1
AAT1147
High Efficiency, Low Noise, Fast Transient 400mA Step-Down Converter
DATA SHEET
Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • sales@skyworksinc.com • www.skyworksinc.com
201986A • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • May 23, 2012
Typical Application
4.7µH
L1
118k
R1
4.7µF
C1
59k
R2
4.7µF
C2 EN
1OUT 2
VIN
3LX 4
AGND
5
PGND 6
PGND 7
PGND
8
AAT1147
U1
VIN VO = 1.8V
General Description
The AAT1147 SwitchReg is a member of Skyworks' Total
Power Management IC™ (TPMIC™) product family. It is
a fixed frequency 1.4MHz step-down converter with an
input voltage range of 2.7V to 5.5V and output voltage
as low as 0.6V.
The AAT1147 is optimized for low noise portable applica-
tions, reacts quickly to load variations, and reaches peak
efficiency at heavy load.
The AAT1147 output voltage is programmable with
external feedback resistors. It can deliver 400mA of load
current while maintaining high power efficiency. The
1.4MHz switching frequency minimizes the size of exter-
nal components while keeping switching losses low.
The AAT1147 is available in a Pb-free, space-saving
2.0x2.1mm SC70JW-8 package and is rated over the
-40°C to +85°C temperature range.
Features
• VIN Range: 2.7V to 5.5V
• VOUT Adjustable from 0.6V to VIN
• 400mA Output Current
• Up to 98% Efficiency
• Low Noise, 1.4MHz Fixed Frequency PWM Operation
• Fast Load Transient
• 150μs Soft Start
• Over-Temperature and Current Limit Protection
• 100% Duty Cycle Low Dropout Operation
• <1μA Shutdown Current
• 8-Pin SC70JW Package
• Temperature Range: -40°C to +85°C
Applications
• Cellular Phones
• Digital Cameras
• Handheld Instruments
• Microprocessor/DSP Core /IO Power
• PDAs and Handheld Computers
• USB Devices
2
AAT1147
High Efficiency, Low Noise, Fast Transient 400mA Step-Down Converter
DATA SHEET
Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • sales@skyworksinc.com • www.skyworksinc.com
201986A • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • May 23, 2012
Pin Descriptions
Pin # Symbol Function
1 EN Enable pin.
2 OUT Feedback input pin. This pin is connected to an external resistive divider for an adjustable output.
3 VIN Input supply voltage for the converter.
4LX
Switching node. Connect the inductor to this pin. It is connected internally to the drain of both high-
and low-side MOSFETs.
5 AGND Non-power signal ground pin.
6, 7, 8 PGND Main power ground return pins. Connect to the output and input capacitor return.
Pin Configuration
SC70JW-8
(Top View)
OUT
VIN
LX
PGND
PGND
PGND
AGND
EN 1
2
3
45
6
7
8
3
AAT1147
High Efficiency, Low Noise, Fast Transient 400mA Step-Down Converter
DATA SHEET
Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • sales@skyworksinc.com • www.skyworksinc.com
201986A • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • May 23, 2012
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 6.25mW/°C above 25°C.
Absolute Maximum Ratings1
Symbol Description Value Units
VIN Input Voltage to GND 6.0 V
VLX LX to GND -0.3 to VIN + 0.3 V
VOUT OUT to GND -0.3 to VIN + 0.3 V
VEN EN to GND -0.3 to 6.0 V
TJOperating Junction Temperature Range -40 to 150 °C
TLEAD Maximum Soldering Temperature (at leads, 10 sec.) 300 °C
Thermal Information2
Symbol Description Value Units
PD Maximum Power Dissipation30.625 W
JA Thermal Resistance 160 °C/W
4
AAT1147
High Efficiency, Low Noise, Fast Transient 400mA Step-Down Converter
DATA SHEET
Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • sales@skyworksinc.com • www.skyworksinc.com
201986A • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • May 23, 2012
1. The AAT1147 is guaranteed to meet performance specifications over the -40°C to +85°C operating temperature range and is assured by design, characterization, and correla-
tion with statistical process controls.
Electrical Characteristics1
TA = -40°C to +85°C, unless otherwise noted. Typical values are TA = 25°C, VIN = 3.6V.
Symbol Description Conditions Min Typ Max Units
Step-Down Converter
VIN Input Voltage 2.7 5.5 V
VUVLO UVLO Threshold
VIN Rising 2.7 V
Hysteresis 100 mV
VIN Falling 1.8 V
VOUT Output Voltage Tolerance IOUT = 0 to 400mA, VIN = 2.7V to 5.5V -3.0 3.0 %
VOUT Output Voltage Range 0.6 VIN V
IQQuiescent Current No Load 160 300 μA
ISHDN Shutdown Current EN = AGND = PGND 1.0 μA
ILIM P-Channel Current Limit 600 mA
RDS(ON)H High Side Switch On Resistance 0.45
RDS(ON)L Low Side Switch On Resistance 0.40
ILXLEAK LX Leakage Current VIN = 5.5V, VLX = 0 to VIN, EN = GND 1 μA
VLINEREG Line Regulation VIN = 2.7V to 5.5V 0.1 %/V
VOUT Out Threshold Voltage Accuracy 0.6V Output, No Load, TA = 25°C 591 600 609 mV
IOUT Out Leakage Current 0.6V Output 0.2 μA
TSStart-Up Time From Enable to Output Regulation 150 μs
FOSC Oscillator Frequency 1.0 1.4 2.0 MHz
TSD Over-Temperature Shutdown Threshold 140 °C
THYS Over-Temperature Shutdown Hysteresis 15 °C
EN
VEN(L) Enable Threshold Low 0.6 V
VEN(H) Enable Threshold High 1.4 V
IEN Input Low Current VIN = VOUT = 5.5V -1.0 1.0 μA
5
AAT1147
High Efficiency, Low Noise, Fast Transient 400mA Step-Down Converter
DATA SHEET
Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • sales@skyworksinc.com • www.skyworksinc.com
201986A • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • May 23, 2012
Typical Characteristics
Efficiency vs. Load
(VOUT = 3.3V; L = 6.8µH)
Output Current (mA)
Efficiency (%)
0
20
40
60
80
100
1 10 100 100
0
VIN = 3.6V
V
IN
= 4.2V
V
IN
= 5.0V
DC Regulation
(VOUT = 3.3V)
Output Current (mA)
Output Error (%)
-2.0
-1.5
-1.0
-0.5
0.0
0.5
1.0
1.5
2.0
0.1 1 10 100 1000
VIN = 3.6V
V
IN
= 4.2V
V
IN
= 5.0V
Efficiency vs. Load
(VOUT = 2.5V; L = 6.8µH)
Output Current (mA)
Efficiency (%)
0
20
40
60
80
100
1 10 100 1000
VIN = 3.6V
V
IN
= 4.2V
V
IN
= 5.0V
DC Regulation
(VOUT = 2.5V)
Output Current (mA)
Output Error (%)
-2.0
-1.5
-1.0
-0.5
0.0
0.5
1.0
1.5
2.0
0.1 1 10 100 1000
VIN = 5.0V
V
IN
= 4.2V
V
IN
= 3.6V
Efficiency vs. Load
(VOUT = 1.8V; L = 4.7µH)
Output Current (mA)
Efficiency (%)
0
20
40
60
80
100
1 10 100 1000
VIN = 3.0V
VIN = 3.6V
VIN = 4.2V
DC Regulation
(VOUT = 1.8V)
Output Current (mA)
Output Error (%)
-2.0
-1.5
-1.0
-0.5
0.0
0.5
1.0
1.5
2.0
0.1 1 10 100 1000
VIN = 3.6V
V
IN
= 4.2V
V
IN
= 3.0V
6
AAT1147
High Efficiency, Low Noise, Fast Transient 400mA Step-Down Converter
DATA SHEET
Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • sales@skyworksinc.com • www.skyworksinc.com
201986A • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • May 23, 2012
Typical Characteristics
Line Regulation
(VOUT = 3.3V)
Input Voltage (V)
Accuracy (%)
-0.5
-0.4
-0.3
-0.2
-0.1
0
0.1
0.2
0.3
0.4
0.5
3.0 3.2 3.4 3.6 3.8 4.0 4.2 4.4 4.6 4.8 5.0
IOUT = 400mA
IOUT = 10mA IOUT = 1mA
Line Regulation
(VOUT = 2.5V)
Input Voltage (V)
Accuracy (%)
-0.5
-0.4
-0.3
-0.2
-0.1
0.0
0.1
0.2
0.3
0.4
0.5
3.0 3.2 3.4 3.6 3.8 4.0 4.2 4.4 4.6 4.8 5.0
IOUT = 400mA
IOUT = 10mA IOUT = 1mA
Line Regulation
(VOUT = 1.8V)
Input Voltage (V)
Accuracy (%)
-0.5
-0.4
-0.3
-0.2
-0.1
0.0
0.1
0.2
0.3
0.4
0.5
3.0 3.2 3.4 3.6 3.8 4.0 4.2 4.4 4.6 4.8 5.0
IOUT = 400mA
IOUT = 10mA IOUT = 1mA
Frequency vs. Input Voltage
Input Voltage (V)
Frequency Variation (%)
-4.0
-3.0
-2.0
-1.0
0.0
1.0
2.0
2.5 2.9 3.3 3.7 4.1 4.5 4.9 5.3
VOUT = 1.8V
VOUT = 3.3V
VOUT = 2.5V
Output Voltage Error vs. Temperature
(VIN = 3.6V; VOUT = 1.8V; IOUT = 400mA)
Temperature (°
°
C)
Output Error (%)
-2.0
-1.5
-1.0
-0.5
0.0
0.5
1.0
1.5
2.0
-40 -25 -10 5 20 35 50 65 80 95
Switching Frequency vs. Temperature
(VIN = 3.6V; VOUT = 1.8V)
Temperature (°
°
C)
Variation (%)
-15
-12
-9
-6
-3
0
3
6
9
12
15
-40 -25 -10 5 20 35 50 65 80 95
7
AAT1147
High Efficiency, Low Noise, Fast Transient 400mA Step-Down Converter
DATA SHEET
Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • sales@skyworksinc.com • www.skyworksinc.com
201986A • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • May 23, 2012
Typical Characteristics
No-Load Quiescent Current
vs. Input Voltage
Input Voltage (V)
Supply Current (µA)
120
130
140
150
160
170
180
190
200
210
220
2.5 3.0 3.5 4.0 4.5 5.0 5.5
85°C
-40°C
25°C
Line Transient Response
(40mA to 400mA; VIN = 3.6V; VOUT = 1.8V;
C1 = 4.7µF; CFF = 100pF)
Output Voltage
(top) (V)
Load and Inductor Current
(bottom) (200mA/div)
Time (25µs/div)
1.0
1.1
1.2
1.3
1.4
1.5
1.6
1.7
1.8
1.9
2.0
-0.6
-0.4
-0.2
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
40 mA
400 mA
Line Transient Response
(40mA to 400mA; VIN = 3.6V;
VOUT = 1.8V; C1 = 4.7µF)
Output Voltage
(top) (V)
Load and Inductor Current
(bottom) (200mA/div)
Time (25µs/div)
1.0
1.1
1.2
1.3
1.4
1.5
1.6
1.7
1.8
1.9
2.0
-0.6
-0.4
-0.2
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
400mA
40mA
Soft Start
(VIN = 3.6V; VOUT = 1.8V; IOUT = 400mA)
Enable and Output Voltage
(top) (V)
Inductor Current
(bottom) (A)
Time (25µs/div)
-2.4
-1.6
-0.8
0.0
0.8
1.6
2.4
3.2
4.0
4.8
5.6
-0.4
0.0
0.4
0.8
1.2
1.6
2.0
2.4
2.8
3.2
3.6
Line Response
(VOUT = 1.8V @ 400mA)
Output Voltage
(top) (V)
Input Voltage
(bottom) (V)
Time (25µs/div)
1.40
1.45
1.50
1.55
1.60
1.65
1.70
1.75
1.80
1.85
1.90
3.2
3.4
3.6
3.8
4.0
4.2
4.4
4.6
4.8
5.0
5.2
Output Ripple
(VIN = 3.6V; VOUT = 1.8V; IOUT = 400mA)
Output Voltage
(AC Coupled) (top) (mV)
Inductor Current
(bottom) (A)
Time (500ns/div)
-120
-100
-80
-60
-40
-20
0
20
40
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
8
AAT1147
High Efficiency, Low Noise, Fast Transient 400mA Step-Down Converter
DATA SHEET
Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • sales@skyworksinc.com • www.skyworksinc.com
201986A • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • May 23, 2012
Functional Description
The AAT1147 is a high performance 400mA 1.4MHz
monolithic step-down converter. It has been designed
with the goal of minimizing external component size and
optimizing efficiency at heavy load. 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 val-
ues).
Only three external power components (CIN, COUT
, and L)
are required. Output voltage is programmed with exter-
nal resistors and ranges from 0.6V to the input voltage.
An additional 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 heavy load conditions
up to 400mA.
The internal error amplifier and compensation provide
excellent transient response, load, and line regulation.
Soft start eliminates any output voltage overshoot when
the enable or the input voltage is applied.
Functional Block Diagram
EN
LX
Err
.
Amp
Logic
DH
DL
PGND
VIN
AGND
Voltage
Reference
INPUT
OUT
9
AAT1147
High Efficiency, Low Noise, Fast Transient 400mA Step-Down Converter
DATA SHEET
Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • sales@skyworksinc.com • www.skyworksinc.com
201986A • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • May 23, 2012
L1 CDRH3D16-4R7
4.7μH
L1
10μF
C1
4.7μF
C2
U1 AAT1147 SC70JW-8
C2 4.7μF 10V 0805 X5R
VOUT =1.8V
GND
VIN
1
2
3
Enable
LX
EN
1
OUT
2
VIN
3
LX
4AGND 5
PGND 6
PGND 7
PGND 8
AAT1147
U1
GND2
118k
R1
59k
R2
C1 10μF 6.3V 0805 X5R
100pF
C4
n/a
C3
Figure 1: Enhanced Transient Response Schematic.
Control Loop
The AAT1147 is a peak current mode step-down con-
verter. 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 compensa-
tion signal is added to the sensed current to maintain
stability for duty cycles greater than 50%. The peak cur-
rent mode loop appears as a voltage-programmed cur-
rent 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. The
error amplifier reference is 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 AAT1147 into a low-power,
non-switching state. The total input current during shut-
down is less than 1μA.
Current Limit and
Over-Temperature Protection
For overload conditions, the peak input current is limit-
ed. 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 hys-
teresis. Once an over-temperature or over-current fault
conditions is removed, the output voltage automatically
recovers.
Under-Voltage Lockout
Internal bias of all circuits is controlled via the VIN input.
Under-voltage lockout (UVLO) guarantees sufficient VIN
bias and proper operation of all internal circuitry prior to
activation.
10
AAT1147
High Efficiency, Low Noise, Fast Transient 400mA Step-Down Converter
DATA SHEET
Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • sales@skyworksinc.com • www.skyworksinc.com
201986A • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • May 23, 2012
Applications Information
Inductor Selection
The step-down converter uses peak current mode con-
trol 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 AAT1147 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.
0.75 ⋅ V
O
m = = = 0.24
L
0.75 ⋅ 1.5V
4.7μH
A
μsec
This is the internal slope compensation. When externally
programming the 0.6V version to 2.5V, the calculated
inductance is 7.5μH.
0.75 ⋅ V
O
L = =
≈
3
⋅ V
O
= 3 ⋅ 2.5V = 7.5μH
m
0.75
⋅
V
O
0.24A
μsec
A
μsec
A
A
μsec
In this case, a standard 6.8μH value is selected.
Table 1 displays inductor values for the AAT1147.
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 satura-
tion 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.
The 4.7μH CDRH3D16 series inductor selected from
Sumida has a 105m DCR and a 900mA DC current rat-
ing. At full load, the inductor DC loss is 17mW which gives
a 2.8% loss in efficiency for a 400mA, 1.5V 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.
⎛⎞
· 1 -
⎝⎠
VO
VIN
CIN =
VO
VIN
⎛⎞
- ESR · FS
⎝⎠
VPP
IO
⎛⎞
· 1 - = for VIN = 2 · V
O
⎝⎠
VO
VIN
VO
VIN
1
4
CIN(MIN) = 1
⎛⎞
- ESR · 4 · FS
⎝⎠
VPP
IO
Always examine the ceramic capacitor DC voltage coef-
ficient characteristics when selecting the proper value.
For example, the capacitance of a 10μF, 6.3V, X5R ceram-
ic capacitor with 5.0V DC applied is actually about 6μF.
Confi guration Output Voltage Inductor
0.6V Adjustable With
External Feedback
1V, 1.2V 2.2μH
1.5V, 1.8V 4.7μH
2.5V, 3.3V 6.8μH
Table 1: Inductor Values.
The maximum input capacitor RMS current is:
⎛⎞
IRMS = IO · · 1 -
⎝⎠
VO
VIN
VO
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.
⎛⎞
· 1 - = D · (1 - D) = 0.52 =
⎝⎠
VO
VIN
VO
VIN
1
2
for VIN = 2 · VO
IO
RMS(MAX)
I2
=
The term
⎛⎞
· 1 -
⎝⎠
VO
VIN
VO
VIN 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.
11
AAT1147
High Efficiency, Low Noise, Fast Transient 400mA Step-Down Converter
DATA SHEET
Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • sales@skyworksinc.com • www.skyworksinc.com
201986A • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • May 23, 2012
Figure 2: AAT1147 Evaluation Board Figure 3: Exploded View of Evaluation
Top Side. Board Top Side Layout.
Figure 4: AAT1147 Evaluation Board
Bottom Side.
The input capacitor provides a low impedance loop for
the edges of pulsed current drawn by the AAT1147. 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.
A laboratory test set-up typically consists of two long
wires running from the bench power supply to the evalu-
ation 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 dur-
ing load transients. Errors in the loop phase and gain
measurements can also result.
12
AAT1147
High Efficiency, Low Noise, Fast Transient 400mA Step-Down Converter
DATA SHEET
Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • sales@skyworksinc.com • www.skyworksinc.com
201986A • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • May 23, 2012
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 induc-
tance cannot be reduced to a level that does not affect
the converter performance, a high ESR tantalum or alu-
minum 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 pro-
vides 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 charac-
teristics necessary for low output ripple.
The output voltage droop due to a load transient is dom-
inated 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 volt-
age droop during the three switching cycles to the output
capacitance can be estimated by:
COUT =
3 · ΔILOAD
VDROOP · FS
Once the average inductor current increases to the DC
load level, the output voltage recovers. The above equa-
tion 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 capac-
itance will reduce the crossover frequency with greater
phase margin.
The maximum output capacitor RMS ripple current is
given by:
1
23
VOUT · (VIN(MAX) - VOUT)
RMS(MAX)
IL · FS · VIN(MAX)
=·
·
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.
Output Resistor Selection
The output voltage of the AAT1147 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 cur-
rent, 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 221k for reduced no load input
current.
⎛⎞
⎝⎠
R1 = -1 · R2 = - 1 · 59kΩ = 88.5kΩ
VOUT
VREF
⎛⎞
⎝⎠
1.5V
0.6V
The AAT1147, combined with an external feedforward
capacitor (C4 in Figure 1), delivers enhanced transient
response for extreme pulsed load applications. The addi-
tion of the feedforward capacitor typically requires a
larger output capacitor C1 for stability.
VOUT (V)
R2 = 59kΩ
R1 (kΩ)
R2 = 221kΩ
R1 (kΩ)
0.8 19.6 75
0.9 29.4 113
1.0 39.2 150
1.1 49.9 187
1.2 59.0 221
1.3 68.1 261
1.4 78.7 301
1.5 88.7 332
1.8 118 442
1.85 124 464
2.0 137 523
2.5 187 715
3.3 267 1000
Table 2: Resistor Values For Use With
0.6V Step-Down Converter.
13
AAT1147
High Efficiency, Low Noise, Fast Transient 400mA Step-Down Converter
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L1 CDRH3D16-4R7
4.7μH
L1
10μF
C1
4.7μF
C2
U1 AAT1147 SC70JW-8
C1 10μF 10V 0805 X5R
C2 4.7μF 10V 0805 X5R
VOUT
GND
VIN
1
2
3
Enable
LX
EN
1
OUT
2
VIN
3
LX
4AGND 5
PGND 6
PGND 7
PGND 8
AAT1147
U1
GND2
118k
R1
59k
R2
Figure 5: AAT1147 Evaluation Board Schematic.
Thermal Calculations
There are three types of losses associated with the
AAT1147 step-down converter: switching losses, conduc-
tion 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 switch-
ing devices. At full load, assuming continuous conduction
mode (CCM), a simplified form of the losses is given by:
IQ is the step-down converter quiescent current. The
term tsw is used to estimate the full load step-down con-
verter switching losses.
PTOTAL
IO
2 · (RDSON(H) · VO + RDSON(L) · [VIN - VO])
VIN
=
+ (tsw · FS · IO + IQ) · VIN
For the condition where the step-down converter is in
dropout at 100% duty cycle, the total device dissipation
reduces to:
PTOTAL = IO
2 · RDSON(H) + IQ · VIN
Since RDS(ON), quiescent current, and switching losses all
vary with input voltage, the total losses should be inves-
tigated over the complete input voltage range.
Given the total losses, the maximum junction tempera-
ture can be derived from the JA for the SC70JW-8 pack-
age which is 160°C/W.
TJ(MAX) = PTOTAL · ΘJA + TAMB
14
AAT1147
High Efficiency, Low Noise, Fast Transient 400mA Step-Down Converter
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Layout
The suggested PCB layout for the AAT1147 is shown in
Figures 2, 3, and 4. The following guidelines should be
used to help ensure a proper layout.
1. The input capacitor (C2) should connect as closely as
possible to VIN (Pin 3) and PGND (Pins 6-8).
2. C1 and L1 should be connected as closely as possi-
ble. The connection of L1 to the LX pin should be as
short as possible.
3. The feedback trace or OUT pin (Pin 2) should be
separate from any power trace and connect as close-
ly as possible to the load point. Sensing along a
high-current load trace will degrade DC load regula-
tion. External feedback resistors should be placed as
closely as possible to the OUT pin (Pin 2) to minimize
the length of the high impedance feedback trace.
4. The resistance of the trace from the load return to
the PGND (Pins 6-8) 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.
A high density, small footprint layout can be achieved
using an inexpensive, miniature, non-shielded, high DCR
inductor. An evaluation board is available with this induc-
tor and is shown in Figure 6. The total solution footprint
area is 40mm2.
Figure 6: Minimum Footprint Evaluation Board
Using 2.0mm x 1.6mm x 0.95mm Inductor.
15
AAT1147
High Efficiency, Low Noise, Fast Transient 400mA Step-Down Converter
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Step-Down Converter Design Example
Specifications
VO = 1.8V @ 400mA (adjustable using 0.6V version), Pulsed Load ILOAD = 300mA
VIN = 2.7V to 4.2V (3.6V nominal)
FS = 1.4MHz
TAMB = 85°C
1.8V Output Inductor
L1 = 3 ⋅ V
O2
= 3 ⋅ 1.8V = 5.4μH
μsec
A
μsec
A
(use 4.7μH; see Table 1)
For Sumida inductor CDRH3D16, 4.7μH, DCR = 105m.
V
O
V
O
1.8
V
1.8V
ΔI
L1
=
⋅ 1 - = ⋅ 1 - = 156mA
L1 ⋅ F
S
V
IN
4.7μH ⋅ 1.4MHz
4.2V
I
PKL1
= I
O
+ ΔI
L1
= 0.4A + 0.068A = 0.468A
2
P
L1
= I
O
2
⋅ DCR = 0.4A
2
⋅ 105mΩ = 17mW
⎛
⎝⎞
⎠
⎛
⎝⎞
⎠
1.8V Output Capacitor
VDROOP = 0.1V
1
23
1 1.8V · (4.2V - 1.8V)
4.7μH · 1.4MHz · 4.2V
23
RMS
IL1 · FS · VIN(MAX)
= ·
·
3 · ΔILOAD
VDROOP · FS
3 · 0.3A
0.1V · 1.4MHz
COUT = = = 6.4μF; use 10µF
· = 45mArms
·
(VO) · (VIN(MAX) - VO)=
Pesr = esr · IRMS2 = 5mΩ · (45mA)2 = 10μW
16
AAT1147
High Efficiency, Low Noise, Fast Transient 400mA Step-Down Converter
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Input Capacitor
Input Ripple VPP = 25mV
CIN = = = 3.11μF; use 4.7μF
1
⎛⎞
- ESR · 4 · FS
⎝⎠
VPP
IO
1
⎛⎞
- 5mΩ · 4 · 1.4MHz
⎝⎠
25mV
0.4A
IO
RMS
I
P = esr · IRMS
2 = 5mΩ · (0.2A)2 = 0.2mW
2
= = 0.2Arms
AAT1147 Losses
PTOTAL
+ (tsw · FS · IO + IQ) · VIN
IO
2 · (RDSON(H) · VO + RDSON(L) · [VIN -VO])
VIN
=
T
J(MAX)
= T
AMB
+ Θ
JA
· P
LOSS
= 85°C +
(
160°C/W
)
· 126mW = 105.1°C
17
AAT1147
High Efficiency, Low Noise, Fast Transient 400mA Step-Down Converter
DATA SHEET
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1. For reduced quiescent current, R2 and R4 = 221kW.
Adjustable Version
(0.6V device)
VOUT (V)
R2 = 59kΩ
R1 (kΩ)
R2 = 221kΩ1
R1 (kΩ) L1 (μH)
0.8 19.6 75.0 2.2
0.9 29.4 113 2.2
1.0 39.2 150 2.2
1.1 49.9 187 2.2
1.2 59.0 221 2.2
1.3 68.1 261 2.2
1.4 78.7 301 4.7
1.5 88.7 332 4.7
1.8 118 442 4.7
1.85 124 464 4.7
2.0 137 523 6.8
2.5 187 715 6.8
3.3 267 1000 6.8
Table 3: Evaluation Board Component Values.
Manufacturer Part Number
Inductance
(μH)
Max DC
Current (A) DCR (Ω)
Size (mm)
LxWxH Type
Sumida CDRH3D16-2R2 2.2 1.20 0.072 3.8x3.8x1.8 Shielded
Sumida CDRH3D16-4R7 4.7 0.90 0.105 3.8x3.8x1.8 Shielded
Sumida CDRH3D16-6R8 6.8 0.73 0.170 3.8x3.8x1.8 Shielded
Murata LQH2MCN4R7M02 4.7 0.40 0.80 2.0x1.6x0.95 Non-Shielded
Murata LQH32CN4R7M23 4.7 0.45 0.20 2.5x3.2x2.0 Non-Shielded
Coilcraft LPO3310-472 4.7 0.80 0.27 3.2x3.2x1.0 1mm
Coiltronics SD3118-4R7 4.7 0.98 0.122 3.1x3.1x1.85 Shielded
Coiltronics SD3118-6R8 6.8 0.82 0.175 3.1x3.1x1.85 Shielded
Coiltronics SDRC10-4R7 4.7 1.30 0.122 5.7x4.4x1.0 1mm Shielded
Table 4: Typical Surface Mount Inductors.
Manufacturer Part Number Value Voltage Temp. Co. Case
Murata GRM219R61A475KE19 4.7μF 10V X5R 0805
Murata GRM21BR60J106KE19 10μF 6.3V X5R 0805
Murata GRM21BR60J226ME39 22μF 6.3V X5R 0805
Table 5: Surface Mount Capacitors.
18
AAT1147
High Efficiency, Low Noise, Fast Transient 400mA Step-Down Converter
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1. XYY = assembly and date code.
2. Sample stock is generally held on part numbers listed in BOLD.
Ordering Information
Package Marking1Part Number (Tape and Reel)2
SC70JW-8 SCXYY AAT1147IJS-0.6-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
SC70JW-8
0.225
±
0.075
0.45
±
0.10
0.05
±
0.05
2.10
±
0.30
2.00
±
0.20
7
°
±
3
°
4
°
±
4
°
1.75
±
0.10
0.85
±
0.15
0.15
±
0.05
1.10 MAX
0.100
2.20
±
0.20
0.048REF
0.50 BSC 0.50 BSC 0.50 BSC
All dimensions in millimeters.
