AAT4618IJS-0.5-1-T1 ANALOGICTECH [Advanced Analogic Technologies], AAT4618IJS-0.5-1-T1 Datasheet - Page 8

• Manufacturer Part Number: AAT4618IJS-0.5-1-T1
• Description: Current Limited Load Switch
• Manufacturer: ANALOGICTECH [Advanced Analogic Technologies]

Applications Information
Input Capacitor
The input capacitor C
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
MOSFET, will flow for less than 1µs before the cur-
rent limit circuitry activates.
curve "Short Circuit Through 0.3Ω.") In this event,
a moderately sized C
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
C
as practically possible. Ceramic, tantalum, or alu-
minum electrolytic capacitors are appropriate for
C
for C
ceramic capacitors are recommended for C
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 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.
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.
8
IN
IN
. There is no specific capacitor ESR requirement
should be located as close to the device VIN pin
IN
.
However, for higher current operation,
IN
IN
will dramatically reduce the
protects the power supply
(See characteristic
DS(ON)
IN
IN
of the
due to
.
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 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.
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.
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.
while current limit is not active can be calculated
using Equation 1.
In Equation 1, I
by the load. R
R
the thermal resistance between the AAT4618 die
and the board onto which it is mounted. T
the maximum temperature that the PCB under the
Eq. 1:
DS(ON)
of the AAT4618 at high temperature. R
T
Current Limited Load Switch
J(MAX)
The maximum internal temperature
MAX
= I
DS(ON)(MAX)
MAX
is the maximum current required
2
· R
DS(ON)(MAX)
is the maximum rated
AAT4618
· R
ΘJA
4618.2006.05.1.2
+ T
A(MAX)
A(MAX)
Since
θJA
is
is