AAT2148 - Skyworks Solutions, Inc.

DATA SHEET
AAT2148
Low-Noise, Fast Transient 1A Step-Down Converter
General Description
Features
The AAT2148 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 AAT2148 incorporates a unique low
noise architecture which reduces ripple and spectral
noise.
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The AAT2148 is available in fixed voltage versions with
internal feedback and a programmable version with
external feedback resistors. It can deliver 1A 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 AAT2148 is designed to maintain high efficiency
throughout the operating range, which is critical for portable applications.
The AAT2148 is available in the Pb-free, thermally
enhanced 3x3mm QFN-16 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
1A 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
QFN33-16 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 (Adjustable Output Voltage)
VIN
2.7V - 5.5V
U1
AAT2148
C1
4.7μF
L1 4.7μH
12
VP
LX
13
11
VP
LX
14
10
VP
LX
15
9
VCC
FB
4
7
EN
PGND
2
3
PGND
PGND
1
R1
118k
VO
1.8V, 1A
C2
4.7μF
R2
59k
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202009A • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • May 28, 2012
1
DATA SHEET
AAT2148
Low-Noise, Fast Transient 1A Step-Down Converter
Pin Descriptions
Pin #
Symbol
1, 2, 3
PGND
4
FB
5, 6, 8, 16
9
10, 11, 12
N/C
VCC
VP
13, 14, 15
LX
EP
Function
Power ground pin
Feedback input. This pin is connected either directly to the converter output or to an external resistive divider for an adjustable output.
Not internally connected.
Bias supply. Supplies power for the internal circuitry. Connect to input power.
Input supply voltage for the converter power stage. Must be closely decoupled to GND.
Switching node. Connect the inductor to this pin. It is internally connected to the drain of both
high- and low-side MOSFETs.
Exposed paddle (bottom); connect to PGND directly beneath package.
Pin Configuration
QFN33-16
(Top View)
LX
LX
LX
N/C
13
14
15
16
PGND
PGND
PGND
FB
1
12
2
11
3
10
4
9
VP
VP
VP
VCC
8
7
6
5
N/C
EN
N/C
N/C
2
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DATA SHEET
AAT2148
Low-Noise, Fast Transient 1A 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
2
50
W
°C/W
Thermal Information
Symbol
PD
JA
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.
Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • sales@skyworksinc.com • www.skyworksinc.com
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3
DATA SHEET
AAT2148
Low-Noise, Fast Transient 1A 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
Step-Down Converter
VIN
Input Voltage
VUVLO
VOUT
VOUT
IQ
ISHDN
ILIM
RDS(ON)H
RDS(ON)L
VLinereg
VFB
IFB
TS
FOSC
TSD
THYS
EN
VEN(L)
VEN(H)
IEN
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
Feedback Threshold Voltage Accuracy
Feedback Leakage Current
Start-Up Time
Oscillator Frequency
Over-Temperature Shutdown Threshold
Over-Temperature Shutdown Hysteresis
Enable Threshold Low
Enable Threshold High
Input Low Current
Min
Typ
2.7
VIN Rising
Hysteresis
IOUT = 0mA to 1A, VIN = 2.7V to 5.5V
1.8
100
-3.0
0.6
No Load, 0.6V Adjustable Version
EN = AGND = PGND
37
1300
VIN = 2.7V to 5.5V; IOUT = 1A
0.6V Output, No Load; TA = 25°C
0.6V Output
From Enable to Output Regulation
TA = 25°C
591
0.9
1700
0.35
0.30
0.1
600
150
2.0
140
15
Max
Units
5.5
2.7
V
V
mV
%
V
μA
μA
mA


%/V
mV
μA
μs
MHz
°C
°C
+3.0
VIN
70
1.0
609
1
2.6
0.6
VIN = VOUT = 5.5V
1.4
-1.0
1.0
V
V
μA
1. The AAT2148 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
Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • sales@skyworksinc.com • www.skyworksinc.com
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DATA SHEET
AAT2148
Low-Noise, Fast Transient 1A Step-Down Converter
Efficiency vs. Output Current
Load Regulation
(VOUT = 1.2V; L = 2.2μH; COUT = 10μF)
(VOUT = 1.2V; L = 2.2μH; COUT = 10μF)
100
1
90
0.8
80
0.6
70
60
50
40
VIN = 2.7V
VIN = 3.6V
VIN = 4.2V
VIN = 5.0V
30
20
10
0
0.1
1
10
100
Output Error (%)
Efficiency (%)
Typical Characteristics
VIN = 2.7V
VIN = 3.6V
VIN = 4.2V
VIN = 5.0V
0.4
0.2
0
-0.2
-0.4
-0.6
-0.8
-1
1000
0.1
1
Load Regulation
(VOUT = 1.8V; L = 4.7μH; COUT = 4.7μF)
1
0.8
80
0.6
70
60
50
40
VIN = 2.7V
VIN = 3.6V
VIN = 4.2V
VIN = 5.0V
30
20
10
0
1
10
100
VIN = 2.7V
VIN = 3.6V
VIN = 4.2V
VIN = 5.0V
0.4
0.2
0
-0.2
-0.4
-0.6
-0.8
-1
1000
0.1
1
Output Current (mA)
10
100
1000
Output Current (mA)
Efficiency vs. Output Current
Load Regulation
(VOUT = 3.3V; L = 6.8μH; COUT = 4.7μF)
(VOUT = 3.3V; L = 6.8μH; COUT = 4.7μF)
100
1
90
0.8
80
0.6
70
60
50
40
30
VIN = 3.6V
VIN = 4.2V
VIN = 5.0V
20
10
0
1
10
Output Current (mA)
100
1000
Output Error (%)
Efficiency (%)
1000
Efficiency vs. Output Current
90
0.1
100
(VOUT = 1.8V; L = 4.7μH; COUT = 4.7μF)
100
0.1
10
Output Current (mA)
Output Error (%)
Efficiency (%)
Output Current (mA)
VIN = 3.6V
VIN = 4.2V
VIN = 5.0V
0.4
0.2
0
-0.2
-0.4
-0.6
-0.8
-1
0.1
1
10
100
1000
Output Current (mA)
Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • sales@skyworksinc.com • www.skyworksinc.com
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DATA SHEET
AAT2148
Low-Noise, Fast Transient 1A Step-Down Converter
Typical Characteristics
Line Regulation
Switching Frequency vs. Temperature
1.00
IOUT = 1A
IOUT = 500mA
IOUT = 1mA
Output Error (%)
0.75
0.50
0.25
0.00
-0.25
-0.50
-0.75
-1.00
2.7
3.1
3.5
3.9
4.3
4.7
5.1
Switching Frequency (MHz)
(VOUT = 1.8V; COUT = 10μF; CFF = 100pF)
2.2
2.15
2.1
2.05
2
1.95
1.9
1.85
1.8
5.5
-40
-20
0
Input Voltage (V)
20
40
60
80
100
Temperature (°C)
Output Error vs. Temperature
Quiescent Current vs. Input Voltage
(IOUT = 500mA)
60
Quiescent Current (μA)
Output Error (%)
0.005
0.000
-0.005
-0.010
-0.015
-40
-20
0
20
40
60
80
55
50
45
40
30
2.7
100
VOUT = 3.3V
VOUT = 1.8V
VOUT = 1.2V
35
3.1
3.5
Temperature (°C)
3.9
4.3
4.7
5.1
Input Voltage (V)
P-Channel RDS(ON) vs. Input Voltage
Quiescent Current vs. Temperature
(VIN = 3.6V)
500
50
40
30
20
-40
VOUT = 3.3V
VOUT = 1.8V
VOUT = 1.2V
-20
0
20
40
Temperature (°C)
6
85°C
25°C
-40°C
450
60
80
100
RDS(ON)_P (mΩ)
Quiescent Current (μA)
60
400
350
300
250
200
2.7
3.3
3.9
4.5
Input Voltage (V)
Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • sales@skyworksinc.com • www.skyworksinc.com
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5.1
5.5
DATA SHEET
AAT2148
Low-Noise, Fast Transient 1A Step-Down Converter
Typical Characteristics
N-Channel RDS(ON) vs. Input Voltage
Load Transient
(VIN = 3.6V; VOUT = 1.8V; COUT = 10μF; CFF = 100pF)
450
85°C
25°C
-40°C
RDS(ON)_N (mΩ)
390
VOUT
(100mV/div)
330
1.8
270
500mA
IOUT
(0.25A/div)
210
0A
150
2.7
3.3
3.9
4.5
5.1
Input Voltage (V)
Time (40μs/div)
Load Transient
Load Transient
(VIN = 3.6V; VOUT = 1.8V; COUT = 10μF; CFF = 100pF)
(VIN = 3.6V; VOUT = 1.8V; COUT = 4.7μF; CFF = 0pF)
VOUT
(100mV/div)
VOUT
(200mV/div)
1.8
1.8
1A
IOUT
(0.5A/div)
300mA
500mA
IOUT
(0.25A/div)
0A
0A
Time (16μs/div)
VOUT
(200mV/div)
Time (40μs/div)
Load Transient
Line Transient
(VIN = 3.6V; VOUT = 1.8V; COUT = 4.7μF; CFF = 0pF)
(VOUT = 1.8V; VIN = 3.6V to 4.2V; IOUT = 500mA; CFF = 100pF)
VIN
(0.5V/div)
1.8
3.6
1A
IOUT
(0.5A/div)
VOUT
(20mV/div)
300mA
0A
Time (16μs/div)
1.8
Time (200μs/div)
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DATA SHEET
AAT2148
Low-Noise, Fast Transient 1A Step-Down Converter
Typical Characteristics
LX
(2V/div)
Output Ripple
Output Ripple
(VIN = 3.6V; VOUT = 1.2V; IOUT = 10mA;
L = 2.2μH; COUT = 10μF; CFF = 100pF)
(VIN = 3.6V; VOUT = 1.2V; IOUT = 1A;
L = 2.2μH; COUT = 10μF; CFF = 100pF)
LX
(2V/div)
0
0
ILX
(0.1A/div)
0A
ILX
(0.2A/div)
1A
VOUT
(20mV/div)
1.2
VOUT
(10mV/div)
1.2
Time (1μs/div)
LX
(2V/div)
ILX
(0.1A/div)
VOUT
(10mV/div)
Output Ripple
Soft Startup
(VIN = 3.6V; VOUT = 1.8V; IOUT = 1A;
L = 4.7μH; COUT = 10μF; CFF = 100pF)
(VIN = 3.6V; VOUT = 1.8V; IOUT = 1A)
VEN
(2V/div)
0
1A
1.8
Time (0.2μs/div)
8
Time (0.2μs/div)
0
IIN
(0.5A/div)
0
VOUT
(1V/div)
0
Time (100μs/div)
Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • sales@skyworksinc.com • www.skyworksinc.com
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DATA SHEET
AAT2148
Low-Noise, Fast Transient 1A Step-Down Converter
Functional Block Diagram
VCC
VP
0.6V REF
FB
OP. AMP
CMP
DH
LOGIC
1MΩ
LX
DL
Temp.
Sensing
OSC
PGND
EN
Functional Description
The AAT2148 is a high performance 1A 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 additional 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.
PGND
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 1A.
The internal error amplifier and compensation provides
excellent transient response, load, and line regulation.
Soft start eliminates any output voltage overshoot and
input inrush current when the enable or the input voltage is applied.
Control Loop
The AAT2148 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.
Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • sales@skyworksinc.com • www.skyworksinc.com
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9
DATA SHEET
AAT2148
Low-Noise, Fast Transient 1A Step-Down Converter
VIN
U1
AAT2148
VIN
12
VP
LX
13
C1
4.7μF
11
VP
LX
14
LX
15
FB
N/C
N/C
4
VIN
10
EN
9
VCC
7
EN
N/C
N/C
EP
PGND
5
JP1
6
0
3
1
2
3
VP
8
16
PGND
2
PGND
1
L1 4.7μH
R1
118k
VOUT
1.8V
C3
100pF
C2
10μF
R2
59k
Figure 1: Enhanced Transient Response Schematic.
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.
Current Limit and
Over-Temperature Protection
Soft Start / Enable
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.
Soft start limits the current surge seen at the input and
eliminates output voltage overshoot. When pulled low,
the enable input forces the AAT2148 into a low-power,
non-switching state. The total input current during shutdown is less than 1μA.
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.
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.
10
Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • sales@skyworksinc.com • www.skyworksinc.com
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DATA SHEET
AAT2148
Low-Noise, Fast Transient 1A Step-Down Converter
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 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 AAT2148 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=
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=
0.75 ⋅ VO
µs
0.75 ⋅ VO
≈ 3 A ⋅ VO
=
m
A
0.24A µs
µs
= 3 ⋅ 2.5V = 7.5µH
A
In this case, a standard 6.8μH value is selected.
For high-voltage fixed versions (2.5V), m = 0.48A/μs.
Table 1 displays inductor values for the AAT2148 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 SD3118 series inductor selected from Coilcraft
has a 162m typical DCR and a 1.31A saturation current. At full load, the inductor DC loss is 162mW which
gives a 9% loss in efficiency for a 1A, 1.8V output.
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.
CIN =
V ⎞
VO ⎛
· 1- O
VIN ⎝
VIN ⎠
⎛ VPP
⎞
- ESR · FS
⎝ IO
⎠
VO ⎛
V ⎞
1
· 1 - O = for VIN = 2 · VO
VIN ⎝
VIN ⎠
4
CIN(MIN) =
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.
Output Voltage
(V)
Inductor
(μH)
Output Capacitor
(μF)
1, 1.2
1.5, 1.8
2.5, 3.3
2.2
4.7
6.8
10
4.7
4.7
Table 1: Inductor and Output Capacitor 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 ⎠
D · (1 - D) =
0.52 =
Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • sales@skyworksinc.com • www.skyworksinc.com
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1
2
11
DATA SHEET
AAT2148
Low-Noise, Fast Transient 1A 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 AAT2148. 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 (C1) 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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DATA SHEET
AAT2148
Low-Noise, Fast Transient 1A Step-Down Converter
Figure 2: AAT2148 Evaluation Board
Top Layer.
12
C1
4.7µF
11
10
VIN
JP1
1
2
3
U1
AAT2148
VIN
VIN
Figure 3: AAT2148 Evaluation Board
Bottom Layer.
EN
LX
13
VP
LX
14
VP
LX
15
FB
N/C
N/C
4
VP
9
VCC
7
EN
N/C
N/C
EP
PGND
5
6
0
3
8
16
PGND
2
PGND
1
L1 4.7µH
R1
267k
VOUT
C3
100pF
C2
10µF
R2
59k
Figure 4: AAT2148 Evaluation Board Schematic.
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 4 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 59k. 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 volt-
ages with R2 set to either 59k for good noise immunity or 221k for reduced no load input current.
1.5V
VOUT
R3 = V
-1 · R4 = 0.6V - 1 · 59kΩ = 88.5kΩ
REF
The adjustable version of the AAT2148, combined with
an external feedforward capacitor (C3 in Figure 1),
delivers enhanced transient response for extreme pulsed
load applications. The addition of the feedforward capacitor typically requires a larger output capacitor C2 for
stability.
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13
DATA SHEET
AAT2148
Low-Noise, Fast Transient 1A Step-Down Converter
R2 = 59k
R2 = 221k
VOUT (V)
R1 (k)
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
Table 2: Adjustable Resistor Values For Use With
0.6V Step-Down Converter.
Thermal Calculations
There are three types of losses associated with the
AAT2148 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:
PTOTAL =
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 temperature can be derived from the JA for the QFN33-16 package which is 50°C/W.
TJ(MAX) = PTOTAL · ΘJA + TAMB
Layout
The suggested PCB layout for the AAT2148 is shown in
Figures 2, 3, and 4. The following guidelines should be
used to help ensure a proper layout.
1.
2.
3.
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.
14
For the condition where the step-down converter is in
dropout at 100% duty cycle, the total device dissipation
reduces to:
4.
The input capacitor (C1) should connect as closely as
possible to VIN (Pins 9-12) and PGND (Pins 1-3).
C2 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 FB pin (Pin 4) should be separate from any power trace and connect as closely as
possible to the load point. Sensing along a highcurrent load trace will degrade DC load regulation.
The external feedback resistors should be placed as
closely as possible to the FB pin (Pin 4) to minimize
the length of the high impedance feedback trace.
The resistance of the trace from the load return to
PGND (Pins 1-3) 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.
Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • sales@skyworksinc.com • www.skyworksinc.com
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DATA SHEET
AAT2148
Low-Noise, Fast Transient 1A Step-Down Converter
Step-Down Converter Design Example
Specifications
VO = 1.8V @ 1A (adjustable using 0.6V version), Pulsed Load ILOAD = 300mA
VIN = 2.7V to 4.2V (3.6V nominal)
FS = 2MHz
TAMB = 85°C
1.8V Output Inductor
L1 = 3
µs
µs
· VO1 = 3
· 1.8V = 5.4µH (use 4.7μH; see Table 1)
A
A
For Coilcraft inductor SD3118, 4.7μH, DCR = 162m.
ΔIL1 =
VO
V
1.8V
1.8V
· 1- O =
· 1L1 · FS
VIN
4.7µH · 2.0MHz
4.2V
IPKL1 = IO +
= 109mA
ΔIL1
= 1A + 0.055A = 1.055A
2
PL1 = IO2 · DCR = 1A2 · 162mΩ = 162mW
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 = 30mV
CIN =
IRMS =
1
=
VPP
- ESR · 4 · FS
IO
1
= 5μF; use 4.7μF
30mV
- 5mW · 4 · 2.0MHz
1A
IO
= 0.5Arms
2
P = ESR · IRMS2 = 5mΩ · (0.5A)2 = 1.25mW
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15
DATA SHEET
AAT2148
Low-Noise, Fast Transient 1A Step-Down Converter
AAT2148 Losses
PTOTAL =
IO2 · (RDSON(HS) · VO + RDSON(LS) · [VIN -VO])
VIN
+ (tsw · F · IO + IQ) · VIN
=
12 · (0.35Ω · 1.8V + 0.3Ω · [4.2V - 1.8V])
4.2V
+ (5ns · 2.0MHz · 1A + 70μA) · 4.2V = 363mW
TJ(MAX) = TAMB + ΘJA · PLOSS = 85°C + (50°C/W) · 363mW = 103.2°C
16
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DATA SHEET
AAT2148
Low-Noise, Fast Transient 1A Step-Down Converter
Adjustable Version
(0.6V device)
VOUT (V)
R2 = 59k
R1 (k)
R2 = 221k1
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
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 (k)
L1 (μH)
0.6-3.3V
0
4.7
Table 3: Evaluation Board Component Values.
Manufacturer
Sumida
Murata
Coilcraft
Part Number
Inductance (μH)
Saturation
Current (A)
DCR ()
Size (mm)
LxWxH
Type
CDRH3D16/HP-2R2
CDRH5D16-4R7
CDRH5D16-6R8
LQH3NPN2R2NG0
LQH32PN2R2NN0
LQH32PN4R7NN0
SD3118-2R2-R
SD3118-4R7-R
SD3118-6R8-R
2.2
4.7
6.8
2.2
2.2
4.7
2.2
4.7
6.8
1.75
2.15
1.8
1.25
1.55
1.0
2.0
1.31
1.12
47
51.3
64.7
73
76
180
74
162
232
4.0x4.0x1.8
5.8x5.8x1.8
5.8x5.8x1.8
3.0x3.0x1.4
3.2x2.7x1.55
3.2x2.7x1.55
3.1x3.1x1.8
3.1x3.1x1.8
3.1x3.1x1.8
Shielded
Shielded
Shielded
Shielded
Shielded
Shielded
Shielded
Shielded
Shielded
Table 4: Typical Surface Mount Inductors.
Manufacturer
Part Number
Value
Voltage
Temp. Co.
Case
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 = 221k.
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17
DATA SHEET
AAT2148
Low-Noise, Fast Transient 1A Step-Down Converter
Ordering Information
Output Voltage1
Package
Marking2
Part Number (Tape and Reel)3
Adj  0.6
QFN33-16
6VXYY
AAT2148IVN-0.6-T1
Skyworks Green™ products are compliant with
all applicable legislation and are halogen-free.
For additional information, refer to Skyworks
Definition of Green™, document number
SQ04-0074.
Package Information
QFN33-16
Pin 1 Dot By Marking
0.230 ± 0.050
Pin 1 Identification
0.500 ± 0.050
1.250 ± 0.050
5
C0.3
13
9
1.250 ± 0.050
Top View
0.025 ± 0.025
Bottom View
0.214 ± 0.036
0.900 ± 0.100
3.000 ± 0.050
0.400 ± 0.100
3.000 ± 0.050
1
Side View
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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