226M
UMK325BJ106KM-T
GRM216R61A225KE24
GRM39X5R225K10H52V
470pF 50V, 20%, X7R, 0603
220nF 25V, 10%, X7R, 0603
4.7nF 50V, 20%, X7R, 0603
22µF 35V Tantalum Case E
10Ω, 0.1W, 0603 5%
Murata
Murata
Murata
Vishay
Vishay/Dale
GRM188R71H471MA01
GRM188R71E224KA88
GRM188R71H472MA01
293D226X9035E2TE3
CRCW060310R0JNEA
10µH 3.9A
9.4mm x 13mm x 5.2mm
100µF, 16V, X case General
Purpose Tantalum
22µF, 10V, X5R, 0805, Ceramic
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AMS4123
3A 20V Step-Down Converter + 1A LDO
R2,R3
R1,R4
D1
U1
10kΩ, 0.1W, 0603 1%
See table 2
3A, 40V Schottky
Step-Down Converter / LDO
Various
Various
Diodes Inc.
AMS
CRCW060310K0FKEA
CRCW0603xxKxFKEA
B340LB
AMS4123
ORDERING INFORMATION
Package Type
SOIC EDP
AMS4123S
TEMP. RANGE
-25°C to 125°C
PACKAGE DIMENSIONS inches (millimeters) unless otherwise noted.
8 LEAD SOIC PLASTIC PACKAGE (S)
3/5/2010
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AMS4123
3A 20V Step-Down Converter + 1A LDO
3/5/2010
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Phone (925) 443-0722
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