4

Date: 5/17/04

SPX2815 1.5A Low Dropout Voltage Regulator

© Copyright 2004 Sipex Corporation

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0

10

100

1000

10000

100000

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 capaci-

tor 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

amplitude of the output ripple will be indepen-

dent 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

mated using 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

into account, the formula becomes

V

where

V

Layout Considerations

Parasitic line resistance can degrade load

regulation. In order to avoid this, connect R

1

directly to V

For the same reason, R

to the negative side of the load.

APPLICATION INFORMATION: Continued