AAT4672 Datasheet, PDF (21/24 Pages) Advanced Analogic Technologies – Power Supply Selector Switch

SmartSwitch^TM
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 spe-
cific capacitor ESR requirement. If desired, C_OUT may be
increased to accommodate any load transient condition.
EN Input
The AAT4672 is enabled when V_EN is ≤ 0.4V (logic ‘0’);
conversely, the AAT4672 is disabled when V_EN is ≥ 1.6V
(logic ‘1’).
PWRSEL Input
When V_PWRSEL is ≤ 0.4V (logic ‘0’) the AUX-OUT current
limit is 20% of the current limit value programmed by
R_IAUX; when V_PWRSEL is ≥ 1.6V (logic ‘1’), the AUX-OUT
current limit is 100% of the current limit value pro-
grammed by R_IAUX.
Current Limit Resistor Selection
The current limits for power supply 1 and power supply
2 inputs are set by resistors between I_MAIN/I_AUX and GND.
The following equation can be used to select the appro-
priate resistor for a particular current limit:
I_CLMAIN/AUX =
⎛ V_IMAIN/IAUX⎞
⎝ R_IMAIN/IAUX⎠
·
200k
I_CLIMAIN/AUX Current limit for MAIN and/or AUX pins
respectively
V_IMAIN/IAUX Internally Regulated Voltage [0.5V ± 20%]
on the IMAIN and IAUX pins respectively
R_IMAIN/IAUX IMAIN and/or IAUX Resistor
200k
Internal Gain Factor
Design Example
A particular application requires that the current limit for
MAIN be set to 2A and the current limit for AUX be set
to 0.2A. What value of resistor is required for the IMAIN
and IAUX pins respectively?
PRODUCT DATASHEET
AAT4672
Power Supply Selector Switch
For MAIN (power supply 1 input):
R_IMAIN
=
⎛ V_IMAIN ⎞
⎝ I_CLMAIN⎠
· 200k
⎛0.5V⎞
= _{⎝ 2A âŽ}  · 200k
= 50kΩ (49.9kΩ standard value)
For AUX (power supply 2 input):
R_IAUX
=
⎛
⎝
V_IAUX
I_CLAUX
⎞
⎠
· 200k
⎛0.5V⎞
= _{⎝0.2AâŽ}  · 200k
= 500kΩ (499.9kΩ standard value)
Thermal Considerations
Since the AAT4672 has an internal current limit and over-
temperature protection(thermal shutdown), junction tem-
perature is rarely a concern. However, if the application
requires large currents in a high temperature environ-
ment, 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
AAT4672 at high temperatures (consult the “R_DSON vs.
Temperature” performance graph in the “Typical
Characteristics” section of this datasheet). For estimat-
ing 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 AAT4672 die 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 AAT4672 would be if the AAT4672 were not dissipat-
ing power. Equation 1 can be rearranged to solve for I_MAX,
into Equation 2 (Eq. 2).
4672.2008.04.1.0
www.analogictech.com
9