AAT1149
3MHz Fast Transient
400mA Step-Down Converter
1149.2006.11.1.0 1
SwitchReg™
General Description
The AAT1149 SwitchReg is a 3.0MHz step-down
converter with an input voltage range of 2.7V to
5.5V and output voltage as low as 1.0V. It is opti-
mized to react quickly to load variations and oper-
ate with a tiny 0603 inductor that is only 1mm tall.
The AAT1149 output voltage is programmable via
external feedback resistors. It can deliver 400mA of
load current while maintaining a low 45μA no load
quiescent current. The 3.0MHz switching frequen-
cy minimizes the size of external components while
keeping switching losses low.
The AAT1149 maintains high efficiency throughout
the operating range, which is critical for portable
applications.
The AAT1149 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
• Ultra-Small 0603 Inductor (Height = 1mm)
•V
IN Range: 2.7V to 5.5V
•V
OUT Adjustable from 1.0V to VIN
• 400mA Max Output Current
• Up to 98% Efficiency
•45μA No Load Quiescent Current
• 3.0MHz Switching Frequency
•70μs Soft Start
• Fast Load Transient
• Over-Temperature Protection
• Current Limit Protection
• 100% Duty Cycle Low-Dropout Operation
•<1μA Shutdown Current
• SC70JW-8 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
L1 1.8µH
C1
4.7µF
R1
118k
R2
59k
V
OUT
= 1.8VV
IN
= 3.6V
C2
4.7µF
EN FB
IN LX
AGND
PGND
PGND
PGND
AAT1149
U1
Pin Descriptions
Pin Configuration
SC70JW-8
(Top View)
FB
IN
LX
PGND
PGND
PGND
AGND
EN
1
2
3
45
6
7
8
Pin # Symbol Function
1 EN Enable pin.
2 FB Feedback input pin. This pin is connected to an external resistive divider for
an adjustable output.
3 IN Input supply voltage for the converter.
4 LX Switching node. Connect the inductor to this pin. It is internally connected 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.
AAT1149
3MHz Fast Transient
400mA Step-Down Converter
21149.2006.11.1.0
Absolute Maximum Ratings1
Thermal Information
Symbol Description Value Units
PDMaximum Power Dissipation2, 3 625 mW
θJA Thermal Resistance2160 °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
VFB FB 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
AAT1149
3MHz Fast Transient
400mA Step-Down Converter
1149.2006.11.1.0 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.
AAT1149
3MHz Fast Transient
400mA Step-Down Converter
41149.2006.11.1.0
Electrical Characteristics1
VIN = 3.6V, TA= -40°C to +85°C, unless otherwise noted. Typical values are TA= 25°C.
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 Adjustable Output Voltage Range 1.0 VIN V
IQQuiescent Current No Load 45 70 μA
ISHDN Shutdown Current VEN = GND 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
VEN = 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 70 μs
Regulation
FOSC Oscillator Frequency TA= 25°C 3.0 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
1. The AAT1149 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
Switching Frequency vs. Input Voltage
Input Voltage (V)
Frequency Variation (%)
-4
-3
-2
-1
0
1
2
2.5 3 3.5 4 4.5 5 5.
5
V
OUT
= 1.1V
V
OUT
= 1.8V V
OUT
= 3V
No Load Quiescent Current vs. Input Voltage
Input Voltage (V)
Supply Current (µA)
0
10
20
30
40
50
60
70
2.5 3 3.5 4 4.5 5 5.5 6
85°C
-40°C
25°C
Load Regulation
(V
OUT
= 1.8V; L = 2.2µH)
Load Current (mA)
Output Error (%)
-1.00
-0.75
-0.50
-0.25
0.00
0.25
0.50
0.75
1.00
0.1 1 10 100 100
0
V
IN
= 2.7V
V
IN
= 4.2V
V
IN
= 3V
V
IN
= 3.6VV
IN
= 5V
Efficiency vs. Load Current
(V
OUT
= 1.8V; L = 2.2µH)
Load Current (mA)
Efficiency (%)
50
60
70
80
90
100
0.1 1 10 100 100
0
V
IN
= 4.2V
V
IN
= 2.7V
V
IN
= 3.6V
V
IN
= 5V
V
IN
= 3V
Load Regulation
(V
OUT
= 3V; L = 3µH)
Load Current (mA)
Output Error (%)
-1.00
-0.75
-0.50
-0.25
0.00
0.25
0.50
0.75
1.00
0.1 1 10 100 1000
V
IN
= 4.2V
V
IN
= 5V
V
IN
= 3.3V
AAT1149
3MHz Fast Transient
400mA Step-Down Converter
1149.2006.11.1.0 5
AAT1149
3MHz Fast Transient
400mA Step-Down Converter
61149.2006.11.1.0
Typical Characteristics
Line Regulation
(V
OUT
= 1.1V)
Input Voltage (V)
Accuracy (%)
-1
-0.8
-0.6
-0.4
-0.2
0
0.2
0.4
0.6
0.8
1
2.5 3 3.5 4 4.5 5 5.5
6
1mA 0mA
600mA 400mA
Line Regulation
(V
OUT
= 1.8V)
Input Voltage (V)
Accuracy (%)
-1
-0.8
-0.6
-0.4
-0.2
0
0.2
0.4
0.6
0.8
1
2.5 3 3.5 4 4.5 5 5.5 6
400mA
600mA
0mA
100mA
Line Regulation
(V
OUT
= 3V)
Input Voltage (V)
Accuracy (%)
-1
-0.8
-0.6
-0.4
-0.2
0
0.2
0.4
0.6
0.8
1
2.5 3 3.5 4 4.5 5 5.
5
0mA
400mA
100mA
1mA
600mA
300mA
Output Voltage Error vs. Temperature
(V
IN
= 3.6V; V
O
= 1.8V; I
OUT
= 400mA)
Temperature (°
°C)
Output Error (%)
-2.0
-1.0
0.0
1.0
2.0
-40 -20 0 20 40 60 80 100
Switching Frequency Variation vs. Temperature
Temperature (°
°
C)
Variation (%)
-10
-8
-6
-4
-2
0
2
4
6
8
10
-40 -20 0 20 40 60 80 100 120
Typical Characteristics
Load Transient
(V
OUT
= 1.1V; C
FF
= 100pF)
Time (50µs/div)
Load and Inductor Current
(bottom) (A)
Output Voltage (top) (V)
0.50
0.60
0.70
0.80
0.90
1.00
1.10
1.20
1.30
0.00
0.25
0.50
0.75
1.00
1.25
1.50
1.75
2.00
1mA
400mA
Load Transient
(V
OUT
= 1.1V; No Feedforward Capacitor)
Time (50µs/div)
Load and Inductor Current
(bottom) (A)
Output Voltage (top) (V)
0.50
0.60
0.70
0.80
0.90
1.00
1.10
1.20
1.30
0.00
0.25
0.50
0.75
1.00
1.25
1.50
1.75
2.00
1mA
400mA
Line Transient
(V
OUT
= 1.8; C
FF
= 100pF)
Time (20µs/div)
Input Voltage (top) (V)
Output Voltage (bottom) (V)
1.78
1.79
1.80
1.81
1.82
1.83
1.84
1.85
1.86
2.50
2.75
3.00
3.25
3.50
3.75
4.00
4.25
4.50
Line Transient
(V
OUT
= 1.8; No Load; No Feedforward Capacitor)
Time (50µs/div)
Input Voltage (top) (V)
Output Voltage (bottom) (V)
2.50
2.75
3.00
3.25
3.50
3.75
4.00
4.25
4.50
1.78
1.80
1.82
1.84
1.86
1.88
1.90
1.92
1.94
P-Channel R
DS(ON)
vs. Input Voltage
Input Voltage (V)
R
DS(ON)
(mΩ
Ω
)
300
350
400
450
500
550
600
650
700
750
2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0
25°C
120°C 100°C
85°C
N-Channel R
DS(ON)
vs. Input Voltage
Input Voltage (V)
R
DS(ON)
(mΩ
Ω
)
300
350
400
450
500
550
600
650
700
750
2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0
25°C
120°C100°C
85°C
AAT1149
3MHz Fast Transient
400mA Step-Down Converter
1149.2006.11.1.0 7
Typical Characteristics
Soft Start
(V
OUT
= 1.8V; C
FF
= 100pF)
Time (50µs/div)
Inductor Current
(bottom) (250mA/div)
Enable and Output Voltage
(top) (V)
-1.50
-1.00
-0.50
0.00
0.50
1.00
1.50
2.00
2.50
0.00
0.25
0.50
0.75
1.00
1.25
1.50
1.75
2.00
Soft Start
(V
OUT
= 1.8V; No Feedforward Capacitor)
Time (50µs/div)
Inductor Current
(bottom) (250mA/div)
Enable and Output Voltage
(top) (V)
-4.00
-3.00
-2.00
-1.00
0.00
1.00
2.00
3.00
4.00
0.00
0.25
0.50
0.75
1.00
1.25
1.50
1.75
2.00
Load Transient
(V
OUT
= 1.8V; C
FF
= 100pF)
Time (50µs/div)
Load and Inductor Current
(bottom) (A)
Output Voltage (top) (V)
1.20
1.30
1.40
1.50
1.60
1.70
1.80
1.90
2.00
0.00
0.25
0.50
0.75
1.00
1.25
1.50
1.75
2.00
400mA
1mA
Load Transient
(V
OUT
= 1.8V; No Feedforward Capacitor)
Time (50µs/div)
Load and Inductor Current
(bottom) (A)
Output Voltage (top) (V)
0.25
0.50
0.75
1.00
1.25
1.50
1.75
2.00
2.25
0.00
0.25
0.50
0.75
1.00
1.25
1.50
1.75
2.00
400mA
1mA
Load Transient
(V
OUT
= 1.8V; C
FF
= 100pF)
Time (50µs/div)
Load and Inductor Current
(bottom) (A)
Output Voltage (top) (V)
1.20
1.30
1.40
1.50
1.60
1.70
1.80
1.90
2.00
0.00
0.25
0.50
0.75
1.00
1.25
1.50
1.75
2.00
10mA
400mA
Load Transient
(V
OUT
= 1.8V; No Feedforward Capacitor)
Time (50µs/div)
Load and Inductor Current
(bottom) (A)
Output Voltage (top) (V)
0.25
0.50
0.75
1.00
1.25
1.50
1.75
2.00
2.25
0.00
0.25
0.75
1.00
1.25
1.50
1.75
2.00
10mA
400mA
AAT1149
3MHz Fast Transient
400mA Step-Down Converter
81149.2006.11.1.0
Typical Characteristics
Soft Start
(V
OUT
= 1.1V; No Feedforward Capacitor)
Time (20µs/div)
Inductor Current
(bottom) (250mA/div)
Enable and Output Voltage
(top) (V)
-0.50
0.00
0.50
1.00
1.50
2.00
2.50
3.00
3.50
-0.75
-0.50
-0.25
0.00
0.25
0.50
0.75
1.00
1.25
Soft Start
(V
OUT
= 3V; No Feedforward Capacitor)
Time (50µs/div)
Inductor Current
(bottom) (250mA/div)
Enable and Output Voltage
(top) (V)
-4.00
-3.00
-2.00
-1.00
0.00
1.00
2.00
3.00
4.00
0.00
0.25
0.50
0.75
1.00
1.25
1.50
1.75
2.00
AAT1149
3MHz Fast Transient
400mA Step-Down Converter
1149.2006.11.1.0 9
AAT1149
3MHz Fast Transient
400mA Step-Down Converter
10 1149.2006.11.1.0
Functional Block Diagram
EN
LX
Err
.
Amp
Logic
DH
DL
PGND
IN
AGND
Voltage
Reference
INPUT
FB
Functional Description
The AAT1149 is a high performance 400mA 3.0MHz
monolithic step-down converter. It minimizes exter-
nal component size, enabling the use of a tiny 0603
inductor that is only 1mm tall, and optimizes effi-
ciency 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 1.8μH 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 feedback resistors, ranging from 1.0V
to the input voltage. An additional feed-forward
capacitor can also be added to the external feed-
back to provide improved transient response (see
Figure 4).
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 all load
conditions, ranging from no load to 400mA.
The internal error amplifier and compensation pro-
vides excellent transient response, load, and line
regulation. Soft start eliminates any output voltage
overshoot when the enable or the input voltage is
applied.
AAT1149
3MHz Fast Transient
400mA Step-Down Converter
1149.2006.11.1.0 11
Control Loop
The AAT1149 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. 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 AAT1149
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 IN
input. Under-voltage lockout (UVLO) guarantees
sufficient VIN bias and proper operation of all inter-
nal circuitry prior to activation.
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.
Table 1 displays suggested inductor
values for various output voltages.
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 1.8μH CDRH2D09 series inductor selected
from Sumida has a 131mΩDCR and a 400mA sat-
uration current rating. At full load, the inductor DC
loss is 21mW which gives a 2.8% loss in efficiency
for a 400mA, 1.8V 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.
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
AAT1149
3MHz Fast Transient
400mA Step-Down Converter
12 1149.2006.11.1.0
Table 1: Inductor Values.
Configuration Output Voltage Typical Inductor Value
1V, 1.2V 1.0μH to 1.2μH
0.6V Adjustable With 1.5V, 1.8V 1.5μH to 1.8μH
External Feedback 2.5V 2.2μH to 2.7μH
3.3V 3.3μH
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.
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
AAT1149. 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 local-
ized, minimizing EMI and input voltage ripple.
The proper placement of the input capacitor (C2)
can be seen in the evaluation board layout in
Figure 1.
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-
⎛⎞
· 1
-
⎝⎠
V
O
V
IN
V
O
V
IN
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
AAT1149
3MHz Fast Transient
400mA Step-Down Converter
1149.2006.11.1.0 13
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.
The maximum output capacitor RMS ripple current
is given by:
1
23
V
OUT
· (V
IN(MAX)
- V
OUT
)
RMS(MAX)
IL
·
F
S
·
V
IN(MAX)
=·
·
C
OUT
=
3
·
ΔI
LOAD
V
DROOP
·
F
S
Figure 1: AAT1149 Evaluation Board Figure 2: Exploded View of Evaluation
Top Side. Board Top Side.
Figure 3: AAT1149 Evaluation Board
Bottom Side.
AAT1149
3MHz Fast Transient
400mA Step-Down Converter
14 1149.2006.11.1.0
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.
Feedback Resistor Selection
Resistors R1 and R2 of Figure 4 program the output
to regulate at a voltage higher than 0.6V. To limit the
bias current required for the external feedback resis-
tor string while maintaining good noise immunity, the
minimum suggested value for R2 is 59kΩ. 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 59kΩ
for good noise immunity or 221kΩfor reduced no
load input current.
The AAT1149, combined with an external feedfor-
ward capacitor (C3 in Figure 4), 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: Feedback Resistor Values.
R2 = 59kΩΩR2 = 221kΩΩ
VOUT (V) R1 (kΩΩ)R1
0.9 29.4 113K
1.0 39.2 150K
1.1 49.9 187K
1.2 59.0 221K
1.3 68.1 261K
1.4 78.7 301K
1.5 88.7 332K
1.8 118 442K
1.85 124 464K
2.0 137 523K
2.5 187 715K
3.3 267 1.00M
⎛⎞
⎝⎠
R1 = -1
·
R2 = - 1
·
59kΩ = 88.5kΩ
V
OUT
V
REF
⎛⎞
⎝⎠
1.5V
0.6V
Figure 4: AAT1149 Evaluation Board Schematic.
L1
4.7μF
C1
4.7μF
C2
V
OUT
V
IN
GND
GND
1
2
3
Enable
LX
EN
1
OUT
2
IN
3
LX
4
AGND
5
PGND
6
PGND
7
PGND
8
AAT1149
U1
R1
59k
R2
C3
AAT1149
3MHz Fast Transient
400mA Step-Down Converter
1149.2006.11.1.0 15
Thermal Calculations
There are three types of losses associated with the
AAT1149 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.
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 AAT1149 is
shown in Figures 1, 2, and 3. The following guide-
lines should be used to help ensure a proper layout.
1. The input capacitor (C2) should connect as close-
ly as possible to IN (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 FB 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. If external feedback resistors
are used, they should be placed as closely as
possible to the FB 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, as shown in Figure 5.
Figure 5: Minimum Footprint Evaluation Board
Using 2.0x1.25x1.0mm Inductor.
T
J(MAX)
=
P
TOTAL
·
Θ
JA
+ T
AMB
P
TOTAL
= I
O
2
· R
DS(ON)H
+ I
Q
· V
IN
P
TOTAL
I
O
2
· (R
DS(ON)H
· V
O
+ R
DS(ON)L
· [V
IN
- V
O
])
V
IN
=
+ (t
sw
· F
S
· I
O
+ I
Q
) · V
IN
AAT1149
3MHz Fast Transient
400mA Step-Down Converter
16 1149.2006.11.1.0
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= 3.0MHz
TAMB = 85°C
1.8V Output Inductor
(use 2.2μH; see Table 1)
For Taiyo Yuden inductor CBC2518T2R2M, 2.2μH, DCR = 130mΩ.
1.8V Output Capacitor
VDROOP = 0.1V
1
23
1 1.8V · (4.2V - 1.8V)
2.2μH · 3.0MHz · 4.2V
23
RMS
IL1 · F
S
· V
IN(MAX)
= ·
·
3 · ΔI
LOAD
V
DROOP
· F
S
3 · 0.3A
0.1V · 3.0MHz
C
OUT
= = = 3.0μF; use 4.7µF
· = 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
2.2μH ⋅ 3.0MHz
4.2V
I
PKL1
= I
O
+ ΔI
L1
= 0.4A + 0.078A = 0.478A
2
P
L1
= I
O
2
⋅ DCR = 0.4A
2
⋅ 130mΩ = 21mW
⎛
⎝⎞
⎠
⎛
⎝⎞
⎠
L1 = 1 ⋅ V
O
= 1 ⋅ 1.8V = 1.8μH
μsec
A
μsec
A
AAT1149
3MHz Fast Transient
400mA Step-Down Converter
1149.2006.11.1.0 17
Input Capacitor
Input Ripple VPP = 25mV
AAT1149 Losses
T
J(MAX)
= T
AMB
+ Θ
JA
· P
LOSS
= 85°C + (160°C/W) · 140mW = 107°C
P
TOTAL
+ (t
sw
· F
S
· I
O
+ I
Q
) · V
IN
I
O
2
· (R
DS(ON)H
· V
O
+ R
DS(ON)L
· [V
IN
-V
O
]
)
V
IN
=
=
+ (5ns · 3MHz · 0.4A + 70μA) · 4.2V = 140m
W
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
= = = 1.45μF; use 2.2μF
1
⎛⎞
- ESR
·
4
·
F
S
⎝⎠
V
PP
I
O
1
⎛⎞
- 5mΩ
·
4
·
3.0MHz
⎝⎠
25mV
0.4A
AAT1149
3MHz Fast Transient
400mA Step-Down Converter
18 1149.2006.11.1.0
Table 3: Evaluation Board Component Values.
Table 4: Typical Surface Mount Inductors.
Part Number/ Inductance Rated DCR Size (mm)
Manufacturer Type (μH) Current (mA) (ΩΩ) LxWxH
0.77 660 110 0603
BRC1608 1.0 520 180 (HMAX = 1mm)
1.5 410 300
Taiyo Yuden 1.5 600 200 0805
BRL2012 2.2 550 250 (HMAX = 1mm)
3.3 450 350
CBC2518 1.0 1000 80 2.5x1.8x1.8
Wire Wound Chip 2.2 890 130
1.2 590 97.5
1.5 520 110
Sumida CDRH2D09 1.8 480 131 3.2x3.2x1.0
Shielded 2.5 440 150
3.0 400 195
1.0 485 300
Murata LQH2MCN4R7M02 1.5 445 400 2.0x1.6x0.95
Unshielded 2.2 425 480
3.3 375 600
0.68 980 31
0.82 830 54
Coiltronics SD3118 1.2 720 75 3.15x3.15x1.2
Shielded 1.5 630 104
2.2 510 116
3.3 430 139
Adjustable Version R2 = 59kΩΩR2 = 221kΩΩ1
(0.6V device)
VOUT (V) R1 (kΩΩ) R1 (kΩΩ) L1 (μH)
1.0 39.2 150 1.0
1.2 59.0 221 1.2
1.5 88.7 332 1.5
1.8 118 442 1.8
2.5 187 715 2.2
3.3 267 1000 3.3
1. For reduced quiescent current, R2 = 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 GRM185R60J475M 4.7μF 6.3V X58 0603
AAT1149
3MHz Fast Transient
400mA Step-Down Converter
1149.2006.11.1.0 19
AAT1149
3MHz Fast Transient
400mA Step-Down Converter
20 1149.2006.11.1.0
Advanced Analogic Technologies, Inc.
830 E. Arques Avenue, Sunnyvale, CA 94085
Phone (408) 737-4600
Fax (408) 737-4611
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.
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
All AnalogicTech products are offered in Pb-free packaging. The term “Pb-free” means
semiconductor products that are in compliance with current RoHS standards, including
the requirement that lead not exceed 0.1% by weight in homogeneous materials. For more
information, please visit our website at http://www.analogictech.com/pbfree.
Output Voltage1Package Marking2Part Number (Tape and Reel)3
0.6; Adj ≥1.0 SC70JW-8 RGXYY AAT1149IJS-0.6-T1
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