Although it may not be immediately obvious best load regu-

lation is obtained when the top of the divider is connected

directly to the output pin not to the load This is illustrated in

Figure 2 If R1 were connected to the load the effective

resistance between the regulator and the load would be

R1

J

Connected as shown Rw is not multiplied by the divider

ratio Rw is about 0004X per foot using 16 gauge wire This

translates to 40 mVft at 10A load current so it is important

to keep the positive lead between regulator and load as

short as possible

TLH9059 – 2

FIGURE 2 Proper Divider Connection

The input resistance of the sense pin is typically 6 kX mod-

eled as a resistor between the sense pin and the output pin

Load regulation will start to degrade if a resistance higher

than 10X is inserted in series with the sense This assumes

a worst-case condition of 05V between output and sense

pins Lower differential voltage will allow higher sense series

resistance

Thermal Load Regulation

Thermal as well as electrical load regulation must be con-

sidered with IC regulators Electrical load regulation occurs

in microseconds thermal regulation due to die thermal gra-

dients occurs in the 02 ms-20 ms time frame and regula-

tion due to overall temperature changes in the die occurs

over a 20 ms to 20 minute period depending on the time

constant of the heat sink used Gradient induced load regu-

lation is calculated from

D

Thermal regulation specified on data sheet

0005%W this yields a 02% change in output voltage

Changes in output voltage due to overall temperature rise

are calculated from

018% Because these two thermal terms can have either

polarity they may subtract from or add to electrical load

regulation For worst-case analysis they must be assumed

to add If the output of the regulator is trimmed under load

only that portion of the load that changes need be used in

the previous calculations significantly improving output ac-

curacy

Line Regulation

Electrical line regulation is very good on the LM196typi-

cally less than 0005% change in output voltage for a 1V

change in input This level of regulation is achieved only for

very low load currents however because of thermal ef-

fects Even with a thermal regulation of 0002%W and a

temperature coefficient of 0003% C DC line regulation

will be dominated by thermal effects as shown by the follow-

ing example

Following a 10% change in input voltage (09) the output

After a much longer time determined by the time constant

of the heat sink the output will change an additional

the temperature coefficient of output voltage and the ther-

mal resistance from die to ambient (2 CW was chosen for

this calculation) The sign of these last two terms varies

from part to part so no assumptions can be made about any

cancelling effects All three terms must be added for a prop-

0062% using

typical values for thermal regulation and tem-

perature coefficient For worst-case analysis the maximum

data sheet specifications for thermal regulation and temper-

ature coefficient should be used along with the

actual ther-

mal resistance of the heat sink being used

Paralleling Regulators

Direct paralleling of regulators is not normally recommend-

ed because they do not share currents equally The regula-

tor with the highest reference voltage will supply all the cur-

rent to the load until it current limits With an 18A load for

instance one regulator might be operating in current limit at

16A while the second device is only carrying 2A Power dis-

sipation in the high current regulator is extremely high with

attendant high junction temperatures Long term reliability

cannot be guaranteed under these conditions

Quasi-paralleling may be accomplished if load regulation is

not critical The connection shown in

Figure 5a will typically

share to within 1A with a worst-case of about 3A Load

regulation is degraded by 150 mV at 20A loads An external

op amp may be used as in

Figure 5b to improve load regula-

tion and provide remote sensing

5