AAT4250
Slew Rate Controlled Load Switch
In applications where there is a possibility of VOUT
exceeding VIN for brief periods of time during nor-
mal operation, the use of a larger value CIN capaci-
tor is highly recommended. A larger value of CIN
with respect to COUT will effect a slower CIN decay
rate during shutdown, thus preventing VOUT from
exceeding VIN. In applications where there is a
greater danger of VOUT exceeding VIN for extended
periods of time, it is recommended to place a schot-
tky diode from VIN to VOUT (connecting the cathode
to VIN and anode to VOUT). The Schottky diode for-
ward voltage should be less then 0.45 volts.
Applications Information
Input Capacitor
Typically a 1µF or larger capacitor is recommend-
ed for CIN in most applications. A CIN capacitor is
not required for basic operation, however, it is use-
ful in preventing load transients from affecting up
stream circuits. CIN should be located as close to
the device VIN pin as practically possible. Ceramic,
tantalum or aluminum electrolytic capacitors may
be selected for CIN. There is no specific capacitor
ESR requirement for CIN. However, for higher cur-
rent operation, ceramic capacitors are recom-
mended for CIN due to their inherent capability over
tantalum capacitors to withstand input current
surges from low impedance sources such as bat-
teries in portable devices.
Thermal Considerations and High
Output Current Applications
The AAT4250 is designed to deliver a continuous
output load current. The limiting characteristic for
maximum safe operating output load current is
package power dissipation. In order to obtain high
operating currents, careful device layout and circuit
operating conditions need to be taken into account.
Output Capacitor
For proper slew operation, a 0.1µF capacitor or
greater between VOUT and GND is required.
The following discussions will assume the load
switch is mounted on a printed circuit board utiliz-
ing the minimum recommended footprint as stated
in the layout considerations section.
Likewise, with the output capacitor, there is no spe-
cific capacitor ESR requirement. If desired, COUT
maybe increased without limit to accommodate any
load transient condition without adversely affecting
the slew rate.
At any given ambient temperature (TA) the maxi-
mum package power dissipation can be deter-
mined by the following equation:
Enable Function
The AAT4250 features an enable / disable function.
This pin (ON) is active high and is compatible with
TTL or CMOS logic. To assure the load switch will
turn on, the ON control level must be greater than
2.0 volts. The load switch will go into shutdown
mode when the voltage on the ON pin falls below
0.8 volts. When the load switch is in shutdown
mode, the OUT pin is tristated, and quiescent cur-
rent drops to leakage levels below 1µA.
PD(MAX) = [TJ(MAX) - TA] / ΘJA
Constants for the AAT4250 are maximum junction
temperature, TJ(MAX) = 125°C, and package thermal
resistance, ΘJA = 150°C/W. Worst case conditions
are calculated at the maximum operating tempera-
ture where TA = 85°C. Typical conditions are cal-
culated under normal ambient conditions where TA
= 25°C. At TA = 85°C, PD(MAX) = 267mW. At TA =
25°C, PD(MAX) = 667mW.
Reverse Output to Input Voltage
Conditions and Protection
The maximum continuous output current for the
AAT4250 is a function of the package power dissi-
pation and the RDS of the MOSFET at TJ(MAX). The
maximum RDS of the MOSFET at TJ(MAX) is calcu-
lated by increasing the maximum room tempera-
ture RDS by the RDS temperature coefficient. The
temperature coefficient (TC) is 2800ppm/°C.
Therefore, at 125°C
Under normal operating conditions a parasitic
diode exists between the output and input of the
load switch. The input voltage should always
remain greater than the output load voltage main-
taining a reverse bias on the internal parasitic
diode. Conditions where VOUT might exceed VIN
should be avoided since this would forward bias
the internal parasitic diode and allow excessive
current flow into the VOUT pin and possibly damage
the load switch.
RDS(MAX) = RDS(25°C) × (1 + TC × ∆T)
RDS(MAX) = 175mΩ × (1 + .002800 × (125°C - 25°C))
RDS(MAX) = 224mΩ
8
4250.2001.12.0.94
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