AAT4680
Electronic Resettable Fuse
ture circuits in any combination are active for more
than approximately three milliseconds, FAULT is
pulled to ground through approximately 100Ω.
Removal of voltage or current transients of less
than two milliseconds prevents capacitive loads
connected to the AAT4680 output from activating
the FAULT flag when they are initially attached.
Pull-up resistances of 1kΩ to 100kΩ are recom-
mended. Since FAULT is an open drain terminal, it
may be pulled up to any voltage rail less than the
maximum operating voltage of 5.5V, allowing for
level shifting between circuits.
Applications Information
Input Capacitor
The input capacitor, CIN, protects the power supply
from current transients generated by the load
attached to the AAT4680. If a short circuit is sud-
denly applied to the output of the AAT4680, there is
a 500 nanosecond long period during which a large
current can flow before current limit circuitry acti-
vates. (See characteristic curve "Short Circuit
Through 0.3Ω.") In this event, a properly sized
input capacitor can dramatically reduce the voltage
transient seen by the power supply and other cir-
cuitry upstream from the AAT4680.
Thermal Considerations
Since the AAT4680 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.
CIN should be located as close to the device VIN pin
as practically possible. Ceramic, tantalum, or alu-
minum electrolytic capacitors may be selected for
CIN. There is no specific capacitor equivalent series
resistance (ESR) requirement for CIN. However, for
higher current operation, ceramic capacitors are
recommended for CIN due to their inherent capabil-
ity over tantalum capacitors to withstand input cur-
rent surges from low impedance sources such as
batteries in portable devices.
Output Capacitor
TJ(MAX) = IMAX2 · RDS(ON)(MAX) · RΘJA + TA(MAX)
Eq. 1:
In order to insure stability while current limit is
active, a small output capacitance of approximately
1µF is required. No matter how big the output
capacitor, output current is limited to the value set
by the AAT4680 current limiting circuitry, allowing
very large output capacitors to be used.
In Equation 1, IMAX is the maximum current
required by the load. RDS(ON)(MAX) is the maximum
rated RDS(ON) of the AAT4680 at high temperature.
RθJA is the thermal resistance between the
AAT4680 die and the board onto which it is mount-
ed. TA(MAX) is the maximum temperature that the
PCB under the AAT4680 would be if the AAT4680
were not dissipating power. Equation 1 can be
rearranged to solve for IMAX; Equation 2.
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 AAT4680 will charge
at a rate no greater than the current limit setting.
TSD(MIN) - TA(MAX)
DS(ON)(MAX) · RΘJA
=
IMAX
Eq. 2:
R
FAULT Output
TSD(MIN) is the minimum temperature required to
activate the AAT4680 over-temperature protection.
With typical specification of 125°C, 115°C is a safe
minimum value to use.
A FAULT flag is provided to alert a system if the
AAT4680 load is not receiving sufficient voltage to
operate properly. If current limit or over-tempera-
8
4680.2006.05.1.2
AAT [ ADVANCED ANALOG TECHNOLOGY, INC. ]
