AAT4285 Datasheet, PDF (8/11 Pages) Advanced Analogic Technologies – 12V Slew Rate Controlled Load Switch

AAT4285
12V 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 useful
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 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 AAT4285 features an enable / disable function.
This pin (ON/OFF) is compatible with both TTL and
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, maintaining 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 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 nor-
mal operation, the use of a larger value C_IN capac-
8
itor is highly recommended. A larger value of C_IN
with respect to C_OUT will affect 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 IN to OUT (connecting the
cathode to IN and anode to OUT). The Schottky
diode forward voltage should be less than 0.45V.
Thermal Considerations and High
Output Current Applications
The AAT4285 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 utilizing
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) -
θ_JA
T_A
Constants for the AAT4285 are maximum junction
temperature, T_J(MAX) = 125°C, and package thermal
resistance, θ_JA = 140°C/W. Worst case conditions
are calculated at the maximum operating tempera-
ture where T_A = 85°C. Typical conditions are calcu-
lated under normal ambient conditions where T_A =
25°C. At T_A = 85°C, P_D(MAX) = 286mW. At T_A =
25°C, P_D(MAX) = 714mW.
The maximum continuous output current for the
AAT4285 is a function of the package power dissipa-
tion and the R_DS of the MOSFET at T_J(MAX). The max-
imum R_DS of the MOSFET at T_J(MAX) is calculated by
increasing the maximum room temperature R_DS by
the R_DS temperature coefficient. The temperature
coefficient (TCR_RDS) is 2800ppm/°C. Therefore,
MAX R_DS125°C = R_DS25°C · (1 + TCR_RDS · ΔT)
MAX R_DS125°C = 240mΩ · (1 + 0.0028 · (125°C - 25°C))
= 307mΩ
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