AAT AAT7301-S-T 2a 15v step-down dc-dc converter Datasheet

Advanced Analog Technology, Inc.
October 2007
AAT7301
Product information presented is for internal use within AAT Inc. only. Details are subject to change without notice.
2A 15V STEP-DOWN DC-DC CONVERTER
FEATURES
GENERAL DESCRIPTION
Buck PWM with Internal PMOS
The AAT7301 is a 2A 15V step-down converter, which
4V to 15V Input Voltage Range
provides an integrated one-channel PWM solution for
Max. 2A Output Current
the power supply of DC-DC system. It offers system
Under-Voltage Lockout (UVLO) Protection
engineers the flexibility to tailor-make the power supply
Internal Short Circuit and Thermal Protection
circuitry for specific applications.
Internal Soft-Start
Fixed Switching Frequency (380kHz / 47kHz)
2 µA Shutdown Current
SOP-8 Package
The buck PWM contains enable control signal, error
amplifier,
PWM
under-voltage
comparator,
protection,
output
oscillator,
short
driver,
circuit
protection and voltage reference circuit.
The AAT7301 contains one buck current mode PWM
with internal PMOS. In addition, a digital soft-start is
PIN CONFIGURATION
also included to prevent inrush current at startup.
AAT7301 comes with a fixed 380kHz oscillator,
however, when the feedback voltage is lower than 0.7V,
the switching frequency changes to 47kHz and returns
to 380kHz after the short-circuit is released.
TYPICAL APPLICATION
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October 2007
AAT7301
ORDERING INFORMATION
DEVICE
TYPE
PART
NUMBER
PACKAGE
PACKING
AAT7301
AAT7301
-S-T
S:SOP8
T: Tape
and Reel
TEMP.
RANGE
MARKING
–40 C to +85 C
AAT7301
XXXXXX
MARKING
DESCRIPTION
Device Type
Lot no.
(6~9 Digits)
NOTE: All AAT products are lead free and halogen free.
ABSOLUTE MAXIMUM RATINGS
PARAMETER
SYMBOL
VALUE
UNIT
Supply Voltage (VDD, SW)
VDD
–0.3 to 15.0
V
Pin Voltage (IN, ENC, EO)
VI
–0.3 to (VDD +0.3)
V
SW Current
ISW
6.5
A
Operating Temperature Range
TC
–40 C to +85 C
C
TSTORAGE
–65 C to +150 C
C
Storage Temperature Range
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Advanced Analog Technology, Inc.
October 2007
AAT7301
ELECTRICAL CHARACTERISTICS
( VDD = 4V~15V, TC = –40 C to 85 C , Tested at TC = 25 C ,VDD = 12V, unless otherwise specified.)
General Item
PARAMETER
Input Voltage Range
SYMBOL
VDD
VUVLO
Under Voltage Lockout Hysteresis
VUHYS
VDD Falling
3.33
3.48
MAX
UNIT
15
V
3.63
V
150
mV
VIN = 1.5V
1
mA
IDD-ON
VIN = 0.85V
5.5
mA
ISHDN
OSC Frequency
fOSC
1.70
VSHORT
EN Low Level
VIL
EN High Level
VIH
Thermal Shutdown
TYP
IDD-OFF
Shutdown Current into VDD
Short Detection Voltage
MIN
4
Under Voltage Lockout
Input Current into VDD
TEST CONDITION
10.00
µA
Normal Operation
380
kHz
Short-Circuit
47.5
kHz
VIN Falling
0.67
0.70
0.73
V
0.4
V
1.6
TSHDN
V
160
C
Buck PWM
PARAMETER
SYMBOL
TEST CONDITION
MIN
TYP
MAX
UNIT
IN Regulation Voltage
VIN
IN = EO
0.88
0.90
0.92
V
IN to EO Transconductance
Gm
IN = EO
35
80
125
µS
Maximum Duty Cycle
90
IN Input Leakage Current
IL
Current-Sense Amplifier
Transresistance
RCS
SW Leakage Current
ILSW
Switch On-Resistance
RON
SW Current Limit
ILIMIT
Soft-Start Time
IN = 0V to 1.5V
0.01
+100.00
0.2
VSW = 15V
–
0.1
2.5
tSS
–
−100.00
%
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nA
V/A
5.0
µA
200
mΩ
3.7
A
700
µs
Advanced Analog Technology, Inc.
October 2007
AAT7301
TYPICAL OPERATING CHARACTERISTICS
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AAT7301
TYPICAL OPERATING CHARACTERISTICS
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AAT7301
PIN DESCRIPTION
PIN NO.
NAME
I/O
FUNCTION
1
NC
-
Not Connected
2
VDD
I
Power Input
3
SW
I/O
4
GND
-
Ground
5
IN
I
Feedback Input
6
EO
I/O
7
ENC
I
Enable Control
8
NC
-
Not Connected
Switching Node
Error Amplifier Compensation Output
FUNCTION BLOCK DIAGRAM
AAT7301
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AAT7301
TYPICAL APPLICATION CIRCUIT
AAT7301
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AAT7301
DETAILED DESCRIPTION
DESIGN PROCEDURE
The AAT7301 is a current-mode buck switch-mode
Programming the Output Voltage
regulator with a fixed switching frequency of 380kHz. It
The output voltage for the AAT7301 is programmed
uses an internal P-channel MOSFET switch to step
using a resistor divider from the output connected to the
down the input voltage to the regulated output voltage.
feedback pins (Figure 1). When setting the output
The converter regulates input voltages from 4V to 15V
voltage, connect a resistive voltage divider from the
down to an output voltage as low as 0.9V, and is able to
output to IN pin and then to GND. Choose the
supply up to 2A of load current.
lower-side (IN-to-GND) resistor, then calculate the
upper-side (output-to-IN) resistor as follows:
ENC Control Input
The AAT7301 contains built-in pull high logic. The IC is
enabled even ENC left floating (ENC pin voltage
>
V

RUPPER = RLOWER  OUT − 1
 VIN

1.6V). When the voltage is lower than 0.4V, the IC is
disabled.
Where VIN is the feedback regulation voltage, 0.9V (typ).
Typical values for RLOWER are in the range of 10kΩ to
Soft-Start
100kΩ.
The AAT7301 has built-in 700µs soft-start time. Upon
power turn-on, if ENC pin is not pulled low; and VDD is
above VUVLO (typ. 3.48V), IN pin will climb from 0V to
0.9V during 700µs soft-start time to reduce inrush
current.
Short Circuit Protection
When short circuit happens, and the feedback voltage
(IN) is less than 0.7V, the switching frequency will
change to 47kHz to reduce the power supply from input
Figure 1. Feedback Network
to output to protect the system. The frequency will
switch back to 380kHz after the short circuit is released.
Thermal Shutdown
The AAT7301 includes a thermal-limit circuit that shuts
down the IC at approximately +160 C . The part turns
on after the IC cools by approximately 20 C .
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AAT7301
Inductor Selection
Input Capacitor
A good rule of thumb when choosing the inductance is
The input capacitor in a DC-to-DC converter reduces
to allow the peak inductor current in the inductor to be
current peaks drawn from the input power source and
approximately 115% of the maximum load current. Also,
reduces
make sure that the peak inductor current is below the
impedance of the input capacitor at the switching
2.5A minimum current limit.
frequency should be less than that of the input source
The inductance value can be calculated by the
so high-frequency switching currents do not pass
equation:
through the input source. Ceramic capacitors are
switching
preferred,
but
noise
tantalum
in
or
the
controller.
low-ESR
The
electrolytic
V
VOUT ⋅ (1 − OUT )
VI
L≈
0.3 ⋅ IOUT ⋅ fSW
capacitors may also satisfy. For insuring stable
Where VOUT is the output voltage, VI is the input voltage,
Output Capacitor
fSW is the switching frequency (380kHz), and IOUT is the
The output capacitor keeps output ripple small and
maximum load current.
ensures control-loop stability. The output capacitor
When the inductance value is determined, then the
must also have low impedance at the switching
peak inductor current can be calculated by the
frequency. Ceramic, polymer, and tantalum or low-ESR
equation:
electrolytic capacitors are suitable.
operation the IC must be bypassed with a 0.22 µF
ceramic capacitor placed close to the VDD.
Output ripple with a ceramic output capacitor is
V
VOUT ⋅ (1 − OUT )
VI
IL(peak) = IOUT +
2 ⋅ fSW ⋅ Lsetting
approximately as follows:
Vripple ≈
Where IOUT is maximum load current, VOUT is the output
voltage, VI is the input voltage, fSW is the switching
frequency (380kHz), and Lsetting is the inductance value.
V
VOUT ⋅ (1 − OUT )
VI
8 ⋅ L setting ⋅ COUT ⋅ fSW 2
Where VOUT is the output voltage, VI is the input voltage,
Choose an inductor that does not saturate under the
fSW is the switching frequency (380kHz), COUT is the
peak inductor current.
output capacitance, and Lsetting is the inductance value.
If the capacitor has significant ESR, the output ripple
component due to capacitor ESR is as follows:
Schottky Diode Selection
Choose a Schottky diode whose maximum reverse
voltage rating is greater than the maximum input
Vripple ≈ 2 ⋅ (IL(peak) − IOUT ) × RESR
voltage, and whose current rating is greater than the
Where IL(peak) is the peak inductor current, IOUT is
peak inductor current.
maximum load current, and RESR is the capacitor’s
ESR.
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AAT7301
Compensation
from EO to GND:
The AAT7301 employs current-mode control, thereby
simplifying the control-loop compensation. When the
AAT7301 operates with continuous inductor current
(typically the case), a RLOAD COUT pole appears in the
CP =
C OUT × RESR
RC
If CP is calculated to be < 10pF, it can be omitted.
loop-gain frequency response. To ensure stability, set
the compensation RCCC to zero to compensate for the
RLOAD COUT pole. Set the loop crossover below the
lower of 1/10 the switching frequency (380kHz). The
compensation resistor and capacitor are then chosen
to optimize control-loop stability.
Choose the compensation resistor RC to set the
desired crossover frequency fC. Determine the value by
the following equation:
RC =
LAYOUT CONSIDERATIONS
Conductors carrying discontinuous currents and any
high-current path should be made as short and wide as
possible. The compensation network should be very
close to the EO pin and avoid through VIA. The IC
must be bypassed with a 0.22 µF ceramic capacitor
placed close to the VDD. Tie the feedback resistor
divider to be very close to output capacitor and far
away from the inductor or Schottky diode. Keep the
2π × COUT × VOUT × RCS × fC
Gm × VIN
feedback network IN close to the IC. Switching nodes
(SW) should be kept as small as possible and should
Where COUT is the output capacitance, VOUT is the
be routed away from high-impedance nodes such as
output voltage, RCS is the current-sense amplifier
IN.
transresistance (0.2V/A), Gm is the error amplifier
transconductance (80 µS ) and VIN is the feedback
threshold voltage (0.9V).
If the value calculated for RC is greater than 100 kΩ ,
recommend 100 kΩ .
Choose the compensation capacitor CC to set the zero
to 1/4 of the crossover frequency.
CC <
COUT × VOUT
4 × RC
If the output filter capacitor COUT has significant ESR, a
zero occurs at the following:
ZESR =
1
2π × C OUT × RESR
If ZESR > fC / 4, it can be ignored, as is typically the case
with ceramic output capacitors. If ZESR < fC / 4, it should
be cancelled with a pole set by capacitor CP connected
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AAT7301
PACKAGE DIMENSION
SOP-8
Symbol
A
A1
A2
b
C
D
E
E1
e
L
y
θ
Dimensions In Millimeters
MIN
TYP
MAX
1.35
1.60
1.73
0.05
-----0.15
-----1.45
-----0.33
0.41
0.51
0.19
0.20
0.25
4.80
4.85
4.95
5.79
5.99
6.20
3.81
3.91
3.99
1.27 BSC
0.406
0.710
1.270
----------0.076
0˚
-----8˚
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