S

7

8

© 2011 Exar Corporation

7/11

Rev. 2.1.0

layer on the backside of the substrate. Refer

to table 1 for the results of the experiment.

The

thermal

interaction

from

other

components in the application can affect the

thermal resistance of the SPX1117. The actual

thermal resistance can be determined with

experimentation.

SPX1117 power dissipation is calculated as

follows:

Maximum Junction Temperature

.

Maximum junction temperature must not

exceed 125°C.

Fig. 19: Substrate Layout for SOT-223

Ripple rejection can be improved by adding a

capacitor between the ADJ pin and ground as

shown in Figure 23. When ADJ pin bypassing

is used, the value of the output capacitor

required increases to its maximum. If the ADJ

pin is not bypassed, the value of the output

capacitor can be lowered to 10μF for an

electrolytic aluminum capacitor or 2.2μF for a

ceramic or solid tantalum capacitor (Fig 22).

However

the

value

of

the

ADJ-bypass

capacitor should be chosen with respect to the

following equation:

1

6.28

Where

C: value of capacitor in Farads

R1: value of resistor R1 in ohms

If an ADJ-bypass capacitor is used, the

amplitude of the output ripple will be

independent of the output voltage. If an ADJ-

bypass capacitor is not used, the output ripple

will be proportional to the ratio of the output

voltage to the reference voltage:

Where M=multiplier for the ripple seen when

the ADJ pin is optimally bypassed.

Ripple rejection for the adjustable version is

shown in Figure 20.

Fig. 20: Ripple Rejection

The output of the adjustable regulator can be

set to any voltage between 1.25V and 15V.

using the formula

1.25

A small correction to this formula is required

depending on the values of resistors R1 and

R2, since the adjustable pin current (approx

50μA) flows through R2. When IADJ is taken

into account, the formula becomes