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AAT4612 Datasheet, PDF (7/10 Pages) Advanced Analogic Technologies – Current Limited Load Switch
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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 AAT4612. When a short circuit is
suddenly applied to the output of the AAT4612, a
large current, limited only by the RDS(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 CIN 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 CIN.
CIN should be located as close to the device VIN pin
as practically possible. Ceramic, tantalum or alu-
minum electrolytic capacitors are appropriate for
CIN. There is no specific capacitor ESR require-
ment for CIN. However, for higher current opera-
tion, 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 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 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.
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
that accommodate this condition. The threshold
voltages are compatible with 5 volt TTL, and 2.5
volt to 5 volt CMOS systems.
AAT4612
Current Limited Load Switch
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.
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.
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 AAT4612 at high temper-
ature. RθJA is the thermal resistance between the
AAT4612 die and the board onto which it is mount-
ed. TA(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 IMAX; Equation 2.
IMAX=
(TSD(MIN) - TA(MAX))
(RDS(ON)(MAX) × RθJA)
TSD(MIN) is the minimum temperature required to
activate the AAT4612 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 -
(188m ×
85)
150))
=
1
A
The AAT4612 is safe to operate at the maximum
current level of 1A at 85°C.
4612.2002.1.0.9
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