AAT1140IGV-0.6-T1 [ANALOGICTECH]
Fast Transient 600mA Step-Down Converter; 快速瞬态600mA降压转换器
| 型号： | AAT1140IGV-0.6-T1 |
| 厂家： | 
ADVANCED ANALOGIC TECHNOLOGIES |
| 描述： | Fast Transient 600mA Step-Down Converter |
| 文件： | 总19页 (文件大小：1021K) |
| 中文： | 中文翻译 | 下载： | 下载PDF数据表文档文件 |
PRODUCT DATASHEET
AAT1140
SwitchRegTM
FastTransient 600mA Step-Down Converter
General Description
Features
The AAT1140 SwitchReg is a 1.4MHz step-down con-
verter 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
• 35μA No Load Quiescent Current
• Up to 98% Efficiency
• 600mA Max Output Current
• 1.4MHz Switching Frequency
• 120μs Soft Start
• Fast Load Transient
• Over-Temperature Protection
• Current Limit Protection
The AAT1140 is available in fixed voltage versions with
internal feedback and a programmable version with
external feedback resistors. It can deliver 600mA of load
current while maintaining a low 35μA no load quiescent
current. The 1.4MHz switching frequency minimizes the
size of external components while keeping switching
losses low.
• 100% Duty Cycle Low-Dropout Operation
• <1μA Shutdown Current
The AAT1140 is designed to maintain high efficiency
throughout the operating range, which is critical for por-
table applications.
• SOT23-5 Package
• Temperature Range: -40°C to +85°C
The AAT1140 is available in a Pb-free SOT23-5 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)
U1
AAT1140
L1
VIN
VOUT
LX
IN
4.7µH
C2
4.7µF
EN
OUT
C1
4.7µF
GND
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PRODUCT DATASHEET
AAT1140
SwitchRegTM
FastTransient 600mA Step-Down Converter
Pin Descriptions
Pin # Symbol Function
1
2
EN
GND
Enable pin.
Ground pin. Connect to the output and input capacitor return.
Switching node. Connect the inductor to this pin. It is internally connected to the drains of both high- and
low-side MOSFETs.
Input supply voltage for the converter.
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
5
LX
IN
OUT
Pin Configuration
SOT23-5
(Top View)
5
1
OUT
EN
2
3
GND
LX
4
IN
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PRODUCT DATASHEET
AAT1140
SwitchRegTM
FastTransient 600mA Step-Down Converter
Absolute Maximum Ratings1
Symbol
Description
Value
Units
VIN
VLX
VOUT
VEN
TJ
Input Voltage to GND
LX to GND
OUT to GND
EN to GND
Operating Junction Temperature Range
Storage Temperature Range
Maximum Soldering Temperature (at leads, 10 sec)
6.0
V
V
V
-0.3 to VIN + 0.3
-0.3 to VIN + 0.3
-0.3 to 6.0
-40 to 150
-65 to 150
300
V
°C
°C
°C
TS
TLEAD
Thermal Information
Symbol
Description
Maximum Power Dissipation2, 3
Thermal Resistance2
Value
Units
PD
θJA
667
150
mW
°C/W
1. Stresses above those listed in Absolute Maximum Ratings may cause permanent damage to the device. Functional operation at conditions other than the operating conditions
specified is not implied. Only one Absolute Maximum Rating should be applied at any one time.
2. Mounted on an FR4 board.
3. Derate 6.67mW/°C above 25°C.
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PRODUCT DATASHEET
AAT1140
SwitchRegTM
FastTransient 600mA Step-Down Converter
Electrical Characteristics1
TA = -40°C to +85°C, unless otherwise noted. Typical values are TA = 25°C, VIN = 3.6V.
Symbol Description
Step-Down Converter
Conditions
Min Typ Max
Units
2.7
5.5
2.7
V
V
mV
V
%
V
μA
μA
mA
Ω
VIN
Input Voltage
VIN Rising
Hysteresis
VIN Falling
160
35
VUVLO
UVLO Threshold
1.8
VOUT
VOUT
IQ
ISHDN
ILIM
RDS(ON)H
RDS(ON)L
ILXLEAK
ΔVLinereg
VOUT
IOUT
ROUT
TS
FOSC
TSD
THYS
Output Voltage Tolerance
Output Voltage Range
Quiescent Current
IOUT = 0 to 600mA, VIN = 2.7V to 5.5V -3.5
+3.5
VIN
70
0.6
No Load, 0.6V Adjustable Version
EN = AGND = PGND
Shutdown Current
1.0
P-Channel Current Limit
High Side Switch On Resistance
Low Side Switch On Resistance
LX Leakage Current
Line Regulation
Out Threshold Voltage Accuracy
Out Leakage Current
Out Impedance
Start-Up Time
Oscillator Frequency
Over-Temperature Shutdown Threshold
Over-Temperature Shutdown Hysteresis
750
0.53
0.40
Ω
VIN = 5.5V, VLX = 0 to VIN, EN = GND
VIN = 2.7V to 5.5V
0.6V Output, No Load, TA = 25°C
0.6V Output
>0.6V Output
From Enable to Output Regulation
TA = 25°C
1
μA
%/V
mV
μA
kΩ
μs
MHz
°C
°C
0.1
600
588
250
1.0
612
0.2
150
1.4
140
15
2.0
EN
VEN(L)
VEN(H)
IEN
Enable Threshold Low
Enable Threshold High
Input Low Current
0.6
1.0
V
V
μA
1.4
-1.0
VIN = VOUT = 5.5V
1. The AAT1140 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.
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PRODUCT DATASHEET
AAT1140
SwitchRegTM
FastTransient 600mA Step-Down Converter
Typical Characteristics
Efficiency vs. Load
(VOUT = 3.3V; L = 6.8μH)
DC Regulation
(VOUT = 3.3V; L = 6.8µH)
100
3.0
2.0
1.0
VIN = 3.6V
90
VIN = 4.2V
VIN = 4.2V
VIN = 5.0V
80
0.0
-1.0
-2.0
-3.0
VIN = 5.0V
70
60
VIN = 5.5V
50
0.1
1
10
100
1000
0
100
200
300
400
500
600
Output Current (mA)
Output Current (mA)
Efficiency vs. Load
(VOUT = 2.5V; L = 6.8μH)
DC Regulation
(VOUT = 2.5V; L = 6.8µH)
100
90
80
70
60
50
3.0
2.0
VIN = 2.7V
VIN = 5.0V
1.0
VIN = 5.0V
VIN = 4.2V
0.0
VIN = 3.6V
-1.0
-2.0
-3.0
VIN = 3.6V
VIN = 3.0V
VIN = 4.2V
0
100
200
300
400
500
600
0.1
1
10
100
1000
Output Current (mA)
Output Current (mA)
Efficiency vs. Load
(VOUT = 1.8V; L = 4.7μH)
DC Regulation
(VOUT = 1.8V; L = 4.7μH)
100
3.0
2.0
VIN = 2.7V
90
80
70
60
50
1.0
VIN = 4.2V
VIN = 2.7V
VIN = 3.6V
VIN = 3.6V
0.0
-1.0
-2.0
-3.0
VIN = 4.2V
0.1
1
10
100
1000
0
100
200
300
400
500
600
Output Current (mA)
Output Current (mA)
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PRODUCT DATASHEET
AAT1140
SwitchRegTM
FastTransient 600mA Step-Down Converter
Typical Characteristics
Soft Start
(VIN = 3.6V; VOUT = 1.8V; Load = 3Ω)
Line Regulation
(VOUT = 1.8V)
0.40
0.30
8.0
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
EN
7.0
IOUT = 10mA
0.20
6.0
0.10
0.00
VOUT
5.0
4.0
-0.10
IOUT = 1mA
-0.20
IOUT = 400mA
3.0
IIN
-0.30
-0.40
2.0
1.0
2.5
3.0
3.5
4.0
4.5
5.0
5.5
6.0
Time (100μs/div)
Input Voltage (V)
Output Voltage Error vs. Temperature
Switching Frequency vs. Temperature
(VIN = 3.6V; VOUT = 1.8V)
(VIN = 3.6V; VO = 1.8V; IOUT = 400mA)
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
(VOUT = 3.0V, L = 6.8µH)
2.0
1.0
60
55
TA = 85°C
VOUT = 1.8V
50
TA = 25°C
0.0
45
40
35
30
-1.0
-2.0
-3.0
-4.0
VOUT = 2.5V
VOUT = 3.3V
TA = -40°C
25
20
2.7
3.1
3.5
3.9
4.3
4.7
5.1
5.5
3.3
3.8
4.3
4.8
5.3
Input Voltage (V)
Input Voltage (V)
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PRODUCT DATASHEET
AAT1140
SwitchRegTM
FastTransient 600mA Step-Down Converter
Typical Characteristics
No Load Quiescent Current vs. Input Voltage
(VOUT = 1.8V, L = 4.7µH)
No Load Quiescent Current vs. Input Voltage
(VOUT = 1.2V, L = 2.2µH)
0.060
0.060
TA = 85°C
0.055
0.055
TA = 85°C
0.050
0.050
0.045
0.040
0.035
0.030
0.025
0.020
TA = 25°C
TA = 25°C
0.045
0.040
0.035
0.030
TA = -40°C
0.025
TA = -40°C
0.020
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)
P-Channel RDS(ON) vs. Input Voltage
N-Channel RDS(ON) vs. Input Voltage
800
700
600
500
400
300
200
700
85°C
600
500
400
300
200
100
85°C
25°C
25°C
-40°C
-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)
Load Transient Response
(1mA to 300mA; VIN = 3.6V; VOUT = 1.8V;
Load Transient Response
(300mA to 400mA; VIN = 3.6V;
VOUT = 1.8V; C1 = 4.7μF)
C1 = 10μF; CFF = 100pF)
2.0
1.9
1.8
1.7
1.90
1.85
1.80
1.75
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)
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PRODUCT DATASHEET
AAT1140
SwitchRegTM
FastTransient 600mA Step-Down Converter
Typical Characteristics
Load Transient Response
(300mA to 400mA; VIN = 3.6V;
VOUT = 1.8V; C1 = 10μF)
1.90
Load Transient Response
(300mA to 400mA; VIN = 3.6V; VOUT = 1.8V;
C1 = 10µF; CFF = 100pF)
1.850
1.85
1.825
VO
VO
1.80
1.800
1.75
1.775
IO
IO
400mA
300mA
400mA
300mA
0.4
0.3
0.2
0.1
0.4
0.3
IL
IL
0.2
0.1
Time (50μs/div)
Time (50µs/div)
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 (25μs/div)
Time (10µs/div)
Output Ripple
(VIN = 3.6V; VOUT = 1.8V; Load = 3Ω)
0.08
0.07
0.06
0.05
0.04
0.03
0.02
0.01
0.00
1.40
1.20
1.00
0.80
0.60
0.40
0.20
0.00
Time (40ns/div)
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PRODUCT DATASHEET
AAT1140
SwitchRegTM
FastTransient 600mA Step-Down Converter
Functional Block Diagram
IN
OUT
See note
Err
Amp
.
DH
LX
Voltage
Logic
Reference
DL
INPUT
EN
GND
Note: For adjustable version, the internal feedback divider is omitted and the OUT pin is tied directly
to the internal error amplifier.
tional feed-forward capacitor can also be added to the
external feedback to provide improved transient response
(see Figure 1).
Functional Description
The AAT1140 is a high performance 600mA 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 recommend-
ed (see table of values).
At dropout, the converter duty cycle increases to 100%
and the output voltage tracks the input voltage minus
the RDS(ON) drop of the P-channel high-side MOSFET.
The input voltage range is 2.7V to 5.5V. The converter
efficiency has been optimized for all load conditions,
ranging from no load to 600mA.
The fixed output version requires only three external
power components (CIN, COUT, and L). The adjustable ver-
sion can be programmed with external feedback to any
voltage, ranging from 0.6V to the input voltage. An addi-
The internal error amplifier and compensation provides
excellent transient response, load, and line regulation.
Soft start eliminates any output voltage overshoot when
the enable or the input voltage is applied.
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PRODUCT DATASHEET
AAT1140
SwitchRegTM
FastTransient 600mA Step-Down Converter
SW
U1
AAT1140
L1
4.7μH
4
2
1
3
IN
LX
VIN
VOUT
C1
4.7μF
C3
100pF
C2
10μF
R1
442k
GND
EN
1
2
3
Enable
5
R2
221k
OUT
Figure 1: Enhanced Transient Response Schematic.
Control Loop
Current Limit and
Over-Temperature Protection
The AAT1140 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.
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.
The output of the voltage error amplifier programs the
current mode loop for the necessary peak switch current
to force a constant output voltage for all load and line
conditions. Internal loop compensation terminates the
transconductance voltage error amplifier output. For
fixed voltage versions, the error amplifier reference volt-
age 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 hys-
teresis. Once an over-temperature or over-current fault
condition is removed, the output voltage automatically
recovers.
Under-Voltage Lockout
Internal bias of all circuits is controlled via the IN input.
Under-voltage lockout (UVLO) guarantees sufficient VIN
bias and proper operation of all internal circuitry prior to
activation.
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 AAT1140 into a low-power,
non-switching state. The total input current during shut-
down is less than 1μA.
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PRODUCT DATASHEET
AAT1140
SwitchRegTM
FastTransient 600mA Step-Down Converter
Applications Information
Configuration
Output Voltage
Inductor
1V, 1.2V
1.5V, 1.8V
2.5V, 3.3V
0.6V to 3.3V
2.2μH
4.7μH
6.8μH
4.7μH
0.6V Adjustable With
External Feedback
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 adjustable and
low-voltage fixed versions of the AAT1140 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.
Fixed Output
Table 1: Inductor Values.
Input Capacitor
Select a 4.7μF to 10μF X7R or X5R ceramic capacitor for
the input. To estimate the required input capacitor size,
determine the acceptable input ripple level (VPP) and solve
for C. The calculated value varies with input voltage and
is a maximum when VIN is double the output voltage.
0.75 VO 0.75 1.5V
=
A
m =
= 0.24
VO
VIN
VO
VIN
L
4.7μH
μsec
· 1 -
This is the internal slope compensation for the adjust-
able (0.6V) version or low-voltage fixed versions. When
externally programming the 0.6V version to 2.5V, the
calculated inductance is 7.5μH.
CIN =
VPP
IO
- ESR ·FS
VO
VIN
VO
VIN
1
· 1 -
=
for VIN = 2 · VO
4
0.75 VO
m
0.75 VO
μsec
A
L =
=
≈
3
VO
A
1
0.24A
CIN(MIN)
=
μsec
VPP
IO
- ESR · 4 · FS
μsec
A
= 3
2.5V = 7.5μH
Always examine the ceramic capacitor DC voltage coeffi-
cient characteristics when selecting the proper value. For
example, the capacitance of a 10μF, 6.3V, X5R ceramic
capacitor with 5.0V DC applied is actually about 6μF.
In this case, a standard 6.8μH value is selected.
For high-voltage fixed versions (≥2.5V), m = 0.48A/
μsec. Table 1 displays inductor values for the AAT1140
fixed and adjustable options.
The maximum input capacitor RMS current is:
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.
VO
VIN
VO
VIN
IRMS = IO ·
· 1 -
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
VO
VIN
1
2
· 1 -
=
D · (1 - D) = 0.52 =
for VIN = 2 · VO
The 4.7μH CDRH2D14 series inductor selected from
Sumida has a 135mΩ typical DCR and a 1A DC current
rating. At full load, the inductor DC loss is 48mW which
gives a 4.5% loss in efficiency for a 600mA, 1.8V output.
IO
IRMS(MAX)
=
2
VO
VIN
VO
VIN
·
1 -
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
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PRODUCT DATASHEET
AAT1140
SwitchRegTM
FastTransient 600mA Step-Down Converter
voltage ripple and the input capacitor RMS current ripple
are a maximum at 50% duty cycle.
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.
The input capacitor provides a low impedance loop for
the edges of pulsed current drawn by the AAT1140. Low
ESR/ESL X7R and X5R ceramic capacitors are ideal for
this function. To minimize stray inductance, the capacitor
should be placed as closely as possible to the IC. This
keeps the high frequency content of the input current
localized, minimizing EMI and input voltage ripple.
The proper placement of the input capacitor (C2) can be
seen in the evaluation board layout in Figure 2.
Figure 2: AAT1140 Evaluation Board
Component Side Layout.
Figure 3: Exploded View of Sample Layout.
Figure 4: AAT1140 Evaluation Board
Solder Side Layout.
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1140.2007.12.1.1
PRODUCT DATASHEET
AAT1140
SwitchRegTM
FastTransient 600mA Step-Down Converter
Since the inductance of a short PCB trace feeding the
input voltage is significantly lower than the power leads
from the bench power supply, most applications do not
exhibit this problem.
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
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.
For applications requiring an adjustable output voltage,
the 0.6V version can be externally programmed. Resistors
R1 and R2 of Figure 5 program the output to regulate at
a voltage higher than 0.6V. To limit the bias current
required for the external feedback resistor string while
maintaining good noise immunity, the minimum sug-
gested 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 sensi-
tive to external noise and interference. Table 2 summa-
rizes the resistor values for various output voltages with
R2 set to either 59kΩ for good noise immunity or 316kΩ
for reduced no load input current.
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
dominated by the capacitance of the ceramic output
capacitor. During a step increase in load current, the
ceramic output capacitor alone supplies the load current
until the loop responds. Within two or three switching
cycles, the loop responds and the inductor current
increases to match the load current demand. The rela-
tionship of the output voltage droop during the three
switching cycles to the output capacitance can be esti-
mated by:
VOUT
VREF
1.5V
0.6V
R1 =
-1 · R2 =
- 1 · 59kΩ = 88.5kΩ
The adjustable version of the AAT1140, combined with
an external feedforward capacitor (C3 in Figure 1),
delivers enhanced transient response for extreme pulsed
load applications. The addition of the feedforward capac-
itor typically requires a larger output capacitor C1 for
stability.
High Noise
Immunity
R2 = 59kΩ
Low Input Current
(Without Load)
R2 = 316kΩ
3 · ΔILOAD
COUT
=
VDROOP · FS
VOUT (V)
R1 (kΩ)
R1 (kΩ)
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.
0.8
0.9
1.0
1.1
1.2
1.3
1.4
1.5
1.8
2.0
2.5
3.0
3.3
19.6
29.4
39.2
49.9
59.0
68.1
78.7
88.7
88.7
137
105
158
210
267
316
365
422
475
634
732
1000
1270
1430
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.
187
237
267
The maximum output capacitor RMS ripple current is
given by:
Table 2: Adjustable Resistor Values For Use With
0.6V Step-Down Converter.
1
V
OUT · (VIN(MAX) - VOUT
)
IRMS(MAX)
=
·
L · F · VIN(MAX)
2 · 3
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1140.2007.12.1.1
13
PRODUCT DATASHEET
AAT1140
SwitchRegTM
FastTransient 600mA Step-Down Converter
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.
Thermal Calculations
There are three types of losses associated with the
AAT1140 step-down converter: switching losses, con-
duction 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 con-
duction mode (CCM), a simplified form of the LDO losses
is given by:
Given the total losses, the maximum junction tempera-
ture can be derived from the θJA for the SOT23-5 pack-
age which is 150°C/W.
TJ(MAX) = PTOTAL · ΘJA + TAMB
Output Dropout
At dropout, the duty cycle of AAT1140 switching is
100%. The minimum dropout voltage is determined by
RDS(ON)H and the inductor copper loss resistor. AAT1140
has 0.53Ω RDS(ON)H. The inductor copper loss resistor var-
ies with different inductor values and manufacturer. The
safe dropout voltage is 0.5V for a 600mA load.
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 con-
verter switching losses.
For example, when load current is 600mA, the voltage
dropped across RDS(ON)H is 0.32V; if the inductor copper
loss resistor is 135mΩ, the voltage drop across the
inductor is 0.08V. So the total voltage drop is 0.4V.
Considering manufacturer’s tolerances, the inductor cop-
per loss resistor and RDS(ON)H will vary from part to part,
a 0.5V dropout window is safe.
For the condition where the step-down converter is in
dropout at 100% duty cycle, the total device dissipation
reduces to:
PTOTAL = IO2 · RDSON(HS) + IQ · VIN
SW
U1
AAT1140
L1
4.7μH
4
3
IN
LX
VIN
VOUT = 1.8V
C1
4.7μF
C3
100pF
C2
10μF
R1
2
1
GND
EN
442k
1
2
3
Enable
5
R2
221k
OUT
Figure 5: AAT1140 Adjustable Evaluation Board Schematic.
w w w . a n a l o g i c t e c h . c o m
14
1140.2007.12.1.1
PRODUCT DATASHEET
AAT1140
SwitchRegTM
FastTransient 600mA Step-Down Converter
Efficiency
Layout
Besides the AAT1140 device losses including switching
losses, conduction losses, and quiescent current losses,
the inductor copper loss also affects the efficiency of the
buck converter. To the buck converter, the average cur-
rent of the inductor is equal to output current IO. So the
loss in the inductor is:
The suggested 2-layer PCB layout for the AAT1140 is
shown in Figures 2, 3 and 4. The following guide lines
should be used to help ensure a proper layout.
1. The power traces (GND, LX, VIN) should be kept
short, direct, and wide to allow large current flow.
Place sufficient multiple-layer pads when needed to
change the trace layer.
2. The input capacitor (C1) should connect as closely
as possible to IN and GND.
PLOSS_L = IO2 · RL
3. The output capacitor C2 and L1 should be connected
as closely as possible. The connection of L1 to the LX
pin should be as short as possible and there should
not be any signal lines under the inductor.
4. The feedback trace or OUT pin should be separate
from any power trace and connect as closely as pos-
sible to the load point. Sensing along a high-current
load trace will degrade DC load regulation. If exter-
nal feedback resistors are used, they should be
placed as closely as possible to the OUT pin to min-
imize the length of the high impedance feedback
trace.
Table 4 shows some recommended inductors. A larger
size inductor usually has smaller DCR. As a example: if
selecting CDRH2D14 4.7μH for 1.8V output, the PLoss_L is
48.6mW when output current is 600mA, so the inductor
loses 4.5% power; if selecting CDRH3D23 4.7μH, the
PLoss_L should be 19.8mW, and the inductor losing power
ratio is only 1.8%. The inductor size and the buck con-
verter efficiency is always a trade-off in the real applica-
tion.
5. The resistance of the trace from the load return to
GND should be kept to a minimum. This will help to
minimize any error in DC regulation due to differ-
ences in the potential of the internal signal ground
and the power ground.
w w w . a n a l o g i c t e c h . c o m
1140.2007.12.1.1
15
PRODUCT DATASHEET
AAT1140
SwitchRegTM
FastTransient 600mA Step-Down Converter
Step-Down Converter Design Example
Specifications
VO = 1.8V @ 600mA (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
μsec
A
μsec
A
(use 4.7μH; see Table 1)
1.8V = 5.4μH
L1 = 3
VO2 = 3
For Sumida inductor CDRH3D16, 4.7μH, DCR = 105mΩ.
VO
L1 F
VO
VIN
1.8
V
1.8V
4.2V
ΔIL1 =
1 -
=
1 -
= 156mA
4.7μH 1.4MHz
ΔIL1
2
IPKL1 = IO +
= 0.6A + 0.068A = 0.668A
2
PL1 = IO DCR = 0.6A2 105mΩ = 38mW
1.8V Output Capacitor
VDROOP = 0.1V
3 · ΔILOAD
3 · 0.3A
COUT
=
=
= 6.4μF; use 10µF
VDROOP · FS
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
Input Capacitor
Input Ripple VPP = 25mV
1
1
CIN =
=
= 4.87µF; use 4.7µF
VPP
IO
25mV
0.6A
- ESR · 4 · FS
- 5mΩ · 4 · 1.4MHz
IO
2
IRMS
=
= 0.3Arms
P = esr · IRMS2 = 5mΩ · (0.3A)2 = 0.45mW
w w w . a n a l o g i c t e c h . c o m
16
1140.2007.12.1.1
PRODUCT DATASHEET
AAT1140
SwitchRegTM
FastTransient 600mA Step-Down Converter
AAT1140 Losses
IO2 · (RDSON(HS) · VO + RDSON(LS) · [VIN -VO])
PTOTAL
=
VIN
+ (tsw · F · IO + IQ) · VIN
0.62 · (0.725Ω · 1.8V + 0.7Ω · [4.2V - 1.8V])
=
4.2V
+ (5ns · 1.4MHz · 0.6A + 70μA) · 4.2V = 118mW
TJ(MAX) = TAMB + ΘJA · PLOSS = 85°C + (150°C/W) · 118mW = 102.7°C
w w w . a n a l o g i c t e c h . c o m
1140.2007.12.1.1
17
PRODUCT DATASHEET
AAT1140
SwitchRegTM
FastTransient 600mA Step-Down Converter
Adjustable Version
(0.6V device)
VOUT (V)
R2 = 59kΩ
R1 (kΩ)
R2 = 316kΩ1
R1 (kΩ)
L1 (μH)
0.8
0.9
1.0
1.1
1.2
1.3
1.4
1.5
1.8
1.85
2.0
2.5
3.3
19.6
29.4
39.2
49.9
59.0
68.1
78.7
88.7
118
124
137
187
267
105
158
210
267
316
365
422
475
634
732
1000
1270
1430
2.2
2.2
2.2
2.2
2.2
2.2
4.7
4.7
4.7
4.7
6.8
6.8
6.8
Fixed Version
VOUT (V)
R2 Not Used
R1 (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
Sumida
Sumida
CDRH3D16-2R2
CDRH3D16-4R7
CDRH3D16-6R8
2.2
4.7
6.8
2.2
4.7
6.8
4.7
4.7
4.7
4.7
6.8
4.7
1.20
0.90
0.73
1.5
0.072
0.105
0.170
75
135
170
0.80
0.20
0.27
0.122
0.175
0.122
3.8x3.8x1.8
3.8x3.8x1.8
3.8x3.8x1.8
Shielded
Shielded
Shielded
Sumida
CDRH2D14
1.0
3.2x3.2x1.55
Shielded
0.85
0.40
0.45
0.80
0.98
0.82
1.30
Murata
Murata
Coilcraft
Coiltronics
Coiltronics
Coiltronics
LQH2MCN4R7M02
LQH32CN4R7M23
LPO3310-472
SD3118-4R7
2.0x1.6x0.95
2.5x3.2x2.0
3.2x3.2x1.0
3.1x3.1x1.85
3.1x3.1x1.85
5.7x4.4x1.0
Non-Shielded
Non-Shielded
1mm
Shielded
Shielded
SD3118-6R8
SDRC10-4R7
1mm Shielded
Table 4: Typical Surface Mount Inductors.
Manufacturer
Part Number
Value
Voltage
Temp. Co.
Case
Murata
Murata
Murata
GRM219R61A475KE19
GRM21BR60J106KE19
GRM21BR60J226ME39
4.7μF
10μF
22μF
10V
6.3V
6.3V
X5R
X5R
X5R
0805
0805
0805
Table 5: Surface Mount Capacitors.
1. For reduced quiescent current, R2 = 316kΩ.
w w w . a n a l o g i c t e c h . c o m
18
1140.2007.12.1.1
PRODUCT DATASHEET
AAT1140
SwitchRegTM
FastTransient 600mA Step-Down Converter
Ordering Information
Output Voltage1
Package
Marking2
Part Number (Tape and Reel)3
Adj 0.6 to VIN
1.8
SOT23-5
SOT23-5
ZKXYY
ZXXYY
AAT1140IGV-0.6-T1
AAT1140IGV-1.8-T1
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.
Package Information
SOT23-5
2.85 ± 0.15
1.90 BSC
0.95
BSC
0.60 REF
0.15 ± 0.07
GAUGE PLANE
0.075
± 0.075
0.45 ± 0.15
0.10 BSC
0.60 REF
10°
±
5°
0.40 ± 0.10
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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Phone (408) 737-4600
Fax (408) 737-4611
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AnalogicTech cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in an AnalogicTech product. No circuit patent licenses, copyrights, mask work rights, or other intellectual
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conditions of sale, AnalogicTech assumes no liability whatsoever, and AnalogicTech disclaims any express or implied warranty relating to the sale and/or use of AnalogicTech products including liability or warranties
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w w w . a n a l o g i c t e c h . c o m
1140.2007.12.1.1
19
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