AAT1155 - Skyworks Solutions, Inc.

DATA SHEET
AAT1155
1MHz 2.5A Step-Down DC/DC Converter
General Description
Features
The AAT1155 is a step-down, SwitchReg™, switching
converter ideal for applications where high efficiency,
small size, and low ripple are critical. Able to deliver 2.5A
with an internal power MOSFET, the current-mode controlled IC provides high efficiency. Fully internally compensated, the AAT1155 simplifies system design and
lowers external parts count.
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The AAT1155 is available in a Pb-free, MSOP-8 package
and is rated over the -40°C to +85°C temperature
range.
5.5V Max Supply Input
Adjustable VOUT: 1.0V to 4.2V
2.5A Output Current
Up to 95% Efficiency
Integrated Low On-Resistance Power Switches
Internally Compensated Current Mode Control
1MHz Switching Frequency
Constant Pulse Width Modulation (PWM) Mode
Low Output Ripple with Light Load
Internal Soft Start
Current Limit Protection
Over-Temperature Protection
MSOP-8 Package
-40°C to +85°C Temperature Range
Applications
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Cable/DSL Modems
Computer Peripherals
High Efficiency Conversion from 5V or 3.3V Supply
Network Cards
Set-Top Boxes
Typical Application
VOUT : 1.0V - 4.2V
100Ω
VP
AAT1155 FB
VP
LX
VCC
LX
ENABLE
10μF
GND
1.5μH
120μF
0.1μF
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1
DATA SHEET
AAT1155
1MHz 2.5A Step-Down DC/DC Converter
Pin Descriptions
Pin #
Symbol
1
FB
2
GND
3
EN
4
5, 8
6, 7
VCC
VP
LX
Function
Feedback input pin. This pin is connected to an external resistive divider for an
adjustable output.
Signal ground.
Enable input pin. When connected high, the AAT1155 is in normal operation. When
connected low, it is powered down. This pin should not be left floating.
Small signal filtered bias supply.
Input supply for converter power stage.
Inductor connection pins. These pins should be connected to the output inductor.
Pin Configuration
MSOP-8
(Top View)
VP
7
LX
3
6
LX
4
5
VP
GND
2
EN
VCC
2
1
1
2
8
FB
Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • sales@skyworksinc.com • www.skyworksinc.com
201994B • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • February 7, 2013
DATA SHEET
AAT1155
1MHz 2.5A Step-Down DC/DC Converter
Absolute Maximum Ratings1
Symbol
VCC, VP
VLX
VFB
VEN
TJ
TLEAD
VESD
Description
VCC, VP to GND
LX to GND
FB to GND
EN to GND
Operating Junction Temperature Range
Maximum Soldering Temperature (at leads, 10 sec)
ESD Rating2 - HBM
Value
Units
6
-0.3 to VP+0.3
-0.3 to VCC+0.3
-0.3 to VCC+0.3
-40 to 150
300
3
kV
Value
Units
150
833
°C/W
mW
V
°C
Thermal Characteristics3
Symbol
JA
PD
Description
Maximum Thermal Resistance
Maximum Power Dissipation
Recommended Operating Conditions
Symbol
T
Description
Ambient Temperature Range
Rating
Units
-40 to +85
°C
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.
2. Human body model is a 100pF capacitor discharged through a 1.5k resistor into each pin.
3. Mounted on a demo board (FR4, in still air).
Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • sales@skyworksinc.com • www.skyworksinc.com
201994B • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • February 7, 2013
3
DATA SHEET
AAT1155
1MHz 2.5A Step-Down DC/DC Converter
Electrical Characteristics
VIN = VCC = VP = 5V, TA = -40°C to +85°C, unless otherwise noted. Typical values are TA = 25°C.
Symbol
VIN
VOUT
VEN(L)
VEN(H)
VUVLO
VUVLO(HYS)
IQ
ISHDN
ILIM
RDS(ON)H

VOUT
(VOUT*VIN)
VOUT/VOUT
fOSC
TSD
THYS
4
Description
Input Voltage Range
Output Voltage Tolerance
Enable Low Voltage
Enable High Voltage
Under-Voltage Lockout
Conditions
Min
VIN = VOUT + 0.2 to 5.5V, IOUT = 0.5A
2.7
-2.5
Typ
Max
Units
5.5
2.5
0.6
V
%
1.4
VIN Rising
VIN Falling
2.5
V
1.2
Under-Voltage Lockout Hysteresis
Quiescent Supply Current
Shutdown Current
Current Limit
High Side Switch On Resistance
Efficiency
No Load, VFB = 0V
VEN = 0V, VIN = 5.5V
TA = 25°C
TA = 25°C
IOUT = 1.0A
250
630
Load Regulation
ILOAD = 0A to 2.5A
2.3
Line Regulation
Oscillator Frequency
Over-Temperature Shutdown Threshold
Over-Temperature Shutdown Hysteresis
VIN = 2.7V to 5.5V
TA = 25°C
0.75
1
140
15
mV
1000
1.0
4.4
60
92
Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • sales@skyworksinc.com • www.skyworksinc.com
201994B • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • February 7, 2013
μA
A
m
%
%/V
MHz
°C
DATA SHEET
AAT1155
1MHz 2.5A Step-Down DC/DC Converter
Typical Characteristics
Efficiency vs. Load Current
Enable Threshold vs. Input Voltage
(VIN = 5.0V; VOUT = 3.3V)
1.2
Enable Threshold (V)
100
Efficiency (%)
95
90
85
80
75
70
65
60
0.01
0.1
1
1.1
EN(H)
1
0.9
0.8
EN(L)
0.7
0.6
10
2.5
3
Output Current (A)
3.5
4
4.5
5
5.5
5
5.5
Input Voltage (V)
RDS(ON) vs. Temperature
RDS(ON) vs. Input Voltage
(IDS = 1A)
80
90
3.6V
RDS(ON) (mΩ)
80
4.2V
75
RDS(ON) (mΩ
Ω)
2.7V
70
60
5.0V
5.5V
70
65
60
55
50
50
45
40
40
-20
0
20
40
60
80
100
2.5
120
3
3.5
4
4.5
Input Voltage (V)
Temperature (°C)
Oscillator Frequency Variation vs. Temperature
Oscillator Frequency Variation
vs. Input Voltage
(VIN = 5V)
(VOUT = 3.3V)
1
0
0.25
Variation (%)
Variation (%)
0.5
0
-0.25
-1
-2
-3
-0.5
3.5
4
4.5
Input Voltage (V)
5
5.5
-4
-20
0
20
40
60
80
100
Temperature (°C)
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DATA SHEET
AAT1155
1MHz 2.5A Step-Down DC/DC Converter
Typical Characteristics
Output Voltage Variation vs. Temperature
Line Regulation
(IOUT = 2A; VO = 3.3V)
(VOUT = 3.3V)
Output Voltage Error (%)
0.4
Variation (%)
0.2
0
-0.2
-0.4
-0.6
-0.8
-20
0
20
40
60
80
100
1
0
IO = 0.3A
-1
-2
-3
-4
IO = 3.0A
-5
3.4
3.7
4
0.0
3.4
-1.0
55°C
Output Error (%)
Output Current (A)
(VOUT = 3.3V)
70°C
2.6
2.4
85°C
2.2
2
100°C
1.8
-2.0
-3.0
-4.0
-5.0
-6.0
-7.0
-8.0
3.75
4
4.25
4.5
4.75
5
5.25
-10.0
0.01
5.5
0.1
Input Voltage (V)
(VOUT = 3.3V; VIN = 5.0V)
5
700
650
2.7V
3.6V
Output Current (A)
Operating Current (mA)
10
Over-Temperature Shutdown
Current vs. Temperature
(FB = 0V)
750
1
Load Current (A)
Non-Switching Operating
Current vs. Temperature
4.2V
600
550
500
5.0V
5.5V
60
80
4.5
4
3.5
3
2.5
2
450
-20
0
20
40
Temperature (°C)
6
5.5
-9.0
1.6
3.5
5.2
(VIN = 5.0V; VOUT = 3.3V)
3.6
2.8
4.9
Load Regulation
AAT1155 Evaluation Board
Over-Temperature Current vs. Input Voltage
3
4.6
Input Voltage (V)
Temperature (°°C)
3.2
4.3
100
120
-20
-10
0
10
20
30
40
50
60
70
Temperature (°C)
Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • sales@skyworksinc.com • www.skyworksinc.com
201994B • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • February 7, 2013
80
90
100
DATA SHEET
AAT1155
1MHz 2.5A Step-Down DC/DC Converter
Inrush and Output
Overshoot Characteristics
Tantalum Output Ripple
14
6
12
4
10
2
8
0
6
-2
4
-4
2
-6
0
-8
0.04
7
0.02
6
0.00
5
-0.02
4
-0.04
3
-0.06
2
-0.08
0.4
0.8
1.2
1.6
1
120μF 6.3V Tantalum
Vishay P/N 594D127X96R3C2T
-0.10
-2
0
2
0
-0.12
-1
0
Time (ms)
Inductor Current
(bottom) (A)
8
Output Ripple
(top) (mV)
(IOUT = 3.0A; VOUT = 3.3V; VIN = 5.0V)
Inductor Current (A)
(bottom trace)
Enalbe and Output Voltage (V)
(top trace)
Typical Characteristics
1
2
3
4
5
Time (μ
μs)
Output Ripple
Output Ripple
(IOUT = 3.0A; VOUT = 3.3V; VIN = 5.0V)
(IOUT = 3.0A; VOUT = 3.3V; VIN = 5.0V)
4
7
2
6
2
6
AC Output Ripple
(top) (mV)
4
-4
3
-6
2
-8
1
200μF 6.3V Ceramic
-10 TDK P/N C3325X5R0J107M
Vishay GRM43SR60J107ME20L
-12
0
1
2
3
4
0
5
-2
4
-4
3
-6
2
-8
0
-10
-1
-12
5
0
2
1
Time (μ
μs)
12
-12
Gain (dB)
16
180
135
12
135
90
8
90
45
0
200μF gain
300μF gain
100μF 6.3 Ceramic
TDK P/N C3225X5R0J107M
Vishay GRM43SR60J107ME20L
-16
10000
-45
-90
-135
-180
100000
Frequency (Hz)
5
4
45
Gain
0
0
-4
-45
-8
-90
-12
120μF 6.3V Tantalum
Vishay P/N 594D127X96R3C2T
-16
10000
100000
Phase (Degrees)
Phase
0
-8
4
180
Phase (Degrees)
8
-4
-1
3
Loop Crossover Gain and Phase
Gain (dB)
L = 1.5μHy
4
0
Time (μ
μs)
Loop Crossover Gain and Phase
16
1
300μF 6.3VCeramic
TDK P/N C3325X5R0J107M
Vishay GRM43SR60J107ME20L
Inductor Current
(bottom) (A)
5
Inductor Current
(bottom) (A)
0
-2
AC Output Ripple
(top) (mV)
7
4
-135
-180
1000000
Frequency (Hz)
Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • sales@skyworksinc.com • www.skyworksinc.com
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7
DATA SHEET
AAT1155
1MHz 2.5A Step-Down DC/DC Converter
Tantalum Transient Response
Transient Response
(IOUT = 0 to 3.0A; VOUT = 3.3V; VIN = 5.0V)
(IOUT = 0 to 3.0A; VOUT = 3.3V; VIN = 5.0V)
3.40
7
6
3.30
6
3.20
5
3.10
4
3.00
3
2.90
2
2.80
1
120μF 6.3V Tantalum
Vishay P/N 594D127X96R3C2T
2.70
2.60
0
100
200
300
400
Output Voltage
(top) (mV)
7
3.30
3.20
2x 100μF 6.3V Ceramic
TDK P/N C3325X5R0J107M
Vishay GRM43SR60J107ME20L
3.10
3
2.90
2
2.80
1
0
2.70
0
-1
2.60
500
-1
0
100
200
300
400
Time (μ
μs)
Transient Response
3.40
7
3.30
6
3.10
5
4
3.00
3
2.90
2
2.80
1
2.70
0
2.60
Inductor Current
(bottom) (A)
Output Voltage
(top) (mV)
(IOUT = 0 to 3.0A; VOUT = 3.3V; VIN = 5.0V)
3x 100μF 6.3V Ceramic
TDK P/N C3325X5R0J107M
Vishay GRM43SR60J107ME20L
-1
0
100
200
300
400
500
Time (μ
μs)
8
4
3.00
Time (μ
μs)
3.20
5
Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • sales@skyworksinc.com • www.skyworksinc.com
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500
Inductor Current
(bottom) (A)
3.40
Inductor Current
(bottom) (A)
Output Voltage
(top) (mV)
Typical Characteristics
DATA SHEET
AAT1155
1MHz 2.5A Step-Down DC/DC Converter
Functional Block Diagram
VCC
VP = 2.5V to 5.5V
REF
FB
OP. AMP
CMP
DH
LOGIC
OSC
LX
Temp.
Sensing
EN
Applications Information
Main Control Loop
The AAT1155 is a peak current mode step-down converter. The inner wide bandwidth loop controls the
inductor peak current. The inductor current is sensed as
it flows through the internal P-channel MOSFET. A fixed
slope compensation signal is then added to the sensed
current to maintain stability for duty cycles greater than
50%. The inner loop appears as a voltage-programmed
current source in parallel with the output capacitor.
The voltage error amplifier output programs the current
loop for the necessary inductor current to force a constant output voltage for all load and line conditions. The
feedback resistive divider is external, dividing the output
voltage to the error amplifier reference voltage of 1.0V.
The error amplifier has a limited DC gain. This eliminates
the need for external compensation components, while
still providing sufficient DC loop gain for good load regulation. The crossover frequency and phase margin are
set by the output capacitor value.
Duty cycle extends to 100% as the input voltage
approaches the output voltage. Thermal shutdown protection disables the device in the event of a short-circuit
or overload condition.
Soft Start/Enable
Soft start controls the current limit when the input voltage or enable is applied. It limits the current surge seen
at the input and eliminates output voltage overshoot.
When pulled low, the enable input forces the device into
a low-power, non-switching state. The total input current
during shutdown is less than 1μA.
Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • sales@skyworksinc.com • www.skyworksinc.com
201994B • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • February 7, 2013
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DATA SHEET
AAT1155
1MHz 2.5A Step-Down DC/DC Converter
Power and Signal Source
The corresponding inductor RMS current is:
Current Limit and
Over-Temperature Protection
Over-temperature and current limit circuitry protects the
AAT1155 and the external Schottky diode during overload, short-circuit, and excessive ambient temperature
conditions. The junction over-temperature threshold is
140°C nominal and has 15°C of hysteresis. Typical
graphs of the over-temperature load current vs. input
voltage and ambient temperature are shown in the
Typical Characteristics section of this document.
2
IRMS = ⎛⎝ IOUT2 + ∆I ≈ IOUT = 2.5A
12
⎛
⎝
Separate small signal ground and power supply pins isolate the internal control circuitry from switching noise. In
addition, the low pass filter R1 and C3 (shown in Figure
1) filters noise associated with the power switching.
I is the peak-to-peak ripple current which is fixed by
the inductor selection above. For a peak-to-peak current
of 40% of the full load current, the peak current at full
load will be 120% of the full load. The 1.5μH inductor
selected from the Sumida CDRH6D38 series has a 11m
DCR and a 4.0A DC current rating with a height of 4mm.
At full load, the inductor DC loss is 70mW for a 0.84%
loss in efficiency.
Schottky Freewheeling Diode
The Schottky average current is the load current multiplied by one minus the duty cycle.
Inductor
⎛
⎝
3.3V
· ⎛⎝1 - 3.3V
2.5A · 0.4 · 1MHz
5.25V
⎛
⎝
=
VOUT
VOUT
· ⎛1 IOUT ∙ k · fS ⎝
VIN(MAX)
= 1.23μH
The factor “k” is the fraction of the full load (40%) selected for the ripple current at the maximum input voltage.
10
VOUT
3.3V
IAVG = IOUT ∙ ⎛1 = 2.5A ∙ ⎛1 ⎝
⎝
VIN
5.0V
⎛
⎝
L=
For VIN at 5V and VOUT at 3.3V, the average diode current
is:
⎛
⎝
For a 2.5A load and the ripple current set to 40% at the
maximum input voltage, the maximum peak-to-peak
ripple current is 1A. Assuming a 5V ±5% input voltage
and 40% ripple, the output inductance required is:
VOUT
⎛
⎝1 - VIN
⎛
⎝
The output inductor is selected to limit the ripple current
to 20% to 40% of the full load current at the maximum
input voltage. 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
inductor saturation characteristics. The inductor should
not show any appreciable saturation under normal load
conditions. During overload and short-circuit conditions,
the inductor can exceed its peak current rating without
affecting the converter performance. Some inductors
may have sufficient peak and average current ratings yet
result in excessive losses due to a high DCR. The losses
associated with the DCR and its effect on the total converter efficiency must be considered.
= 0.85A
With a 125°C maximum junction temperature and a
120°C/W thermal resistance, the maximum average current is:
IAVG =
TJ(MAX) - TAMB
θJA · VF
=
125°C - 70°C
= 1.14A
120°C / W ∙ 0.4V
VF is the forward voltage drop of the Schottky diode.
For overload, short-circuit, and excessive ambient temperature conditions, the AAT1155 enters over-temperature shutdown mode protecting the AAT1155 as well as
the output Schottky. In this mode, the output current is
limited internally until the junction temperature reaches
the temperature limit (see over-temperature characteristics graphs). The diode reverse voltage must be rated
to withstand the input voltage.
Input Capacitor Selection
The primary function of the input capacitor is to provide
a low impedance loop for the edges of pulsed current
Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • sales@skyworksinc.com • www.skyworksinc.com
201994B • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • February 7, 2013
DATA SHEET
AAT1155
1MHz 2.5A Step-Down DC/DC Converter
VOUT
VOUT ⎛
∙ 1- V
VIN ⎝
IN
⎛
⎝
IRMS = IOUT ∙
A high ESR tantalum capacitor with a value about 10
times the input ceramic capacitor may also be required
when using a 10μF or smaller ceramic input bypass
capacitor. This dampens any input oscillations that may
occur due to the source inductance resonating with the
converter input impedance.
Due to the ESR zero associated with the tantalum capacitor, smaller values than those required with ceramic
capacitors provide more phase margin with a greater
loop crossover frequency.
Thermal
Losses associated with the AAT1155 output switching
MOSFET are due to switching losses and conduction
losses. The conduction losses are associated with the
RDS(ON) characteristics of the output switching device. At
the full load condition, assuming continuous conduction
mode (CCM), an accurate calculation of the RDS(ON) losses
can be derived from the following equations:
PON = I RMS2 · RDS(ON)
RDS(ON) losses
2
IRMS = ⎛⎝ IOUT2 + ∆I ∙ D
12
⎛
⎝
drawn by the AAT1155. A low ESR/ESL ceramic capacitor
is ideal for this function. To minimize stray inductance,
the capacitor should be placed as closely as possible to
the IC. This also keeps the high frequency content of the
input current localized, minimizing the radiated and conducted EMI while facilitating optimum performance of
the AAT1155. Proper placement of the input capacitor C1
is shown in the layout in Figure 1. Ceramic X5R or X7R
capacitors are ideal. The size required will vary depending on the load, output voltage, and input voltage source
impedance characteristics. Typical values range from 1μF
to 10μF. The input capacitor RMS current varies with the
input voltage and the output voltage. It is highest when
the input voltage is double the output voltage where it is
one half of the load current.
where, IRMS is internal switch RMS current.
D is the duty cycle and VF is the forward voltage drop of
the Schottky diode.
D=
Output Capacitor
With no external compensation components, the output
capacitor has a strong effect on the loop stability. Larger
output capacitance will reduce the crossover frequency
with greater phase margin. A 200μF ceramic capacitor
provides sufficient bulk capacitance to stabilize the output during large load transitions and has ESR and ESL
characteristics necessary for very low output ripple. The
RMS ripple current is given by:
IRMS =
(V
- VF) · (VIN - VOUT)
1
· OUT
L · fS · VIN
2· 3
For a ceramic output capacitor, the dissipation due to the
RMS current and associated output ripple are negligible.
Tantalum capacitors with sufficiently low ESR to meet
output ripple requirements generally have an RMS current rating much greater than that actually seen in this
application. The maximum tantalum output capacitor
ESR is:
ESR ≤
VOUT+ VF
VIN + VF
I is the peak-to-peak inductor ripple current. A simplified form of calculating the RDS(ON) and switching losses
is given by:
P=
IOUT2 · RDS(ON) ∙ VOUT
+ (tSW · fS · IOUT + IQ) · VIN
VIN
where IQ is the AAT1155 quiescent current.
Once the total losses have been determined, the junction
temperature can be derived. The thermal resistance
(JA) for the MSOP-8 package mounted on an FR4 printed circuit board in still air is 150°C/W.
TJ = P · ΘJA + TAMB
TAMB is the maximum ambient temperature and TJ is the
resultant maximum junction temperature.
VRIPPLE
ΔI
where I is the peak-to-peak inductor ripple current.
Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • sales@skyworksinc.com • www.skyworksinc.com
201994B • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • February 7, 2013
11
DATA SHEET
AAT1155
1MHz 2.5A Step-Down DC/DC Converter
Part Number
Manufacturer
C4532X5ROJ107M
GRM43-2 X5R 107M 6.3
GRM43-2 X5R 476K 6.3
GRM42-6 X5R 106K 6.3
594D127X_6R3C2T
595D107X0016C
TDK
Capacitance (μF)
Voltage (V)
Temp Co.
100
Murata
47
10
120
100
Vishay
X5R
6.3
Case
1812
1206
C
16
Table 1: AAT1155 Recommended Capacitors.
Part Number
Manufacturer
CDRH6D38-4763-T055
N05D B1R5M
NP06DB B1R5M
LQH55DN1R5M03
LQH66SN1R5M03
Sumida
Taiyo Yuden
Inductance (μH)
I (A)
DCR (Ω)
Height (mm)
Type
0.014
0.025
1.5
4.0
3.2
3.0
3.7
3.8
4.0
2.8
3.2
Shielded
Non-Shielded
Shielded
Non-Shielded
Shielded
Murata
0.022
0.016
4.7
Table 2: AAT1155 Recommended Inductors.
Manufacturer
Part Number
VF
Diodes Inc.
ROHM
Micro Semi
B340LA
RB050L-40
5820SM
0.45V @ 3A
0.45 @ 3A
0.46V @ 3A
Table 3: AAT1155 Recommended Diodes.
12
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DATA SHEET
AAT1155
1MHz 2.5A Step-Down DC/DC Converter
Evaluation Board Description
Layout
The AAT1155 evaluation board demonstrates the performance, along with the suggested size and placement of
external components for the AAT1155 integrated buck
regulator. The external components are selected for
minimum size and optimum operation up to 2.5A output
current.
Figure 1 shows the evaluation board schematic. Figures
2 and 3 display the suggested PCB layout for the
AAT1155. The following guidelines should be used to
help ensure a proper layout. Table 4 and 5 provide the
AAT1155 evaluation board specifications and the component list respectively.
1.
Components Summary
Capacitor C4 is a high equivalent series resistance (ESR)
tantalum capacitor for damping the high Q input network
associated with the inductance of the leads feeding the
input power to the evaluation board and the low ESR
10μF ceramic input capacitor (C1). R2 is a pull-up resistor to allow the enable pin to be used to enable and disable the IC by either shorting the enable pin to GND or
leaving the pin open. Both R2 and C4 are for demonstration purposes only and are not necessary in the final
solution. Resistors R3 and R4 are used with the 1V model
to set the output voltage to any desired value from 1.0V
to as high as 200mV below the minimum input voltage.
They are not required for the fixed output voltage versions of the AAT1155. For fixed output voltage versions,
R4 is omitted and R3 is replaced with a short.
2.
3.
4.
5.
6.
The connection from the input capacitor to the
Schottky anode should be as short as possible.
The input capacitor should connect as closely as possible to VP (Pins 5 and 8) and GND (Pin 2).
C1, L1, and CR1 should be connected as closely as
possible. The connection from the cathode of the
Schottky to the LX node should be as short as possible.
The feedback trace (Pin 1) should be separate from
any power trace and connect as closely as possible
to the load point. Sensing along a high-current load
trace can degrade DC load regulation.
The resistance of the trace from the load return to
GND (Pin 2) 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 reference
ground and the load return.
R1 and C3 are required in order to provide a cleaner
power source for the AAT1155 control circuitry.
VIN+
VOUT+
R1
100
C4
100μF
U1
AAT1155
R3
1
R2
100k
Enable
C1
10μF
C3
0.1μF
VP
8
2
GND LX
7
3
EN
LX
6
4
VCC VP
5
FB
R4
10.0k
LX
L1
1.5μH
C2
CR1
B340LA 120μF
GND
Figure 1: AAT1155 Evaluation Board with Adjustable Output.
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13
DATA SHEET
AAT1155
1MHz 2.5A Step-Down DC/DC Converter
Specification
Test Conditions
Input Voltage
Output Error Voltage
Output Current
Min
Typ
Max
Units
2.7
3.6
±3
5.5
V
%
A
0
2.5
Table 4: AAT1155 Evaluation Board Specifications.
Figure 2: Evaluation Board Top Side.
Component
Part#
U1
U1
U1
U1
CR1
C2
C4
C1
C3
R1
R2
L1
AAT1155IKS-1.0-T1
AAT1155IKS-1.8-T1
AAT1155IKS-2.5-T1
AAT1155IKS-3.3-T1
B340LA
594D127X96R6R3C2T
565D107X0016C
GRM319R60J106KE01
GRM155R61A104KA01
Chip Resistor
Chip Resistor
CDRH6D28-1R5
R3
Chip Resistor
R4
Chip Resistor
Figure 3: Evaluation Board Bottom Side.
Description
1.0V 850kHz 2.5A Buck Converter
1.8V 850kHz 2.5A Buck Converter
2.5V 850kHz 2.5A Buck Converter
3.3V 850kHz 2.5A Buck Converter
Low VF 3A, 40V Schottky Diode
120μF, 6.3V, Tantalum
100μF, 16V, Tantalum
10μF, 6.3V, X5R, 10%, 1206
0.1μF, 25V, X5R, 10%, 0402
100kΩ, 5%, 1/16W, 0402
100kΩ, 5%, 1/16W, 0402
1.5μH Shielded
AAT1155IKS-1.0-T1
see Table 6, 1%, 1/16W, 0402
AAT1155IKS-1.1-T1 thru
0Ω, 0402
AAT1155IKS-3.3 -T1
AAT1155IKS-1.0-T1 (when
output is > 1.0V only,
10kΩ, 1%, 1/16W; 0402
otherwise not needed)
AAT1155IKS-1.1-T1 thru
AAT1155IKS-3.3 -T1
Manufacturer
Skyworks
Skyworks
Skyworks
Skyworks
Diodes Inc.
Vishay
Vishay
MuRata
MuRata
Vishay
Vishay
Sumida
Vishay
Vishay
Vishay
N/A
Table 5: AAT1155 Evaluation Board Component Listing.
14
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DATA SHEET
AAT1155
1MHz 2.5A Step-Down DC/DC Converter
For an adjustable output, set R4 to 10kΩ and select R3 according to table 6.
Vo (V)
R3 (kΩ)
1.0
1.1
1.2
1.25
1.3
1.4
1.5
1.6
1.7
1.8
1.85
1.9
2.0
2.1
2.2
2.3
2.4
2.5
3.3
0.00 (short)
1.00
2.00
2.55
3.01
4.02
4.99
6.04
6.98
8.06
8.45
9.09
10.0
11.0
12.1
13.0
14.0
15.0
23.2
Table 6. Resistor selection for adjustable output voltage (R4=10.0kΩ)
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15
DATA SHEET
AAT1155
1MHz 2.5A Step-Down DC/DC Converter
Design Example
Efficiency vs. Load Current
IOUT 2.5A
IRIPPLE 40% of Full Load at Max VIN
VOUT 2.5V
VIN 5V 5%
fS 1MHz
TMAX 70°C
(VIN = 5.0V; VOUT = 3.3V)
100
Efficiency (%)
95
Inductor Selection
V
VOUT
· ⎛1 - OUT
IOUT ∙ k · fS ⎝
VIN
⎛
⎝
TJ(MAX) = TAMB + ΘJA · PON
= 70°C + 150°C/W · 0.42W
= 133°C
Diode
V
IDIODE = IOUT ∙ ⎛⎝1 - OUT
VIN
V
= 2.5A ∙ ⎛1 - OUT = 0.93A
⎝
VIN
⎛
⎝
IPK is the peak current.
(2.5A)2 · 70mΩ · 3.3V ⎛20ns · 1MHz · 2.5A + 690μA⎞
+
· 5V
⎝
⎠
5V
2
⎛
⎝
= 2.91A
IOUT2 · RDS(ON) · VOUT ⎛tSW · FS · IOUT
+ IQ⎞ · VIN
+
2
⎝
VIN
⎠
= 0.42 W
⎛
⎝
= 2.5A +0.41A
10
Output Current (A)
=
⎛
⎝
3.3V
· ⎛1 - 3.3V
1.5μH · 1MHz ⎝
5.25V
∆I
IPK = IOUT + 12
1
Figure 4: 5V Input, 3.3V Output.
PON =
Sumida inductor Series CDRH6D38.
= 0.82A
0.1
AAT1155 Junction Temperature
Use standard value of 1.5μH
=
70
65
= 1.23μH
V
VOUT ⎛
· 1 - OUT
⎝
L · fS
VIN
75
60
0.01
3.3V
=
· ⎛⎝1 - 3.3V
2.5A · 0.4 · 1MHz
5.25V
∆I =
85
80
⎛
⎝
L=
90
PDIODE = VF · IDIODE
= 0.35V · 0.93A = 0.33W
Given an ambient thermal resistance of 120°C/W from
the manufacturer’s datasheet, TJ(MAX) of the diode is:
TJ(MAX) = TAMB + ΘJA · P
= 70°C + 120°C / W · 0.33W
= 109°C
16
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DATA SHEET
AAT1155
1MHz 2.5A Step-Down DC/DC Converter
Input Capacitor
The output capacitor value required for sufficient loop
phase margin depends on the type of capacitor selected.
For a low ESR ceramic capacitor, a minimum value of
200μF is required. For a low ESR tantalum capacitor,
lower values are acceptable. While the relatively higher
ESR associated with the tantalum capacitor will give
more phase margin and a more dampened transient
response, the output voltage ripple will be higher.
The input capacitor ripple is:
The 120μF Vishay 594D tantalum capacitor has an ESR
of 85m and a ripple current rating of 1.48Arms in a C
case size. Although smaller case sizes are sufficiently
rated for this ripple current, their ESR level would result
in excessive output ripple.
The ESR requirement for a tantalum capacitor can be
estimated by :
ESR ≤
IRMS =
=
VRIPPLE
∆I
=
100 mV
= 111mΩ
0.9A
(VOUT + VF) · (VIN + VOUT)
L · fS · VIN
2· 3
1
1
2
∙
3.65V ∙1.7 V
= 240mArms
3 1.5μH· 1MHz ∙ 5V
·
Two or three 1812 X5R 100μF 6.3V ceramic capacitors in
parallel also provide sufficient phase margin. The low
ESR and ESL associated with ceramic capacitors also
reduces output ripple significantly over that seen with
tantalum capacitors. Temperature rise due to ESR ripple
current dissipation is also reduced.
IRMS = IOUT ∙
VOUT
VOUT ⎛
∙ 1- V
VIN ⎝
IN
⎛
⎝
Output Capacitor
= 1.82Arms
In the examples shown, C1 is a ceramic capacitor located as closely to the IC as possible. C1 provides the low
impedance path for the sharp edges associated with the
input current. C4 may or may not be required, depending upon the impedance characteristics looking back into
the source. It serves to dampen any input oscillations
that may arise from a source that is highly inductive. For
most applications, where the source has sufficient bulk
capacitance and is fed directly to the AAT1155 through
large PCB traces or planes, it is not required. When operating the AAT1155 evaluation board on the bench, C4 is
required due to the inductance of the wires running from
the laboratory power supply to the evaluation board.
Adjustable Output
For applications requiring an output other than 1V, the
AAT1155 can be externally programmed. Resistors R3
and R4 of Figure 1 force the output to regulate higher
than 1V. For accurate results (less than 1% error for all
outputs), select R4 to be 10k. Once R4 has been selected, R3 can be calculated. For a 1.25V output with R4 set
to 10k, R3 is 2.5k.
R3 = (VOUT - 1) · R4 = 0.25 · 10kΩ = 2.5kΩ
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17
DATA SHEET
AAT1155
1MHz 2.5A Step-Down DC/DC Converter
Ordering Information
Output Voltage
Package
Marking1
Part Number (Tape & Reel)2
1.0V (Adj. VOUT ≥ 1.0V)
MSOP-8
KXXYY
AAT1155IKS-1.0-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
MSOP-8
4° ± 4°
4.90 ± 0.10
3.00 ± 0.10
1.95 BSC
0.95 REF
0.60 ± 0.20
PIN 1
3.00 ± 0.10
0.85 ± 0.10
0.95 ± 0.15
10° ± 5°
GAUGE PLANE
0.254 BSC
0.155 ± 0.075
0.075 ± 0.075
0.65 BSC
0.30 ± 0.08
All dimensions in millimeters.
1. XYY = assembly and date code.
2. Sample stock is generally held on part numbers listed in BOLD.
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