AAT1155IKS-1.0-T1 [ANALOGICTECH]
1MHz 2.5A Step-Down DC/DC Converter; 1MHz的2.5A降压型DC / DC转换器
| 型号： | AAT1155IKS-1.0-T1 |
| 厂家： | 
ADVANCED ANALOGIC TECHNOLOGIES |
| 描述： | 1MHz 2.5A Step-Down DC/DC Converter |
| 文件： | 总17页 (文件大小：389K) |
| 中文： | 中文翻译 | 下载： | 下载PDF数据表文档文件 |
AAT1155
1MHz 2.5A Step-Down DC/DC Converter
™
SwitchReg
General Description
Features
The AAT1155 SwitchReg™ is a member of
AnalogicTech's Total Power Management IC™
(TPMIC™) product family. The step-down switch-
ing converter is ideal for applications where high
efficiency, small size, and low ripple are critical.
Able to deliver 2.5A with an internal power MOS-
FET, the current-mode controlled IC provides high
efficiency. Fully internally compensated, the
AAT1155 simplifies system design and lowers
external parts count.
•
•
•
•
•
5.5V Max Supply Input
Fixed or 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 PWM Mode
Low Output Ripple With Light Load
Internal Soft Start
Current Limit Protection
Over-Temperature Protection
MSOP-8 Package
•
•
•
•
•
•
•
•
•
The AAT1155 is available in a Pb-free MSOP-8
package and is rated over the -40°C to +85°C tem-
perature range.
-40°C to +85°C Temperature Range
Applications
•
•
•
Cable/DSL Modems
Computer Peripherals
High Efficiency Conversion from 5V or 3.3V
Supply
•
•
Network Cards
Set-Top Boxes
Typical Application
INPUT
VP
10µF
FB
LX
LX
AAT1155
1.5µH
ENABLE
VCC
100Ω
0.1µF
OUTPUT
120µF
GND
1155.2005.11.1.6
1
AAT1155
1MHz 2.5A Step-Down DC/DC Converter
Pin Descriptions
Pin #
Symbol
Function
1
2
FB
GND
EN
Feedback input pin.
Signal ground.
3
Converter enable pin.
4
VCC
VP
Small signal filtered bias supply.
Input supply for converter power stage.
Inductor connection pin.
5, 8
6, 7
LX
Pin Configuration
MSOP-8
1
8
VP
FB
2
3
4
7
6
5
GND
EN
LX
LX
VP
VCC
2
1155.2005.11.1.6
AAT1155
1MHz 2.5A Step-Down DC/DC Converter
Absolute Maximum Ratings1
TA = 25°C, unless otherwise noted.
Symbol
Description
Value
Units
VCC, VP
VLX
VCC, VP to GND
6
V
V
LX to GND
-0.3 to VP+0.3
-0.3 to VCC+0.3
-0.3 to VCC+0.3
-40 to 150
300
VFB
FB to GND
V
VEN
EN to GND
V
TJ
Operating Junction Temperature Range
Maximum Soldering Temperature (at leads, 10 sec)
ESD Rating2 - HBM
°C
°C
V
TLEAD
VESD
3000
Thermal Characteristics3
Symbol
Description
Value
Units
ΘJA
PD
Maximum Thermal Resistance
Maximum Power Dissipation
150
833
°C/W
mW
Recommended Operating Conditions
Symbol
Description
Rating
Units
T
Ambient Temperature Range
-40 to +85
°C
1. Stresses above those listed in Absolute Maximum Ratings may cause permanent damage to the device. Functional operation at condi-
tions other than the operating conditions specified is not implied. Only one Absolute Maximum Rating should be applied at any one time.
2. Human body model is a 100pF capacitor discharged through a 1.5kΩ resistor into each pin.
3. Mounted on a demo board (FR4, in still air).
1155.2005.11.1.6
3
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
Description
Conditions
Min Typ Max Units
VIN
Input Voltage Range
2.7
5.5
2.5
V
VOUT
Output Voltage Tolerance
VIN = VOUT + 0.2 to 5.5V,
IOUT = 0.5A
-2.5
%
VIL
VIH
Input Low Voltage
Input High Voltage
0.6
2.5
V
V
1.4
1.2
V
IN Rising
VUVLO
Under-Voltage Lockout
V
VIN Falling
VUVLO(HYS)
IQ
ISHDN
ILIM
RDS(ON)L
η
∆VOUT (VOUT*∆VIN) Load Regulation
Under-Voltage Lockout Hysteresis
Quiescent Supply Current
Shutdown Current
250
mV
No Load, VFB = 0V
VEN = 0V, VIN = 5.5V
TA = 25°C
630 1000 µA
1.0
µA
A
Current Limit
4.4
High Side Switch On Resistance TA = 25°C
60
92
mΩ
%
Efficiency
IOUT = 1.0A
ILOAD = 0A to 2.5A
VIN = 2.7V to 5.5V
TA = 25°C
2.3
0.75
1
%
∆VOUT/VOUT
FOSC
Line Regulation
%/V
MHz
°C
Oscillator Frequency
Over-Temperature Shutdown
Threshold
TSD
140
THYS
Over-Temperature Shutdown
Hysteresis
15
°C
4
1155.2005.11.1.6
AAT1155
1MHz 2.5A Step-Down DC/DC Converter
Typical Characteristics
RDS(ON) vs. Temperature
Oscillator Frequency Variation
vs. Input Voltage
(VOUT = 3.3V)
90
80
70
60
50
40
0.5
2.7V
3.6V
4.2V
0.25
0
5.5V
5.0V
-0.25
-0.5
-20
0
20
40
60
80
100
120
3.5
4
4.5
5
5.5
Temperature (°C)
Input Voltage (V)
Oscillator Frequency Variation vs. Temperature
(VIN = 5V)
RDS(ON) vs. Input Voltage
(IDS = 1A)
80
75
70
65
60
55
50
45
40
1
0
-1
-2
-3
-4
2.5
3
3.5
4
4.5
5
5.5
-20
0
20
40
60
80
100
Input Voltage (V)
Temperature (°C)
Enable Threshold vs. Input Voltage
Output Voltage Variation vs. Temperature
(IOUT = 2A; VO = 3.3V)
1.2
0.4
0.2
0
1.1
1
EN(H)
0.9
0.8
0.7
0.6
-0.2
-0.4
-0.6
-0.8
EN(L)
2.5
3
3.5
4
4.5
5
5.5
-20
0
20
40
60
80
100
Input Voltage (V)
Temperature (°°C)
1155.2005.11.1.6
5
AAT1155
1MHz 2.5A Step-Down DC/DC Converter
Typical Characteristics
AAT1155 Evaluation Board
Over-Temperature Current vs. Input Voltage
(VOUT = 3.3V)
Line Regulation
(VOUT = 3.3V)
1
0
3.6
3.4
55°C
3.2
I
O = 0.3A
3
-1
-2
-3
-4
-5
70°C
2.8
2.6
2.4
85°C
2.2
2
100°C
IO = 3.0A
1.8
1.6
3.5
3.75
4
4.25
4.5
4.75
5
5.25
5.5
3.4
3.7
4
4.3
4.6
4.9
5.2
5.5
Input Voltage (V)
Input Voltage (V)
Load Regulation
(VIN = 5.0V; VOUT = 3.3V)
Non-Switching Operating
Current vs. Temperature
(FB = 0V)
750
0.0
-1.0
-2.0
-3.0
-4.0
-5.0
-6.0
-7.0
-8.0
-9.0
700
650
600
550
500
450
2.7V 3.6V 4.2V
5.0V 5.5V
-10.0
0.01
0.10
1.00
10.00
-20
0
20
40
60
80
100
120
Load Current (A)
Temperature (°C)
Over-Temperature Shutdown
Current vs. Temperature
(VOUT = 3.3V; VIN = 5.0V)
Inrush and Output
Overshoot Characteristics
8
6
14
12
10
8
5
4.5
4
2
4
3.5
3
0
6
-2
-4
-6
-8
4
2
2.5
2
0
-2
0
0.4
0.8
1.2
1.6
2
-20 -10
0
10 20
30
40
50 60
70
80
90 100
Time (ms)
Temperature (°C)
6
1155.2005.11.1.6
AAT1155
1MHz 2.5A Step-Down DC/DC Converter
Typical Characteristics
Tantalum Output Ripple
(IOUT = 3.0A; VOUT = 3.3V; VIN = 5.0V)
Loop Crossover Gain and Phase
16
12
8
180
135
90
L = 1.5µHy
0.04
0.02
7
6
5
4
3
2
1
0
-1
Phase
0.00
4
45
-0.02
-0.04
-0.06
-0.08
-0.10
-0.12
0
0
200µF gain
-4
-45
-90
-135
-180
300µF gain
-8
100µF 6.3 Ceramic
TDK P/N C3225X5R0J107M
Vishay GRM43SR60J107ME20L
-12
-16
120µF 6.3V Tantalum
Vishay P/N 594D127X96R3C2T
10000
100000
0
1
2
3
4
5
Frequency (Hz)
Time (µµs)
Output Ripple
(IOUT = 3.0A; VOUT = 3.3V; VIN = 5.0V)
Loop Crossover Gain and Phase
16
12
8
180
135
90
4
7
6
5
4
3
2
1
0
-1
2
0
4
45
Gain
-2
0
0
-4
-4
-45
-90
-135
-6
-8
-8
200µF 6.3V Ceramic
TDK P/N C3325X5R0J107M
Vishay GRM43SR60J107ME20L
-10
-12
120µF 6.3V Tantalum
Vishay P/N 594D127X96R3C2T
-12
-16
-180
0
1
2
3
4
5
10000
100000
1000000
Time (µs)
Frequency (Hz)
Tantalum Transient Response
(IOUT = 0 to 3.0A; VOUT = 3.3V; VIN = 5.0V)
Output Ripple
(IOUT = 3.0A; VOUT = 3.3V; VIN = 5.0V)
4
2
0
7
3.40
3.30
3.20
3.10
3.00
2.90
2.80
2.70
2.60
7
6
5
4
3
2
1
0
-1
6
5
4
3
2
1
0
-1
-2
-4
-6
-8
300µF 6.3VCeramic
TDK P/N C3325X5R0J107M
Vishay GRM43SR60J107ME20L
-10
-12
120µF 6.3V Tantalum
Vishay P/N 594D127X96R3C2T
0
1
2
3
4
5
0
100
200
300
400
500
Time (µµs)
Time (µµs)
1155.2005.11.1.6
7
AAT1155
1MHz 2.5A Step-Down DC/DC Converter
Typical Characteristics
Transient Response
(IOUT = 0 to 3.0A; VOUT = 3.3V; VIN = 5.0V)
Transient Response
(IOUT = 0 to 3.0A; VOUT = 3.3V; VIN = 5.0V)
3.40
3.30
7
6
5
4
3
2
1
0
-1
3.40
3.30
3.20
3.10
3.00
2.90
2.80
2.70
2.60
7
6
5
4
3
2
1
0
-1
3.20
2x 100µF 6.3V Ceramic
TDK P/N C3325X5R0J107M
Vishay GRM43SR60J107ME20L
3x 100µF 6.3V Ceramic
TDK P/N C3325X5R0J107M
Vishay GRM43SR60J107ME20L
3.10
3.00
2.90
2.80
2.70
2.60
0
100
200
300
400
500
0
100
200
300
400
500
Time (µs)
Time (µµs)
8
1155.2005.11.1.6
AAT1155
1MHz 2.5A Step-Down DC/DC Converter
Functional Block Diagram
VCC
VP = 2.5V to 5.5V
REF
CMP
DH
OP. AMP
FB
LOGIC
LX
Temp.
Sensing
OSC
EN
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.
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 sta-
bility for duty cycles greater than 50%. The inner
loop appears as a voltage-programmed current
source in parallel with the output capacitor.
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 volt-
age 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.
The voltage error amplifier output programs the cur-
rent loop for the necessary inductor current to force
a constant output voltage for all load and line con-
ditions. The feedback resistive divider is internal,
dividing the output voltage to the error amplifier ref-
erence voltage of 1.0V. The error amplifier has a
limited DC gain. This eliminates the need for exter-
nal 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.
Power and Signal Source
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.
1155.2005.11.1.6
9
AAT1155
1MHz 2.5A Step-Down DC/DC Converter
Vin 2.7V-5.5V
VOUT 1.25V @ 2.5A
R3
2.55k
U1
AAT1155-1.0
R1 R2
100 100k
FB
VP
C4
100µF
L1
1.5µH
GND LX
EN LX
VCC VP
C1
C2
D1
B340LA 120µF
10µF
C3
0.1µF
R4
10.0k
rtn
C1 Murata 10µF 6.3V X5R GRM42-6X5R106K6.3
C2 Vishay 120µF 6.3V 594D127X96R6R3C2T
C3 0.1µF 0603ZD104M AVX
C4 Vishay Sprague 100µF 16V 595D107X0016C 100µF 16V
D1 B340LA Diodes Inc.
L1 CDRH6D28-1.5µH Sumida
Options
C2 MuRata 100uF 6.3V GRM43-2 X5R 107M 100µF 6.3V (two or three in parallel)
C2 TDK 100µF 6.3V C3325X5R0J107M 100µF 6.3V (two or three in parallel)
Figure 1: AAT1155 Evaluation Board with Adjustable Output.
peak ripple current is 1A. Assuming a 5V ±5% input
voltage and 40% ripple, the output inductance
required is:
Current Limit and Over-Temperature
Protection
Over-temperature and current limit circuitry pro-
tects the AAT1155 and the external Schottky diode
during overload, short-circuit, and excessive ambi-
ent temperature conditions. The junction over-tem-
perature threshold is 140°C nominal and has 15°C
of hysteresis. Typical graphs of the over-tempera-
ture load current vs. input voltage and ambient
temperature are shown in the Typical Character-
istics section of this document.
VOUT
IOUT · k · FSW
VOUT
VIN(MAX)
⎛
⎞
⎠
L =
· 1 -
⎝
3.3V
2.5A · 0.4 · 1MHz
⎞
⎛
⎝
⎞
⎠
⎛
⎝
3.3V
5.25V⎠
=
· 1 -
= 1.23µH
The factor "k" is the fraction of the full load (40%)
selected for the ripple current at the maximum input
voltage.
Inductor
The output inductor is selected to limit the ripple cur-
rent to 20% to 40% of the full load current at the max-
imum input voltage. Manufacturer's specifications list
both the inductor DC current rating, which is a ther-
mal limitation, and the peak current rating, which is
determined by the inductor saturation characteristics.
The inductor should not show any appreciable satu-
ration 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 exces-
sive losses due to a high DCR. The losses associat-
ed with the DCR and its effect on the total converter
efficiency must be considered.
The corresponding inductor RMS current is:
∆I2
12
⎛
⎝
⎞
⎠
2
IRMS
=
IO
+
I = 2.5A
≈
O
∆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 11mΩ 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.
For a 2.5A load and the ripple current set to 40% at
the maximum input voltage, the maximum peak-to-
10
1155.2005.11.1.6
AAT1155
1MHz 2.5A Step-Down DC/DC Converter
3A Surface Mount Schottky Diodes
Schottky Freewheeling Diode
The Schottky average current is the load current
multiplied by one minus the duty cycle.
Diodes Inc.
ROHM
B340LA
RB050L-40
5820SM
0.45V @ 3A
0.45 @ 3A
0.46V @ 3A
Micro Semi
VO
⎛
⎝
⎞
⎠
1-
V
IN
Input Capacitor Selection
The primary function of the input capacitor is to pro-
vide a low impedance loop for the edges of pulsed
current drawn by the AAT1155. A low ESR/ESL
ceramic capacitor is ideal for this function. To mini-
mize stray inductance, the capacitor should be
placed as closely as possible to the IC. This also
keeps the high frequency content of the input cur-
rent localized, minimizing the radiated and con-
ducted EMI while facilitating optimum performance
of the AAT1155. Proper placement of the input
capacitor C1 is shown in the layout in Figure 2.
Ceramic X5R or X7R capacitors are ideal. The
size required will vary depending on the load, out-
put 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 high-
est when the input voltage is double the output volt-
age where it is one half of the load current.
For VIN at 5V and VOUT at 3.3V, the average diode
current is:
VO
3.3V
⎛
⎝
⎞
⎛
⎞
=
IAVG = IO ·
·
1-
= 2.5A 1-
0.85A
V ⎠
⎝
5.0V⎠
IN
With a 125°C maximum junction temperature and a
120°C/W thermal resistance, the maximum aver-
age current is:
T
J(MAX)- T
125°C - 70°C
AMB
IAVG
=
=
= 1.14A
θ
J-A · VFWD 120°C/W 0.4V
·
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 with-
stand the input voltage.
V
VO
⎛
⎞
⎠
O
IRMS = IO ·
· 1-
V
V
⎝
IN
IN
Vin 3.5V-5.5V
Vout 3.3V @2.5A
U1
AAT1155-3.3
R1 R2
100 100k
FB VP
C4
100µF
L1
1.5µH
GND LX
EN LX
VCC VP
C1
+
-
C2
D1
B340LA 120µF
10µF
C3
0.1µF
rtn
C1 Murata 10µF 6.3V X5R GRM42-6X5R106K6.3
C2 Vishay120µF 6.3V 594D127X96R6R3C2T
C3 0.1µF 0603ZD104M AVX
C4 Vishay Sprague 100µF 16V 595D107X0016C 100µF 16V
D1 B340LA Diodes Inc.
L1 CDRH6D28-1.5µH Sumida
Options
C2 MuRata 100µF 6.
3V (two or three in parallel)
3V GRM43-2 X5R 107M 100µF 6.
C2 TDK 100µF 6.3V C3325X5R0J107M 100µF 6.3V (two or three in parallel)
Figure 2: 3.3V, 3A Output Efficiency.
1155.2005.11.1.6
11
AAT1155
1MHz 2.5A Step-Down DC/DC Converter
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 oscilla-
tions 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.
Layout
Figures 3 and 4 display the suggested PCB layout
for the AAT1155. The following guidelines should
be used to help ensure a proper layout.
Output Capacitor
With no external compensation components, the out-
put 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:
1. The connection from the input capacitor to the
Schottky anode should be as short as possible.
2. The input capacitor should connect as closely
as possible to VP (Pins 5 and 8) and GND
(Pin 2).
3. 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.
4. 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.
(VOUT+VFWD)·(V - VOUT
)
1
IN
IRMS
=
·
L·F·V
2 · 3
IN
For a ceramic output capacitor, the dissipation due
to the RMS current and associated output ripple
are negligible.
5. The resistance of the trace from the load
return to the ground (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.
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:
6. R1 and C3 are required in order to provide
a cleaner power source for the AAT1155 con-
trol circuitry.
VRIPPLE
ESR ≤
∆I
where ∆I is the peak-to-peak inductor ripple current.
Figure 3: Evaluation Board Top Side.
Figure 4: Evaluation Board Bottom Side.
12
1155.2005.11.1.6
AAT1155
1MHz 2.5A Step-Down DC/DC Converter
Thermal
Design Example
Losses associated with the AAT1155 output switch-
ing MOSFET are due to switching losses and con-
duction losses. The conduction losses are associ-
ated with the RDS(ON) characteristics of the output
switching device. At the full load condition, assum-
ing continuous conduction mode (CCM), an accu-
rate calculation of the RDS(ON) losses can be
derived from the following equations:
(see Figures 2 and 5 for reference)
IOUT
IRIPPLE
VOUT
VIN
2.5A
40% of Full Load at Max VIN
2.5V
5V 5%
1MHz
70°C
FS
2
P = IRMS · RDS(ON)
TMAX
ON
RDS(ON) losses
Inductor Selection
VOUT
V
V
⎛
⎞
OUT
∆I 2
12
L =
· 1-
⎛
⎝
⎞
⎠
2
IO
⎝
⎠
I
k F
O · ·
IRMS
=
+
·D
IN
3.3V
3.3V
⎛
⎞
⎠
=
· 1-
= 1.23µH
⎝
Internal switch RMS current
2.5A · 0.4 · 1MHz
5.25V
D is the duty cycle and VF is the forward voltage
drop of the Schottky diode.
Use standard value of 1.5µH
Sumida inductor Series CDRH6D38.
VO + VF
D =
V +VF
IN
V
V
O
⎛
⎝
⎞
⎠
O
∆I =
1-
=
·
L F
V
IN
∆I is the peak-to-peak inductor ripple current.
3.3V
3.3V
⎛
⎝
⎞
= 0.82A
A simplified form of calculating the RDS(ON) and
switching losses is given by:
1-
⎠
5.25V
1.5µH 1MHz
·
∆I
2
2
IPK = IOUT
+
=
IO
R
V
DS(ON) o
·
P =
+ t F I + I
SW · ·
V
·
Q IN
O
V
IN
2.5A + 0.41 = 2.91A
where IQ is the AAT1155 quiescent current.
Efficiency vs. Load Current
(VIN = 5.0V; VOUT = 3.3V)
Once the total losses have been determined, the
junction temperature can be derived. The thermal
resistance (ΘJA) for the MSOP-8 package mounted
on an FR4 printed circuit board in still air is 150°C/W.
100
95
90
85
80
75
70
65
60
TJ = P QJA + TAMB
TAMB is the maximum ambient temperature and TJ
is the resultant maximum junction temperature.
0.01
0.1
1
10
Output Current (A)
Figure 5: 5V Input, 3.3V Output.
1155.2005.11.1.6
13
AAT1155
1MHz 2.5A Step-Down DC/DC Converter
The 120µF Vishay 594D tantalum capacitor has an
ESR of 85mΩ and a ripple current rating of 1.48Arms
in a C case size. Although smaller case sizes are suf-
ficiently rated for this ripple current, their ESR level
would result in excessive output ripple.
AAT1155 Junction Temperature
IO2 · RDS(ON) · VO
tSW · F · IO
⎛
⎝
⎞
P
+ IQ · V =
=
+
ON
IN
V
2
⎠
IN
2.52 · 70mΩ · 3.3V 20ns · 1MHz · 2.5A
⎛
⎝
⎞
+ 690 A
µ
The ESR requirement for a tantalum capacitor can
be estimated by :
+
⎠
5V
2
0.42 Watts
VRIPPLE
100mV
0.9A
ESR≤
=
= 111mΩ
TJ(MAX) = TAMB + θJA · P =
∆I
70°C + 150°C/W · 0.42W = 133°C
(
)
(V - V
·
VOUT+V
)
1
·
2· 3
FWD
IN
OUT
IRMS
=
=
L·F·V
IN
1
2· 3
3.65V 1.7V
·
·
= 240mArms
Diode
1.5µH· 1MHz·5V
VO
⎛
IDIODE = IO· 1-
⎝
⎞
⎠
=
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 sig-
nificantly over that seen with tantalum capacitors.
Temperature rise due to ESR ripple current dissi-
pation is also reduced.
V
IN
3.3V
⎛
2.5A· 1-
⎝
⎞
⎠
= 0.93A
5.25V
V = 0.35V
FW
PDIODE = VFW · IDIODE
=
Input Capacitor
0.35V ·0.93A = 0.33A
The input capacitor ripple is:
Given an ambient thermal resistance of 120°C/W
from the manufacturer's data sheet, TJ(MAX) of the
diode is:
V
V
O
⎛
⎞
⎠
O
IRMS = IO ·
· 1-
= 1.82Arms
⎝
V
V
IN
IN
T
J(MAX)= TAMB +ΘJA P =
·
In the examples shown, C1 is a ceramic capacitor
located as closely to the IC as possible. C1 pro-
vides 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 laborato-
ry power supply to the evaluation board.
70°C +120°C/W 0.33W =
109°C
·
Output Capacitor
The output capacitor value required for sufficient
loop phase margin depends on the type of capaci-
tor selected. For a low ESR ceramic capacitor, a
minimum value of 200µF is required. For a low
ESR tantalum capacitor, lower values are accept-
able. 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.
14
1155.2005.11.1.6
AAT1155
1MHz 2.5A Step-Down DC/DC Converter
1 volt. For accurate results (less than 1% error for
all outputs), select R4 to be 10kΩ. Once R4 has
been selected, R3 can be calculated. For a 1.25
volt output with R4 set to 10kΩ, R3 is 2.5kΩ.
Adjustable Output
For applications requiring an output other than the
fixed outputs available, the 1V version can be pro-
grammed externally. Resistors R3 and R4 of
Figure 1 force the output to regulate higher than
R3 = (VO - 1) · R4 = 0.25 · 10kΩ = 2.5kΩ
Capacitors
Capacitance
Voltage
(V)
Part Number
Manufacturer
(µF)
Temp Co.
Case
C4532X5ROJ107M
TDK
100
100
47
6.3
6.3
6.3
6.3
6.3
16
X5R
X5R
X5R
X5R
1812
1812
1812
1206
C
GRM43-2 X5R 107M 6.3
GRM43-2 X5R 476K 6.3
GRM42-6 X5R 106K 6.3
594D127X_6R3C2T
595D107X0016C
MuRata
MuRata
MuRata
Vishay
Vishay
10
120
100
C
Inductors
Inductance
I
DCR
(Ω)
Height
(mm)
Part Number
Manufacturer
(µH)
(Amps)
Type
CDRH6D38-4763-T055 Sumida
1.5
1.5
1.5
1.5
1.5
4.0
3.2
3.0
3.7
3.8
0.014
0.025
0.022
0.022
0.016
4.0
2.8
3.2
4.7
4.7
Shielded
Non-Shielded
Shielded
N05D B1R5M
Taiyo Yuden
NP06DB B1R5M
LQH55DN1R5M03
LQH66SN1R5M03
Taiyo Yuden
MuRata
Non-Shielded
Shielded
MuRata
Diodes
Manufacturer
Part Number
VF
Diodes Inc.
ROHM
B340LA
RB050L-40
5820SM
0.45V @ 3A
0.45 @ 3A
0.46V @ 3A
Micro Semi
1155.2005.11.1.6
15
AAT1155
1MHz 2.5A Step-Down DC/DC Converter
Ordering Information
Output Voltage
Package
Marking1
Part Number (Tape and Reel)2
1.0V (Adj. VOUT ≥ 1.0V)
MSOP-8
MSOP-8
MSOP-8
MSOP-8
KXXYY
KYXYY
ILXYY
IKXYY
AAT1155IKS-1.0-T1
AAT1155IKS-1.8-T1
AAT1155IKS-2.5-T1
AAT1155IKS-3.3-T1
1.8V
2.5V
3.3V
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semiconductor products that are in compliance with current RoHS standards, including
the requirement that lead not exceed 0.1% by weight in homogeneous materials. For more
information, please visit our website at http://www.analogictech.com/pbfree.
Package Information
MSOP-8
4° 4°
1.95 BSC
0.60 0.20
PIN 1
0.254 BSC
0.155 0.075
3.00 0.10
10° 5°
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.
16
1155.2005.11.1.6
AAT1155
1MHz 2.5A Step-Down DC/DC Converter
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Phone (408) 737-4600
Fax (408) 737-4611
1155.2005.11.1.6
17
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