Skyworks AAT1106 600ma step-down converter Datasheet

DATA SHEET
AAT1106
600mA Step-Down Converter
General Description
Features
The AAT1106 is a 1.5MHz constant frequency current
mode PWM step-down SwitchReg™ converter with a
unique adaptive slope compensation scheme allowing
the device to operate with a lower range of inductor values to optimize size and provide efficient operation. It is
ideal for portable equipment powered by single-cell
Lithium-ion batteries and is optimized for high efficiency,
achieving levels up to 96%.
•
•
•
•
•
•
•
The AAT1106 can supply up to 600mA load current from
a 2.5V to 5.5V input voltage and the output voltage can
be regulated as low as 0.6V. The device also can run at
100% duty cycle for low dropout operation, extending
battery life in portable systems. In addition, light load
operation provides very low output ripple for noise sensitive applications and the 1.5MHz switching frequency
minimizes the size of external components while keeping
switching losses low.
The AAT1106 is available in a Pb-free, low-profile 5-pin
TSOT23 package, and is rated over the -40°C to +85°C
temperature range.
•
•
•
•
VIN Range: 2.5V to 5.5V
VOUT: Adjustable 0.6V to VIN
Up to 600mA Output Current
Up to 96% Efficiency
1.5MHz Switching Frequency
100% Duty Cycle Dropout Operation
Adaptive Slope Compensated Current Mode Control
for Excellent Line and Load Transient Response
<1μA Shutdown Current
Short-Circuit and Thermal Fault Protection
TSOT23-5 Package
-40°C to +85°C Temperature Range
Applications
•
•
•
•
•
•
•
Cellular Phones, Smartphones
Digital Still Cameras
Digital Video Cameras
Microprocessor and DSP Core Supplies
MP3 and Portable Media Players
PDAs
Wireless and DSL Modems
Typical Application
L1
2.2μH
VIN
2.5V to 5.5V
C2
22pF
AAT1106
C1
4.7μF
VOUT
1.8V
LX
IN
C3
10μF
FB
EN
GND
R2
634K
R1
316K
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1
DATA SHEET
AAT1106
600mA Step-Down Converter
Pin Descriptions
Pin #
Symbol
1
EN
2
GND
3
LX
4
IN
5
FB
Function
Enable pin. Active high. In shutdown, all functions are disabled drawing <1μA supply current. Do not leave EN
floating.
Ground pin.
Switching node. Connect the output inductor to this pin. Connects to the drains of the internal P- and N-channel MOSFET switches.
Supply input pin. Must be closely decoupled to GND with a 2.2μF or larger ceramic capacitor.
Feedback input pin. Connect FB to the center point of the external resistor divider. The feedback threshold
voltage is 0.6V.
Pin Configuration
TSOT23-5
(Top View)
2
EN
1
GND
2
LX
3
5
FB
4
IN
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DATA SHEET
AAT1106
600mA Step-Down Converter
Absolute Maximum Ratings
Symbol
VIN
VEN, VFB
VLX
TJ
TS
TLEAD
Description
Input Supply Voltage
EN, FB Voltages
LX Voltages
Operating Temperature Range
Storage Temperature Range
Lead Temperature (soldering, 10s)
Value
Units
-0.3 to 6.0
-0.3 to VIN + 0.3
V
-40 to +85
-65 to +150
300
°C
Value
Units
150
667
°C/W
mW
Recommended Operating Conditions
Symbol
JA
PD
Description
Thermal Resistance (TSOT23-5)
Maximum Power Dissipation at TA = 25°C
1. Absolute Maximum Ratings are those values beyond which the life of a device may be impaired.
2. TJ is calculated from the ambient temperature TA and power dissipation PD according to the following formula: TJ = TA + PD · JA.
3. Thermal resistance is specified with approximately 1 square inch of 1 oz copper.
Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • [email protected] • www.skyworksinc.com
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3
DATA SHEET
AAT1106
600mA Step-Down Converter
Electrical Characteristics
VIN = VEN = 3.6V, TA = 25°C, unless otherwise noted.
Symbol
Description
Conditions
Step-Down Converter
VIN
Input Voltage Range
FB Input Bias Current
Output Voltage Line Regulation
VIN = 2.5V to 5.5V, IOUT = 10mA
Output Voltage Load Regulation
IOUT = 10mA to 600mA
Maximum Output Current
Oscillator Frequency
Startup Time
P-Channel MOSFET
N-Channel MOSFET
VIN = 3.0V
VFB = 0.6V
From Enable to Output Regulation
ILX = 300mA
ILX = 300mA
VIN = 3V, VFB = 0.5V,
Duty Cycle <35%
VOVL = VOVL - VFB
Input DC Supply Current
VFB
Regulated Feedback Voltage
IFB
VOUT/
VOUT/VIN
VOUT/
VOUT/IOUT
ILIM
FOSC
TS
RDS(ON)
Peak Inductor Current
VEN(L)
VEN(H)
IEN
TSD
THYS
Output Over-Voltage Lockout
Enable Threshold Low
Enable Threshold High
Input Low Current
Over-Temperature Shutdown Threshold
Over-Temperature Shutdown Hysteresis
Typ
2.5
Active Mode, VFB = 0.5V
Shutdown Mode, VFB = 0V, VIN = 4.2V
TA = 25°C
TA = 0°C  TA  +85°C
TA = -40°C  TA  +85°C
VFB = 0.65V
IQ
Min
270
0.08
0.5880
0.5865
0.5850
-30
0.6000
0.11
Max
Units
5.5
400
1.0
0.6120
0.6135
0.6150
30
V
0.40
0.0015
600
1.2
1.5
100
0.30
0.20
%/V
mA
MHz
μs
0.50
0.45

A
60
mV
0.4
1.0
150
15
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nA
1.8
1. 100% production test at +25°C. Specifications over the temperature range are guaranteed by design and characterization.
4
V
%/mA
1.20
1.4
-1.0
μA
V
μA
°C
DATA SHEET
AAT1106
600mA Step-Down Converter
Typical Characteristics
Efficiency vs. Output Current
Efficiency vs. Output Current
(VOUT = 2.5V; L = 2.2µH; TA = 25°°C)
(VIN = 3.6V; VOUT = 2.5V; TA = 25°°C)
100
Efficiency (%)
80
VIN = 2.7V
90
80
Efficiency (%)
90
100
VIN = 4.2V
70
60
50
VIN = 3.6V
40
30
70
40
10
10
1
10
100
L = 2.2µH
30
20
0.1
L = 4.7µH
50
20
0
L = 10µH
60
L = 1.4µH
0
0.1
1000
1
10
Output Current (mA)
Efficiency vs. Output Current
(VOUT = 1.8V; L = 2.2µH; TA = 25°°C)
VIN = 3.6V
(VIN = 3.6V; VOUT = 1.8V; TA = 25°°C)
100
VIN = 2.7V
90
80
Efficiency (%)
Efficiency (%)
90
70
60
50
VIN = 4.2V
40
30
80
L = 2.2µH
70
L = 1.4µH
60
L = 10µH
50
40
L = 4.7µH
30
20
20
10
0.1
10
0.1
1
10
100
1000
1
10
Output Current (mA)
100
90
90
VIN = 2.7V
Efficiency (%)
Efficiency (%)
(VOUT = 1.2V; L = 2.2µH; TA = 25°°C)
100
VIN = 4.2V
60
50
VIN = 3.6V
40
30
80
60
40
10
10
100
Output Current (mA)
1000
VIN = 3.6V
30
10
1
VIN = 4.2V
50
20
0.1
VIN = 2.7V
70
20
0
1000
Efficiency vs. Output Current
(VOUT = 1.5V; L = 2.2µH; TA = 25°°C)
70
100
Output Current (mA)
Efficiency vs. Output Current
80
1000
Output Current (mA)
Efficiency vs. Output Current
100
100
0
0.1
1
10
100
1000
Output Current (mA)
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5
DATA SHEET
AAT1106
600mA Step-Down Converter
Typical Characteristics
Efficiency vs. Input Voltage
Output Voltage vs. Output Current
(VIN = 3.6V; L = 2.2µH; VOUT = 1.8V)
(VIN = 3.6V; VOUT = 1.8V; L = 2.2µH)
1.84
100
ILOAD = 500mA
1.82
Output Voltage (V)
95
Efficiency (%)
90
85
ILOAD = 100mA
80
75
70
ILOAD = 10mA
65
60
55
50
1.8
1.78
1.76
1.74
1.72
1.7
1.68
1.66
2
3
4
5
1.64
6
0
200
400
Input Voltage (V)
600
800
1000
1200
Load Current (mA)
Frequency vs. Input Voltage
RDS(ON) vs. Input Voltage
(VOUT = 1.8V; ILOAD = 150mA; L = 2.2µH)
1.560
0.400
0.350
1.540
1.530
RDS(ON) (Ω
Ω)
Frequency (MHz)
1.550
1.520
1.510
1.500
1.490
P-Channel MOSFET
0.300
0.250
0.200
N-Channel MOSFET
1.480
1.470
2.7 2.9 3.1 3.3 3.5 3.7 3.9 4.1 4.3 4.5 4.7 4.9 5.1 5.3 5.5
0.150
2
2.5
3
Input Voltage (V)
3.5
4
4.5
5
5.5
6
Input Voltage (V)
RDS(ON) vs. Temperature
Feedback Voltage vs. Temperature
(VIN = 3.6V)
(VIN = 3.6V)
0.604
P-Channel
0.33
0.602
RDS(ON) (Ω)
Feedback Voltage (V)
0.36
0.603
0.601
0.600
0.27
0.24
0.599
0.21
0.598
0.18
N-Channel
0.15
0.597
-40
-20
0
20
40
Temperature (°°C)
6
0.30
60
80
100
-45
-30
-15
0
15
30
45
60
Temperature (°C)
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75
90
DATA SHEET
AAT1106
600mA Step-Down Converter
Typical Characteristics
Frequency vs. Temperature
Input Supply Current vs. Temperature
Input Supply Current (µA)
OSC Frequency (MHz)
1.60
1.55
1.50
1.45
1.40
1.35
1.30
1.25
1.20
1.15
1.10
-50
-25
0
25
50
100
150
320
300
280
260
240
220
200
-50
Temperature (°C)
-30
-10
10
30
50
70
90
Temperature (°C)
Load Transient Response
Load Transient Response
(PWM Mode Only; ILOAD = 100mA to 400mA; L = 2.2µH;
CIN = 10µF; COUT = 10µF; VIN = 3.6V; VOUT = 1.8V)
(Light Load Mode to PWM Mode; ILOAD = 28mA to 400mA;
L = 2.2µH; CIN = 10µF; COUT = 10µF; VIN = 3.6V; VOUT = 1.8V)
PWM
VSW
2V/div
VSW
2V/div
VOUT
100mV/div
VOUT
200mV/div
ILOAD
500mA/div
ILOAD
500mA/div
40µs/div
Light Load
4µs/div
Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • [email protected] • www.skyworksinc.com
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7
DATA SHEET
AAT1106
600mA Step-Down Converter
Typical Characteristics
Startup Waveform
Startup Waveform
6
1.75
5
1.50
3
VEN = 3.0V
VOUT = 1.8V
2
1.25
1.00
0.75
1
IIN
0
0.50
-1
0.25
-2
0.00
-3
-0.25
1.75
6
5
4
3
1.50
VEN = 3.0V
1.25
VOUT = 1.8V
2
0.75
1
0.50
0
0.25
-1
-2
IIN
-3
Time (20µs/div)
Time (20µs/div)
Startup Waveform
4
3
2
1
0
1.50
VEN = 3.0V
1.25
VOUT = 1.8V
1.00
0.75
IIN
0.50
-1
0.25
-2
0.00
-3
-0.25
Input Current
(bottom) (A)
Output Voltage (top) (V)
1.75
5
Time (20µs/div)
8
0.00
-0.25
(VOUT = 1.8V; CFF = 100pF; RLOAD = 3Ω
Ω;
CIN = 4.7µF; COUT = 10µF; L = 2.2µH)
6
1.00
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Input Current
(bottom) (A)
4
Output Voltage (mid) (V)
(VOUT = 1.8V; CFF = 22pF; RLOAD = 3Ω
Ω;
CIN = 4.7µF; COUT = 10µF; L = 2.2µH)
Input Current
(bottom) (A)
Output Voltage (top) (V)
(VOUT = 1.8V; CFF = 0pF; RLOAD = 3Ω
Ω;
CIN = 4.7µF; COUT = 10µF; L = 2.2µH)
DATA SHEET
AAT1106
600mA Step-Down Converter
Functional Block Diagram
SLOPE
COMP
OSC
VIN
4
+
ISENSE
COMP
-
BLANKING
FB
5
+
EA
-
S
Q
R
Q
RS LATCH
PWM
LOGIC
R
NON-OV ERLA P
CONTROL
0.6V
COMP
+
DRV
-
LX
•
3
COUT
•
+
IZERO
COMP
-
V IN
V OUT
R1
+
OVDET
-
0.65V
VIN
2.7 - 5.5V
+
R2
GND
2
•
0.6V
EN
1
REF
SHUTDOWN
Functional Description
The AAT1106 is a high performance 600mA, 1.5MHz fixed
frequency monolithic switch-mode step-down converter
which uses a current mode architecture with an adaptive
slope compensation scheme. It minimizes external component size and optimizes efficiency over the complete
load range. The adaptive slope compensation allows the
device to remain stable over a wider range of inductor
values so that smaller values (1μH to 4.7μH) with associated lower DCR can be used to achieve higher efficiency.
Apart from the small bypass input capacitor, only a small
L-C filter is required at the output. The AAT1106 can be
programmed with external feedback to any voltage,
ranging from 0.6V to the input voltage. It uses internal
MOSFETs to achieve high efficiency and can generate
very low output voltage by using an internal reference of
0.6V. At dropout, the converter duty cycle increases to
100% and the output voltage tracks the input voltage
minus the low RDS(ON) drop of the P-channel high-side
MOSFET. The input voltage range is 2.5V to 5.5V. The
converter efficiency has been optimized for all load condi-
tions, ranging from no load to 600mA at VIN = 3V. The
internal error amplifier and compensation provides excellent transient response, load, and line regulation.
Current Mode PWM Control
Slope compensated current mode PWM control provides
stable switching and cycle-by-cycle current limit for
excellent load and line response and protection of the
internal main switch (P-channel MOSFET) and synchronous rectifier (N-channel MOSFET). During normal
operation, the internal P-channel MOSFET is turned on
for a specified time to ramp the inductor current at each
rising edge of the internal oscillator, and is switched off
when the feedback voltage is above the 0.6V reference
voltage. The current comparator, ICOMP, limits the peak
inductor current. When the main switch is off, the synchronous rectifier turns on immediately and stays on
until either the inductor current starts to reverse, as
indicated by the current reversal comparator, IZERO, or
the beginning of the next clock cycle.
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9
DATA SHEET
AAT1106
600mA Step-Down Converter
Control Loop
Dropout Operation
The AAT1106 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. An adaptive 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 the
adjustable output, the error amplifier reference is fixed
at 0.6V.
When the input voltage decreases toward the value of
the output voltage, the AAT1106 allows the main switch
to remain on for more than one switching cycle and
increases the duty cycle until it reaches 100%.
Enable
The enable pin is active high. When pulled low, the
enable input forces the AAT1106 into a low-power, nonswitching state. The total input current during shutdown
is less than 1μA.
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 150°C with 15°C of hysteresis. Once an
over-temperature or over-current fault conditions is
removed, the output voltage automatically recovers.
10
The duty cycle D of a step-down converter is defined as:
D = TON · FOSC · 100% ≈
VOUT
· 100%
VIN
Where TON is the main switch on time and FOSC is the
oscillator frequency (1.5MHz).
The output voltage then is the input voltage minus the
voltage drop across the main switch and the inductor. At
low input supply voltage, the RDS(ON) of the P-channel
MOSFET increases and the efficiency of the converter
decreases. Caution must be exercised to ensure the heat
dissipated does not exceed the maximum junction temperature of the IC.
Maximum Load Current
The AAT1106 will operate with an input supply voltage
as low as 2.5V; however, the maximum load current
decreases at lower input due to the large IR drop on the
main switch and synchronous rectifier. The slope compensation signal reduces the peak inductor current as a
function of the duty cycle to prevent sub-harmonic oscillations at duty cycles greater than 50%. Conversely, the
current limit increases as the duty cycle decreases.
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DATA SHEET
AAT1106
600mA Step-Down Converter
Applications Information
Figure 1 shows the basic application circuit of AAT1106.
L1
2.2μH
VIN
2.5V to 5.5V
LX
IN
C2
22pF
AAT1106
C1
4.7μF
VOUT
1.8V
C3
10μF
FB
EN
R2
634K
GND
R1
316K
Figure 1: Basic Application Circuit
Setting the Output Voltage
For applications requiring an adjustable output voltage,
the AAT1106-0.6 can be externally programmed.
Resistors R1 and R2 of Figure 2 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 R1 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 1 summarizes the resistor values for various output voltages
with R1 set to either 59kΩ for good noise immunity or
316kΩ for reduced no load input current.
The AAT1106, combined with an external feed forward
capacitor (C2 in Figure 2), delivers enhanced transient
response for extreme pulsed load applications. The addition of the feed forward capacitor typically requires a
larger output capacitor C3 for stability. The external
resistor sets the output voltage according to the following equation:
⎝
⎛
⎝
VOUT = 0.6 V · ⎛1 +
VOUT (V)
R1 = 59k
R1 = 316k
R2 (k)
R2 (k)
19.6
29.4
39.2
49.9
59.0
68.1
78.7
88.7
118
124
137
187
267
105
158
210
261
316
365
422
475
634
655
732
1000
1430
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
Table 1: Resistor Selection for Output Voltage
Setting; Standard 1% Resistor Values
Substituted Closest to the Calculated Values.
Inductor Selection
For most designs, the AAT1106 operates with inductor
values of 1μH to 4.7μH. Low inductance values are
physically smaller, but require faster switching, which
results in some efficiency loss. The inductor value can be
derived from the following equation:
L=
VOUT · (VIN - VOUT)
VIN · ∆IL · fOSC
Where IL is inductor ripple current. Large value inductors lower ripple current and small value inductors result
in high ripple currents. Choose inductor ripple current
approximately 35% of the maximum load current
600mA, or IL = 210mA.
R2
R1
or
V
-1
[⎛⎝ 0.6V
OUT
[⎛⎝
R2 =
· R1
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11
DATA SHEET
AAT1106
600mA Step-Down Converter
Part
Sumida
CR43
Sumida
CDRH4D18
Toko
D312C
L (μH)
Max
DCR
(m)
Rated DC
Current
(A)
1.4
2.2
3.3
4.7
1.0
2.2
3.3
4.7
1.5
2.2
3.3
4.7
56.2
71.2
86.2
108.7
4.5
75
110
162
120
140
180
240
2.52
1.75
1.44
1.15
1,72
1.32
1.04
0.84
1.29
1.14
0.98
0.79
Size
WxLxH
(mm)
4.5x4.0x3.5
4.7x4.7x2.0
for duty cycles greater than 50%. Using lower value
inductors provides better overall efficiency and also
makes it easier to standardize on one inductor for different required output voltage levels. In order to do this
and keep the step-down converter stable when the duty
cycle is greater than 50%, the AAT1106 separates the
slope compensation into 2 phases. The required slope
compensation is automatically detected by an internal
circuit using the feedback voltage VFB before the error
amp comparison to VREF.
VREF
3.6x3.6x1.2
Error Amp
VFB
Table 2: Typical Surface Mount Inductors.
For output voltages above 2.0V, when light-load efficiency is important, the minimum recommended inductor size is 2.2μH. For optimum voltage-positioning load
transients, choose an inductor with DC series resistance
in the 50m to 150m range. For higher efficiency at
heavy loads (above 200mA), or minimal load regulation
(with some transient overshoot), the resistance should
be kept below 100m. The DC current rating of the
inductor should be at least equal to the maximum load
current plus half the ripple current to prevent core saturation (600mA + 105mA). Table 2 lists some typical
surface mount inductors that meet target applications
for the AAT1106.
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. For example, the 2.2μH CR43 series inductor selected from Sumida has a 71.2mΩ DCR and a
1.75ADC current rating. At full load, the inductor DC loss
is 25mW which gives a 2.8% loss in efficiency for a
600mA, 1.5V output.
Slope Compensation
The AAT1106 step-down converter uses peak current
mode control with a unique adaptive slope compensation
scheme to maintain stability with lower value inductors
12
When below 50% duty cycle, the slope compensation is
0.284A/μs; but when above 50% duty cycle, the slope
compensation is set to 1.136A/μs. The output inductor
value must be selected so the inductor current down slope
meets the internal slope compensation requirements.
Below 50% duty cycle, the slope compensation requirement is:
m=
1.25
= 0.284A/µs
2·L
Therefore:
L =
0.625
= 2.2µH
m
Above 50% duty cycle,
m=
5
= 1.136A/µs
2·L
Therefore:
L =
2.5
= 2.2µH
m
With these adaptive settings, a 2.2μH inductor can be
used for all output voltages from 0.6V to 5V.
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DATA SHEET
AAT1106
600mA Step-Down Converter
Input Capacitor Selection
The input capacitor reduces the surge current drawn
from the input and switching noise from the device. The
input capacitor impedance at the switching frequency
shall be less than the input source impedance to prevent
high frequency switching current passing to the input. A
low ESR input capacitor sized for maximum RMS current
must be used. Ceramic capacitors with X5R or X7R
dielectrics are highly recommended because of their low
ESR and small temperature coefficients. A 4.7μF ceramic capacitor is sufficient for most applications.
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.
The maximum input capacitor RMS current is:
IRMS = IO ·
VO ⎛
V ⎞
· 1- O
VIN ⎝
VIN ⎠
for VIN = 2 · VO.
D · (1 - D) =
0.52 =
VO
IO
2
⎛
V ⎞
· 1- O
The term VIN ⎝ VIN ⎠ appears in both the input voltage
ripple and input capacitor RMS current equations and is
at maximum when VO is twice VIN. This is why the input
voltage ripple and the input capacitor RMS current ripple
are a maximum at 50% duty cycle. The input capacitor
provides a low impedance loop for the edges of pulsed
current drawn by the AAT1106. 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 (C1) can be seen in the
evaluation board layout in Figure 2. A laboratory test setup 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 lowESR 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.
Output Capacitor Selection
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 ⎠
IRMS(MAX) =
1
2
The output capacitor is required to keep the output voltage ripple small and to ensure regulation loop stability.
The output capacitor must have low impedance at the
switching frequency. Ceramic capacitors with X5R or
X7R dielectrics are recommended due to their low ESR
and high ripple current. The output ripple VOUT is determined by:
ΔVOUT ≤
⎞
VOUT · (VIN - VOUT) ⎛
1
· ESR +
⎝
VIN · fOSC · L
8 · fOSC · C3⎠
The output capacitor limits the output ripple and provides holdup during large load transitions. A 4.7μF to
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13
DATA SHEET
AAT1106
600mA Step-Down Converter
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:
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.
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:
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 TSOT23-5 package which is
150°C/W.
TJ(MAX) = PTOTAL · ΘJA + TA
Layout Guidance
When laying out the PC board, the following steps should
be taken to ensure proper operation of the AAT1106.
These items are also illustrated graphically in Figure 3.
1.
2.
3.
4.
Thermal Calculations
There are three types of losses associated with the
AAT1106 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 =
5.
The power traces (GND, LX, IN) should be kept short,
direct, and wide to allow large current flow. Place sufficient multiple-layer pads when needed to change
the trace layer.
The input capacitor (C1) should connect as closely as
possible to IN (Pin 4) and GND (Pin 2).
The output capacitor C3 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.
The feedback FB trace (Pin 5) 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. The
external feedback resistors should be placed as close
as possible to the FB pin (Pin 5) to minimize the
length of the high impedance feedback trace.
The resistance of the trace from the load return to
the 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 signal
ground and the power ground.
IO2 · (RDSON(HS) · VO + RDSON(LS) · [VIN - VO])
VIN
+ (tsw · F · IO + IQ) · VIN
14
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DATA SHEET
AAT1106
600mA Step-Down Converter
a: Top Layer
c: Bottom Layer
b: Internal GND Plane
d: Middle Layer
Figure 2: AAT1106 Four-Layer Layout Example with Internal GND Plane.
Evaluation Board Description
The AAT1106 evaluation board contains a fully tested
600mA, 1.5MHz Step-Down DC/DC Regulator. The circuit
has an input voltage range of 2.5V to 5.5V and four preset selectable outputs (1.2V, 1.5V, 1.8V and 2.5V).
The AAT1106 comes in a small 5-pin TSOT23 package
and the board has been optimized to fit small form factor
designs. An optional TVS (SM6T6V8A) is connected
between VIN and GND so that the evaluation board can
be used in a hot-plug application. These features, plus
the nominal operating frequency of 1.5MHz allowing the
use of low profile surface mount components, make the
AAT1106 evaluation board an ideal circuit for use in
battery-powered, hand-held applications.
A schematic of the complete circuit is shown in Figure 3.
The evaluation board layer details are provided in
Figures 4, 5, 6 and 7. Table 3 provides the component
list for the AAT1106 evaluation board.
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15
DATA SHEET
AAT1106
600mA Step-Down Converter
JP1
U1
AAT1106
1
2.5V ~ 5.5V
4
VIN
C1
4.7µH
5
EN
GND
2
IN
FB/VOUT
LX
L1
2.2µH
3
1.2V, 1.5V, 1.8V, 2.5V
VOUT
LX
C3
10µH
R2A 316k
R2B 470k
R2C 634k
R2D 1M
JP2
1 2
3 4
5 6
7 8
C2 22pF
R1
316k
Figure 3: AAT1106 Evaluation Board Schematic.
Component
Part#
U1
L1
C1
C2
C3
R1, R2A
R2B
R2C
R2D
No Designator
AAT1106
SF32-2R2M-R
GRM42-6X7R475K16PT
C1005COG1H220JT000P
GRM31BR71C106KA01L
Chip Resistor
Chip Resistor (optional)
Chip Resistor (optional)
Chip Resistor (optional)
SM6T6V8A (optional)
Description
1.5 MHz, 600mA Synchronous Step-Down Converter
INDUCTOR 2.2μH 1.8A SMD
CAP CERAMIC 4.7μF16V X7R 10% 1206
CAP CERAMIC 22pF 50V C0G 5% 0402
CAP CERAMIC 10μF 16V X7R 10% 1206
RES 316kΩ 1/16W 1% 0402 SMD
RES 470kΩ 1/16W 1% 0402 SMD
RES 634kΩ 1/16W 1% 0402 SMD
RES 1MΩ 1/16W 1% 0402 SMD
6.8V TVS
Manufacturer
Skyworks
Fenfa
MURATA
TDK
MURATA
ST
Table 3: AAT1106 Evaluation Board Component Listing.
16
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DATA SHEET
AAT1106
600mA Step-Down Converter
Figure 4: PCB Top Side.
Figure 6: PCB Midlayer 1 Side.
Figure 5: PCB Bottom Side.
Figure 7: PCB Midlayer 2 Side.
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17
DATA SHEET
AAT1106
600mA Step-Down Converter
Ordering Information
Output Voltage
Package
Marking1
Part Number (Tape & Reel)2
Adj. 0.6 to VIN
TSOT23-5
VVXYY
AAT1106ICB-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 Information3
TSOT23-5
1.900 BSC
0.450 ± 0.150
0.950 BSC
0.127 BSC
1.600 BSC
2.800 BSC
Detail "A"
End View
Top View
0.950 ± 0.150
2.900 BSC
0°
+10°
-0°
0.450 ± 0.150
0.050 ± 0.050
Side View
Detail "A"
All dimensions in millimeters.
1. XYY = assembly and date code.
2. Sample stock is generally held on part numbers listed in BOLD.
3. Package outline exclusive of mold flash and metal burr.
18
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DATA SHEET
AAT1106
600mA Step-Down Converter
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