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AAT4680 Datasheet, PDF (7/8 Pages) Advanced Analogic Technologies – Electronic Resettable Fuse
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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 tran-
sient seen by the power supply and other circuitry
upstream from the AAT4680.
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 ESR requirement
for CIN. However, for higher current operation,
ceramic capacitors are recommended for CIN due to
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 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.
For example, USB ports are specified to have at
least 120µF of capacitance down stream 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.
FAULT Output
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-
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
AAT4680
Electronic Resettable Fuse
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.
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 appli-
cations, the maximum current available without 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.
TJ(MAX) = IMAX2 × RDS(ON)(MAX) × RθJA + TA(MAX)
In Equation 1, IMAX is the maximum current
required by the load. RDS(ON)(MAX) is the maxi-
mum rated RDS(ON) of the AAT4680 at high temper-
ature. 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.
IMAX=
TSD(MIN) - TA(MAX)
RDS(ON)(MAX) × RθJA
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.
For example, if an application is specified to oper-
ate in 50°C environments, the PCB operates at
temperatures as high as 85°C. The application is
sealed and its PCB is small, causing RθJA to be
approximately 150°C/W. Using Equation 2,
IMAX=
115 - 85
600 × 150
=
575
mA
To prevent thermal limiting, the operating load cur-
rent in the application must be less than 575mA.
4680.2001.11.0.91
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