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參數(shù)資料
| 型號： | AAT4618IJS-T1 |
| 廠商： | Advanced Analogic Technologies, Inc. |
| 英文描述： | Current Limited Load Switch |
| 中文描述： | 電流限制負荷開關(guān) |
| 文件頁數(shù)： | 7/10頁 |
| 文件大?。?/td> | 225K |
| 代理商： | AAT4618IJS-T1 |
Applications Information
Input Capacitor
The input capacitor, C
IN
protects the power supply
from current transients generated by the load
attached to the AAT4618. When a short circuit is
suddenly applied to the output of the AAT4618, a
large current, limited only by the R
DS(ON)
of the
MOSFET, will flow for less than 1μs before the cur-
rent limit circuitry activates. (See characteristic
curve "Short Circuit Through 0.3
.") In this event,
a moderately sized C
IN
will dramatically reduce the
voltage transient seen by the power supply and by
other circuitry upstream from the AAT4618. The
extremely fast short circuit response time of the
AAT4618 reduces the size requirement of C
IN
.
C
IN
should be located as close to the device VIN pin
as practically possible. Ceramic, tantalum or alu-
minum electrolytic capacitors are appropriate for
C
IN
. There is no specific capacitor ESR requirement
for C
IN
. However, for higher current operation,
ceramic capacitors are recommended for C
IN
due to
their inherent capability over tantalum capacitors to
withstand input current surges from low impedance
sources such as batteries in portable devices.
Output Capacitor
In order to insure stability while current limit is
active, a small capacitance of approximately
0.47μF is required. No matter how large the output
capacitor however, output current is limited to the
value set by the AAT4618 current limiting circuitry,
so very large output capacitors can be used.
For example, USB ports are specified to have at
least 120μF of capacitance down stream from their
controlling power switch. The current limiting circuit
will allow an output capacitance of 1000μF or more
without disturbing the upstream power supply.
ON (Enable Input)
In many systems, power planes are controlled by
integrated circuits which run at lower voltages than
the power plane itself. The enable input (ON) of
the AAT4618 has low and high threshold voltages
that accommodate this condition. The threshold
voltages are compatible with 5 volt TTL, and 2.5
volt to 5 volt CMOS systems.
When switching the AAT4618 on into a capacitive
load, the AAT4618 will charge the output capacitive
load at a rate no greater than the current limit setting.
Attaching Loads
Capacitive loads attached to an enabled AAT4618
will charge at a rate no greater than the current limit
setting.
FAULT Output
The FAULT flag is provided to alert the system if an
AAT4618 load is not receiving sufficient voltage to
operate properly. If current limit or over tempera-
ture circuits in any combination are active for more
than approximately four milliseconds, the FAULT
flag is pulled to ground through approximately
100
. The filtering of voltage or current transients
of less than four milliseconds prevents capacitive
loads connected to the AAT4618 output from acti-
vating the FAULT flag when they are initially
attached. Pull up resistances of 1k
to 100k
are
recommended. Since FAULT is an open drain ter-
minal, it may be pulled up to any unrelated voltage
less than the maximum operating voltage of 5.5V,
allowing for level shifting between circuits.
Thermal Considerations
Since the AAT4618 has internal current limit and
over temperature protection, junction temperature
is rarely a concern. However, if the application
requires large currents in a hot environment, it is
possible that temperature rather than current limit
will be the dominant regulating condition. In these
applications, the maximum current available with-
out risk of an over temperature condition must be
calculated. The maximum internal temperature
while current limit is not active can be calculated
using Equation 1.
T
J
(MAX) = I
MAX
In Equation 1, I
MAX
is the maximum current
required by the load. R
DS(ON)
(MAX) is the maxi-
mum rated R
DS(ON)
of the AAT4618 at high temper-
ature. R
θ
JA
is the thermal resistance between the
AAT4618 die and the board onto which it is mount-
ed. T
A
(MAX) is the maximum temperature that the
PCB under the AAT4618 would be if the AAT4618
were not dissipating power. Equation 1 can be
rearranged to solve for I
MAX
; Equation 2.
T
SD
(MIN) - T
A
(MAX)
R
DS(ON)
(MAX)
×
R
θ
JA
2
×
R
DS(ON)
(MAX)
×
R
θ
JA
+ T
A
(MAX)
I
MAX
=
AAT4618
Current Limited Load Switch
4618.2002.2.0.9
7
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