AAT4601
1.8A Current Limited P-Channel Switch
load. In these applications, the maximum current
available without risk of activation of the over-tem-
perature circuit can be calculated. The maximum
internal temperature while current limit is not active
can be calculated using Equation 1:
Operation in Current Limit
If an excessive load is applied to the output of an
AAT4601, the load current will be limited by the
device's current limit circuitry. Refer to Figure 1,
Overload Operation. If a short circuit were to occur
on the load, it would demand more current than
allowed by the internal current limiting circuit and the
voltage at the AAT4601's output would drop. This
causes the AAT4601 to dissipate more power than in
normal operation, causing the die temperature to
increase. When die temperature exceeds the inter-
nal over-temperature threshold, the AAT4601 will
shut down. After shutting down, the AAT4601 cools
to a level below the over-temperature threshold, at
which point it will start up again. The AAT4601 will
continue to cycle off and on until one of the following
events occurs: the load current is reduced to a level
below the AAT4601's current limit setting; the input
power is removed; or the output is turned off by a
logic high level applied to the ON pin.
2
Eq. 1:
TJ(MAX) = IMAX × RDS(ON)(MAX) × RθJA + TA(MAX)
In Equation 1, IMAX is the maximum current required
by the load. RDS(ON)(MAX) is the maximum rated
RDS(ON) of the AAT4601 at high temperature. RθJA is
the thermal resistance between the AAT4601's die
and the board onto which it is mounted. TA(MAX) is
the maximum ambient temperature for the printed
circuit board assembly under the AAT4601 when the
load switch is not dissipating power. Equation 1 can
be transformed to provide IMAX; Refer to Equation 2:
TSD(MIN) - TA(MAX)
RDS(ON)(MAX) × RΘJA
=
Thermal Considerations
IMAX
Eq. 2:
Since the AAT4601 has internal current limit and
over-temperature protection, junction temperature is
rarely a concern. If an application requires a large
load current in a high-temperature operating envi-
ronment, there is the possibility that the over-tem-
perature protection circuit, rather than the current
limit circuit, will regulate the current available to the
TSD(MIN) is the minimum temperature required to
activate the AAT4601's over-temperature protec-
tion. With a typical specification of 125°C, 115°C is
a safe minimum value to use.
6
5
Input Voltage
4
3
1Ω is Applied to Output at t = 0
RSET is 2kΩ
2
Output Voltage
1
0
-0.5
0
0.5
1
1.5
2
Time (s)
Figure 1: Overload Operation.
10
4601.2006.05.1.3
AAT [ ADVANCED ANALOG TECHNOLOGY, INC. ]
