AAT4601 Datasheet, PDF (11/16 Pages) Advanced Analogic Technologies – 1.5A Current Limited P-Channel Switch

AAT4601
1.8A Current Limited P-Channel Switch
For example, a portable device is specified to oper-
ate in a 50°C environment. The printed circuit
board assembly will operate at temperatures as
high as 85°C. This portable device has a sealed
case and the area of the printed board assembly is
relatively small, causing R_θJA to be approximately
120°C/W. Using Equation 2:
Eq. 3: I_MAX ⁼
115 - 85
130 · 120
= 1.4A
If this system requires less than 1.4A, the thermal
limit will not activate during normal operation.
Input Capacitor
The input capacitor serves two purposes. First, it
protects the source power supply from transient
current effects generated by the application load
circuit. If a short circuit is suddenly applied to the
output of an AAT4601, there is a microsecond-long
period during which a large current can flow before
the current limit circuitry activates; refer to the char-
acteristic curve, "Short-Circuit Through 0.3Ω." A
properly sized input capacitor can dramatically
reduce the load switch input transient response
effects seen by the power supply and other circuit-
ry upstream from the AAT4601.
The second purpose of the input capacitor is to pre-
vent transient events generated by the load circuit
from affecting operation of the AAT4601. For exam-
ple, if an AAT4601 is used in a circuit that operates
from a 3V power supply with poor step load
response, it is possible that turning on the load switch
could cause the input power supply to droop below
the AAT4601's under-voltage lockout threshold. This
drop in voltage would cause the AAT4601 to turn off
until the input power supply voltage recovers. Since
this cycle would be self-perpetuating, the entire circuit
could be seen to be unstable. In the very rare case
where capacitor cost is prohibitive, the output load cir-
cuit should be slew rate limited when turned on.
Output Capacitor
In order to insure stability while the device current
limit is active, a small capacitance of approximately
1µF should be used. When the AAT4601 is activat-
4601.2006.05.1.3
ed using the ON function, there are no momentary
current transients, as in the case when a short circuit
is suddenly applied to a device that is already on;
refer to the characteristic curve, "Switch Turn-On
Time." No matter how big the output capacitor, out-
put current is limited to the value allowed by the
threshold determined by R_SET and the internal cur-
rent limiting circuitry. This permits very large output
capacitors to be used.
For example, USB ports are specified to have at
least 120µF of downstream capacitance from their
controlling power switch. An output capacitance as
large as 1000µF would not disturb the input power
supply to the AAT4601 used to control the USB port.
ON Input
When the AAT4601 is in the off state, the output is
an open circuit and the device quiecent current
consumption is reduced to less than 1µA. The ON
threshold voltage is set to allow the AAT4601 to be
controlled by 5V TTL levels, as well as CMOS
power from 2.5V to 5V. The ON function control
voltage level should not exceed the input supply
level applied to the IN pin.
FAULT Output
A FAULT flag is provided to alert a system if the load
switch is not receiving a sufficient voltage level to
properly operate. If either the current limit or over-
temperature circuits in any combination are active for
more than approximately 2ms continuously, the
FAULT pin is pulled to ground internally through a
100Ω resistance. The 2ms delay on the FAULT func-
tion is intended to prevent capacitive loads connect-
ed to the load switch output from activating FAULT
when the device is turned on. The placement of a
pull-up resistor between the FAULT pin and the IN
pin is recommended. Reasonable values for the
pull-up resistor should range from 10kΩ to 100kΩ.
Since FAULT is an open drain terminal, it may be
pulled up to any voltage that is not greater than the
level present on the IN pin. This is done to allow the
AAT4601 to signal ancillary circuitry that is powered
by a voltage level less than the level on the IN pin.
Reverse Voltage
The AAT4601 is designed to control current flowing
from IN to OUT. If a voltage is applied to OUT
11