Thermal Resistance

The thermal resistance between two points of a conductive

system is expressed as

CW

where subscript order indicates the direction of heat flow A

simplified heat transfer circuit for a cased semiconductor

and heat sink system is shown in the figure below

The circuit is valid only if the system is in thermal equilibrium

(constant heat flow) and there are indeed single specific

junction case or heat sink) Nevertheless this is a reason-

able approximation of actual performance

TLK5558 – 8

FIGURE 2 Semiconductor-Heat Sink Thermal Circuit

data sheet depends upon the material and size of the pack-

age die size and thickness and quality of the die bond to

the case or lead frame The case-to-heat sink thermal re-

heat sink and upon the area and quality of the contact sur-

03 to 05 CW using silicone grease

on the quality of the heat sink and the ambient conditions

Cooling is normally required to maintain the worst case op-

from known operating conditions Rewriting the above equa-

tion we find

CW

for a DC Signal

Stability and Compensation

As with most amplifiers care should be taken with lead

dress component placement and supply decoupling in or-

der to ensure stability For example resistors from the out-

put to an input should be placed with the body close to the

input to minimize ‘‘pickup’’ and maximize the frequency of

the feedback pole by minimizing the capacitance from the

input to ground

A feedback pole is created when the feedback around any

amplifier is resistive The parallel resistance and capaci-

tance from the input device (usually the inverting input) to ac

ground set the frequency of the pole In many instances the

frequency of this pole is much greater than the expected 3

dB frequency of the closed loop gain and consequently

there is negligible effect on stability margin However if the

feedback pole is less than approximately six times the ex-

pected 3 dB frequency a lead capacitor should be placed

from the output to the input of the op amp The value of the

added capacitor should be such that the RC time consistant

of this capacitor and the resistance it parallels is greater

than or equal to the original feedback pole time constant

Some inductive loads may cause output stage oscillation A

01 mF ceramic capacitor in series with a 10X resistor from

the output to ground will usually remedy this situation

TLK5558 – 9

FIGURE 3 Driving Inductive Loads

Capacitive loads may be compensated for by traditional

techniques (See ‘‘Operational Amplifiers Theory and Prac-

tice’’ by Roberge published by Wiley)

TLK5558 – 10

Compensate for Capacitive Load

A similar but alternative technique may be used for the

LH0101

TLK5558 – 11

FIGURE 5 Alternate Compensation for Capacitive Load

7