Analogic AAT1155IKS-3.3-T1 1mhz 2.5a step-down dc/dc converter Datasheet

AAT1155
1MHz 2.5A Step-Down DC/DC Converter
General Description
Features
The AAT1155 SwitchReg™ is a member of
AnalogicTech's Total Power Management IC™
(TPMIC™) product family. The step-down switching converter is 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.
SwitchReg™
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
-40°C to +85°C Temperature Range
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Applications
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Cable/DSL Modems
Computer Peripherals
High Efficiency Conversion from 5V or 3.3V
Supply
Network Cards
Set-Top Boxes
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Typical Application
INPUT
VP
10µF
FB
AAT1155
LX
1.5µH
LX
ENABLE
100Ω
VCC
OUTPUT
GND
120µF
0.1µF
1155.2005.11.1.6
1
AAT1155
1MHz 2.5A Step-Down DC/DC Converter
Pin Descriptions
Pin #
Symbol
Function
1
FB
2
GND
3
EN
4
VCC
5, 8
VP
Input supply for converter power stage.
6, 7
LX
Inductor connection pin.
Feedback input pin.
Signal ground.
Converter enable pin.
Small signal filtered bias supply.
Pin Configuration
MSOP-8
VP
7
LX
3
6
LX
4
5
VP
GND
2
EN
VCC
2
1
1
2
8
FB
1155.2005.11.1.6
AAT1155
1MHz 2.5A Step-Down DC/DC Converter
Absolute Maximum Ratings1
TA = 25°C, unless otherwise noted.
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
3000
V
V
V
V
°C
°C
V
Value
Units
150
833
°C/W
mW
Rating
Units
-40 to +85
°C
Thermal Characteristics3
Symbol
ΘJA
PD
Description
Maximum Thermal Resistance
Maximum Power Dissipation
Recommended Operating Conditions
Symbol
T
Description
Ambient Temperature Range
1. Stresses above those listed in Absolute Maximum Ratings may cause permanent damage to the device. Functional operation at conditions other than the operating conditions specified is not implied. Only one Absolute Maximum Rating should be applied at any one time.
2. 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
VIN
VOUT
VIL
VIH
VUVLO
VUVLO(HYS)
IQ
ISHDN
ILIM
RDS(ON)L
η
∆VOUT (VOUT*∆VIN)
∆VOUT/VOUT
FOSC
TSD
THYS
4
Description
Input Voltage Range
Output Voltage Tolerance
Conditions
VIN = VOUT + 0.2 to 5.5V,
IOUT = 0.5A
Input Low Voltage
Input High Voltage
Under-Voltage Lockout
Under-Voltage Lockout Hysteresis
Quiescent Supply Current
Shutdown Current
Current Limit
High Side Switch On Resistance
Efficiency
Load Regulation
Line Regulation
Oscillator Frequency
Over-Temperature Shutdown
Threshold
Over-Temperature Shutdown
Hysteresis
Min Typ Max Units
2.7
-2.5
5.5
2.5
V
%
0.6
V
V
1.4
VIN Rising
VIN Falling
No Load, VFB = 0V
VEN = 0V, VIN = 5.5V
TA = 25°C
TA = 25°C
IOUT = 1.0A
ILOAD = 0A to 2.5A
VIN = 2.7V to 5.5V
TA = 25°C
2.5
1.2
250
630
V
60
92
±2.3
0.75
1
140
mV
µA
µA
A
mΩ
%
%
%/V
MHz
°C
15
°C
4.4
1000
1.0
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
2.7V
3.6V
0.5
4.2V
Variation (%)
RDS(ON) (mΩ)
80
70
60
5.0V
5.5V
50
40
-20
0
20
40
60
80
100
0.25
0
-0.25
-0.5
3.5
120
4
Temperature (°C)
4.5
5
5.5
Input Voltage (V)
Oscillator Frequency Variation vs. Temperature
RDS(ON) vs. Input Voltage
(IDS = 1A)
(VIN = 5V)
80
1
70
0
Variation (%)
RDS(ON) (mΩ
Ω)
75
65
60
55
50
-1
-2
-3
45
40
2.5
3
3.5
4
4.5
5
-4
5.5
-20
Input Voltage (V)
0
20
40
60
80
100
Temperature (°C)
Output Voltage Variation vs. Temperature
Enable Threshold vs. Input Voltage
(IOUT = 2A; VO = 3.3V)
1.2
0.2
EN(H)
Variation (%)
Enable Threshold (V)
0.4
1.1
1
0.9
0.8
EN(L)
0.7
0
-0.2
-0.4
-0.6
-0.8
0.6
2.5
3
3.5
4
Input Voltage (V)
1155.2005.11.1.6
4.5
5
5.5
-20
0
20
40
60
80
100
Temperature (°°C)
5
AAT1155
1MHz 2.5A Step-Down DC/DC Converter
Typical Characteristics
AAT1155 Evaluation Board
Over-Temperature Current vs. Input Voltage
Line Regulation
(VOUT = 3.3V)
3.6
Output Current (A)
Output Voltage Error (%)
(VOUT = 3.3V)
1
0
IO = 0.3A
-1
-2
-3
-4
IO = 3.0A
3.4
55°C
3.2
3
70°C
2.8
2.6
2.4
85°C
2.2
2
100°C
1.8
1.6
-5
3.4
3.7
4
4.3
4.6
4.9
5.2
3.5
5.5
3.75
4
4.25
4.5
4.75
5
5.25
5.5
Input Voltage (V)
Input Voltage (V)
Load Regulation
Non-Switching Operating
Current vs. Temperature
(VIN = 5.0V; VOUT = 3.3V)
(FB = 0V)
Operating Current (mA)
0.0
Output Error (%)
-1.0
-2.0
-3.0
-4.0
-5.0
-6.0
-7.0
-8.0
-9.0
-10.0
0.01
750
700
650
2.7V
3.6V
550
500
1.00
10.00
-20
0
20
Output Current (A)
60
80
4.5
4
3.5
3
2.5
2
0
10
20
30
40
50
60
Temperature (°C)
40
100
120
70
80
90
100
Inrush and Output
Overshoot Characteristics
8
14
6
12
4
10
2
8
0
6
-2
4
-4
2
-6
0
-8
Inductor Current (A)
(bottom trace)
Enalbe and Output Voltage (V)
(top trace)
(VOUT = 3.3V; VIN = 5.0V)
5
6
5.5V
Temperature (°C)
Over-Temperature Shutdown
Current vs. Temperature
-10
5.0V
450
0.10
Load Current (A)
-20
4.2V
600
-2
0
0.4
0.8
1.2
1.6
2
Time (ms)
1155.2005.11.1.6
AAT1155
1MHz 2.5A Step-Down DC/DC Converter
Typical Characteristics
Loop Crossover Gain and Phase
Tantalum Output Ripple
(IOUT = 3.0A; VOUT = 3.3V; VIN = 5.0V)
0.02
6
8
0.00
5
-0.02
4
-0.04
3
-0.06
2
-0.08
1
120µF 6.3V Tantalum
Vishay P/N 594D127X96R3C2T
-0.10
Gain (dB)
12
-0.12
1
2
3
4
90
Phase
45
0
0
200µF gain
-4
300µF gain
-8
5
-45
-90
100µF 6.3 Ceramic
TDK P/N C3225X5R0J107M
Vishay GRM43SR60J107ME20L
-135
-16
10000
-1
0
135
4
-12
0
180
L = 1.5µHy
Phase (Degrees)
7
Inductor Current
(bottom) (A)
Output Ripple
(top) (mV)
16
0.04
-180
100000
Frequency (Hz)
Time (µ
µs)
Output Ripple
Loop Crossover Gain and Phase
7
2
6
5
-2
4
-4
3
-6
2
-8
1
200µF 6.3V Ceramic
-10 TDK P/N C3325X5R0J107M
Vishay GRM43SR60J107ME20L
-12
0
1
2
3
0
12
135
8
90
4
45
Gain
0
0
-4
-45
-8
-90
120µF 6.3V Tantalum
Vishay P/N 594D127X96R3C2T
-16
10000
5
Time (µ
µs)
-135
-180
1000000
100000
Frequency (Hz)
Output Ripple
Tantalum Transient Response
(IOUT = 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
0
5
3.20
5
-2
4
-4
3
-6
2
-8
1
300µF 6.3VCeramic
TDK P/N C3325X5R0J107M
Vishay GRM43SR60J107ME20L
-10
0
-12
-1
0
1
2
3
Time (µ
µs)
1155.2005.11.1.6
4
5
Output Voltage
(top) (mV)
7
2
3.10
4
3.00
3
2.90
2
2.80
1
120µF 6.3V Tantalum
Vishay P/N 594D127X96R3C2T
2.70
Inductor Current
(bottom) (A)
4
Inductor Current
(bottom) (A)
AC Output Ripple
(top) (mV)
180
-12
-1
4
16
Phase (Degrees)
0
Gain (dB)
4
Inductor Current
(bottom) (A)
AC Output Ripple
(top) (mV)
(IOUT = 3.0A; VOUT = 3.3V; VIN = 5.0V)
0
2.60
-1
0
100
200
300
400
500
Time (µ
µs)
7
AAT1155
1MHz 2.5A Step-Down DC/DC Converter
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
2x 100µF 6.3V Ceramic
TDK P/N C3325X5R0J107M
Vishay GRM43SR60J107ME20L
3.10
3.00
3
2.90
2
2.80
1
2.70
2.60
0
100
200
300
Time (µ
µs)
8
4
400
500
Output Voltage
(top) (mV)
7
3.30
3.20
3x 100µF 6.3V Ceramic
TDK P/N C3325X5R0J107M
Vishay GRM43SR60J107ME20L
3.10
5
4
3.00
3
2.90
2
2.80
1
0
2.70
0
-1
2.60
Inductor Current
(bottom) (A)
3.40
Inductor Current
(bottom) (A)
Output Voltage
(top) (mV)
Typical Characteristics
-1
0
100
200
300
400
500
Time (µ
µs)
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
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 internal,
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.
1155.2005.11.1.6
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.
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.
9
AAT1155
1MHz 2.5A Step-Down DC/DC Converter
Vin 2.7V-5.5V
VOUT 1.25V @ 2.5A
R1
100
R2
100k
R3
2.55k
U1
AAT1155-1.0
FB
C4
100µF
VP
L1
1.5µH
GND LX
EN
C1
10µF
C3
0.1µF
rtn
LX
VCC VP
D1
B340LA
R4
10.0k
C2
120µF
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.
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.
Inductor
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.
For a 2.5A load and the ripple current set to 40% at
the maximum input voltage, the maximum peak-to10
peak ripple current is 1A. Assuming a 5V ±5% input
voltage and 40% ripple, the output inductance
required is:
VOUT
VOUT ⎞
⎛
· 1- V
·
k
·
F
⎝
OUT
SW
IN(MAX)⎠
L =I
=
3.3V
⎞
⎛
⎞
⎛
· 1 - 3.3V
⎝ 2.5A · 0.4 · 1MHz ⎠
⎝ 5.25V⎠
= 1.23µH
The factor "k" is the fraction of the full load (40%)
selected for the ripple current at the maximum input
voltage.
The corresponding inductor RMS current is:
IRMS =
⎛ 2 ∆I 2 ⎞
I +
≈ I O = 2.5A
⎝ O
12 ⎠
∆I is the peak-to-peak ripple current which is fixed
by the inductor selection above. For a peak-topeak 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.
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
Micro Semi
⎛ VO ⎞
1⎝ VIN ⎠
The primary function of the input capacitor is to provide 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 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 2.
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.
V
3.3V ⎞
= 0.85A
IAVG = IO · ⎛1 - O ⎞ = 2.5A · ⎛1 ⎝ VIN ⎠
⎝ 5.0V ⎠
With a 125°C maximum junction temperature and a
120°C/W thermal resistance, the maximum average current is:
TJ(MAX)- TAMB
θJ-A · VFWD
=
0.45V @ 3A
0.45 @ 3A
0.46V @ 3A
Input Capacitor Selection
For VIN at 5V and VOUT at 3.3V, the average diode
current is:
IAVG =
B340LA
RB050L-40
5820SM
125°C - 70°C
= 1.14A
120 °C / W · 0.4V
For overload, short-circuit, and excessive ambient
temperature conditions, the AAT1155 enters overtemperature 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.
IRMS = IO ·
VO ⎛
V
· 1- O ⎞
VIN ⎝ VIN ⎠
Vin 3.5V-5.5V
Vout 3.3V @ 2.5A
R1
100
R2
100k
C4
100µF
U1
AAT1155-3.3
FB
VP
L1
1.5µH
GND LX
EN
C1
10µF
VCC VP
C3
0.1µF
rtn
LX
D1
B340LA
C2
120µF
+
-
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 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 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 oscillations that may occur due to the source inductance
resonating with the converter input impedance.
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 =
1
2· 3
·
( VOUT+ VFWD) · (VIN - VOUT)
L · F · VIN
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 ≤
VRIPPLE
∆I
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.
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.
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.
6. R1 and C3 are required in order to provide
a cleaner power source for the AAT1155 control circuitry.
where ∆I is the peak-to-peak inductor ripple current.
Figure 3: Evaluation Board Top Side.
12
Figure 4: Evaluation Board Bottom Side.
1155.2005.11.1.6
AAT1155
1MHz 2.5A Step-Down DC/DC Converter
Thermal
Design Example
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:
(see Figures 2 and 5 for reference)
PON = I RMS2 · RDS(ON)
RDS(ON) losses
IOUT
2.5A
IRIPPLE
40% of Full Load at Max VIN
VOUT
2.5V
VIN
5V ±5%
FS
1MHz
TMAX
70°C
Inductor Selection
L=
∆I 2 ⎞
⎛
·D
IRMS = I O2 +
⎝
12 ⎠
=
Internal switch RMS current
D is the duty cycle and VF is the forward voltage
drop of the Schottky diode.
VOUT ⎛ VOUT⎞
· 1IO · k · F ⎝
VIN ⎠
3.3V
3.3V ⎞
⎛
· 1= 1.23µH
2.5A · 0.4 · 1MHz ⎝ 5.25V⎠
Use standard value of 1.5µH
Sumida inductor Series CDRH6D38.
D=
VO + VF
VIN + VF
∆I =
∆I is the peak-to-peak inductor ripple current.
VO ⎛
V ⎞
1- O =
L · F ⎝ VIN ⎠
3.3V
⎛ 3.3V ⎞
= 0.82A
11.5µH · 1MHz ⎝ 5.25V⎠
A simplified form of calculating the RDS(ON) and
switching losses is given by:
I 2 · R DS(ON) Vo
+ tSW ·F · I O + I Q · VIN
P= O
VIN
I PK = IOUT +
∆I
=
2
2.5A + 0.41 = 2.91A
where IQ is the AAT1155 quiescent current.
Efficiency vs. Load Current
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.
TAMB is the maximum ambient temperature and TJ
is the resultant maximum junction temperature.
100
95
Efficiency (%)
TJ = P QJA + TAMB
(VIN = 5.0V; VOUT = 3.3V)
90
85
80
75
70
65
60
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
IO2 · RDS(ON) · VO ⎛ tSW · F · IO
+ IQ⎞ · VIN =
+
VIN
2
⎝
⎠
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 sufficiently rated for this ripple current, their ESR level
would result in excessive output ripple.
⎞
2.52 · 70mΩ · 3.3V ⎛ 20ns · 1MHz · 2.5A
+
+ 690µA
⎝
⎠
5V
2
The ESR requirement for a tantalum capacitor can
be estimated by :
AAT1155 Junction Temperature
PON =
0.42 Watts
TJ(MAX) = TAMB + θJA · P =
70°C + 150°C/W · 0.42W = 133°C
ESR ≤
IRMS =
VRIPPLE 100 mV
=
= 111mΩ
∆I
0.9A
1
2· 3
·
(VOUT+ VFWD) · (VIN - VOUT)
L · F · VIN
=
1
3.65V ·1.7 V
·
= 240mArms
2 · 3 1.5µH · 1MHz · 5V
Diode
⎛ V ⎞
IDIODE = IO · 1 - O =
⎝ VIN ⎠
3.3V ⎞
⎛
= 0.93A
2.5A · 1 ⎝ 5.25V⎠
VFW = 0.35V
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.
PDIODE = VFW · IDIODE =
0.35V · 0.93A = 0.33A
Input Capacitor
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:
IRMS = I O ·
VO ⎛
V ⎞
· 1 - O = 1.82Arms
VIN ⎝ VIN ⎠
TJ(MAX) = TAMB + ΘJA · P =
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 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.
14
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.
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 programmed 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
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
MuRata
MuRata
MuRata
Vishay
Vishay
Capacitance
(µF)
Voltage
(V)
100
100
47
10
120
100
6.3
6.3
6.3
6.3
6.3
16
Temp Co.
Case
X5R
X5R
X5R
X5R
1812
1812
1812
1206
C
C
Inductors
Part Number
Manufacturer
CDRH6D38-4763-T055
N05D B1R5M
NP06DB B1R5M
LQH55DN1R5M03
LQH66SN1R5M03
Sumida
Taiyo Yuden
Taiyo Yuden
MuRata
MuRata
Inductance
(µH)
I
(Amps)
DCR
Ω)
(Ω
Height
(mm)
Type
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
Non-Shielded
Shielded
Diodes
Manufacturer
Part Number
VF
Diodes Inc.
ROHM
Micro Semi
B340LA
RB050L-40
5820SM
0.45V @ 3A
0.45 @ 3A
0.46V @ 3A
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)
1.8V
2.5V
3.3V
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
All AnalogicTech products are offered in Pb-free packaging. The term “Pb-free” means
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°
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.
16
1155.2005.11.1.6
AAT1155
1MHz 2.5A Step-Down DC/DC Converter
© Advanced Analogic Technologies, Inc.
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or other intellectual property rights are implied. AnalogicTech reserves the right to make changes to their products or specifications or to discontinue any product or service without notice.
Customers are advised to obtain the latest version of relevant information to verify, before placing orders, that information being relied on is current and complete. All products are sold
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830 E. Arques Avenue, Sunnyvale, CA 94085
Phone (408) 737-4600
Fax (408) 737-4611
1155.2005.11.1.6
17
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