AAT4682 Datasheet, PDF (7/8 Pages) Advanced Analogic Technologies – Dual Electronic Resettable Switches

AAT4682
Dual Electronic Resettable Switches
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 current can flow before
current limit circuitry activates. (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 sup-
ply and other circuitry upstream from the AAT4682.
C_IN should be located as close to the device V_IN pin
as practically possible. Ceramic, tantalum or alu-
minum electrolytic capacitors may be selected 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.
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 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 AAT4682 will charge
at a rate no greater than the current limit setting.
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 temperature
circuits in any combination are active for more than
approximately two milliseconds, the corresponding
FAULT output is pulled to ground through approxi-
mately 100Ω. Removal of voltage or current tran-
sients of less than two milliseconds prevents capaci-
tive loads connected to the AAT4682 output from acti-
vating the FAULT flag when they are initially attached.
Pull up resistances of 10kΩ to 100kΩ are recom-
mended. Since the FAULT outputs are open drain
terminals, they 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 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 tem-
perature rather than current limit will be the dominant
regulating condition. In these applications, the max-
imum combined current available without risk of an
over temperature condition can be calculated. The
maximum internal temperature while current limit is
not active can be calculated using Equation 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 maxi-
mum rated R_DS(ON) of the AAT4682 at high temper-
ature. 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.
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 AAT4682 over temperature protection.
The typical over temperature threshold specifica-
tion is 125°C, therefore 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 R_θJA to be
approximately 150°C/W. Using Equation 2,
I_MAX=
115 - 85
600 × 150
=
575mA
To prevent thermal limiting, the combined operating
load current of each output in the application must
be less than 575mA.
4682.2001.11.0.9
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