10
LT1185
1185ff
Thermal Regulation
IC regulators have a regulation term not found in discrete
designs because the power transistor is thermally coupled
to the reference. This creates a shift in the output voltage
which is proportional to power dissipation in the regulator.
∆V
OUT
= P(K1 + K2 θ
JA
)
= (I
OUT
)(V
IN
– V
OUT
)(K1 + K2 θ
JA
)
K1 and K2 are constants. K1 is a fast time constant effect
caused by die temperature
gradients
which are estab-
lished within 50ms of a power change. K1 is specified on
the data sheet as thermal regulation, in percent per watt.
K2 is a long time constant term caused by the temperature
drift of the regulator reference voltage. It is also specified,
but in percent per degree centigrade. It must be multiplied
by overall thermal resistance, junction-to-ambient, θ
JA
.
As an example, assume a 5V regulator with an input
voltage of 8V, load current of 2A, and a total thermal
resistance of 4°C/W, including junction-to-case, (use
control area specification), interface, and heat sink resis-
tance. K1 and K2, respectively, from the data sheet are
0.014%/W and 0.01%/°C.
∆V
OUT
= (2A)(8V – 5V)(0.014 + 0.01 • 4)
= 0.32%
This shift in output voltage could be in either direction
because K1 and K2 can be either positive or negative.
Thermal regulation is already included in the worst case
reference specification.
Output Voltage Reversal
Some IC regulators suffer from a latch-up state when their
output is forced to a reverse voltage of as little as one diode
drop. The latch-up state can be triggered without a fault
condition when the load is connected to an opposite
polarity supply instead of to ground. If the second supply
is turned on first, it will pull the output of the first supply
to a reverse voltage through the load. The first supply may
then latch off when turned on. This problem is particularly
annoying because the diode clamps which should always
be used to protect against polarity reversal do not usually
stop the latch-up problem.
The LT1185 is designed to allow output reverse polarity of
several volts without damage or latch-up, so that a simple
diode clamp can be used.
APPLICATIO S I FOR ATIO
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