Acutechnology Semiconductor Inc.
The AQ533 provides
Reverse Current Protection.
When the AQ533 is turned off, no
current will flow from the output to input. A small current will flow from output to ground
through the Vout set resistors R1 and R2. The need for protection diodes is eliminated.
AQ533
6. Thermal Considerations
When an integrated circuit operates with an appreciable current, its junction temperature is
elevated. It is important to quantify its thermal limits in order to achieve acceptable
performance and reliability. This limit is determined by summing the individual parts
consisting of a series of temperature rises from the semiconductor junction to the operating
environment. The heat generated at the device junction flows through the die to the die
attach pad, through the lead frame to the surrounding case material, to the printed circuit
board, and eventually to the ambient environment.
The AQ533 regulators have internal thermal shutdown to protect the device from over-
heating. Under all possible operating conditions, the junction temperature of the AQ533
must be lower than 125°C. A heatsink may be required depending on the maximum power
dissipation and maximum ambient temperature of the application.
To determine if a heatsink is needed, the power dissipated by the regulator, P
D
, must be
calculated:
P
D
= (V
IN
-V
OUT
) I
L
where the I
L
is the load current.
The next parameter, which must be calculated, is the max. allowable temperature rise, T(max):
T(max)=T
J
(max)-T
A
(max)
where T
J
(max) is the maximum allowable junction temperature (125°C), and T
A
(max) is the
maximum ambient temperature, which will be encountered in the application.
Using the calculated values for T(max) and P
D
, the maximum allowable value for the
junction to ambient thermal resistance (θ
JA
) can be calculated:
θ
JA
=T(max)/ P
D
If the maximum allowable value for
θ
JA
is found to be greater than the junction to ambient
thermal resistance for the package used, no heatsink is needed since the package alone will
dissipate enough heat to satisfy these requirements.
7. Ultra low Dropout Operation
The AQ533 allows for ultra low dropout operation by connecting the pin
ENABLE
to >0.95V + Vout.
This results in a dropout of
350 mV
at
500 mA
of load current, between Vin and V
OUT
.
This feature is important when the supply voltage is dropping near the output regulated voltage.
Furthermore if the voltage dropout is low, the internal power dissipation is also reduced and the
thermal requirements of the device are less stringent.
www.acutechnology.com
8
Rev. 1.7
September 13, 2006
ACUTECH [ ACUTECHNOLOGY SEMICONDUCTOR ]
