Analogic AAT1145 1.5a step-down converter Datasheet

PRODUCT DATASHEET
AAT1145
SwitchRegTM
1.5A Step-Down Converter
General Description
Features
The AAT1145 SwitchReg™ is a 1.2MHz constant frequency current mode PWM step-down converter. It is
ideal for portable equipment requiring very high current
up to 1.5A from single-cell Lithium-ion batteries while
still achieving over 90% efficiency during peak load conditions. The AAT1145 also can run at 100% duty cycle
for low dropout operation, extending battery life in portable systems while light load operation provides very
low output ripple for noise sensitive applications.
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The AAT1145 can supply up to 1.5A output load current
from a 2.5V to 5.5V input voltage and the output voltage
can be regulated as low as 0.6V. The high switching frequency minimizes the size of external components while
keeping switching losses low. The internal slope compensation setting allows the device to operate with smaller
inductor values to optimize size and provide efficient
operation.
The AAT1145 is available in adjustable (0.6V to VIN) and
fixed (1.8V) output voltage versions. The device is available in a Pb-free, 3x3mm 10-lead TDFN package and is
rated over the -40°C to +85°C temperature range.
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Input Voltage Range: 2.5V to 5.5V
Output Voltages from 0.6V to VIN
1.5A Output Current
High Efficiency: Up to 95%
1.2MHz Constant Switching Frequency
Low RDS(ON) Internal Switches: 135mΩ, 95mΩ
Allows Use of Ceramic Capacitors
Current Mode Operation for Excellent Line and Load
Transient Response
Short-Circuit and Thermal Fault Protection
Soft Start
Low Dropout Operation: 100% Duty Cycle
Low Shutdown Current: ISHUTDOWN < 1μA
TDFN33-10 Package
-40°C to +85°C Temperature Range
Applications
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Cellular Phones
Digital Cameras
DSP Core Supplies
PDAs
Portable Instruments
Smart Phones
Typical Application
VIN 2.5V-5.5V
C1
10μF
1145.2008.01.1.2
L1
2.2μH
1
2
3
8
EN
LX
IN
LX
7
AAT1145-1.8 OUT
5
AIN
6
AGND
PGND
4
AGND
PGND
VOUT
1.8V,1.5A
C2
22μF
10
9
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1
PRODUCT DATASHEET
AAT1145
SwitchRegTM
1.5A Step-Down Converter
Pin Descriptions
Pin #
Symbol
1
EN
2
3
4, 6
IN
AIN
AGND
5
FB/OUT
7, 8
9, 10
LX
PGND
EP
Function
Enable pin. Active high. In shutdown, all functions are disabled drawing <1μA supply current. Do not
leave EN floating.
Power supply input pin. Must be closely decoupled to AGND with a 2.2μF or greater ceramic capacitor.
Analog supply input pin. Provides bias for internal circuitry.
Analog ground pin
FB pin (AAT1145IDE-0.6): Adjustable version feedback input. Connect FB to the center point of the
external resistor divider. The feedback threshold voltage is 0.6V.
OUT pin (AAT1145IDE-1.8): Fixed version feedback input. Connect OUT to the output voltage, VOUT.
Switching node pin. Connect the output inductor to this pin.
Power ground pin
Power ground exposed pad. Must be connected to bare copper ground plane.
Pin Configuration
TDFN-10
(Top View)
EN
IN
AIN
AGND
FB/OUT1
1
10
2
9
3
8
4
7
5
6
PGND
PGND
LX
LX
AGND
1. FB pin for the adjustable voltage version (AAT1145IDE-0.6), OUT pin for the fixed voltage version (AAT1145IDE-1.8).
2
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1145.2008.01.1.2
PRODUCT DATASHEET
AAT1145
SwitchRegTM
1.5A Step-Down Converter
Absolute Maximum Ratings1
Symbol
IN, AIN
VFB, VLX
VEN
PGND, AGND
TA
TSTORAGE
TLEAD
Description
Input Supply Voltages
FB, LX Voltages
EN Voltage
Ground Voltages
Operating Temperature Range
Storage Temperature
Lead Temperature (Soldering, 10s)
Value
Units
-0.3 to 6.0
-0.3 to VIN + 0.3
-0.3 to VIN + 0.3
-0.3 to 6.0
-40 to +85
-65 to 150
300
V
V
V
V
°C
°C
°C
Value
Units
45
2.2
°C/W
W
Thermal Information3
Symbol
θJA
PD
Description
Thermal Resistance2
Maximum Thermal 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 · θJA.
3. Thermal Resistance is specified with approximately 1 square inch of 1 oz. copper.
1145.2008.01.1.2
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3
PRODUCT DATASHEET
AAT1145
SwitchRegTM
1.5A Step-Down Converter
Electrical Characteristics1
VIN = 3.6V, TA = -40°C to +85°C unless otherwise noted; typical values are TA = 25°C.
Symbol
VIN
VOUT
Description
Conditions
IFB
Input Voltage Range
Output Voltage Range
Input DC Supply Active Mode
Current
Shutdown Mode
Feedback Input Bias Current
VFB
Regulated Feedback Voltage3
IQ
ΔVLINEREG/
ΔVIN
ΔVLOADREG/
ΔIOUT
VFB
VOUT
FOSC
TS
TSD
THYS
ILIM
RDS(ON)
VEN(L)
VEN(H)
IEN
Min
Typ
Max
Units
V
V
μA
μA
nA
0.6000
0.6000
0.6000
5.5
VIN
500
1
30
0.6120
0.6135
0.6150
0.20
%/V
2.5
0.6
2
VFB = 0.5V
VEN = 0V, VAIN = 5.5V
VFB = 0.65V
TA = 25°C
TA = 0°C ≤ TA ≤ 85°C
TA = -40°C ≤ TA ≤ 85°C
300
0.1
0.5880
0.5865
0.5850
Line Regulation
VIN = 2.5V to 5.5V, IOUT = 10mA
0.10
Load Regulation
IOUT = 10mA to 1500mA
0.20
Output Voltage Accuracy
VIN = 2.5 to 4.2V, IOUT = 10 to 1500mA
Oscillator Frequency
Startup Time
Over-Temperature Shutdown
Threshold
Over-Temperature Shutdown
Hysteresis
Peak Switch Current
P-CH MOSFET
N-CH MOSFET
Enable Threshold Low
Enable Threshold High
Input Low Current
VFB = 0.6V
From Enable to Output Regulation
-3
0.96
1.5
-1.0
%/A
+3
%
1.44
MHz
ms
170
°C
10
°C
2.5
135
95
VIN = 3.6V
VIN = 3.6V
VIN = VEN = 5.5V
1.2
1.3
V
A
200
150
0.3
1.0
mΩ
V
V
μA
1. The AAT1145 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.
2. VIN should be not less than VOUT + VDROPOUT, where VDROPOUT = IOUT · (RDS(ON)PMOS + ESRINDUCTOR), typically VDROPOUT = 0.3V.
3. The regulated feedback voltage is tested in an internal test mode that connects VFB to the output of the error amplifier.
4
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1145.2008.01.1.2
PRODUCT DATASHEET
AAT1145
SwitchRegTM
1.5A Step-Down Converter
Typical Characteristics
Efficiency vs. Output Current
Efficiency vs. Output Current
(VOUT = 3.3V; TA = 25°°C; L = 2.2µH)
(VOUT = 1.8V; TA = 25°°C; L = 2.2µH)
90
90
80
80
Efficiency (%)
100
Efficiency (%)
100
70
60
50
40
VIN = 3.7V
VIN = 4.2V
VIN = 5V
VIN = 5.5V
30
20
10
0
0.1
1
10
100
1000
70
60
50
VIN = 2.5V
VIN = 3.6V
VIN = 4.2V
VIN = 5V
VIN = 5.5V
40
30
20
10
0
0.1
10000
1
Output Current (mA)
80
Efficiency (%)
90
80
Efficiency (%)
100
90
70
60
50
VIN = 2.7V
VIN = 3.6V
VIN = 4.2V
VIN = 5V
VIN = 5.5V
10
0
0.1
1
10
100
1000
70
60
50
40
VIN = 2.5V
VIN = 3.6V
VIN = 4.2V
VIN = 5V
30
20
10
0
10000
0.1
1
100
1000
10000
DC Regulation
DC Regulation
(VOUT = 3.3V; TA = 25°C; L = 2.2µH; COUT = 22µF)
(VOUT = 1.8V; TA = 25°C; L = 2.2µH; COUT = 22µF)
1.854
VIN = 5.5V
VIN = 5V
VIN = 4.2V
VIN = 3.7V
3.366
3.333
3.300
3.267
3.234
0
300
600
900
1200
1500
Output Voltage (V)
3.399
Output Voltage (V)
10
Output Current (mA)
Output Current (mA)
3.201
10000
(VOUT = 1.2V; TA = 25°C; L = 2.2µH)
100
20
1000
Efficiency vs. Output Current
(VOUT = 1.5V; TA = 25°°C; L = 2.2µH)
30
100
Output Current (mA)
Efficiency vs. Output Current
40
10
1.818
1.800
1.782
1.764
1.746
Output Current (mA)
1145.2008.01.1.2
VIN = 5.5V
VIN = 5V
VIN = 4.2V
VIN = 3.6V
VIN = 2.5V
1.836
0
300
600
900
1200
1500
Output Current (mA)
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5
PRODUCT DATASHEET
AAT1145
SwitchRegTM
1.5A Step-Down Converter
Typical Characteristics
DC Regulation
DC Regulation
(VOUT = 1.5V; TA = 25°C; L = 2.2µH; COUT = 22µF)
(VOUT = 1.2V; TA = 25°C; L = 2.2µH; COUT = 22µF)
1.236
VIN = 5.5V
VIN = 5V
VIN = 4.2V
VIN = 3.6V
VIN = 2.5V
1.530
1.515
Output Voltage (V)
Output Voltage (V)
1.545
1.500
1.485
1.470
1.455
1.224
1.212
1.200
1.188
1.164
0
300
600
900
1200
VIN = 5V
VIN = 4.2V
VIN = 3.6V
VIN = 2.5V
1.176
1500
0
300
600
Output Current (mA)
Load Transient Response
(VIN = 3.6V; VOUT = 1.8V; COUT = 22µF; CFF = 22pF)
Output Voltage (top) (V)
Input Current (mA)
0.20
0.10
4.0
4.4
4.8
2.2
1.8
1.4
1.0
0.6
0.2
-0.2
1.5A
0.2A
5.2
Time (200µs/div)
Soft Start
(VIN = 3.6V; VOUT = 1.8V; IOUT = 1.5A; CFF = 100pF)
VOUT
(1V/div)
6
4
2
0
-2
1.8
1.3
0.8
Inductor Current
(bottom) (A)
Enable Voltage (top) (V)
Output Voltage (middle) (V)
Start-Up Response
(VOUT = 1.8V; VIN = 3.6V; No Load;
CIN = 10µF; COUT = 22µF; L = 2.2µH)
VENABLE
(2V/div)
Output Current (bottom) (A)
0.30
2.2
2.0
1.8
1.6
Input Voltage (V)
0.3
-0.2
Time (400µs/div)
Time (1ms/div)
6
1500
(VOUT = 3.3V; L = 2.2µH)
0.40
3.6
1200
Input Current vs. Input Voltage
0.50
0.00
900
Output Current (mA)
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1145.2008.01.1.2
PRODUCT DATASHEET
AAT1145
SwitchRegTM
1.5A Step-Down Converter
Typical Characteristics
Output Ripple
(VOUT = 1.8V; VIN = 3.6V; IOUT = 0A; L = 2.2µH)
5.00
4.00
3.00
3.6V
2.00
0.40
1.00
0.20
0.00
0.00
-1.00
Output Voltage (top) (V)
4.2V
1.81
1.80
1.79
0.300
0.200
0.100
0.000
-0.100
Inductor Current (bottom) (A)
1.82
6.00
Line Transient (top) (V)
Output Voltage (bottom) (VAC)
Line Transient Response
(VOUT = 1.8V; IOUT = 1.5A; L = 2.2µH; CIN = 10µF; COUT = 22µF)
Time (10µs/div)
Time (40µs/div)
Output Ripple
(VOUT = 1.8V; VIN = 3.6V; IOUT = 1.5A; L = 2.2µH)
Output Voltage (top) (V)
1.81
1.80
1.79
2.0
1.8
1.6
1.4
1.2
1.0
Inductor Current (bottom) (A)
1.82
Time (400ns/div)
1145.2008.01.1.2
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7
PRODUCT DATASHEET
AAT1145
SwitchRegTM
1.5A Step-Down Converter
Functional Block Diagram
OSC
SLOPE
COMP
IN
VIN 2.5V to 5.5V
ISENSE
AMP
0.6V
Softstart
SET
RESET
ICOMP
PWM
LOGIC
NON-OVERLAP
CONTROL
VOUT
LX
L1
OUT
R1*
R1*
COUT
OVDET
0.65V
Over-Temperature
and Short-Circuit
Protection
R2*
R2*
VIN
IZERO
COMP
PGND
0.6V
EN
REF
SHUTDOWN
AIN AGND
*The resistor divider R1 + R2 is internally set for the fixed output versions, and is externally set for the adjustable output versions.
Functional Description
The AAT1145 is a high output current monolithic switchmode step-down DC-DC converter. The device operates
at a fixed 1.2MHz switching frequency, and uses a slope
compensated current mode architecture. This step-down
DC-DC converter can supply up to 1500mA output current at VIN = 3.6V and has an input voltage range from
2.5V to 5.5V. It minimizes external component size and
optimizes efficiency at the heavy load range. The slope
compensation allows the device to remain stable over a
wider range of inductor values so that smaller values
(1μH to 4.7μH) with 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 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 near the input voltage. It
uses internal MOSFETs to achieve high efficiency and can
generate very low output voltages by using an internal
reference of 0.6V. At dropout, the converter duty cycle
8
increases to100% and the output voltage tracks the
input voltage minus the low RDS(ON) drop of the P-channel
high-side MOSFET and the inductor DCR. The internal
error amplifier and compensation provides excellent
transient response, load and line regulation. Internal
soft start eliminates any output voltage overshoot when
the enable or the input voltage is applied.
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 with 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 switched off
when the peak inductor current is above the error 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
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1145.2008.01.1.2
PRODUCT DATASHEET
AAT1145
SwitchRegTM
1.5A Step-Down Converter
until either the inductor current starts to reverse, as
indicated by the current reversal comparator, IZERO, or
the beginning of the next clock cycle.
Control Loop
The AAT1145 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 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. The enable pin is
active high. When pulled low, the enable input (EN)
forces the AAT1145 into a low-power, non-switching
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 2.5A. 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. The termination lasts for seven consecutive
clock cycles after a current limit has been sensed during
a series of four consecutive clock cycles.
1145.2008.01.1.2
Thermal protection completely disables switching when
internal dissipation becomes excessive. The junction
over-temperature threshold is 170°C with 10°C of hysteresis. Once an over-temperature or over-current fault
conditions is removed, the output voltage automatically
recovers.
Dropout Operation
When the battery input voltage decreases near the value
of the output voltage, the AAT1145 allows the main
switch to remain on for more than one switching cycle
and increases the duty cycle until it reaches 100%. 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. 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 AAT1145 will operate with an input supply voltage as
low as 2.5V, however, the maximum load current
decreases at lower input voltages due to a 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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PRODUCT DATASHEET
AAT1145
SwitchRegTM
1.5A Step-Down Converter
Applications Information
VIN 2.5V-5.5V
1
2
C1
10μF
3
EN
LX
IN
LX
AIN
AAT1145-0.6
6 AGND
4 AGND
FB
PGND
PGND
L1
2.2μH
8
7
VOUT
1.8V,1.5A
C3
22pF
5
10
R1
634kΩ
C2
22μF
delivers enhanced transient response for extreme pulsed
load applications. The addition of the feed forward
capacitor typically requires a larger output capacitor C2
for stability. The external resistor sets the output voltage
according to the following equation:
⎛
R1 ⎞
VOUT = 0.6V · 1 +
⎝
R2 ⎠
R2
316kΩ
9
R1 =
Figure 1: Basic Application Circuit for the
Adjustable Output Version.
VIN 2.5V-5.5V
C1
10μF
1
2
3
EN
LX
IN
LX
AIN
AAT1145-1.8 OUT
6 AGND
4 AGND
PGND
PGND
8
L1
2.2μH
VOUT
1.8V,1.5A
Table 1 shows the resistor selection for different output
voltage settings.
VOUT (V)
R2 = 59kΩ
R1 (kΩ)
R2 = 316kΩ
R1 (kΩ)
0.8
0.9
1.0
1.1
1.2
1.3
1.4
1.5
1.8
1.85
2.0
2.5
3.3
19.6
29.4
39.2
49.9
59.0
68.1
78.7
88.7
118
124
137
187
267
105
158
210
261
316
365
422
475
634
655
732
1000
1430
7
5
C2
22μF
10
9
Figure 2: Basic Application Circuit for the
Fixed Output Versions.
Setting the Output Voltage
Figure 1 shows the basic application circuit with the
AAT1145 adjustable output version while Figure 2 shows
the application circuit with the AAT1145 fixed output
version. For applications requiring an adjustable output
voltage, the AAT1145-0.6 adjustable version can be
externally programmed. Resistors R1 and R2 in Figure 1
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 1 summarizes the resistor values
for various output voltages with R2 set to either 59kΩ
for good noise immunity or 316kΩ for reduced no load
input current.
Table 1: Resistor Selections for Different Output
Voltage Settings (Standard 1% Resistors
Substituted For Calculated Values).
Inductor Selection
For most designs, the AAT1145 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:
The adjustable version of the AAT1145, combined with
an external feed forward capacitor (C3 in Figure 1),
10
⎛ VOUT ⎞
- 1 · R2
⎝ 0.6V ⎠
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L=
VOUT · (VIN - VOUT)
VIN · ΔIL · fOSC
1145.2008.01.1.2
PRODUCT DATASHEET
AAT1145
SwitchRegTM
1.5A Step-Down Converter
Where ΔIL is inductor ripple current. Large value inductors lower ripple current and small value inductors result
in high ripple currents. Choose inductor ripple current
approximately 30% of the maximum load current
1500mA, or
ΔIL = 450mA
For output voltages above 2.0V, when light-load efficiency is important, the minimum recommended inductor is 2.2μH.
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 optimum voltage-positioning load transients, choose an inductor with DC series resistance in
the 20mΩ to 100mΩ range. For higher efficiency at
heavy loads (above 200mA), or minimal load regulation
(but some transient overshoot), the resistance should be
kept below 100mΩ. 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
(1500mA + 225mA). Table 2 lists some typical surface
mount inductors that meet target applications for the
AAT1145.
The worst case external current slope (m) using the
2.2μH inductor is when VOUT = 3.3V and is:
m=
To keep the power supply stable when the duty cycle is
above 50%, the internal slope compensation (mA)
should be:
ma ≥
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
should be less than the input source impedance to prevent high frequency switching current passing to the
input. The calculated value varies with input voltage and
is a maximum when VIN is double the output voltage.
CIN =
CIN(MIN) =
Slope Compensation
1145.2008.01.1.2
1
· m = 0.75A/µs
2
Therefore, to guarantee current loop stability, the slope
of the compensation ramp must be greater than one-half
of the down slope of the current waveform. So the internal slope compensated value of 1A/μs will guarantee
stability using a 2.2μH inductor value for all output voltages from 0.6V to 3.3V.
For example, the 2.2μH SD3118-2R2-R inductor selected
from Coiltronics has a 74mΩ DCR and a 2.00ADC current
rating. At full load, the inductor DC loss is 106mW which
gives a 5% loss in efficiency for a 1200mA, 1.8V output.
The AAT1145 step-down converter uses peak current
mode control with slope compensation for stability when
duty cycles are greater than 50%. The slope compensation is set to maintain stability with lower value inductors
which provide better overall efficiency. The output inductor value must be selected so the inductor current down
slope meets the internal slope compensation requirements. As an example, the value of the slope compensation is set to 1A/μs which is large enough to guarantee
stability when using a 2.2μH inductor for all output voltage levels from 0.6V to 3.3V.
VOUT 3.3
=
= 1.5A/µs
L
2.2
V ⎞
VO ⎛
· 1- O
VIN ⎝
VIN ⎠
⎛ VPP
⎞
- ESR · fS
⎝ IO
⎠
1
⎛ VPP
⎞
- ESR · 4 · fS
⎝ IO
⎠
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 10μF ceramic capacitor for most applications is sufficient. A large
value may be used for improved input voltage filtering.
The maximum input capacitor RMS current is:
www.analogictech.com
IRMS = IO ·
VO ⎛
V ⎞
· 1- O
VIN ⎝
VIN ⎠
11
PRODUCT DATASHEET
AAT1145
SwitchRegTM
1.5A Step-Down Converter
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.
IRMS(MAX) =
1
· IO
2
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 Figures 3 and 4.
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.
Output Capacitor Selection
The function of output capacitance is to store energy to
attempt to maintain a constant voltage. The energy is
stored in the capacitor’s electric field due to the voltage
applied.
The value of output capacitance is generally selected to
limit output voltage ripple to the level required by the
specification. Since the ripple current in the output inductor is usually determined by L, VOUT and VIN, the series
impedance of the capacitor primarily determines the output voltage ripple. The three elements of the capacitor
that contribute to its impedance (and output voltage
12
ripple) are equivalent series resistance (ESR), equivalent
series inductance (ESL), and capacitance (C).
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 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 two switching cycles to
the output capacitance can be estimated by:
COUT =
2 · ΔILOAD
VDROOP · fS
In many practical designs, to get the required ESR, a
capacitor with much more capacitance than is needed
must be selected.
For both continuous or discontinuous inductor current
mode operation, the ESR of the COUT needed to limit the
ripple to ∆VO, V peak-to-peak is:
ESR ≤
ΔVO
ΔIL
Ripple current flowing through a capacitor’s ESR causes
power dissipation in the capacitor. This power dissipation
causes a temperature increase internal to the capacitor.
Excessive temperature can seriously shorten the expected life of a capacitor. Capacitors have ripple current ratings that are dependent on ambient temperature and
should not be exceeded. The output capacitor ripple current is the inductor current, IL, minus the output current,
IO. The RMS value of the ripple current flowing in the
output capacitance (continuous inductor current mode
operation) is given by:
IRMS = ΔIL ·
3
= ΔIL · 0.289
6
ESL can be a problem by causing ringing in the low
megahertz region but can be controlled by choosing low
ESL capacitors, limiting lead length (PCB and capacitor),
and replacing one large device with several smaller ones
connected in parallel.
www.analogictech.com
1145.2008.01.1.2
PRODUCT DATASHEET
AAT1145
SwitchRegTM
1.5A Step-Down Converter
In conclusion, in order to meet the requirement of output voltage ripple small and regulation loop stability,
ceramic capacitors with X5R or X7R dielectrics are recommended due to their low ESR and high ripple current
ratings. The output ripple VOUT is determined by:
Layout Guidance
When laying out the PC board, the following layout
guideline should be followed to ensure proper operation
of the AAT1145:
1.
1
VOUT · (VIN - VOUT) ⎛
⎞
ΔVOUT ≤
· ⎝ESR +
8 · fOSC · COUT ⎠
VIN · fOSC · L
2.
A 22μF ceramic capacitor can satisfy most applications.
Thermal Calculations
There are three types of losses associated with the
AAT1145 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 =
3.
4.
5.
IO2 · (RDSON(HS) · VO + RDSON(LS) · [VIN - VO])
VIN
6.
+ (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:
7.
The exposed pad (EP) must be reliably soldered to
the GND plane. A PGND pad below EP is strongly
recommended.
The power traces, including the GND trace, the LX
trace and the IN trace should be kept short, direct
and wide to allow large current flow. The L1 connection to the LX pins should be as short as possible.
Use several VIA pads when routing between layers.
The input capacitor (C1) should connect as closely as
possible to IN (Pin 2) and AGND (Pins 4 and 6) to get
good power filtering.
Keep the switching node, LX (Pins 7 and 8), away
from the sensitive FB/OUT node.
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 FB pin
(Pin 5) to minimize the length of the high impedance
feedback trace.
The output capacitor C2 and L1 should be connected
as closely as possible. The connection of L1 to the LX
pin should be as short as possible and there should
not be any signal lines under the inductor.
The resistance of the trace from the load return to
PGND 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.
Figures 4 and 5 show an example of a layout with 2
layers.
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 θJA for the DFN-10 package which is
45°C/W.
TJ(MAX) = PTOTAL · ΘJA + TAMB
1145.2008.01.1.2
www.analogictech.com
13
PRODUCT DATASHEET
AAT1145
SwitchRegTM
1.5A Step-Down Converter
Manufacturer
Part Number
Inductance
(μH)
Max DC
Current (A)
DCR (mΩ)
Size (mm)
LxWxH
Type
Sumida
Sumida
Sumida
Coiltronics
Coiltronics
CDRH3D16
CDRH4D15
CDRH5D16
SD3118-2R2-R
SD3114-2R2-R
2.2
3.3
4.7
2.2
2.2
1.75
1.6
2.15
2.00
1.74
47
71
51.3
74
110
4.0x4.0x1.8
4.7x4.7x1.7
5.8x5.8x1.8
3.1x3.1x1.8
3.1x3.1x1.4
Shielded
Shielded
Shielded
Shielded
Shielded
Manufacturer
Part Number
Value
Voltage (V)
Temp. Co.
Case
Murata
Murata
Murata
GRM219R60J106KE19
GRM21BR60J226ME39
GRM1551X1E220JZ01B
10μF
22μF
22pF
6.3
6.3
25
X5R
X5R
JIS
0805
0805
0402
Table 2. Suggested Component Selection Information
L1
2.2μH
VIN 2.5V-5.5V
C1
10μF
EN
LX
IN
LX
AIN
AAT1145
C3
22pF
R1
634k
FB
EP
C2
22μF
R2
316k
PGND
AGND
AGND
VOUT
1.8V,1.5A
PGND
U1: AAT1145 TDFN33-10
L1: SD3118-2R2-R
C1: 10μF 6.3V 0805 X5R
C2: 22μF 6.3V 0805 X5R
Figure 3: AAT1145 Adjustable Voltage Version Recommended Evaluation Board Schematic.
Figure 4: AAT1145 Evaluation
Board Top Layer.
14
Figure 5: Exploded View of AAT1145
Evaluation Board Top Layer.
www.analogictech.com
1145.2008.01.1.2
PRODUCT DATASHEET
AAT1145
SwitchRegTM
1.5A Step-Down Converter
Step-Down Converter Design Example
Specifications
VO = 1.8V @1.5A
VIN = 2.7V to 4.2V (3.6V nominal)
fS = 1.2MHz
Transient droop = 100mV
∆VO = 50mV
1.8V Output Inductor
ΔIL = 30% ⋅ IO = 0.3 · 1.5 = 450mA
L=
VOUT · (VIN(MAX) - VOUT)
1.8 · (4.2 - 1.8)
=
=1.90µH
VIN(MAX) ⋅ ΔIL ⋅ fOSC
4.2 ⋅ 0.45 · 1.2 · 106
For Cooper 2.2μH inductor (SD3118-2R2-R) with DCR 74mΩ, the ∆IL should be
ΔIL =
VO ⎛ VO ⎞
⋅ 1· T = 341mA
L ⎝ VIN ⎠
IPKL = IO +
0.341
ΔIL
= 1.5 +
= 1.67A
2
2
PL = IO2 ⋅ DCR = 1.52 ⋅ 0.074 = 166.5mW
1.8V Output Capacitor
COUT =
2 · ΔILOAD
2 · 1.2
=
= 20µF; use 22µF
VDROOP · fS
0.1 · 1.2 · 106
ESR ≤
ΔVO
0.05
=
= 0.15Ω
ΔIL
0.341
Select a 22μF, 10mΩ ESR ceramic capacitor to meet the ripple 50mV requirement.
ΔVOUT ≤
=
1
VOUT · (VIN - VOUT) ⎛
⎞
· ⎝ESR +
8 · fOSC · COUT ⎠
VIN · fOSC · L
1.8 · (4.2 - 1.8)
1
⎛
⎞
· ⎝ 0.01 +
= 6mV
6
-6
6
-6 ⎠
4.2 · 1.2 · 10 · 2.2 · 10
8 · 1.2 · 10 · 22 · 10
IRMS = ΔIL ·0.289 = 0.341 · 0.289 = 98.5mArms
PCOUT = ESR · IRMS2 = 0.01 · 0.09852 = 97μW
1145.2008.01.1.2
www.analogictech.com
15
PRODUCT DATASHEET
AAT1145
SwitchRegTM
1.5A Step-Down Converter
Input Capacitor
Input ripple VPP = 50mV
CIN(MIN) =
IRMS =
1
⎛ VPP
⎞
- ESR · 4 · fS
⎝ IO
⎠
=
1
⎛ 0.05
⎞
- 0.01 · 4 · 1.2 · 106
⎝ 1.5
⎠
= 8.9µF; use 10µF
IO
1.5
=
= 750mArms
2
2
PCIN = ESR · IRMS2 = 0.01 · 0.752 = 5.6mW
AAT1145 Losses
PTOTAL = IO2 · RDS(ON)P · D + IO2 · RDS(ON)N · (1 - D) + (tSW · fS · IO) · VIN
= 1.52 · 0.135 ·
16
1.8
1.8⎞
⎛
+ 1.52 · 0.095 · 1 + (5 · 10-9 · 1.2 · 106 · 1.5) · 4.2 = 290mW
⎝
4.2
4.2⎠
www.analogictech.com
1145.2008.01.1.2
PRODUCT DATASHEET
AAT1145
SwitchRegTM
1.5A Step-Down Converter
Ordering Information
Output Voltage
Package
Marking1
Part Number (Tape and Reel)2
Adj. 0.6V to VIN
Fixed 1.8V
TDFN33-10
TDFN33-10
QNXYY
WUXYY
AAT1145IDE-0.6-T1
AAT1145IDE-1.8-T1
Package Information3
TDFN33-10
Pin 1 dot by marking
0.500 BSC
1.70 ± 0.05
3.00 ± 0.05
0.23 ± 0.05
Pin 1 identification
R0.200
0.40 ± 0.05
3.00 ± 0.05
2.40 ± 0.05
Top View
0.05 ± 0.05
0.203 REF
0.75 ± 0.05
Bottom View
Side View
All dimensions in millimeters.
1. XYY = assembly and date code.
2. Sample stock is generally held on all part numbers listed in BOLD.
3. The leadless package family, which includes QFN, TQFN, DFN, TDFN and STDFN, has exposed copper (unplated) at the end of the lead terminals due to the manufacturing
process. A solder fillet at the exposed copper edge cannot be guaranteed and is not required to ensure a proper bottom solder connection.
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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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relating to fitness for a particular purpose, merchantability, or infringement of any patent, copyright or other intellectual property right. In order to minimize risks associated with the customer’s applications, adequate
design and operating safeguards must be provided by the customer to minimize inherent or procedural hazards. Testing and other quality control techniques are utilized to the extent AnalogicTech deems necessary to
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1145.2008.01.1.2
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17