AAT4282A Datasheet, PDF (10/14 Pages) Advanced Analogic Technologies – Dual Slew Rate Controlled Load Switch

AAT4282A
Dual Slew Rate Controlled Load Switch
the internal parasitic diode and allow excessive
current flow into the VOUT pin, possibly damaging
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 highly recommended. A larg-
er value of C_IN with respect to C_OUT will effect a
slower C_IN decay rate during shutdown, thus pre-
venting 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 recommend-
ed to place a Schottky diode from V_IN to V_OUT (con-
necting 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 AAT4282A 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 must 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 Printed Circuit Board Layout Recommendations
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 AAT4282A are maximum junction
temperature (T_J(MAX) = 125°C) and package ther-
mal resistance (θ_JA = 70°C/W). Worst case condi-
tions are calculated at the maximum operating tem-
perature, 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) = 571mW. At T_A =
25°C, P_D(MAX) = 1429mW.
The maximum continuous output current for the
AAT4282A is a function of the package power dis-
sipation and the R_DS of the MOSFET at T_J(MAX). The
maximum RDS of the MOSFET at T_J(MAX) is calcu-
lated by increasing the maximum room tempera-
ture R_DS by the R_DS temperature coefficient. The
temperature coefficient (TC) is 2800ppm/°C.
Therefore, at 125°C:
R_DS(MAX) = R_DS(25°C) · (1 + TC · ΔT)Ω
R_DS(MAX) = 130mΩ · + 0.002800 · (125°C - 25°C))
R_DS(MAX) = 166.4mΩ
For maximum current, refer to the following equation:
I_OUT(MAX) <
P_D(MAX)
R_DS
For example, if V_IN = 5V, R_DS(MAX) = 166.4mΩ, and
T_A = 25°C, I_OUT(MAX) = 2.93A. If the output load cur-
rent were to exceed 2.93A or if the ambient tem-
perature were to increase, the internal die temper-
ature would increase and the device would be
damaged. Higher peak currents can be obtained
with the AAT4282A. To accomplish this, the device
thermal resistance must be reduced by increasing
the heat sink area or by operating the load switch
in a duty cycle manner. Duty cycles with peaks less
than 2ms in duration can be considered using the
method described in the High Peak Current
Applications section of this datasheet.
1 The actual maximum junction temperature of AAT4282A is 150°C. However, good designed practice is to derate the maximum die tem-
perature down to 125 °C to prevent the possibility of over-temperature damage.
10
4282A.2007.09.1.1