7

Apr 20-07 Rev B

SPX1117 800mA Low Dropout Voltage Regulator

© 2007 Sipex Corporation

Thermal Characteristics

The thermal resistance of SPX1117 (SOT-223

Package) is 15

°C/W from junction to tab and 31

°C/W from tab to ambient for a total of 46 °C/W

from junction to ambient (Table 1). The SPX1117

features the internal thermal limiting to protect

the device during overload conditions. Special

care needs to be taken during continuous load

conditions such that the maximum junction tem-

perature does not exceed 125

°C. Thermal pro-

tection is activated at >155

°C and deactiviated

at <140

°C.

Taking the FR-4 printed circuit board and 1/16

thick with 1 ounce copper foil as an experiment

(fig.13), the PCB material is effective at trans-

mitting heat with the tab attached to the pad area

and a ground plane 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 effect the thermal resis-

tance of the SPX1117.

The actual thermal

resistance can be determined with experimenta-

tion.

SPX1117 power dissipation is calculated as

follows:

P

Maximum Junction Temperature range:

T

(junction-to-ambient)

Maximum junction temperature must not ex-

ceed the 125

°C.

Ripple Rejection

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 re-

quired increases to its maximum. If the ADJ pin

is not bypassed, the value of the output capacitor

can be lowered to 10

µF for an electrolytic alu-

minum capacitor or 2.2

µF 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:

C = 1 / ( 6.28 * F

Where

C = value of the capacitor in Farads

(select an equal or larger standard value),

F

R

If an ADJ-bypass capacitor is used, the ampli-

tude 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:

M = V

Where M = multiplier for the ripple seen when

the ADJ pin is optimally bypassed.

V

Ripple rejection for the adjustable version is

shown in Figure 20.

Figure 19. Substrate Layout for SOT-223

50 X 50mm

35 X 17mm

16 X 10mm

-90

-80

-70

-60

-50

-40

-30

-20

-10

0

10

100

1000

10000

100000

1000000

Frequency, Hz

Figure 20. Ripple Rejection; Vin=3.3V, Vout=1.8V

(adj.), Iload=200mA

APPLICATION INFORMATION: Continued