AAT4674 Datasheet, PDF (9/13 Pages) Advanced Analogic Technologies – Power Supply Selector Switch

Applications Information
Input Capacitors
A 1µF or greater capacitor is generally recom-
mended between IN1 and GND (C_IN1), and
between IN2 and GND (C_IN2). An input capacitor is
not required for basic operation; however, it is use-
ful in preventing load transients from affecting up-
stream circuits. Ceramic, tantalum, or aluminum
electrolytic capacitors may be selected for
C_IN1/C_IN2. There is no specific capacitor equivalent
series resistance (ESR) requirement for C_IN1/C_IN2.
However, for higher current operation, ceramic
capacitors are recommended for C_IN1/C_IN2 due to
their inherent capability over tantalum capacitors to
withstand input current surges from low impedance
sources such as batteries in portable devices.
Output Capacitor
A 1µF or greater capacitor is required between
OUT and GND (C_OUT). As with the input capacitor,
there is no specific capacitor ESR requirement. If
desired, C_OUT may be increased to accommodate
any load transient condition.
EN Input
The AAT4674/4674-1 is enabled when V_EN is ≤
0.4V (logic ‘0’), conversely the AAT4674/4674-1 is
disabled when V_EN is ≥ 1.6V (logic ‘1’).
SEL Input
When V_SEL is ≤ 0.4V (logic ‘0’) the output voltage
equals power supply 1 input (V_OUT = V_IN1), and con-
versely when V_SEL is ≥ 1.6V (logic ‘1’) the output
voltage equals power supply 2 input (V_OUT = V_IN2).
Current Limit Resistor Selection
The current limits for power supply 1 and power
supply 2 inputs are set by resistors connected
between IIN1/IIN2 and GND. The following equa-
tion can be used to select the appropriate resistor
for a particular current limit:
I_CLINX
=
⎛V_IINX ⎞
⎝R_IINX⎠
·
200k
4674.2007.09.1.0
AAT4674
Power Supply Selector Switch
I_CLINX
V_IINX
R_IINX
200k
Current limit for IN1 and/or IN2 pins
respectively
Internally Regulated Voltage
[0.5V ± 20%] on the IIN1 and IIN2 pins
respectively
IIN1 and/or IIN2 Resistor
Internal Gain Factor
Design Example
A particular application requires that the current
limit for IN1 be set to 2A and the current limit for IN2
be set to 0.2A. What value of resistor is required
for the IIN1 and IIN2 pins respectively?
For IN1 (power supply 1 input):
R_IIN1
=
⎛ V_IIN1⎞
⎝ I_CLIN1⎠
·
200k
⎛0.5V⎞
= _{⎝ 2A âŽ}  · 200k
= 50kΩ (49.9kΩ standard value)
For IN2 (power supply 2 input):
R_IIN2
=
⎛ V_IIN2 ⎞
⎝ I_CLIN2 ⎠
·
200k
⎛0.5V⎞
= _{⎝0.2AâŽ}  · 200k
= 500kΩ (499.9kΩ standard value)
Thermal Considerations
Since the AAT4674/4674-1 have an internal current
limit and over-temperature protection (thermal shut-
down), junction temperature is rarely a concern.
However, if the application requires large currents in
a high temperature environment, it is possible that
temperature rather than current limit will be the dom-
inant regulating condition. In these applications, the
maximum current available without risk of an over-
temperature condition must be calculated. The max-
imum internal temperature while current limit is not
active can be calculated using Equation 1 (Eq. 1).
Eq. 1: T_J(MAX) = I_MAX² · R_DS(ON)(MAX) · R_θJA + T_A(MAX)
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