AAT4280 Datasheet, PDF (10/14 Pages) Advanced Analogic Technologies – Slew Rate Controlled Load Switch

AAT4280
Slew Rate Controlled Load Switch
Applications Information
Input Capacitor
A 1µF or larger capacitor is typically recommended
for C_IN in most applications. A C_IN capacitor is not
required for basic operation. However, C_IN is use-
ful in preventing load transients from affecting
upstream circuits. C_IN should be located as close
to the device V_IN pin as practically possible.
Ceramic, tantalum, or aluminum electrolytic capac-
itors may be selected for C_IN. There is no specific
capacitor ESR requirement for C_IN. However, for
higher current operation, ceramic capacitors are
recommended for C_IN 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.
Output Capacitor
For proper slew operation, a 0.1µF capacitor or
greater between V_OUT and GND is recommended.
The output capacitor has no specific capacitor type
or ESR requirement. If desired, C_OUT may be
increased without limit to accommodate any load
transient condition without adversely affecting the
device turn-on slew rate time.
Enable Function
The AAT4280 features an enable / disable function.
This pin (ON/OFF) is compatible with both TTL or
CMOS logic.
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 V_OUT might exceed V_IN
should be avoided since this would forward bias
the internal parasitic diode and allow excessive
current flow into the V_OUT pin and possibly damage
the load switch.
In applications where there is a possibility of V_OUT
exceeding V_IN for brief periods of time during normal
operation, the use of a larger value C_IN capacitor is
10
highly recommended. A larger value of C_IN with
respect to C_OUT will effect a slower C_IN decay rate
during shutdown, thus preventing V_OUT from
exceeding V_IN. In applications where there is a
greater danger of V_OUT exceeding V_IN for extended
periods of time, it is recommended to place a
Schottky diode from V_IN to V_OUT (connecting the
cathode to V_IN and anode to V_OUT). The Schottky
diode forward voltage should be less than 0.45V.
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.
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.
At any given ambient temperature (T_A), the maxi-
mum package power dissipation can be deter-
mined by the following equation:
P_D(MAX) = [T_J(MAX) - T_A] / Θ_JA
Constants for the AAT4280 are maximum junction
temperature, T_J(MAX) = 125°C, and package thermal
resistance, Θ_JA = 120°C/W. Worst case conditions
are calculated at the maximum operating tempera-
ture where T_A = 85°C. Typical conditions are cal-
culated under normal ambient conditions where T_A
= 25°C. At T_A = 85°C, P_D(MAX) = 333mW. At T_A =
25°C, P_D(MAX) = 833mW.
The maximum continuous output current for the
AAT4280 is a function of the package power dissi-
pation and the R_DS of the MOSFET at T_J(MAX). The
maximum R_DS of the MOSFET at T_J(MAX) is calcu-
lated by increasing the maximum room temperature
R_DS by the R_DS temperature coefficient. The tem-
perature coefficient (T_C) is 2800ppm/°C. Therefore,
MAX R_DS125°C = R_DS25°C · (1 + T_C · ΔT)
MAX R_DS125°C = 120mΩ · (1 + 0.0028 ·
(125°C - 25°C)) = 154mΩ
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