ANALOGICTECH AAT2146

PRODUCT DATASHEET
AAT2146
SwitchRegTM
Low Noise, Fast Transient 600mA Step-Down Converter
General Description
Features
The AAT2146 SwitchReg is a 2MHz 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. The AAT2146 incorporates a unique low
noise architecture which reduces ripple and spectral
noise.
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The AAT2146 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 37μA no load quiescent
current. The 2MHz switching frequency minimizes the
size of external components while keeping switching
losses low.
The AAT2146 is designed to maintain high efficiency
throughout the operating range, which is critical for portable applications.
The AAT2146 is available in the Pb-free, space-saving
2.0x2.1mm SC70JW-8 package and is rated over the
-40°C to +85°C temperature range.
VIN Range: 2.7V to 5.5V
Low Noise Light Load Mode
Low Ripple PWM Mode
VOUT Fixed or Adjustable from 0.6V to VIN
37μA No Load Quiescent Current
Up to 98% Efficiency
600mA Max Output Current
2MHz Switching Frequency
150μs 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
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Cellular Phones
Digital Cameras
Handheld Instruments
Microprocessor / DSP Core / IO Power
PDAs and Handheld Computers
USB Devices
Typical Application (Fixed Output Voltage)
U1
AAT2146
VIN
C2
2.2µF
2146.2008.04.1.1
VIN
LX
EN
OUT
AGND
PGND
PGND
PGND
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L1
4.7µH
VO
C1
4.7µF
1
PRODUCT DATASHEET
AAT2146
SwitchRegTM
Low Noise, Fast Transient 600mA Step-Down Converter
Pin Descriptions
Pin #
Symbol
1
EN
2
OUT
3
VIN
4
LX
5
6, 7, 8
AGND
PGND
Function
Enable pin.
Feedback input pin. This pin is connected either directly to the converter output or to an external
resistive divider for an adjustable output.
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.
Non-power signal ground pin.
Main power ground return pins. Connect to the output and input capacitor return.
Pin Configuration
SC70JW-8
(Top View)
2
EN
1
8
PGND
OUT
2
7
PGND
VIN
3
6
PGND
LX
4
5
AGND
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2146.2008.04.1.1
PRODUCT DATASHEET
AAT2146
SwitchRegTM
Low Noise, Fast Transient 600mA Step-Down Converter
Absolute Maximum Ratings1
Symbol
VIN
VLX
VOUT
VEN
TJ
TLEAD
Description
Input Voltage GND
LX to GND
OUT to GND
EN to GND
Operating Junction Temperature Range
Maximum Soldering Temperature (at leads, 10 sec)
Value
Units
6.0
-0.3 to VIN + 0.3
-0.3 to VIN + 0.3
-0.3 to VIN + 0.3
-40 to 150
300
V
V
V
V
°C
°C
Value
Units
625
160
mW
°C/W
Thermal Information
Symbol
PD
θJA
Description
Maximum Power Dissipation
Thermal Resistance2
2, 3
1. Stresses above those listed in Absolute Maximum Ratings may cause permanent damage to the device. Functional operation at conditions other than the operating conditions
specified is not implied. Only one Absolute Maximum Rating should be applied at any one time.
2. Mounted on an FR4 board.
3. Derate 6.25mW/°C above 25°C.
2146.2008.04.1.1
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3
PRODUCT DATASHEET
AAT2146
SwitchRegTM
Low Noise, Fast Transient 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
Conditions
Min
Typ
Max
Units
5.5
2.7
V
V
mV
V
%
V
μA
μA
mA
Ω
Ω
%/V
mV
μA
kΩ
μs
MHz
°C
°C
Step-Down Converter
VIN
VUVLO
VOUT
VOUT
IQ
ISHDN
ILIM
RDS(ON)H
RDS(ON)L
ΔVLinereg
VOUT
IOUT
ROUT
TS
FOSC
TSD
THYS
EN
VEN(L)
VEN(H)
IEN
2.7
Input Voltage
UVLO Threshold
Output Voltage Tolerance
Output Voltage Range
Quiescent Current
Shutdown Current
P-Channel Current Limit
High Side Switch On Resistance
Low Side Switch On Resistance
Line Regulation
Out Threshold Voltage Accuracy
Out Leakage Current
Out Impedance
Start-Up Time
Oscillator Frequency
Over-Temperature Shutdown Threshold
Over-Temperature Shutdown Hysteresis
Enable Threshold Low
Enable Threshold High
Input Low Current
VIN Rising
Hysteresis
VIN Falling
IOUT = 0 to 600mA, VIN = 2.7V to 5.5V
100
1.8
-3.0
0.6
No Load, 0.6V Adjustable Version
EN = AGND = PGND
+3.0
VIN
70
1.0
37
800
VIN = 2.7V to 5.5V; IOUT = 600mA
0.6V Output, No Load; TA = 25°C
0.6V Output
>0.6V Output
From Enable to Output Regulation
TA = 25°C
591
0.35
0.30
0.1
600
609
0.2
250
0.9
150
2.0
140
15
2.6
0.6
VIN = VOUT = 5.5V
1.4
-1.0
1.0
V
V
μA
1. The AAT2146 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.
4
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2146.2008.04.1.1
PRODUCT DATASHEET
AAT2146
SwitchRegTM
Low Noise, Fast Transient 600mA Step-Down Converter
Typical Characteristics
Efficiency vs. Output Current
Load Regulation
(VOUT = 1.8V)
(VOUT = 1.8V)
100
1
0.8
Output Error (%)
Efficiency (%)
90
80
70
60
50
VIN = 2.7V
VIN = 3.6V
VIN = 4.2V
40
30
0.1
1
10
100
0.6
0.4
0.2
0
-0.2
-0.4
VIN = 2.7V
VIN = 3.6V
VIN = 4.2V
-0.6
-0.8
-1
1000
0.1
1
10
Output Current (mA)
Efficiency vs. Output Current
Load Regulation
(VOUT = 2.5V)
(VOUT = 2.5V)
1
100
Output Error (%)
Efficiency (%)
80
70
60
VIN = 3V
VIN = 3.6V
VIN = 4.2V
VIN = 5V
50
40
1
10
100
0.6
0.4
0.2
0
-0.2
-0.4
-0.6
-0.8
-1
0.1
1000
1
10
Output Current (mA)
Efficiency vs. Output Current
Load Regulation
(VOUT = 3.3V)
(VOUT = 3.3V)
1
Output Error (%)
80
70
60
50
VIN = 3.6V
VIN = 4.2V
VIN = 5V
40
1
10
100
0.6
0.4
0.2
0
-0.2
-0.4
-0.6
-0.8
1000
-1
0.1
Output Current (mA)
2146.2008.04.1.1
1000
VIN = 3.6V
VIN = 4.2V
VIN = 5V
0.8
90
Efficiency (%)
100
Output Current (mA)
100
30
0.1
1000
VIN = 3V
VIN = 3.6V
VIN = 4.2V
VIN = 5V
0.8
90
30
0.1
100
Output Current (mA)
1
10
100
1000
Output Current (mA)
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5
PRODUCT DATASHEET
AAT2146
SwitchRegTM
Low Noise, Fast Transient 600mA Step-Down Converter
Typical Characteristics
Line Regulation
Switching Frequency vs. Temperature
(VOUT = 1.8V)
1mA
400mA
600mA
0.4
0.3
Accuracy (%)
Switching Frequency (MHz)
(VOUT = 1.8V; IOUT = 1A)
0.5
0.2
0.1
0
-0.1
-0.2
-0.3
-0.4
-0.5
2.7
3.1
3.5
3.9
4.3
4.7
5.1
5.5
2.08
2.06
2.04
2.02
2
1.98
1.96
1.94
1.92
-40
-15
10
Input Voltage (V)
35
60
85
Input Voltage (V)
Frequency Variation vs. Input Voltage
Output Voltage Error vs. Temperature
(VIN = 3.6V; VO = 1.8V, IOUT = 400mA)
2.0
Output Voltage Error (%)
Frequency Variation (%)
4
3
2
1
0
-1
-2
VOUT = 1.8V
VOUT = 3V
-3
-4
2.7
3.1
3.5
3.9
4.3
4.7
5.1
1.0
0.0
-1.0
-2.0
-40
5.5
-20
0
550
60
55
45
40
35
30
25
85°C
25C
-40°C
20
15
3.1
3.5
3.9
4.3
80
100
120°C
100°C
85°C
25°C
500
50
RDS(ON) (mΩ
Ω)
Supply Current (µA)
60
P-Channel RDS(ON) vs. Input Voltage
No Load Quiescent Current vs. Input Voltage
4.7
5.1
450
400
350
300
250
5.5
200
2.5
3
3.5
4
4.5
5
5.5
6
Input Voltage (V)
Input Voltage (V)
6
40
Temperature (°C)
Input Voltage (V)
10
2.7
20
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2146.2008.04.1.1
PRODUCT DATASHEET
AAT2146
SwitchRegTM
Low Noise, Fast Transient 600mA Step-Down Converter
Typical Characteristics
N-Channel RDS(ON) vs. Input Voltage
Load Transient
(VIN = 3.6V; VOUT = 1.8V; COUT = 10µF; CFF = 100pF)
RDS(ON) (mΩ
Ω)
500
450
Output Voltage (top) (V)
120°C
100°C
85°C
25°C
400
350
300
250
200
2.5
1.8
1.7
1.5
3.5
4
4.5
5
5.5
1.3
300mA
1.2
1mA
6
Time (50µs/div)
Load Transient
Load Transient
(VIN = 3.6V; VOUT = 1.8V; COUT = 4.7µF; CFF = 0pF)
(VIN = 3.6V; VOUT = 1.8V; COUT = 10µF; CFF = 0pF)
1.7
400mA
300mA
1.65
1.6
400mA
1.55
300mA
1.5
Output Voltage (top) (V)
1.75
1.85
1.8
1.75
1.7
400mA
300mA
1.65
1.6
1.55
400mA
300mA
1.5
Time (50µs/div)
Output and Inductor Current
(100mA/div)
1.8
Output and Inductor Current
(100mA/div)
1.85
Output Voltage (top) (V)
1mA
1.4
Input Voltage (V)
Time (50µs/div)
Line Transient
(VOUT = 1.8V; VIN = 3.6V to 4.2V; IOUT = 400mA; CFF = 0pF)
1.7
1.65
400mA
300mA
1.6
1.55
300mA
400mA
1.5
4.8
1.92
4.2
1.9
3.6
1.88
3
1.86
2.4
1.84
1.8
1.82
1.2
1.8
0.6
1.78
0
1.76
Time (50µs/div)
2146.2008.04.1.1
Output Voltage (bottom) (V)
1.75
Output and Inductor Current
(100mA/div)
1.8
Input Voltage (top) (V)
Load Transient
(VIN = 3.6V; VOUT = 1.8V; COUT = 10µF; CFF = 100pF)
1.85
Output Voltage (top) (V)
300mA
1.6
1.1
3
Output and Inductor Current
(100mA/div)
1.9
550
Time (50µs/div)
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7
PRODUCT DATASHEET
AAT2146
SwitchRegTM
Low Noise, Fast Transient 600mA Step-Down Converter
Output Ripple
(VOUT = 1.8V; VIN = 3.6V; IOUT = 400mA; CFF = 0pF)
0.02
0.35
0.01
0.3
-0.01
0.2
-0.02
0.15
-0.03
0.1
-0.04
0.05
-0.05
0
-0.06
-0.05
0.02
1.4
0.01
1.2
0
1
-0.01
0.8
-0.02
0.6
-0.03
0.4
-0.04
0.2
-0.05
0
-0.06
-0.2
Time (10µs/div)
Inductor Current
(bottom) (A)
0.25
0
Output Voltage (top) (V)
Output Ripple
(VOUT = 1.8V; VIN = 3.6V; IOUT = 1mA; CFF = 0pF)
Inductor Current
(bottom) (A)
Output Voltage (top) (V)
Typical Characteristics
Time (10µs/div)
Soft Start
4
3
2
1
0
0.5
0
-0.5
Input Current (bottom) (A)
Enable Voltage (top) (V)
Output Voltage (middle) (V)
(VIN = 3.6V; VOUT = 1.8V; IOUT = 400mA)
Time (100µs/div)
8
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2146.2008.04.1.1
PRODUCT DATASHEET
AAT2146
SwitchRegTM
Low Noise, Fast Transient 600mA Step-Down Converter
Functional Block Diagram
VIN
OUT
See note
Err
Amp
.
DH
Voltage
Reference
EN
LX
Logic
DL
INPUT
PGND
AGND
Note: For adjustable version, the internal feedback divider is omitted and the OUT pin is tied directly
to the internal error amplifier.
Functional Description
The AAT2146 is a high performance 600mA 2MHz monolithic step-down converter. It has been designed with the
goal of minimizing external component size and optimizing efficiency over the complete load range, and produces reduced ripple and spectral noise. 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).
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 input voltage. An addi-
2146.2008.04.1.1
tional feed-forward capacitor can also be added to the
external feedback to provide improved transient response
(see Figure 1).
At dropout, the converter duty cycle increases to 100%
and the output voltage tracks the input voltage minus
the RDSON 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 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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9
PRODUCT DATASHEET
AAT2146
SwitchRegTM
Low Noise, Fast Transient 600mA Step-Down Converter
1
2
3
VIN
Enable
U1
AAT2146
CFF
VOUT
C1
10µF
1
R1
2
118k
3
L1
4.7µH
4
EN
PGND
OUT
PGND
VIN
PGND
LX
AGND
8
7
6
5
C2
4.7µF
R2
59k
GND
GND2
LX
U1 AAT2146 SC70JW-8
L1 CDRH3D16-4R7
C1 10µF 10V 0805 X5R
C2 4.7µF 10V 0805 X5R
CFF 100pF 0402 X5R
Figure 1: Enhanced Transient Response Schematic.
Control Loop
The AAT2146 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 protection. 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 compensation 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.
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 AAT2146 into a low-power,
non-switching state. The total input current during shutdown is less than 1μA.
10
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 internal circuitry prior to
activation.
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2146.2008.04.1.1
PRODUCT DATASHEET
AAT2146
SwitchRegTM
Low Noise, Fast Transient 600mA Step-Down Converter
Applications Information
Input Capacitor
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 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 AAT2146 is 0.24A/μs.
This equates to a slope compensation that is 75% of the
inductor current down slope for a 1.5V output and 4.7μH
inductor.
m=
CIN =
0.75 ⋅ VO
µs
0.75 ⋅ VO
≈ 3 A ⋅ VO
=
m
A
0.24A µs
=3
µs
⋅ 2.5V = 7.5µH
A
For high-voltage fixed versions (≥2.5V), m = 0.48A/μs.
Table 1 displays inductor values for the AAT2146 fixed
and adjustable options.
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 losses due to a high DCR.
Always consider the losses associated with the DCR and
its effect on the total converter efficiency when selecting
an inductor.
The 4.7μH CDRH3D16 series inductor selected from
Sumida has a 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.
2146.2008.04.1.1
CIN(MIN) =
⎛ VPP
⎞
- ESR · FS
⎝ IO
⎠
1
⎛ VPP
⎞
- ESR · 4 · FS
⎝ IO
⎠
Always examine the ceramic capacitor DC voltage coefficient 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.
Configuration
In this case, a standard 6.8μH value is selected.
V ⎞
VO ⎛
· 1- O
VIN ⎝
VIN ⎠
VO ⎛
V ⎞
1
· 1 - O = for VIN = 2 · VO
VIN ⎝
VIN ⎠
4
0.75 ⋅ VO 0.75 ⋅ 1.5V
A
=
= 0.24
L
4.7µH
µs
This is the internal slope compensation for the adjustable (0.6V) version or low-voltage fixed versions. When
externally programming the 0.6V version to 2.5V, the
calculated inductance is 7.5μH.
L=
Select a 2.2μF 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.
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
Fixed Output
Table 1: Inductor Values.
The maximum input capacitor RMS current is:
IRMS = IO ·
VO ⎛
V ⎞
· 1- O
VIN ⎝
VIN ⎠
The input capacitor RMS ripple current varies with the
input and output voltage and will always be less than or
equal to half of the total DC load current.
VO ⎛
V ⎞
· 1- O =
VIN ⎝
VIN ⎠
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D · (1 - D) =
0.52 =
1
2
11
PRODUCT DATASHEET
AAT2146
SwitchRegTM
Low Noise, Fast Transient 600mA Step-Down Converter
Output Capacitor
for VIN = 2 · VO
I
IRMS(MAX) = O
2
VO
⎛
V ⎞
· 1- O
The term V ⎝ V ⎠ 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.
IN
IN
The input capacitor provides a low impedance loop for
the edges of pulsed current drawn by the AAT2146. 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 output capacitor limits the output ripple and provides 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 characteristics 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 relationship of the output voltage droop during the three
switching cycles to the output capacitance can be estimated by:
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 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 during load transients. Errors in the loop phase and gain
measurements 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.
12
COUT =
3 · ΔILOAD
VDROOP · FS
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:
IRMS(MAX) =
1
VOUT · (VIN(MAX) - VOUT)
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.
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2146.2008.04.1.1
PRODUCT DATASHEET
AAT2146
SwitchRegTM
Low Noise, Fast Transient 600mA Step-Down Converter
Figure 2: AAT2146 Evaluation Board
Component Side Layout.
Figure 3: Exploded View of AAT2146
Evaluation Board Component Side Layout.
Figure 4: AAT2146 Evaluation Board
Solder Side Layout.
Adjustable Output Resistor Selection
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 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
2146.2008.04.1.1
R2 set to either 59kΩ for good noise immunity or 221kΩ
for reduced no load input current.
⎛ VOUT ⎞
⎛ 1.5V ⎞
R1 = V
-1 · R2 = 0.6V - 1 · 59kΩ = 88.5kΩ
⎝ REF ⎠
⎝
⎠
The adjustable version of the AAT2146, combined 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 larger output capacitor C1 for
stability.
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13
PRODUCT DATASHEET
AAT2146
SwitchRegTM
Low Noise, Fast Transient 600mA Step-Down Converter
R2 = 59kΩ
R2 = 221kΩ
VOUT (V)
R1 (kΩ)
R1
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
75K
113K
150K
187K
221K
261K
301K
332K
442K
464K
523K
715K
1.00M
switching devices. At full load, assuming continuous conduction mode (CCM), a simplified form of the losses is
given by:
PTOTAL =
IO2 · (RDSON(HS) · VO + RDSON(LS) · [VIN - VO])
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.
For the condition where the step-down converter is in
dropout at 100% duty cycle, the total device dissipation
reduces to:
Table 2: Adjustable Resistor Values For Use With
0.6V Step-Down Converter.
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.
Thermal Calculations
There are three types of losses associated with the
AAT2146 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
Given the total losses, the maximum junction temperature can be derived from the θJA for the SC70JW-8 package which is 160°C/W.
TJ(MAX) = PTOTAL · ΘJA + TAMB
1
2
3
Enable
VIN
C4
100pF
U1
AAT2146
1
VOUT =1.8V
R1
2
L1 118k
4.7μH
3
4
C1
10μF
C3
n/a
R2
59k
EN
PGND
OUT
PGND
VIN
PGND
LX
AGND
8
7
6
5
C2
4.7μF
GND
LX
GND2
U1 AAT2146 SC70JW-8
L1 CDRH3D16-4R7
C2 4.7μF 10V 0805 X5R
C1 10μF 6.3V 0805 X5R
Figure 5: AAT2146 Adjustable Evaluation Board Schematic.
14
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2146.2008.04.1.1
PRODUCT DATASHEET
AAT2146
SwitchRegTM
Low Noise, Fast Transient 600mA Step-Down Converter
Layout
The suggested PCB layout for the AAT2146 is shown in
Figures 2, 3, and 4. The following guidelines should be
used to help ensure a proper layout.
1.
2.
3.
4.
The input capacitor (C2) should connect as closely as
possible to VIN (Pin 3) and PGND (Pins 6-8).
C1 and L1 should be connected as closely as possible. The connection of L1 to the LX pin should be as
short as possible.
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 feedback 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.
The resistance of the trace from the load return to
the PGND (Pins 6-8) should be kept to a minimum.
This will help to minimize any error in DC regulation
due to differences in the potential of the internal
signal ground and the power ground.
2146.2008.04.1.1
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.
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15
PRODUCT DATASHEET
AAT2146
SwitchRegTM
Low Noise, Fast Transient 600mA Step-Down Converter
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 = 2MHz
TAMB = 85°C
1.8V Output Inductor
L1 = 3
µs
µs
⋅ VO2 = 3
⋅ 1.8V = 5.4µH (use 4.7μH; see Table 1)
A
A
For Sumida inductor CDRH3D16, 4.7μH, DCR = 105mΩ.
ΔIL1 =
⎛
VO
V ⎞
1.8V
1.8V ⎞
⎛
⋅ 1- O =
⋅ ⎝1 = 109mA
L1 ⋅ FS ⎝
VIN ⎠ 4.7µH ⋅ 2.0MHz
4.2V ⎠
IPKL1 = IO +
ΔIL1
= 0.4A + 0.055A = 0.455A
2
PL1 = IO2 ⋅ DCR = 0.4A2 ⋅ 105mΩ = 17mW
1.8V Output Capacitor
VDROOP = 0.1V
COUT =
3 · ΔILOAD
3 · 0.3A
=
= 4.5µF; use 4.7µF
VDROOP · FS
0.1V · 2.0MHz
IRMS =
(VO) · (VIN(MAX) - VO)
1
1.8V · (4.2V - 1.8V)
·
= 32mArms
=
4.7µH
· 2.0MHz · 4.2V
L1
·
F
·
V
2· 3
2· 3
IN(MAX)
1
·
Pesr = esr · IRMS2 = 5mΩ · (32mA)2 = 6µW
Input Capacitor
Input Ripple VPP = 25mV
CIN =
IRMS =
⎛ VPP
⎝ IO
1
1
=
= 2.17µF; use 2.2µF
⎞
⎛ 25mV
⎞
- 5mΩ · 4 · 2.0MHz
- ESR · 4 · FS
⎠
⎝ 0.4A
⎠
IO
= 0.2Arms
2
P = esr · IRMS2 = 5mΩ · (0.2A)2 = 0.2mW
16
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2146.2008.04.1.1
PRODUCT DATASHEET
AAT2146
SwitchRegTM
Low Noise, Fast Transient 600mA Step-Down Converter
AAT2146 Losses
PTOTAL =
IO2 · (RDSON(HS) · VO + RDSON(LS) · [VIN -VO])
VIN
+ (tsw · F · IO + IQ) · VIN
=
0.42 · (0.725Ω · 1.8V + 0.7Ω · [4.2V - 1.8V])
4.2V
+ (5ns · 2.0MHz · 0.4A + 70µA) · 4.2V = 131mW
TJ(MAX) = TAMB + ΘJA · PLOSS = 85°C + (160°C/W) · 131mW = 105.9°C
2146.2008.04.1.1
www.analogictech.com
17
PRODUCT DATASHEET
AAT2146
SwitchRegTM
Low Noise, Fast Transient 600mA Step-Down Converter
Adjustable Version
(0.6V device)
VOUT (V)
R2 = 59kΩ
R1 (kΩ)
R2 = 221kΩ1
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
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
Fixed Version
VOUT (V)
R2, R4 Not Used
R1 (kΩ)
L1 (μH)
0.6-3.3V
0
4.7
Table 3: Evaluation Board Component Values.
Manufacturer
Sumida
Murata
Coilcraft
Coiltronics
TDK
Wurth
Part Number
Inductance (μH)
Max DC
Current (A)
DCR (Ω)
Size (mm)
LxWxH
Type
CDRH3D16-2R2
CDRH3D16-4R7
CDRH3D16-6R8
LQH2MCN4R7M02
LQH32CN4R7M23
LPO3310-472
SD3118-4R7
SD3118-6R8
SDRC10-4R7
VLS3015T-4R7MR99
VLS3015T-6R8MR86
744042006
2.2
4.7
6.8
4.7
4.7
4.7
4.7
6.8
4.7
4.7
6.8
6.8
1.20
0.90
0.73
0.40
0.45
0.80
0.98
0.82
1.30
0.99
0.86
1.25
0.072
0.105
0.170
0.80
0.20
0.27
0.122
0.175
0.122
0.136
0.176
0.100
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
3.0x3.0x1.5
3.0x3.0x1.5
4.8x4.8x1.8
Shielded
Shielded
Shielded
Non-Shielded
Non-Shielded
1mm
Shielded
Shielded
1mm Shielded
Shielded
Shielded
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 and R4 = 221kΩ.
18
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2146.2008.04.1.1
PRODUCT DATASHEET
AAT2146
SwitchRegTM
Low Noise, Fast Transient 600mA Step-Down Converter
Ordering Information
Output Voltage1
Package
Marking2
Part Number (Tape and Reel)3
Adj ≥ 0.6
SC70JW-8
YXXYY
AAT2146IJS-0.6-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/about/quality.aspx.
Package Information
SC70JW-8
2.20 ± 0.20
1.75 ± 0.10
0.50 BSC 0.50 BSC 0.50 BSC
0.225 ± 0.075
2.00 ± 0.20
0.100
7° ± 3°
0.45 ± 0.10
4° ± 4°
0.05 ± 0.05
0.15 ± 0.05
1.10 MAX
0.85 ± 0.15
0.048REF
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 typically held on part numbers listed in BOLD.
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3230 Scott Boulevard, Santa Clara, CA 95054
Phone (408) 737-4600
Fax (408) 737-4611
© Advanced Analogic Technologies, Inc.
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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