AAT4280
Slew Rate Controlled Load Switch
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
Schottky diode from VIN to VOUT (connecting the
cathode to VIN and anode to VOUT). The Schottky
diode forward voltage should be less than 0.45V.
Applications Information
Input Capacitor
A 1µF or larger capacitor is typically recommended
for CIN in most applications. A CIN capacitor is not
required for basic operation. However, CIN is use-
ful in preventing load transients from affecting
upstream circuits. CIN should be located as close
to the device VIN pin as practically possible.
Ceramic, tantalum, or aluminum electrolytic capac-
itors may be selected for CIN. There is no specific
capacitor ESR requirement for CIN. However, for
higher current operation, ceramic capacitors are
recommended for CIN due to their inherent capabil-
ity over tantalum capacitors to withstand input cur-
rent surges from low impedance sources, such as
batteries in portable devices.
Thermal Considerations and
High Output Current Applications
The AAT4280 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
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 of this
datasheet.
For proper slew operation, a 0.1µF capacitor or
greater between VOUT and GND is recommended.
The output capacitor has no specific capacitor type
or ESR requirement. If desired, COUT may be
increased without limit to accommodate any load
transient condition without adversely affecting the
device turn-on slew rate time.
At any given ambient temperature (TA), the maxi-
mum package power dissipation can be deter-
mined by the following equation:
PD(MAX) = [TJ(MAX) - TA] / ΘJA
Enable Function
The AAT4280 features an enable / disable function.
This pin (ON/OFF) is compatible with both TTL or
CMOS logic.
Constants for the AAT4280 are maximum junction
temperature, TJ(MAX) = 125°C, and package thermal
resistance, ΘJA = 120°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) = 333mW. At TA =
25°C, PD(MAX) = 833mW.
Reverse Output-to-Input Voltage
Conditions and Protection
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.
The maximum continuous output current for the
AAT4280 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 temperature
RDS by the RDS temperature coefficient. The tem-
perature coefficient (TC) is 2800ppm/°C. Therefore,
MAX RDS125°C = RDS25°C · (1 + TC · ΔT)
In applications where there is a possibility of VOUT
exceeding VIN for brief periods of time during normal
operation, the use of a larger value CIN capacitor is
MAX RDS125°C = 120mΩ · (1 + 0.0028 ·
(125°C - 25°C)) = 154mΩ
10
4280.2006.11.1.4
AAT [ ADVANCED ANALOG TECHNOLOGY, INC. ]
