4

Date: 6/21/06 Rev C

SPX2815 1.5A Low Dropout Voltage Regulator

© 2006 Sipex Corporation

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0

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10000

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1000000

Frequency, Hz

Figure 5. Ripple Rejection; Vin=3.3V, Vout=1.8V (adj.),

Iload=200mA

Figure 6. Current Source

LOAD

ADJ

SPX2815

V

IN

C

1

OUT

IN

4.7μF

C

2

I

OUT

R

1

V

REF

R

1

I

+

V

OUT

Ripple Rejection

Ripple rejection can be improved by adding a

capacitor between the ADJ pin and ground as

shown in Figure 8. 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 22

μF for an electrolytic aluminum

capacitor or 10

μF for a solid tantalum capacitor

(Fig 7). However, the value of the ADJ-bypass

capacitor should be chosen with respect to the

following equation:

_______1______-_

C =

( 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 5.

Output Voltage

The output of the adjustable regulator can be set

to any voltage between 1.25V and 15V. The

value of V

the formula

V

A small correction to this formula is required

depending on the values of resistors R

since the adjustable pin current (approx 50

μA)

flows through R

When I

count, the formula becomes

V

where

V

Layout Considerations

Parasitic line resistance can degrade load regu-

lation. In order to avoid this, connect R

to V

reason, R

side of the load.

APPLICATION INFORMATION: Continued