AAT1141_201979B.pdf

DATA SHEET
AAT1141
Fast Transient 600mA Step-Down Converter
General Description
Features
The AAT1141 SwitchReg is a 2MHz step-down converter
with an input voltage range of 2.7V to 5.5V and output
voltage as low as 0.6V. It is optimized to react quickly to
a load variation.
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The AAT1141 is available in fixed voltage versions with
internal feedback and a programmable version with
external feedback resistors. It can deliver 600mA of load
current while maintaining a low 35μA no load quiescent
current. The 2MHz switching frequency minimizes the
size of external components while keeping switching
losses low.
The AAT1141 is designed to maintain high efficiency
throughout the operating range, which is critical for portable applications.
VIN Range: 2.7V to 5.5V
VOUT Fixed or Adjustable from 0.6V to VIN
35μA No Load Quiescent Current
Up to 98% Efficiency
600mA Max Output Current
2MHz Switching Frequency
150μs Soft Start
Fast Load Transient
Over-Temperature Protection
Current Limit Protection
100% Duty Cycle Low-Dropout Operation
<1μA Shutdown Current
SOT23-5 Package
Temperature Range: -40°C to +85°C
Applications
The AAT1141 is available in a Pb-free SOT23-5 package
and is rated over the -40°C to +85°C temperature
range.
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Cellular Phones
Digital Cameras
Handheld Instruments
Microprocessor / DSP Core / IO Power
PDAs and Handheld Computers
USB Devices
Typical Application (Fixed Output Voltage)
U1
AAT1141
VIN
IN
C2
4.7µF
EN
GND
LX
L1
4.7µH
VOUT
OUT
C1
4.7µF
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1
DATA SHEET
AAT1141
Fast Transient 600mA Step-Down Converter
Pin Descriptions
Pin #
Symbol
1
2
3
IN
GND
EN
4
OUT
5
LX
Function
Input supply voltage for the converter.
Ground pin. Connect to the output and input capacitor return.
Enable pin.
Feedback input pin. This pin is connected either directly to the converter output or to an external
resistive divider for an adjustable output.
Switching node. Connect the inductor to this pin. It is internally connected to the drains of both
high- and low-side MOSFETs.
Pin Configuration
SOT23-5
(Top View)
2
IN
1
GND
2
EN
3
5
LX
4
OUT
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DATA SHEET
AAT1141
Fast Transient 600mA Step-Down Converter
Absolute Maximum Ratings1
Symbol
VIN
VLX
VOUT
VEN
TJ
TS
TLEAD
Description
Input Voltage to GND
LX to GND
OUT to GND
EN to GND
Operating Junction Temperature Range
Storage Temperature Range
Maximum Soldering Temperature (at leads, 10 sec)
Value
Units
6.0
-0.3 to VIN + 0.3
-0.3 to VIN + 0.3
-0.3 to 6.0
-40 to 150
-65 to 150
300
V
V
V
V
°C
°C
°C
Value
Units
667
150
mW
°C/W
Thermal Information
Symbol
PD
θJA
Description
Maximum Power Dissipation2, 3
Thermal Resistance2
1. Stresses above those listed in Absolute Maximum Ratings may cause permanent damage to the device. Functional operation at conditions other than the operating conditions
specified is not implied. Only one Absolute Maximum Rating should be applied at any one time.
2. Mounted on an FR4 board.
3. Derate 6.67mW/°C above 25°C.
Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • sales@skyworksinc.com • www.skyworksinc.com
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3
DATA SHEET
AAT1141
Fast Transient 600mA Step-Down Converter
Electrical Characteristics1
TA = -40°C to +85°C, unless otherwise noted. Typical values are TA = 25°C, VIN = 3.6V.
Symbol
Description
Conditions
Step-Down Converter
VIN
Input Voltage
VUVLO
VOUT
VOUT
IQ
ISHDN
ILIM
RDS(ON)H
RDS(ON)L
ILXLEAK
VLinereg
VOUT
IOUT
ROUT
TS
FOSC
TSD
THYS
EN
VEN(L)
VEN(H)
IEN
UVLO Threshold
Output Voltage Tolerance
Output Voltage Range
Quiescent Current
Shutdown Current
P-Channel Current Limit
High Side Switch On Resistance
Low Side Switch On Resistance
LX Leakage Current
Line Regulation
Out Threshold Voltage Accuracy
Out Leakage Current
Out Impedance
Start-Up Time
Oscillator Frequency
Over-Temperature Shutdown Threshold
Over-Temperature Shutdown Hysteresis
Enable Threshold Low
Enable Threshold High
Input Low Current
Min
Typ
2.7
VIN Rising
Hysteresis
VIN Falling
IOUT = 0 to 600mA, VIN = 2.7V to 5.5V
Max
Units
5.5
2.7
V
V
mV
V
%
V
μA
μA
mA


μA
%/V
mV
μA
k
μs
MHz
°C
°C
100
1.8
-3.5
0.6
No Load, 0.6V Adjustable Version
EN = AGND = PGND
35
+3.5
VIN
70
1.0
800
0.35
0.30
VIN = 5.5V, VLX = 0 to VIN, EN = GND
VIN = 2.7V to 5.5V
0.6V Output, No Load, TA = 25°C
0.6V Output
>0.6V Output
From Enable to Output Regulation
TA = 25°C
1
588
0.1
600
612
0.2
250
1.2
150
2.0
140
15
2.6
0.6
VIN = VOUT = 5.5V
1.4
-1.0
1.0
V
V
μA
1. The AAT1141 is guaranteed to meet performance specifications over the -40°C to +85°C operating temperature range and is assured by design, characterization, and correlation with statistical process controls.
4
Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • sales@skyworksinc.com • www.skyworksinc.com
201979B • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • March 15, 2013
DATA SHEET
AAT1141
Fast Transient 600mA Step-Down Converter
Typical Characteristics
Efficiency vs. Load
DC Regulation
(VOUT = 3.3V; L = 6.8µ
µH)
100
(VOUT = 3.3V; L = 6.8µH)
3.0
VIN = 3.6V
Output Error (%)
Efficiency (%)
90
VIN = 4.2V
80
VIN = 5.0V
70
60
50
0.1
1
10
100
2.0
1.0
-1.0
VIN = 5.5V
-2.0
0
100
Output Current (mA)
Output Error (%)
Efficiency (%)
VIN = 5.0V
VIN = 4.2V
VIN = 3.6V
60
50
0.1
600
1
10
100
2.0
0.0
-1.0
VIN = 3.6V
VIN = 3.0V
VIN = 4.2V
-2.0
-3.0
1000
VIN = 5.0V
1.0
0
100
Output Current (mA)
200
300
400
500
600
Output Current (mA)
Efficiency vs. Load
DC Regulation
(VOUT = 1.8V; L = 4.7µ
µH)
(VOUT = 1.8V; L = 4.7μH)
100
3.0
VIN = 2.7V
80
VIN = 3.6V
Output Error (%)
Efficiency (%)
500
3.0
VIN = 2.7V
80
VIN = 4.2V
70
60
50
0.1
400
(VOUT = 2.5V; L = 6.8µH)
90
90
300
DC Regulation
(VOUT = 2.5V; L = 6.8µ
µH)
70
200
Output Current (mA)
Efficiency vs. Load
100
VIN = 5.0V
0.0
-3.0
1000
VIN = 4.2V
2.0
1.0
VIN = 2.7V
-1.0
-2.0
1
10
Output Current (mA)
100
1000
VIN = 3.6V
0.0
VIN = 4.2V
-3.0
0
100
200
300
400
500
600
Output Current (mA)
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5
DATA SHEET
AAT1141
Fast Transient 600mA Step-Down Converter
Typical Characteristics
Line Regulation
Soft Start
(VOUT = 1.8V)
(VIN = 3.6V; VOUT = 1.8V; Load = 3Ω; CFF = 100pF)
0.5
Output Voltage (V)
VOUT
(2V/div)
0V
0V
IIN
(200mA/div)
IOUT = 10mA
IOUT = 600mA
0.4
EN
(2V/div)
0.3
0.2
0.1
0.0
-0.1
-0.2
-0.3
-0.4
0mA
-0.5
2.5
3.0
3.5
5.0
5.5
6.0
Switching Frequency vs. Temperature
Output Voltage Error vs. Temperature
(VIN = 3.6V; VOUT = 1.8V)
(VIN = 3.6V; VO = 1.8V; IOUT = 400mA)
10.00
Frequency Variation (%)
2.0
Output Error (%)
4.5
Input Voltage (V)
Time (100μs/div)
1.0
0.0
-1.0
-2.0
-40
4.0
-20
0
20
40
60
80
8.00
6.00
4.00
2.00
0.00
-2.00
-4.00
-6.00
-8.00
-10.00
-40
100
-20
0
20
40
60
80
100
Temperature (°°C)
Temperature (°°C)
Frequency vs. Input Voltage
No Load Quiescent Current vs. Input Voltage
(VOUT = 3.0V, L = 6.8µH)
60
VOUT = 1.8V
1.0
0.0
-1.0
VOUT = 2.5V
-2.0
VOUT = 3.3V
-3.0
-4.0
2.7
3.1
3.5
3.9
4.3
Input Voltage (V)
6
Supply Current (µA)
Frequency Variation (%)
2.0
4.7
5.1
5.5
55
TA = 85°C
50
TA = 25°C
45
40
35
30
TA = -40°C
25
20
3.3
3.8
4.3
4.8
Input Voltage (V)
Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • sales@skyworksinc.com • www.skyworksinc.com
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5.3
DATA SHEET
AAT1141
Fast Transient 600mA Step-Down Converter
Typical Characteristics
No Load Quiescent Current vs. Input Voltage
No Load Quiescent Current vs. Input Voltage
(VOUT = 1.8V, L = 4.7µH)
(VOUT = 1.2V, L = 2.2µH)
0.060
TA = 85°C
0.055
0.050
Supply Current (mA)
Supply Current (mA)
0.060
TA = 25°C
0.045
0.040
0.035
0.030
TA = -40°C
0.025
0.020
2.7
3.1
3.5
3.9
4.3
4.7
5.1
TA = 85°C
TA = 25°C
0.050
0.045
0.040
0.035
0.030
TA = -40°C
0.025
0.020
5.5
2.7
3.1
3.5
3.9
4.3
4.7
5.1
Input Voltage (V)
Input Voltage (V)
P-Channel RDS(ON) vs. Input Voltage
N-Channel RDS(ON) vs. Input Voltage
700
600
85°C
25°C
-40°C
500
500
400
300
5.5
85°C
25°C
-40°C
550
RDS(ON)L (mΩ
Ω)
600
RDS(ON)H (mΩ
Ω)
0.055
450
400
350
300
250
200
200
150
100
2.7
3.1
3.5
3.9
4.3
4.7
5.1
5.5
100
2.7
3.1
3.5
Input Voltage (V)
3.9
4.3
4.7
5.1
5.5
Input Voltage (V)
Step-Down Converter Load Transient Response
Step-Down Converter Load Transient Response
(1mA to 300mA; VIN = 3.6V; VOUT = 1.8V;
COUT = 4.7µF; CFF = 100pF)
(300mA to 400mA; VIN = 3.6V;
VOUT = 1.8V; COUT = 4.7µF; CFF = 100pF)
VOUT
(100mV/div)
VOUT
(50mV/div)
1.8V
1.8V
300mA
IOUT
(100mA/div)
IOUT
(100mA/div)
1mA
Time (40µs/div)
400mA
300mA
Time (40µs/div)
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7
DATA SHEET
AAT1141
Fast Transient 600mA Step-Down Converter
Typical Characteristics
Step-Down Converter Load Transient Response
Step-Down Converter Load Transient Response
(1mA to 300mA; VIN = 3.6V; VOUT = 1.0V;
COUT = 10µF; CFF = 0pF)
(300mA to 400mA; VIN = 3.6V;
VOUT = 1.0V; COUT = 10µF; CFF = 0pF)
VOUT
(100mV/div)
VOUT
(50mV/div)
1.0V
1.0V
400mA
300mA
IOUT
(100mA/div)
300mA
IOUT
(100mA/div)
1mA
Time (40µs/div)
Line Response
Step-Down Converter Output Ripple
(VOUT = 1.8V @ 400mA)
(VOUT = 1.8V; VIN = 3.6V; IOUT = 1mA; L = 4.7µH;
CFF = 100pF; COUT = 4.7µF)
1.82
6.0
1.81
5.5
1.80
5.0
1.79
4.5
1.78
4.0
1.77
3.5
1.76
Input Voltage
(bottom) (V)
Output Voltage
(top) (V)
Time (40µs/div)
3.0
Time (25μ
μs/div)
Output
Ripple
(20mV/div)
1.8V
LX
(2V/div)
1.8V
Inductor
Current
(100mA/div)
0mA
Time (10µs/div)
Step-Down Converter Output Ripple
(VOUT = 1.8V; VIN = 3.6V; IOUT = 600mA;
L = 4.7µH; CFF = 100pF; COUT = 4.7µF)
Output
Ripple
(20mV/div)
LX
(2V/div)
Inductor
Current
(100mA/div)
1.8V
0V
600mA
Time (0.5µs/div)
8
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DATA SHEET
AAT1141
Fast Transient 600mA Step-Down Converter
Functional Block Diagram
IN
OUT
See note
Err
Amp
.
DH
Voltage
Reference
EN
INPUT
LX
Logic
DL
GND
Note: For adjustable version, the internal feedback divider is omitted and the OUT pin is tied directly
to the internal error amplifier.
Functional Description
The AAT1141 is a high performance 600mA 2MHz monolithic step-down converter. It has been designed with the
goal of minimizing external component size and optimizing efficiency over the complete load range. Apart from
the small bypass input capacitor, only a small L-C filter
is required at the output. Typically, a 4.7μH inductor and
a 4.7μF ceramic capacitor are recommended (see table
of values).
The fixed output version requires only three external
power components (CIN, COUT, and L). The adjustable version can be programmed with external feedback to any
voltage, ranging from 0.6V to the input voltage. An addi-
tional feed-forward capacitor can also be added to the
external feedback to provide improved transient response
(see Figure 1).
At dropout, the converter duty cycle increases to 100%
and the output voltage tracks the input voltage minus
the RDS(ON) drop of the P-channel high-side MOSFET.
The input voltage range is 2.7V to 5.5V. The converter
efficiency has been optimized for all load conditions,
ranging from no load to 600mA.
The internal error amplifier and compensation provides
excellent transient response, load, and line regulation.
Soft start eliminates any output voltage overshoot when
the enable or the input voltage is applied.
Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • sales@skyworksinc.com • www.skyworksinc.com
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9
DATA SHEET
AAT1141
Fast Transient 600mA Step-Down Converter
SW
U1
AAT1141
L1
4.7μH
1
V IN
C1
4.7μF
Enable
2
IN
LX
5
VOUT
C3
100pF
GND
R1
118k
C2
4.7μF
1
3
2
EN
OUT
4
3
R2
59k
Figure 1: Enhanced Transient Response Schematic.
Control Loop
The AAT1141 is a peak current mode step-down converter. The current through the P-channel MOSFET (high
side) is sensed for current loop control, as well as short
circuit and overload protection. A fixed slope compensation signal is added to the sensed current to maintain
stability for duty cycles greater than 50%. The peak current mode loop appears as a voltage-programmed current source in parallel with the output capacitor.
The output of the voltage error amplifier programs the
current mode loop for the necessary peak switch current
to force a constant output voltage for all load and line
conditions. Internal loop compensation terminates the
transconductance voltage error amplifier output. For
fixed voltage versions, the error amplifier reference voltage is internally set to program the converter output
voltage. For the adjustable output, the error amplifier
reference is fixed at 0.6V.
Soft Start / Enable
Soft start limits the current surge seen at the input and
eliminates output voltage overshoot. When pulled low,
the enable input forces the AAT1141 into a low-power,
non-switching state. The total input current during shutdown is less than 1μA.
10
Current Limit and
Over-Temperature Protection
For overload conditions, the peak input current is limited. To minimize power dissipation and stresses under
current limit and short-circuit conditions, switching is
terminated after entering current limit for a series of
pulses. Switching is terminated for seven consecutive
clock cycles after a current limit has been sensed for a
series of four consecutive clock cycles.
Thermal protection completely disables switching when
internal dissipation becomes excessive. The junction
over-temperature threshold is 140°C with 15°C of hysteresis. Once an over-temperature or over-current fault
condition is removed, the output voltage automatically
recovers.
Under-Voltage Lockout
Internal bias of all circuits is controlled via the IN input.
Under-voltage lockout (UVLO) guarantees sufficient VIN
bias and proper operation of all internal circuitry prior to
activation.
Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • sales@skyworksinc.com • www.skyworksinc.com
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DATA SHEET
AAT1141
Fast Transient 600mA Step-Down Converter
Applications Information
Inductor Selection
The step-down converter uses peak current mode control with slope compensation to maintain stability for
duty cycles greater than 50%. The output inductor
value must be selected so the inductor current down
slope meets the internal slope compensation requirements. The internal slope compensation for the adjustable and low-voltage fixed versions of the AAT1141 is
0.24A/μsec. This equates to a slope compensation that
is 75% of the inductor current down slope for a 1.5V
output and 4.7μH inductor.
The 4.7μH CDRH2D14 series inductor selected from
Sumida has a 135mΩ typical DCR and a 1A DC current
rating. At full load, the inductor DC loss is 48mW which
gives a 4.5% loss in efficiency for a 600mA, 1.8V output.
Input Capacitor
Select a 4.7μF to 10μF X7R or X5R ceramic capacitor for
the input. To estimate the required input capacitor size,
determine the acceptable input ripple level (VPP) and
solve for C. The calculated value varies with input voltage and is a maximum when VIN is double the output
voltage.
0.75 · VO 0.75 · 1.5V
A
m=
=
= 0.24
L
4.7μH
μs
This is the internal slope compensation for the adjustable (0.6V) version or low-voltage fixed versions. When
externally programming the 0.6V version to 2.5V, the
calculated inductance is 7.5μH.
L=
CIN =
μs
· 2.5V = 7.5μH
A
In this case, a standard 6.8μH value is selected.
For high-voltage fixed versions (2.5V), m = 0.48A/
μsec. Table 1 displays inductor values for the AAT1141
fixed and adjustable options.
Manufacturer's specifications list both the inductor DC
current rating, which is a thermal limitation, and the
peak current rating, which is determined by the saturation characteristics. The inductor should not show any
appreciable saturation under normal load conditions.
Some inductors may meet the peak and average current
ratings yet result in excessive losses due to a high DCR.
Always consider the losses associated with the DCR and
its effect on the total converter efficiency when selecting
an inductor.
Output
Voltage (V)
Inductor (μH)
Output
Capacitor (μF)
1, 1.2
1.5, 1.8
2.5, 3.3
2.2
4.7
6.8
10
4.7
4.7
Table 1: Inductor and Output Capacitor Values.
⎛ VPP
⎞
- ESR · FS
⎝ IO
⎠
VO ⎛
V ⎞
1
· 1 - O = for VIN = 2 · VO
VIN ⎝
VIN ⎠
4
0.75 · VO
μs
0.75 · VO
≈ 3 A · VO
=
m
A
0.24 μs
=3
V ⎞
VO ⎛
· 1- O
VIN ⎝
VIN ⎠
CIN(MIN) =
1
⎛ VPP
⎞
- ESR · 4 · FS
⎝ IO
⎠
Always examine the ceramic capacitor DC voltage coefficient characteristics when selecting the proper value.
For example, the capacitance of a 10μF, 6.3V, X5R
ceramic capacitor with 5.0V DC applied is actually about
6μF.
The maximum input capacitor RMS current is:
IRMS = IO ·
VO ⎛
V ⎞
· 1- O
VIN ⎝
VIN ⎠
The input capacitor RMS ripple current varies with the
input and output voltage and will always be less than or
equal to half of the total DC load current.
VO ⎛
V ⎞
· 1- O =
VIN ⎝
VIN ⎠
D · (1 - D) =
for VIN = 2 · VO
IRMS(MAX) =
VO
0.52 =
1
2
IO
2
⎛
V ⎞
· 1- O
The term V ⎝ V ⎠ appears in both the input voltage
ripple and input capacitor RMS current equations and is
a maximum when VO is twice VIN. This is why the input
voltage ripple and the input capacitor RMS current ripple
are a maximum at 50% duty cycle.
IN
IN
Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • sales@skyworksinc.com • www.skyworksinc.com
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11
DATA SHEET
AAT1141
Fast Transient 600mA Step-Down Converter
The input capacitor provides a low impedance loop for
the edges of pulsed current drawn by the AAT1141. Low
ESR/ESL X7R and X5R ceramic capacitors are ideal for
this function. To minimize stray inductance, the capacitor
should be placed as closely as possible to the IC. This
keeps the high frequency content of the input current
localized, minimizing EMI and input voltage ripple.
The proper placement of the input capacitor (C2) can be
seen in the evaluation board layout in Figure 2.
Figure 2: AAT1141 Sample Layout
Top Side.
A laboratory test set-up typically consists of two long
wires running from the bench power supply to the evaluation board input voltage pins. The inductance of these
wires, along with the low-ESR ceramic input capacitor,
can create a high Q network that may affect converter
performance. This problem often becomes apparent in
the form of excessive ringing in the output voltage during load transients. Errors in the loop phase and gain
measurements can also result.
Figure 3: Exploded View of Sample Layout.
Figure 4: AAT1141 Sample Layout
Bottom Side.
12
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DATA SHEET
AAT1141
Fast Transient 600mA Step-Down Converter
Since the inductance of a short PCB trace feeding the
input voltage is significantly lower than the power leads
from the bench power supply, most applications do not
exhibit this problem.
In applications where the input power source lead inductance cannot be reduced to a level that does not affect
the converter performance, a high ESR tantalum or aluminum electrolytic should be placed in parallel with the
low ESR, ESL bypass ceramic. This dampens the high Q
network and stabilizes the system.
Output Capacitor
The output capacitor limits the output ripple and provides holdup during large load transitions. A 4.7μF to
10μF X5R or X7R ceramic capacitor typically provides
sufficient bulk capacitance to stabilize the output during
large load transitions and has the ESR and ESL characteristics necessary for low output ripple.
The output voltage droop due to a load transient is
dominated by the capacitance of the ceramic output
capacitor. During a step increase in load current, the
ceramic output capacitor alone supplies the load current
until the loop responds. Within two or three switching
cycles, the loop responds and the inductor current
increases to match the load current demand. The relationship of the output voltage droop during the three
switching cycles to the output capacitance can be estimated by:
Dissipation due to the RMS current in the ceramic output
capacitor ESR is typically minimal, resulting in less than
a few degrees rise in hot-spot temperature.
Adjustable Output Resistor Selection
For applications requiring an adjustable output voltage,
the 0.6V version can be externally programmed. Resistors
R1 and R2 of Figure 5 program the output to regulate at
a voltage higher than 0.6V. To limit the bias current
required for the external feedback resistor string while
maintaining good noise immunity, the minimum suggested value for R2 is 59k. Although a larger value will
further reduce quiescent current, it will also increase the
impedance of the feedback node, making it more sensitive to external noise and interference. Table 2 summarizes the resistor values for various output voltages with
R2 set to either 59k for good noise immunity or 316k
for reduced no load input current.
⎛ VOUT ⎞
⎛ 1.5V ⎞
R1 = V
-1 · R2 = 0.6V - 1 · 59kΩ = 88.5kΩ
⎝ REF ⎠
⎝
⎠
The adjustable version of the AAT1141, combined with
an external feedforward capacitor (C3 in Figure 1),
delivers enhanced transient response for extreme pulsed
load applications. The addition of the feedforward capacitor typically requires a larger output capacitor C1 for
stability.
VOUT (V)
High Noise
Immunity
R2 = 59k
R1 (k)
Low Input
Current
(Without Load)
R2 = 316k
R1 (k)
0.8
0.9
1.0
1.1
1.2
1.3
1.4
1.5
1.8
2.0
2.5
3.0
3.3
19.6
29.4
39.2
49.9
59.0
68.1
78.7
88.7
88.7
137
187
237
267
105
158
210
267
316
365
422
475
634
732
1000
1270
1430
3 · ΔILOAD
COUT =
VDROOP · FS
Once the average inductor current increases to the DC
load level, the output voltage recovers. The above equation establishes a limit on the minimum value for the
output capacitor with respect to load transients.
The internal voltage loop compensation also limits the
minimum output capacitor value to 4.7μF. This is due to
its effect on the loop crossover frequency (bandwidth),
phase margin, and gain margin. Increased output capacitance will reduce the crossover frequency with greater
phase margin.
The maximum output capacitor RMS ripple current is
given by:
IRMS(MAX) =
1
VOUT · (VIN(MAX) - VOUT)
L · F · VIN(MAX)
2· 3
·
Table 2: Adjustable Resistor Values For Use With
0.6V Step-Down Converter.
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13
DATA SHEET
AAT1141
Fast Transient 600mA Step-Down Converter
Thermal Calculations
Since RDS(ON), quiescent current, and switching losses all
vary with input voltage, the total losses should be investigated over the complete input voltage range.
There are three types of losses associated with the
AAT1141 step-down converter: switching losses, conduction losses, and quiescent current losses. Conduction
losses are associated with the RDS(ON) characteristics of
the power output switching devices. Switching losses are
dominated by the gate charge of the power output
switching devices. At full load, assuming continuous conduction mode (CCM), a simplified form of the LDO losses
is given by:
Given the total losses, the maximum junction temperature can be derived from the JA for the SOT23-5 package which is 150°C/W.
TJ(MAX) = PTOTAL · ΘJA + TAMB
Output Dropout
PTOTAL =
IO2 · (RDSON(HS) · VO + RDSON(LS) · [VIN - VO])
At dropout, the duty cycle of AAT1141 switching is
100%. The minimum dropout voltage is determined by
RDS(ON)H and the inductor copper loss resistor. AAT1141
has 0.35Ω RDS(ON)H. The inductor copper loss resistor varies with different inductor values and manufacturer. The
safe dropout voltage is 0.5V for a 600mA load.
VIN
+ (tsw · F · IO + IQ) · VIN
IQ is the step-down converter quiescent current. The
term tsw is used to estimate the full load step-down converter switching losses.
For example, when load current is 600mA, the voltage
dropped across RDS(ON)H is 0.21V; if the inductor copper
loss resistor is 135mΩ, the voltage drop across the
inductor is 0.08V. So the total voltage drop is 0.29V.
Considering manufacturer’s tolerances, the inductor copper loss resistor and RDS(ON)H will vary from part to part,
a 0.4V dropout window is safe.
For the condition where the step-down converter is in
dropout at 100% duty cycle, the total device dissipation
reduces to:
PTOTAL = IO2 · RDSON(HS) + IQ · VIN
SW
U1
AAT1141
1
V IN
C1
4.7μF
Enable
2
IN
LX
5
L1
4.7μH
VOUT = 1.8V
C3
100pF
GND
R1
634k
C2
4.7μF
1
3
2
3
EN
OUT
4
R2
316k
Figure 5: AAT1141 Adjustable Evaluation Board Schematic.
14
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DATA SHEET
AAT1141
Fast Transient 600mA Step-Down Converter
Efficiency
Layout
Besides the AAT1141 device losses including switching
losses, conduction losses, and quiescent current losses,
the inductor copper loss also affects the efficiency of the
buck converter. To the buck converter, the average current of the inductor is equal to output current IO. So the
loss in the inductor is:
The suggested 2-layer PCB layout for the AAT1141 is
shown in Figures 2, 3 and 4. The following guide lines
should be used to help ensure a proper layout.
1.
PLOSS_L = IO2 · RL
2.
Table 4 shows some recommended inductors. A larger
size inductor usually has lower DCR. For example, if
selecting CDRH2D14 4.7μH for 1.8V output, the PLoss_L is
48.6mW when the output current is 600mA, so the
inductor loses 4.5% power; if selecting CDRH3D23
4.7μH, the PLoss_L should be 19.8mW, and the inductor
power loss ratio is only 1.8%. The inductor size and the
buck converter efficiency is always a trade-off in the real
application.
3.
4.
5.
The power traces (GND, LX, VIN) should be kept
short, direct, and wide to allow large current flow.
Place sufficient multiple-layer pads when needed to
change the trace layer.
The input capacitor (C1) should connect as closely
as possible to IN and GND.
The output capacitor C2 and L1 should be connected
as closely as possible. The connection of L1 to the LX
pin should be as short as possible and there should
not be any signal lines under the inductor.
The feedback trace or OUT pin should be separate
from any power trace and connect as closely as possible to the load point. Sensing along a high-current
load trace will degrade DC load regulation. If external feedback resistors are used, they should be
placed as closely as possible to the OUT pin to minimize the length of the high impedance feedback
trace.
The resistance of the trace from the load return to
GND should be kept to a minimum. This will help to
minimize any error in DC regulation due to differences in the potential of the internal signal ground
and the power ground.
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15
DATA SHEET
AAT1141
Fast Transient 600mA Step-Down Converter
Step-Down Converter Design Example
Specifications
VO =
VIN =
FS =
TAMB =
1.8V @ 600mA (adjustable using 0.6V version), Pulsed Load ILOAD = 300mA
2.7V to 4.2V (3.6V nominal)
2MHz
85°C
1.8V Output Inductor
µs
µs
L1 = 3 A · VO2 = 3 A · 1.8V = 5.4µH (use 4.7μH; see Table 1)
For Sumida inductor CDRH3D16, 4.7μH, DCR = 105m.
ΔIL1 =
VO
V
1.8V
· 1- O =
L1 · F
VIN
4.7µH · 2MHz
IPKL1 = IO +
· 1-
1.8V
= 109.2mA
4.2V
ΔIL1
= 0.6A + 0.055A = 0.655A
2
PL1 = IO2 · DCR = 0.6A2 · 105mΩ = 38mW
1.8V Output Capacitor
VDROOP = 0.1V
COUT =
3 · ΔILOAD
3 · 0.3A
=
= 4.48µF; use 10µF
0.1V · 2MHz
VDROOP · FS
IRMS =
(VO) · (VIN(MAX) - VO)
1
1.8V · (4.2V - 1.8V)
·
= 31.5mArms
=
L1 · F · VIN(MAX)
2 · 3 4.7µH · 2MHz · 4.2V
2· 3
1
·
Pesr = esr · IRMS2 = 5mΩ · (31.5mA)2 = 5µW
Input Capacitor
Input Ripple VPP = 25mV
CIN =
IRMS =
1
VPP
- ESR · 4 · FS
IO
=
1
25mV
- 5mW · 4 · 2MHz
0.6A
= 3.4µF; use 4.7µF
IO
= 0.3Arms
2
P = esr · IRMS2 = 5mΩ · (0.3A)2 = 0.45mW
16
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DATA SHEET
AAT1141
Fast Transient 600mA Step-Down Converter
AAT1141 Losses
PTOTAL =
IO2 · (RDSON(HS) · VO + RDSON(LS) · [VIN -VO])
VIN
+ (tsw · F · IO + IQ) · VIN
=
0.62 · (0.35Ω · 1.8V + 0.3Ω · [4.2V - 1.8V])
4.2V
+ (5ns · 2MHz · 0.6A + 70μA) · 4.2V = 141mW
TJ(MAX) = TAMB + ΘJA · PLOSS = 85°C + (150°C/W) · 141mW = 106.2°C
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17
DATA SHEET
AAT1141
Fast Transient 600mA Step-Down Converter
Adjustable Version
(0.6V device)
VOUT (V)
R2 = 59k
R1 (k)
R2 = 316k1
R1 (k)
L1 (μH)
0.8
0.9
1.0
1.1
1.2
1.3
1.4
1.5
1.8
1.85
2.0
2.5
3.3
19.6
29.4
39.2
49.9
59.0
68.1
78.7
88.7
118
124
137
187
267
105
158
210
267
316
365
422
475
634
732
1000
1270
1430
2.2
2.2
2.2
2.2
2.2
2.2
4.7
4.7
4.7
4.7
6.8
6.8
6.8
Fixed Version
VOUT (V)
R2 Not Used
R1 (k)
L1 (μH)
0.6-3.3V
0
4.7
Table 3: Evaluation Board Component Values.
Manufacturer
Part Number
Inductance (μH)
Max DC
Current (A)
DCR ()
Sumida
Sumida
Sumida
CDRH3D16-2R2
CDRH3D16-4R7
CDRH3D16-6R8
Sumida
CDRH2D14
Murata
Murata
Coilcraft
Coiltronics
Coiltronics
Coiltronics
LQH2MCN4R7M02
LQH32CN4R7M23
LPO3310-472
SD3118-4R7
SD3118-6R8
SDRC10-4R7
2.2
4.7
6.8
2.2
4.7
6.8
4.7
4.7
4.7
4.7
6.8
4.7
1.20
0.90
0.73
1.5
1.0
0.85
0.40
0.45
0.80
0.98
0.82
1.30
0.072
0.105
0.170
75
135
170
0.80
0.20
0.27
0.122
0.175
0.122
Size (mm)
LxWxH
Type
3.8x3.8x1.8
3.8x3.8x1.8
3.8x3.8x1.8
Shielded
Shielded
Shielded
3.2x3.2x1.55
Shielded
2.0x1.6x0.95
2.5x3.2x2.0
3.2x3.2x1.0
3.1x3.1x1.85
3.1x3.1x1.85
5.7x4.4x1.0
Non-Shielded
Non-Shielded
1mm
Shielded
Shielded
1mm Shielded
Table 4: Typical Surface Mount Inductors.
Manufacturer
Part Number
Value
Voltage
Temp. Co.
Case
Murata
Murata
Murata
GRM219R61A475KE19
GRM21BR60J106KE19
GRM21BR60J226ME39
4.7μF
10μF
22μF
10V
6.3V
6.3V
X5R
X5R
X5R
0805
0805
0805
Table 5: Surface Mount Capacitors.
1. For reduced quiescent current, R2 = 316k.
18
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DATA SHEET
AAT1141
Fast Transient 600mA Step-Down Converter
Ordering Information
Output Voltage1
Package
Marking2
Part Number (Tape and Reel)3
Adj 0.6 to VIN
Adj 0.6 to VIN
1.0
1.2
1.5
1.8
2.0
3.0
3.3
TSOT23-5
SOT23-5
SOT23-5
SOT23-5
SOT23-5
SOT23-5
SOT23-5
SOT23-5
SOT23-5
YJXYY
1AXYY
5AXYY
4VXYY
5BXYY
ZEXYY
C4XYY
5CXYY
5DXYY
AAT1141ICB-0.6-T1
AAT1141IGV-0.6-T1
AAT1141IGV-1.0-T1
AAT1141IGV-1.2-T1
AAT1141IGV-1.5-T1
AAT1141IGV-1.8-T1
AAT1141IGV-2.0-T1
AAT1141IGV-3.0-T1
AAT1141IGV-3.3-T1
Skyworks Green™ products are compliant with
all applicable legislation and are halogen-free.
For additional information, refer to Skyworks
Definition of Green™, document number
SQ04-0074.
Package Information
SOT23-5
2.85 ± 0.15
1.90 BSC
0.40 ± 0.10
0.075 ± 0.075
0.15 ± 0.07
4°° ± 4°°
10°° ± 5°°
1.10 ± 0.20
0.60 REF
1.20 ± 0.25
2.80 ± 0.20
1.575 ± 0.125
0.95
BSC
0.60 REF
0.45 ± 0.15
GAUGE PLANE
0.10 BSC
All dimensions in millimeters.
1. Contact Sales for other voltage options.
2. XYY = assembly and date code.
3. Sample stock is generally held on part numbers listed in BOLD.
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19
DATA SHEET
AAT1141
Fast Transient 600mA Step-Down Converter
TSOT23-5
1.900 BSC
0.450 ± 0.150
0.950 BSC
0.127 BSC
1.600 BSC
2.800 BSC
Detail "A"
End View
Top View
0.950 ± 0.150
2.900 BSC
0°
+10°
-0°
0.450 ± 0.150
0.050 ± 0.050
Side View
Detail "A"
All dimensions in millimeters.
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