Current Limited Load Switch
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 0.3Ω.") 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.
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.
AAT4618
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 4ms, the FAULT flag is pulled
to ground through approximately 100Ω. The filter-
ing of voltage or current transients of less than 4ms
prevents capacitive loads connected to the
AAT4618 output from activating the FAULT flag
when they are initially attached. Pull-up resistanc-
es of 1kΩ to 100kΩ are recommended. Since
FAULT is an open drain terminal, it may be pulled
up to any unrelated voltage less than the maximum
operating voltage of 5.5V, allowing for level shifting
between circuits.
Output Capacitor
In order to insure stability while current limit is
active, a low capacitance (approximately 0.47µF) is
required. No matter how large the output capaci-
tor, 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 downstream from their
controlling power switch. The current limiting circuit
will allow an output capacitance of 1000µF or more
without disturbing the upstream power supply.
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.
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 5V TTL and 2.5V to
5V CMOS systems.
Eq. 1:
T
J(MAX)
= I
MAX2
· R
DS(ON)(MAX)
· R
ΘJA
+ T
A(MAX)
In Equation 1, I
MAX
is the maximum current required
by the load. R
DS(ON)(MAX)
is the maximum rated
R
DS(ON)
of the AAT4618 at high temperature. R
θJA
is
the thermal resistance between the AAT4618 die
and the board onto which it is mounted. T
A(MAX)
is
the maximum temperature that the PCB under the
8
4618.2006.05.1.2
AAT [ ADVANCED ANALOG TECHNOLOGY, INC. ]
