Acutechnology Semiconductor Inc.
AQ1541
5. Protection Diodes
Unlike older regulators, the AQ1541 family does not need any protection diodes between
the adjustment pin and output or from the output to the input to prevent over-stressing the
die. Internal resistors are limiting the internal current paths on the adjustment pin.
Therefore, even with capacitors on the adjustment pin, no protection diode is needed to
ensure device safety under short circuit conditions. External diodes between the input and
output are not usually needed. Only if high value output capacitors are used (> 1000uF)
and the input is instantaneously shorted to ground, can damage occur.
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 AQ1541 regulators have internal thermal shutdown to protect the device from over-
heating. Under all possible operating conditions, the junction temperature of the AQ1541
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 maximum 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.
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Rev. 2.2
September 13, 2005
ACUTECH [ ACUTECHNOLOGY SEMICONDUCTOR ]
