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 AAT4612. When a short circuit is
suddenly applied to the output of the AAT4612, 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 AAT4612. The
extremely fast short-circuit response time of the
AAT4612 reduces the size requirement of C
IN
.
C
IN
should be located as close to the device IN 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.
that accommodate this condition. The threshold
voltages are compatible with 5V TTL and 2.5V to
5V CMOS systems.
When switching the AAT4612 on into a capacitive
load, the AAT4612 will charge the output capacitive
load at a rate no greater than the current limit setting.
AAT4612
Attaching Loads
Capacitive loads attached to an enabled AAT4612
will charge at a rate no greater than the current limit
setting.
Thermal Considerations
Since the AAT4612 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.
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 AAT4612 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.
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 AAT4612 at high temperature.
R
θJA
is the thermal resistance between the
AAT4612 die and the board onto which it is mount-
ed. T
A(MAX)
is the maximum temperature that the
PCB under the AAT4612 would be if the AAT4612
were not dissipating power. Equation 1 can be
rearranged to solve for I
MAX
:
ON
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 AAT4612 has low and high threshold voltages
Eq. 2:
I
MAX
=
T
SD(MIN)
- T
A(MAX)
R
DS(ON)(MAX)
· R
ΘJA
T
SD(MIN)
is the minimum temperature required to
activate the AAT4612 over-temperature protection.
8
4612.2006.09.1.3
AAT [ ADVANCED ANALOG TECHNOLOGY, INC. ]
