Dual Electronic Resettable Switch
Applications Information
Input Capacitor
The input capacitor protects the power supply from
current transients generated by the loads attached
to the AAT4682. If a short circuit is suddenly
applied to a AAT4682 output, there is a 500
nanosecond long period during which a large cur-
rent can flow before current limit circuitry activates.
(See characteristic curve "Short Circuit Through
0.3Ω.") In this event, a properly sized input capac-
itor can dramatically reduce the voltage transient
seen by the power supply and other circuitry
upstream from the AAT4682.
C
IN
should be located as close to the device VIN
pin as practically possible. Ceramic, tantalum, or
aluminum electrolytic capacitors may be selected
for C
IN
. There is no specific capacitor equivalent
series resistance (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 with-
stand input current surges from low impedance
sources such as batteries in portable devices.
AAT4682
FAULT Outputs
FAULT flags are provided to alert a system if a
AAT4682 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 two milliseconds, the corre-
sponding FAULT output is pulled to ground through
approximately 100Ω. Removal of voltage or cur-
rent transients of less than two milliseconds pre-
vents capacitive loads connected to the AAT4682
output from activating the FAULT flag when they
are initially attached. Pull-up resistances of 10kΩ
to 100kΩ are recommended. Since the FAULT out-
puts are open drain terminals, they may be pulled
up to any voltage rail less than the maximum oper-
ating voltage of 5.5V, allowing for level shifting
between circuits.
Thermal Considerations
Since the AAT4682 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 combined current avail-
able without risk of an over-temperature condition
can be calculated. The maximum internal temper-
ature while current limit is not active can be calcu-
lated using Equation 1.
Eq. 1:
T
J(MAX)
= I
MAX2
· R
DS(ON)(MAX)
· R
ΘJA
+ T
A(MAX)
Output Capacitors
In order to insure stability while current limit is
active, a small output capacitance of approximately
1µF is required at each output. No matter how big
the output capacitor, output current is limited to the
value set by the AAT4682 current limiting circuitry,
allowing very large output capacitors to 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.
Attaching Loads
Capacitive loads attached to the AAT4682 will charge
at a rate no greater than the current limit setting.
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 AAT4682 at high temperature.
R
θJA
is the thermal resistance between the
AAT4682 die and the board onto which it is mount-
ed. T
A(MAX)
is the maximum temperature that the
PCB under the AAT4682 would be if the AAT4682
were not dissipating power. Equation 1 can be
rearranged to solve for I
MAX
; Equation 2.
Eq. 2:
I
MAX
=
T
SD(MIN)
- T
A(MAX)
R
DS(ON)(MAX)
· R
ΘJA
8
4682.2006.05.1.2
AAT [ ADVANCED ANALOG TECHNOLOGY, INC. ]
