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AAT4280A_08 Datasheet, PDF (11/16 Pages) Advanced Analogic Technologies – Slew Rate Controlled Load Switch
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SmartSwitchTM
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 useful in preventing
load transients from affecting upstream circuits. CIN
should be located as close to the device VIN pin as practi-
cally possible. Ceramic, tantalum, or aluminum electro-
lytic capacitors may be selected for CIN. There is no
specific capacitor ESR requirement for CIN; however, for
higher current operation, ceramic capacitors are recom-
mended for CIN due to their inherent capability over tan-
talum capacitors to withstand input current surges from
low impedance sources, such as batteries in portable
devices.
Output Capacitor
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 require-
ment. If desired, COUT may be increased without limit to
accommodate any load transient condition without
adversely affecting the device turn-on slew rate time.
Enable Function
The AAT4280A 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, maintaining a reverse bias on the internal para-
sitic diode. Conditions where VOUT might exceed VIN should
be avoided since this would forward bias the internal para-
sitic diode and allow excessive current flow into the VOUT
pin and possibly damage the load switch.
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
highly recommended. A larger value of CIN with respect
to COUT will effect a slower CIN decay rate during shut-
down, thus preventing VOUT from exceeding VIN. In appli-
cations where there is a greater danger of VOUT exceeding
PRODUCT DATASHEET
AAT4280A
Slew Rate Controlled Load Switch
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.
Thermal Considerations and
High Output Current Applications
The AAT4280A is designed to deliver a continuous output
load current. The limiting characteristic for maximum
safe operating output load current is package power dis-
sipation. 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 (TA), the maximum
package power dissipation can be determined by the fol-
lowing equation:
PD(MAX)
=
[TJ(MAX) -
θJA
TA]
Constants for the AAT4280A are maximum junction tem-
perature, TJ(MAX) = 125°C, and package thermal resis-
tance, ΘJA = 120°C/W. Worst case conditions are calcu-
lated at the maximum operating temperature where TA =
85°C. Typical conditions are calculated under normal
ambient conditions where TA = 25°C. At TA = 85°C,
PD(MAX) = 333mW. At TA = 25°C, PD(MAX) = 833mW.
The maximum continuous output current for the AAT4280A
is a function of the package power dissipation and the RDS
of the MOSFET at TJ(MAX). The maximum RDS of the MOSFET
at TJ(MAX) is calculated by increasing the maximum room
temperature RDS by the RDS temperature coefficient. The
temperature coefficient (TC) is 2800ppm/°C. Therefore,
MAX RDS125°C = RDS25°C · (1 + TC · ΔT)
MAX RDS125°C = 120mΩ · (1 + 0.0028 ·
(125°C - 25°C)) = 154mΩ
For maximum current, refer to the following equation:
IOUT(MAX) <
PD(MAX)
RDS
4280A.2008.04.1.5
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