AAT1147
High Efficiency, Low Noise, Fast Transient
400mA Step-Down Converter
1147.2006.05.1.0 1
SwitchReg™
General Description
The AAT1147 SwitchReg is a member of
AnalogicTech's Total Power Management IC™
(TPMIC™) product family. It is a fixed frequency
1.4MHz step-down converter with an input volt-
age range of 2.7V to 5.5V and output voltage as
low as 0.6V.
The AAT1147 is optimized for low noise portable
applications, 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 effi-
ciency. The 1.4MHz switching frequency mini-
mizes the size of external components while keep-
ing switching losses low.
The AAT1147 is available in a Pb-free, space-sav-
ing 2.0x2.1mm SC70JW-8 package and is rated
over the -40°C to +85°C temperature range.
Features
•V
IN Range: 2.7V to 5.5V
•V
OUT 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
Typical Application
4.7μH
L1
118k
R1
4.7μF
C1
59k
R2
4.7μF
C2 EN
1
OUT
2
VIN
3
LX
4
AGND
5
PGND
6
PGND
7
PGND
8
AAT1147
U1
V
IN
V
O
= 1.8V
Pin Descriptions
Pin Configuration
SC70JW-8
(Top View)
OUT
VIN
LX
PGND
PGND
PGND
AGND
EN
1
2
3
45
6
7
8
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.
4 LX 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.
AAT1147
High Efficiency, Low Noise, Fast Transient
400mA Step-Down Converter
21147.2006.05.1.0
Absolute Maximum Ratings1
Thermal Information2
Symbol Description Value Units
PDMaximum Power Dissipation30.625 W
θJA Thermal Resistance 160 °C/W
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
AAT1147
High Efficiency, Low Noise, Fast Transient
400mA Step-Down Converter
1147.2006.05.1.0 3
1. Stresses above those listed in Absolute Maximum Ratings may cause permanent damage to the device. Functional operation at condi-
tions 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.
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
VIN Rising 2.7 V
VUVLO UVLO Threshold Hysteresis 100 mV
VIN Falling 1.8 V
VOUT Output Voltage Tolerance IOUT = 0 to 400mA, -3.0 3.0 %
VIN = 2.7V to 5.5V
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 Ω
RDS(ON)L Low Side Switch On Resistance 0.40 Ω
ILXLEAK LX Leakage Current VIN = 5.5V, VLX = 0 to VIN, 1µA
EN = GND
ΔVLINEREG Line Regulation VIN = 2.7V to 5.5V 0.1 %/V
VOUT Out Threshold Voltage Accuracy 0.6V Output, No Load, 591 600 609 mV
TA= 25°C
IOUT Out Leakage Current 0.6V Output 0.2 µA
TSStart-Up Time From Enable to Output 150 µs
Regulation
FOSC Oscillator Frequency 1.0 1.4 2.0 MHz
TSD Over-Temperature Shutdown 140 °C
Threshold
THYS Over-Temperature Shutdown 15 °C
Hysteresis
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
AAT1147
High Efficiency, Low Noise, Fast Transient
400mA Step-Down Converter
41147.2006.05.1.0
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 correlation with statistical process controls.
Typical Characteristics
DC Regulation
(V
OUT
= 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
V
IN
= 3.6V
V
IN
= 4.2V
V
IN
= 3.0V
Efficiency vs. Load
(V
OUT
= 1.8V; L = 4.7µH)
Output Current (mA)
Efficiency (%)
0
20
40
60
80
100
1 10 100 1000
V
IN
= 3.0V
V
IN
= 3.6V
V
IN
= 4.2V
DC Regulation
(V
OUT
= 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
V
IN
= 5.0V
V
IN
= 4.2V
V
IN
= 3.6V
Efficiency vs. Load
(V
OUT
= 2.5V; L = 6.8µH)
Output Current (mA)
Efficiency (%)
0
20
40
60
80
100
1 10 100 1000
V
IN
= 3.6V
V
IN
= 4.2V
V
IN
= 5.0V
DC Regulation
(V
OUT
= 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
V
IN
= 3.6V
V
IN
= 4.2V
V
IN
= 5.0V
Efficiency vs. Load
(V
OUT
= 3.3V; L = 6.8µH)
Output Current (mA)
Efficiency (%)
0
20
40
60
80
100
1 10 100 100
0
V
IN
= 3.6V
V
IN
= 4.2V
V
IN
= 5.0V
AAT1147
High Efficiency, Low Noise, Fast Transient
400mA Step-Down Converter
1147.2006.05.1.0 5
Typical Characteristics
Switching Frequency vs. Temperature
(V
IN
= 3.6V; V
OUT
= 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
Output Voltage Error vs. Temperature
(V
IN
= 3.6V; V
OUT
= 1.8V; I
OUT
= 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
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
V
OUT
= 1.8V
V
OUT
= 3.3V
V
OUT
= 2.5V
Line Regulation
(V
OUT
= 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
I
OUT
= 400mA
I
OUT
= 10mA I
OUT
= 1mA
Line Regulation
(V
OUT
= 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
I
OUT
= 400mA
I
OUT
= 10mA I
OUT
= 1mA
Line Regulation
(V
OUT
= 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
I
OUT
= 400mA
I
OUT
= 10mA I
OUT
= 1mA
AAT1147
High Efficiency, Low Noise, Fast Transient
400mA Step-Down Converter
61147.2006.05.1.0
Typical Characteristics
Output Ripple
(V
IN
= 3.6V; V
OUT
= 1.8V; I
OUT
= 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
Line Response
(V
OUT
= 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
Soft Start
(V
IN
= 3.6V; V
OUT
= 1.8V; I
OUT
= 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 Transient Response
(40mA to 400mA; V
IN
= 3.6V;
V
OUT
= 1.8V; C
1
= 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
Line Transient Response
(40mA to 400mA; V
IN
= 3.6V; V
OUT
= 1.8V;
C
1
= 4.7µF; C
FF
= 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
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
AAT1147
High Efficiency, Low Noise, Fast Transient
400mA Step-Down Converter
1147.2006.05.1.0 7
AAT1147
High Efficiency, Low Noise, Fast Transient
400mA Step-Down Converter
81147.2006.05.1.0
Functional Block Diagram
EN
LX
Err
.
Amp
Logic
DH
DL
PGND
VIN
AGND
Voltage
Reference
INPUT
OUT
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 values).
Only three external power components (CIN, COUT,
and L) are required. Output voltage is programmed
with external resistors and ranges from 0.6V to the
input voltage. An additional feed-forward capacitor
can also be added to the external feedback to pro-
vide improved transient response (see Figure 1).
At dropout, the converter duty cycle increases to
100% and the output voltage tracks the input volt-
age minus the RDS(ON) drop of the P-channel high-
side MOSFET.
The input voltage range is 2.7V to 5.5V. The con-
verter efficiency has been optimized for heavy load
conditions up to 400mA.
The internal error amplifier and compensation pro-
vide excellent transient response, load, and line
regulation. Soft start eliminates any output voltage
overshoot when the enable or the input voltage is
applied.
Control Loop
The AAT1147 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 pro-
tection. 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 compen-
sation 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 shutdown is less than 1µA.
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 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 inter-
nal circuitry prior to activation.
AAT1147
High Efficiency, Low Noise, Fast Transient
400mA Step-Down Converter
1147.2006.05.1.0 9
Figure 1: Enhanced Transient Response Schematic.
L1 CDRH3D16-4R7
4.7μH
L1
10μF
C1
4.7μF
C2
U1 AAT1147 SC70JW-8
C2 4.7μF 10V 0805 X5R
V
OUT
=1.8V
GND
V
IN
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
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 induc-
tor 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.
This is the internal slope compensation. When
externally programming the 0.6V version to 2.5V,
the calculated inductance is 7.5µH.
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
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 loss-
es 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 105mΩDCR and a 900mA DC
current rating. 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 capac-
itor for the input. To estimate the required input
capacitor size, determine the acceptable input rip-
ple level (VPP) and solve for C. The calculated
value varies with input voltage and is a maximum
when VIN is double the output voltage.
Always examine the ceramic capacitor DC voltage
coefficient characteristics when selecting the prop-
er value. For example, the capacitance of a 10µF,
6.3V, X5R ceramic capacitor with 5.0V DC applied
is actually about 6µF.
C
IN(MIN)
= 1
⎛⎞
- ESR
·
4
·
F
S
⎝⎠
V
PP
I
O
⎛⎞
· 1
-
= for V
IN
= 2 · V
O
⎝⎠
V
O
V
IN
V
O
V
IN
1
4
⎛⎞
· 1
-
⎝⎠
V
O
V
IN
C
IN
=
V
O
V
IN
⎛⎞
- ESR
·
F
S
⎝⎠
V
PP
I
O
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
0.75 ⋅ V
O
m = = = 0.24
L
0.75 ⋅ 1.5V
4.7μH
A
μsec
AAT1147
High Efficiency, Low Noise, Fast Transient
400mA Step-Down Converter
10 1147.2006.05.1.0
Table 1: Inductor Values.
Configuration Output Voltage Inductor
0.6V Adjustable With
1V, 1.2V 2.2µH
External Feedback 1.5V, 1.8V 4.7µH
2.5V, 3.3V 6.8µH
The maximum input capacitor RMS current is:
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.
for VIN = 2 · VO
The term appears in both the input
voltage ripple and input capacitor RMS current
equations and is a maximum when VOis twice VIN.
This is why the input voltage ripple and the input
capacitor RMS current ripple are a maximum at
50% duty cycle.
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 evaluation board input voltage pins. The induc-
tance of these wires, along with the low-ESR
ceramic input capacitor, can create a high Q net-
work 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 meas-
urements can also result.
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 typi-
cally provides sufficient bulk capacitance to stabi-
lize 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 out-
put capacitor. During a step increase in load cur-
rent, 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:
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.
C
OUT
=
3
·
ΔI
LOAD
V
DROOP
·
F
S
I
O
RMS(MAX)
I2
=
⎛⎞
· 1
-
= D
· (1 - D) = 0.5
2
=
⎝⎠
V
O
V
IN
V
O
V
IN
1
2
⎛⎞
I
RMS
= I
O
· · 1
-
⎝⎠
V
O
V
IN
V
O
V
IN
AAT1147
High Efficiency, Low Noise, Fast Transient
400mA Step-Down Converter
1147.2006.05.1.0 11
The maximum output capacitor RMS ripple current
is given 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.
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 maintain-
ing good noise immunity, the minimum suggested
value for R2 is 59kΩ. Although a larger value will fur-
ther 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 59kΩfor good noise
immunity or 221kΩfor reduced no load input current.
⎛⎞
⎝⎠
R1 = -1
·
R2 = - 1
·
59kΩ = 88.5kΩ
V
OUT
V
REF
⎛⎞
⎝⎠
1.5V
0.6V
1
23
V
OUT
· (V
IN(MAX)
- V
OUT
)
RMS(MAX)
IL
·
F
S
·
V
IN(MAX)
=·
·
AAT1147
High Efficiency, Low Noise, Fast Transient
400mA Step-Down Converter
12 1147.2006.05.1.0
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 AAT1147, combined with an external feedfor-
ward capacitor (C4 in Figure 1), delivers enhanced
transient response for extreme pulsed load appli-
cations. The addition of the feedforward capacitor
typically requires a larger output capacitor C1 for
stability.
Table 2: Resistor Values For Use With 0.6V
Step-Down Converter.
Thermal Calculations
There are three types of losses associated with the
AAT1147 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 losses is given by:
IQis the step-down converter quiescent current.
The term tsw is used to estimate the full load step-
down converter switching losses.
P
TOTAL
I
O
2
· (R
DSON(H)
· V
O
+ R
DSON(L)
· [V
IN
- V
O
])
V
IN
=
+ (t
sw
· F
S
· I
O
+ I
Q
) · V
IN
R2 = 59kΩΩR2 = 221kΩΩ
VOUT (V) R1 (kΩΩ) R1 (kΩΩ)
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
AAT1147
High Efficiency, Low Noise, Fast Transient
400mA Step-Down Converter
1147.2006.05.1.0 13
Figure 5: AAT1147 Evaluation Board Schematic.
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
V
OUT
GND
V
IN
1
2
3
Enable
LX
EN
1
OUT
2
VIN
3
LX
4
AGND
5
PGND
6
PGND
7
PGND
8
AAT1147
U1
GND2
118k
R1
59k
R2
For the condition where the step-down converter is
in dropout at 100% duty cycle, the total device dis-
sipation reduces to:
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.
Given the total losses, the maximum junction tem-
perature can be derived from the θJA for the
SC70JW-8 package which is 160°C/W.
Layout
The suggested PCB layout for the AAT1147 is
shown in Figures 2, 3, and 4. The following guide-
lines 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
possible. 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 closely as possible to the load point.
Sensing along a high-current load trace will
degrade DC load regulation. External feedback
resistors should be placed as closely as possi-
ble 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 poten-
tial 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 inductor 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.
T
J(MAX)
=
P
TOTAL
·
Θ
JA
+ T
AMB
P
TOTAL
= I
O
2
· R
DSON(H)
+ I
Q
· V
IN
AAT1147
High Efficiency, Low Noise, Fast Transient
400mA Step-Down Converter
14 1147.2006.05.1.0
Step-Down Converter Design Example
Specifications
VO= 1.8V @ 400mA (adjustable using 0.6V version), Pulsed Load ΔILOAD = 300mA
VIN = 2.7V to 4.2V (3.6V nominal)
FS= 1.4MHz
TAMB = 85°C
1.8V Output Inductor
(use 4.7µH; see Table 1)
For Sumida inductor CDRH3D16, 4.7µH, DCR = 105mΩ.
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 · F
S
· V
IN(MAX)
= ·
·
3 · ΔI
LOAD
V
DROOP
· F
S
3 · 0.3A
0.1V · 1.4MHz
C
OUT
= = = 6.4μF; use 10µF
· = 45mArms
·
(V
O
) · (V
IN(MAX)
- V
O
)
=
P
esr
= esr · I
RMS2
= 5mΩ · (45mA)
2
= 10μW
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
⎛
⎝⎞
⎠
⎛
⎝⎞
⎠
L1 = 3 ⋅ V
O2
= 3 ⋅ 1.8V = 5.4μH
μsec
A
μsec
A
AAT1147
High Efficiency, Low Noise, Fast Transient
400mA Step-Down Converter
1147.2006.05.1.0 15
Input Capacitor
Input Ripple VPP = 25mV
AAT1147 Losses
T
J(MAX)
= T
AMB
+ Θ
JA
· P
LOSS
= 85°C + (160°C/W) · 126mW = 105.1°C
P
TOTAL
+ (t
sw
· F
S
· I
O
+ I
Q
) · V
IN
I
O
2
· (R
DSON(H)
· V
O
+ R
DSON(L)
· [V
IN
-V
O
]
)
V
IN
=
=
+ (5ns · 1.4MHz · 0.4A + 70μA) · 4.2V = 126mW
0.4
2
· (0.725
Ω
·
1.8V + 0.7Ω
·
[4.2V - 1.8V])
4.2V
I
O
RMS
I
P = esr
·
I
RMS
2
= 5mΩ
·
(0.2A)
2
= 0.2mW
2
= = 0.2Arms
C
IN
= = = 3.11μF; use 4.7μF
1
⎛⎞
- ESR
·
4
·
F
S
⎝⎠
V
PP
I
O
1
⎛⎞
- 5mΩ
·
4
·
1.4MHz
⎝⎠
25mV
0.4A
AAT1147
High Efficiency, Low Noise, Fast Transient
400mA Step-Down Converter
16 1147.2006.05.1.0
Table 3: Evaluation Board Component Values.
Table 4: Typical Surface Mount Inductors.
Inductance Max DC DCR Size (mm)
Manufacturer Part Number (µH) Current (A) (ΩΩ) 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
Adjustable Version R2 = 59kΩΩR2 = 221kΩΩ1
(0.6V device)
VOUT (V) R1 (kΩΩ) R1 (kΩΩ) 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
AAT1147
High Efficiency, Low Noise, Fast Transient
400mA Step-Down Converter
1147.2006.05.1.0 17
1. For reduced quiescent current, R2 and R4 = 221kΩ.
Table 5: Surface Mount Capacitors.
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
AAT1147
High Efficiency, Low Noise, Fast Transient
400mA Step-Down Converter
18 1147.2006.05.1.0
AAT1147
High Efficiency, Low Noise, Fast Transient
400mA Step-Down Converter
1147.2006.05.1.0 19
Ordering Information
Package Information
SC70JW-8
All dimensions in millimeters.
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
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Package Marking1Part Number (Tape and Reel)2
SC70JW-8 SCXYY AAT1147IJS-0.6-T1
Advanced Analogic Technologies, Inc.
830 E. Arques Avenue, Sunnyvale, CA 94085
Phone (408) 737-4600
Fax (408) 737-4611
1. XYY = assembly and date code.
2. Sample stock is generally held on part numbers listed in BOLD.
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