AAT4674_08 Datasheet, PDF (9/12 Pages) Advanced Analogic Technologies – Power Supply Selector Switch

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Output Capacitor
A 1μF or greater capacitor is required between OUT and
GND (C_OUT). As with the input capacitor, there is no spe-
cific 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 conversely 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 equation can be used to select
the appropriate resistor for a particular current limit:
I_CLINX
=
⎛V_IINX ⎞
⎝R_IINX⎠
·
200k
I_CLINX Current limit for IN1 and/or IN2 pins respectively
V_IINX Internally Regulated Voltage [0.5V ± 20%] on the
IIN1 and IIN2 pins respectively
R_IINX IIN1 and/or IIN2 Resistor
200k 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)
PRODUCT DATASHEET
AAT4674
Power Supply Selector Switch
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 shutdown),
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 dominant regulating condition.
In these applications, the maximum current available
without risk of an over-temperature condition must be
calculated. The maximum 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)
In Equation 1, I_MAX is the maximum current required by
the load. R_DS(ON)(MAX) is the maximum rated R_DS(ON) of the
AAT4674/4674-1 at high temperatures (consult the
“R_DSON vs. Temperature” performance graph in the
“Typical Characteristics” section of this datasheet). For
estimating the R_DS(ON)(MAX) use the data on the “R_DSON vs
Temperature” performance graph and increase the value
from the performance graph by 50%. R_θJA is the thermal
resistance between the AAT4674/4674-1 and the printed
circuit board (PCB) onto which it is mounted; R_θJA is the
thermal resistance of the TSOPJW-12 package. T_A(MAX) is
the maximum ambient temperature that the PCB under
the AAT4674/4674-1 would be if the AAT4674/4674-1
were not dissipating power. Equation 1 can be rear-
ranged to solve for I_MAX, into Equation 2 (Eq. 2).
Eq. 2: I_MAX =
T_SD(MIN) - T_A(MAX)
R · R _DS(ON)(MAX)
ΘJA
T_SD(MIN) is the minimum temperature required to activate
the AAT4674/4674-1 over-temperature protection (ther-
mal shutdown). With typical specification of 140°C,
125°C is a safe minimum value to use.
4674.2008.02.1.1
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