11

LT1185

APPLICATIO S I FOR ATIO

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.

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,

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.

= 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.