AAT1146 [ANALOGICTECH]
Fast Transient 400mA Step-Down Converter; 快速瞬态400毫安降压转换器
| 型号： | AAT1146 |
| 厂家： | 
ADVANCED ANALOGIC TECHNOLOGIES |
| 描述： | Fast Transient 400mA Step-Down Converter |
| 文件： | 总20页 (文件大小：768K) |
| 中文： | 中文翻译 | 下载： | 下载PDF数据表文档文件 |
AAT1146
Fast Transient 400mA Step-Down Converter
™
SwitchReg
General Description
Features
The AAT1146 SwitchReg is a member of
AnalogicTech's Total Power Management IC™
(TPMIC™) product family. It is a 1.4MHz 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.
•
•
•
•
•
•
•
•
•
•
•
•
•
•
VIN Range: 2.7V to 5.5V
VOUT Fixed or Adjustable from 0.6V to VIN
27μA No Load Quiescent Current
Up to 98% Efficiency
400mA Max Output Current
1.4MHz Switching Frequency
120μs Soft Start
Fast Load Transient
Over-Temperature Protection
Current Limit Protection
The AAT1146 is available in fixed voltage versions
with internal feedback and a programmable ver-
sion with external feedback resistors. It can deliver
400mA of load current while maintaining a low
27μA no load quiescent current. The 1.4MHz
switching frequency minimizes the size of external
components while keeping switching losses low.
100% Duty Cycle Low-Dropout Operation
<1μA Shutdown Current
SC70JW-8 Package
Temperature Range: -40°C to +85°C
The AAT1146 is designed to maintain high efficien-
cy throughout the operating range, which is critical
for portable applications.
The AAT1146 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.
Applications
•
•
•
•
•
•
Cellular Phones
Digital Cameras
Handheld Instruments
Microprocessor / DSP Core / IO Power
PDAs and Handheld Computers
USB Devices
Typical Application (Fixed Output Voltage)
VO
VIN
U1
AAT1146
L1
3
4
VIN
EN
LX
4.7μH
1
5
8
2
OUT
C2
4.7μF
C1
4.7μF
7
AGND PGND
PGND PGND
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AAT1146
Fast Transient 400mA Step-Down Converter
Pin Descriptions
Pin #
Symbol
EN
Function
1
2
Enable pin.
OUT
Feedback input pin. This pin is connected either directly to the converter
output or to an external resistive divider for an adjustable output.
3
4
VIN
LX
Input supply voltage for the converter.
Switching node. Connect the inductor to this pin. It is internally connected to
the drain of both high- and low-side MOSFETs.
5
AGND
PGND
Non-power signal ground pin.
6, 7, 8
Main power ground return pins. Connect to the output and input capacitor
return.
Pin Configuration
SC70JW-8
(Top View)
8
7
6
5
1
2
3
4
PGND
PGND
PGND
AGND
EN
OUT
VIN
LX
2
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AAT1146
Fast Transient 400mA Step-Down Converter
Absolute Maximum Ratings1
Symbol
Description
Value
Units
VIN
VLX
Input Voltage GND
6.0
V
V
LX to GND
-0.3 to VIN + 0.3
-0.3 to VIN + 0.3
-0.3 to 6.0
-40 to 150
300
VOUT
VEN
TJ
OUT to GND
V
EN to GND
V
Operating Junction Temperature Range
Maximum Soldering Temperature (at leads, 10 sec)
°C
°C
TLEAD
Thermal Information
Symbol
Description
Value
Units
PD
Maximum Power Dissipation2, 3
Thermal Resistance2
625
160
mW
θJA
°C/W
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.
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AAT1146
Fast Transient 400mA 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
Min
Typ
Max Units
Step-Down Converter
VIN
Input Voltage
2.7
5.5
2.7
V
V
VIN Rising
Hysteresis
VIN Falling
VUVLO
UVLO Threshold
100
mV
V
1.8
-3.0
0.6
I
OUT = 0 to 400mA,
VOUT
VOUT
IQ
Output Voltage Tolerance
Output Voltage Range
Quiescent Current
+3.0
VIN
70
%
V
VIN = 2.7V to 5.5V
No Load, 0.6V Adjustable
Version
27
μA
ISHDN
ILIM
RDS(ON)H
RDS(ON)L
Shutdown Current
EN = AGND = PGND
1.0
μA
mA
Ω
P-Channel Current Limit
600
High Side Switch On Resistance
Low Side Switch On Resistance
0.45
0.40
Ω
V
IN = 5.5V, VLX = 0 to VIN,
ILXLEAK
ΔVLinereg
VOUT
LX Leakage Current
1
μA
%/V
mV
EN = GND
Line Regulation
VIN = 2.7V to 5.5V
0.6V Output, No Load
TA = 25°C
0.1
Out Threshold Voltage Accuracy
591
250
600
609
0.2
IOUT
Out Leakage Current
Out Impedance
0.6V Output
μA
kΩ
ROUT
>0.6V Output
From Enable to Output
Regulation
TS
Start-Up Time
150
μs
FOSC
TSD
Oscillator Frequency
TA = 25°C
1.0
1.4
140
15
2.0
MHz
°C
Over-Temperature Shutdown Threshold
Over-Temperature Shutdown Hysteresis
THYS
°C
EN
VEN(L)
VEN(H)
IEN
Enable Threshold Low
Enable Threshold High
Input Low Current
0.6
1.0
V
V
1.4
VIN = VOUT = 5.5V
-1.0
μA
1. The AAT1146 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.
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AAT1146
Fast Transient 400mA Step-Down Converter
Typical Characteristics
Efficiency vs. Load
(VOUT = 1.8V; L = 4.7μH)
DC Regulation
(VOUT = 1.8V)
100
1.0
VIN = 2.7V
90
0.5
VIN = 4.2V
VIN = 4.2V
80
VIN = 3.6V
0.0
70
60
50
VIN = 3.6V
-0.5
VIN = 2.7V
-1.0
0.1
1
10
100
1000
1000
1000
0.1
1
10
100
1000
Output Current (mA)
Output Current (mA)
Efficiency vs. Load
(VOUT = 2.5V; L = 6.8μμH)
DC Regulation
(VOUT = 2.5V)
100
90
80
70
60
50
1.0
0.5
VIN = 2.7V
VIN = 4.2V
VIN = 5.0V
VIN = 5.0V
VIN = 4.2V
0.0
VIN = 3.6V
VIN = 3.6V
-0.5
-1.0
VIN = 3.0V
0.1
1
10
100
0.1
1
10
100
1000
Output Current (mA)
Output Current (mA)
Efficiency vs. Load
(VOUT = 3.3V; L = 6.8μH)
DC Regulation
(VOUT = 3.3V; L = 6.8µH)
100
1.0
0.5
VIN = 3.6V
VIN = 5.0V
VIN = 4.2V
90
80
70
60
50
VIN = 4.2V
0.0
VIN = 5.0V
-0.5
-1.0
VIN = 3.6V
0.1
1
10
100
0.1
1
10
100
1000
Output Current (mA)
Output Current (mA)
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AAT1146
Fast Transient 400mA Step-Down Converter
Typical Characteristics
Soft Start
(VIN = 3.6V; VOUT = 1.8V; IOUT = 400mA)
Line Regulation
(VOUT = 1.8V)
0.40
0.30
5.0
1.6
1.4
VEN
VO
4.0
3.0
IOUT = 10mA
1.2
1.0
0.8
0.6
0.4
0.2
0.0
-0.2
-0.4
0.20
0.10
2.0
1.0
0.00
0.0
-0.10
-0.20
-0.30
-0.40
-1.0
-2.0
-3.0
-4.0
-5.0
IOUT = 1mA
IOUT = 400mA
IL
2.5
3.0
3.5
4.0
4.5
5.0
5.5
6.0
Time (100μμs/div)
Input Voltage (V)
Output Voltage Error vs. Temperature
(VIN = 3.6V; VO = 1.8V; IOUT = 400mA)
Switching Frequency vs. Temperature
(VIN = 3.6V; VOUT = 1.8V)
2.0
1.0
15.0
12.0
9.0
6.0
3.0
0.0
0.0
-3.0
-6.0
-9.0
-12.0
-15.0
-1.0
-2.0
-40
-20
0
20
40
60
80
100
-40
-20
0
20
40
60
80
100
Temperature (°C)
Temperature (°C)
Frequency vs. Input Voltage
No Load Quiescent Current vs. Input Voltage
2.0
1.0
50
45
40
35
VOUT = 1.8V
0.0
25°C
85°C
-1.0
-2.0
-3.0
-4.0
30
25
20
15
10
VOUT = 2.5V
VOUT = 3.3V
-40°C
2.7
3.1
3.5
3.9
4.3
4.7
5.1
5.5
2.7
3.1
3.5
3.9
4.3
4.7
5.1
5.5
Input Voltage (V)
Input Voltage (V)
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AAT1146
Fast Transient 400mA Step-Down Converter
Typical Characteristics
P-Channel RDS(ON) vs. Input Voltage
N-Channel RDS(ON) vs. Input Voltage
750
700
650
600
550
500
450
400
350
300
750
700
650
120°C
100°C
120°C
100°C
600
550
500
450
400
350
300
85°C
85°C
25°C
25°C
2.5
3.0
3.5
4.0
4.5
5.0
5.5
6.0
2.5
3.0
3.5
4.0
4.5
5.0
5.5
6.0
Input Voltage (V)
Input Voltage (V)
Load Transient Response
(1mA to 300mA; VIN = 3.6V; VOUT = 1.8V;
C1 = 10μF; CFF = 100pF)
Load Transient Response
(300mA to 400mA; VIN = 3.6V;
V
OUT = 1.8V; C1 = 4.7μμF)
2.0
1.90
1.85
1.80
1.75
1.9
1.8
1.7
VO
VO
IO
IO
300mA
400mA
300mA
1mA
IL
0.4
0.3
0.2
0.1
IL
0
Time (50μs/div)
Time (50μs/div)
Load Transient Response
(300mA to 400mA; VIN = 3.6V;
Load Transient Response
(300mA to 400mA; VIN = 3.6V; VOUT = 1.8V;
V
OUT = 1.8V; C1 = 10μμF)
C1 = 10μμF; C4 = 100pF)
1.850
1.90
1.85
1.80
1.75
1.825
1.800
1.775
VO
IO
VO
IO
400mA
400mA
300mA
300mA
0.4
0.4
0.3
0.2
0.1
0.3
0.2
0.1
IL
IL
Time (50μs/div)
Time (50μs/div)
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AAT1146
Fast Transient 400mA Step-Down Converter
Typical Characteristics
Line Response
(VOUT = 1.8V @ 400mA)
Output Ripple
(VIN = 3.6V; VOUT = 1.8V; IOUT = 1mA)
40
20
0.30
0.25
0.20
0.15
0.10
0.05
0.00
-0.05
-0.10
1.82
1.81
1.80
1.79
1.78
1.77
6.0
5.5
5.0
4.5
4.0
3.5
3.0
VO
0
-20
-40
-60
-80
-100
-120
IL
1.76
Time (25μμs/div)
Time (10µs/div)
Output Ripple
(VIN = 3.6V; VOUT = 1.8V; IOUT = 400mA)
40
0.9
20
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
VO
0
-20
-40
-60
-80
IL
-100
-120
Time (500ns/div)
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AAT1146
Fast Transient 400mA Step-Down Converter
Functional Block Diagram
OUT
VIN
See note
Err
Amp
.
DH
LX
Voltage
Logic
Reference
DL
INPUT
EN
PGND
AGND
Note: For adjustable version, the internal feedback divider is omitted and the OUT pin is tied directly
to the internal error amplifier.
input voltage. An additional feed-forward capacitor
can also be added to the external feedback to pro-
vide improved transient response (see Figure 1).
Functional Description
The AAT1146 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 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).
At dropout, the converter duty cycle increases to
100% and the output voltage tracks the input volt-
age minus the RDSON 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.
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
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AAT1146
Fast Transient 400mA Step-Down Converter
1
2
3
Enable
VIN
C4
100pF
U1
AAT1146
1
2
3
4
8
7
6
5
EN
PGND
VOUT =1.8V
R1
118k
4.7μH
OUT PGND
L1
VIN
LX
PGND
AGND
C1
10μF
R2
59k
C2
4.7μF
C3
n/a
GND
LX
GND2
U1 AAT1146 SC70JW-8
L1 CDRH3D16-4R7
C2 4.7μF 10V 0805 X5R
C1 10μF 6.3V 0805 X5R
Figure 1: Enhanced Transient Response Schematic.
into a low-power, non-switching state. The total
input current during shutdown is less than 1μA.
Control Loop
The AAT1146 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.
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.
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 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.
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
Soft Start / Enable
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.
Soft start limits the current surge seen at the input
and eliminates output voltage overshoot. When
pulled low, the enable input forces the AAT1146
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AAT1146
Fast Transient 400mA Step-Down Converter
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.
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 adjustable and low-voltage fixed versions of the
AAT1146 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 CDRH3D16 series inductor selected
from Sumida has a 105mΩ DCR 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.
0.75 ⋅ VO 0.75 ⋅ 1.5V
= 0.24
A
m =
=
L
4.7μH
μsec
This is the internal slope compensation for the
adjustable (0.6V) version or low-voltage fixed ver-
sions. When externally programming the 0.6V ver-
sion to 2.5V, the calculated inductance is 7.5μH.
VO
VIN
⎛
· 1
⎝
VO
VIN
⎞
⎠
-
CIN =
⎛
⎝
VPP
IO
⎞
0.75 ⋅ VO
0.75
⋅
VO
A
μsec
A
- ESR
·
FS
L =
=
≈
3
⋅ VO
⎠
m
0.24A
μsec
VO
VIN
⎛
VO
VIN
⎞
⎠
1
4
· 1
-
=
for VIN = 2 × VO
μsec
A
⎝
= 3
⋅ 2.5V = 7.5μH
1
In this case, a standard 6.8μH value is selected.
CIN(MIN)
=
⎛
⎝
VPP
IO
⎞
⎠
- ESR
·
4
·
FS
For high-voltage fixed versions (≥2.5V), m = 0.48A/
μsec. Table 1 displays inductor values for the
AAT1146 fixed and adjustable options.
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.
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
Configuration
Output Voltage
Inductor
2.2μH
1V, 1.2V
1.5V, 1.8V
2.5V, 3.3V
0.6V to 3.3V
0.6V Adjustable With
External Feedback
4.7μH
6.8μH
Fixed Output
4.7μH
Table 1: Inductor Values.
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AAT1146
Fast Transient 400mA Step-Down Converter
The maximum input capacitor RMS current is:
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.
VO
VIN
⎛
· 1
⎝
VO
VIN
⎞
⎠
IRMS = IO
·
-
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.
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
VIN
⎛
· 1
⎝
VO
VIN
⎞
⎠
1
2
-
=
D
· (1 - D) = 0.52 =
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.
for VIN = 2 x VO
IO
IRMS(MAX)
=
2
VO
VIN
⎛
1 -
VO
VIN
⎞
⎠
·
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:
⎝
The term
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.
The input capacitor provides a low impedance loop
for the edges of pulsed current drawn by the
AAT1146. 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.
3
·
VDROOP FS
ΔILOAD
COUT
=
·
The proper placement of the input capacitor (C2)
can be seen in the evaluation board layout in
Figure 2.
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.
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.
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.
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AAT1146
Fast Transient 400mA Step-Down Converter
Figure 2: AAT1146 Evaluation Board
Top Side.
Figure 3: Exploded View of Evaluation
Board Top Side Layout.
Figure 4: AAT1146 Evaluation Board
Bottom Side.
The maximum output capacitor RMS ripple current
is given by:
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 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 221kΩ for reduced no load input current.
1
VOUT · (VIN(MAX) - VOUT)
IRMS(MAX)
=
·
L · F · VIN(MAX)
2 · 3
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
V
V
1.5V
0.6V
⎛
⎝
⎞
⎛
⎝
⎞
- 1 ·
R1 =
OUT -1
·
R2 =
59kΩ = 88.5kΩ
For applications requiring an adjustable output volt-
age, the 0.6V version can be externally pro-
grammed. Resistors R1 and R2 of Figure 5 program
⎠
⎠
REF
1146.2006.04.1.3
13
AAT1146
Fast Transient 400mA Step-Down Converter
The adjustable version of the AAT1146, combined
Thermal Calculations
with an external feedforward capacitor (C4 in
Figure 1), delivers enhanced transient response for
extreme pulsed load applications. The addition of
the feedforward capacitor typically requires a larg-
er output capacitor C1 for stability.
There are three types of losses associated with the
AAT1146 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:
Ω
Ω
R2 = 59k
R2 = 221k
R1
Ω
VOUT (V)
R1 (k )
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
75K
113K
150K
187K
221K
261K
301K
332K
442K
464K
523K
715K
1.00M
IO2 · (RDSON(HS) · VO + RDSON(LS) · [VIN - VO])
PTOTAL
=
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.
124
137
187
267
Table 2: Adjustable Resistor Values For Use
With 0.6V Step-Down Converter.
1
2
3
Enable
VIN
U1
AAT1146
1
2
3
4
8
7
6
5
EN
PGND
R1
OUT PGND
118k
VIN
LX
PGND
AGND
VOUT
L1
4.7μH
C1
C2
4.7μF
R2
59k
10μF
GND
GND2
LX
U1 AAT1146 SC70JW-8
L1 CDRH3D16-4R7
C1 10μF 10V 0805 X5R
C2 4.7μF 10V 0805 X5R
Figure 5: AAT1146 Adjustable Evaluation Board Schematic.
14
1146.2006.04.1.3
AAT1146
Fast Transient 400mA Step-Down Converter
For the condition where the step-down converter is
in dropout at 100% duty cycle, the total device dis-
sipation reduces to:
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. If external feed-
back resistors are used, they 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 poten-
tial of the internal signal ground and the power
ground.
PTOTAL = IO2 · RDSON(HS) + IQ · VIN
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.
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.
TJ(MAX)
=
PTOTAL
·
Θ
JA + TAMB
Layout
The suggested PCB layout for the AAT1146 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.
Figure 6: Minimum Footprint Evaluation Board
Using 2.0mm x 1.6mm x 0.95mm Inductor.
1146.2006.04.1.3
15
AAT1146
Fast Transient 400mA Step-Down Converter
Step-Down Converter Design Example
Specifications
VO
VIN
FS
= 1.8V @ 400mA (adjustable using 0.6V version), Pulsed Load ΔILOAD = 300mA
= 2.7V to 4.2V (3.6V nominal)
= 1.4MHz
TAMB = 85°C
1.8V Output Inductor
μsec
A
μsec
A
L1 = 3
⋅ VO2 = 3
⋅ 1.8V = 5.4μH
(use 4.7μH; see Table 1)
For Sumida inductor CDRH3D16, 4.7μH, DCR = 105mΩ.
⎛
⎞
⎠
VO
L1 ⋅ F
VO
VIN
1.8
V
1.8V
4.2V
⎛
⋅ 1 -
⎝
⎞
⎠
ΔIL1 =
=
⋅ 1 -
= 156mA
⎝
4.7μH ⋅ 1.4MHz
ΔI
IPKL1 = IO + L1 = 0.4A + 0.068A = 0.468A
2
PL1 = IO2 ⋅ DCR = 0.4A2 ⋅ 105mΩ = 17mW
1.8V Output Capacitor
VDROOP = 0.1V
3 · ΔILOAD
VDROOP · FS
3 · 0.3A
COUT
=
=
= 6.4μF; use 10µF
0.1V · 1.4MHz
(VO) · (VIN(MAX) - VO)
L1 · F · VIN(MAX)
1
1.8V · (4.2V - 1.8V)
1
·
= 45mArms
IRMS
=
·
=
4.7μH · 1.4MHz · 4.2V
2· 3
2· 3
Pesr = esr · IRMS2 = 5mΩ · (45mA)2 = 10μW
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1146.2006.04.1.3
AAT1146
Fast Transient 400mA Step-Down Converter
Input Capacitor
Input Ripple VPP = 25mV
1
1
CIN =
=
= 3.11μF; use 4.7μF
⎛
⎝
VPP
IO
⎞
⎛
⎝
25mV
0.4A
⎞
- ESR
·
4
·
FS
- 5mΩ
· 4 · 1.4MHz
⎠
⎠
IO
IRMS
=
= 0.2Arms
2
P = esr
·
IRMS2 = 5mΩ
·
(0.2A)2 = 0.2mW
AAT1146 Losses
IO2 · (RDSON(HS) · VO + RDSON(LS) · [VIN -VO
])
PTOTAL
=
VIN
+ (tsw · F · IO + IQ) · VIN
0.42 · (0.725
Ω
·
1.8V + 0.7Ω
4.2V
· [4.2V - 1.8V])
=
+ (5ns · 1.4MHz · 0.4A + 70μA) · 4.2V = 126mW
TJ(MAX) = TAMB + ΘJA · PLOSS = 85°C + (160°C/W) · 126mW = 105.1°C
1146.2006.04.1.3
17
AAT1146
Fast Transient 400mA Step-Down Converter
Adjustable Version
(0.6V device)
1
Ω
Ω
R2 = 221k
R2 = 59k
Ω
Ω
VOUT (V)
R1 (k )
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
75.0
113
150
187
221
261
301
332
442
464
523
715
1000
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
124
137
187
267
Fixed Version
R2, R4 Not Used
VOUT (V)
R1 (kΩ)
L1 (μH)
0.6-3.3V
0
4.7
Table 3: Evaluation Board Component Values.
Inductance
Max DC
Current (A)
DCR
(Ω)
Size (mm)
LxWxH
Manufacturer
Part Number
(μH)
Type
Sumida
CDRH3D16-2R2
CDRH3D16-4R7
CDRH3D16-6R8
LQH2MCN4R7M02
LQH32CN4R7M23
LPO3310-472
2.2
4.7
6.8
4.7
4.7
4.7
4.7
6.8
4.7
1.20
0.90
0.73
0.40
0.45
0.80
0.98
0.82
1.30
0.072
0.105
0.170
0.80
3.8x3.8x1.8
3.8x3.8x1.8
3.8x3.8x1.8
2.0x1.6x0.95
2.5x3.2x2.0
3.2x3.2x1.0
3.1x3.1x1.85
3.1x3.1x1.85
5.7x4.4x1.0
Shielded
Shielded
Sumida
Sumida
Shielded
MuRata
Non-Shielded
Non-Shielded
1mm
MuRata
0.20
Coilcraft
Coiltronics
Coiltronics
Coiltronics
0.27
SD3118-4R7
0.122
0.175
0.122
Shielded
SD3118-6R8
Shielded
SDRC10-4R7
1mm Shielded
Table 4: Typical Surface Mount Inductors.
1. For reduced quiescent current, R2 and R4 = 221kΩ.
18
1146.2006.04.1.3
AAT1146
Fast Transient 400mA Step-Down Converter
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.
1146.2006.04.1.3
19
AAT1146
Fast Transient 400mA Step-Down Converter
Ordering Information
Output Voltage1
Package
Marking2
Part Number (Tape and Reel)3
1.875
Adj ≥ 0.6
SC70JW-8
SC70JW-8
QMXYY
OXXYY
AAT1146IJS-1.875-T1
AAT1146IJS-0.6-T1
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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.
Package Information
SC70JW-8
0.50 BSC 0.50 BSC 0.50 BSC
0.225 0.075
2.00 0.20
0.048REF
0.100
0.45 0.10
4° 4°
7° 3°
2.10 0.30
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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830 E. Arques Avenue, Sunnyvale, CA 94085
Phone (408) 737-4600
Fax (408) 737-4611
20
1146.2006.04.1.3
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