SPX2815T(2004) Ver la hoja de datos (PDF) - Signal Processing Technologies

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Fabricante

SPX2815T
(Rev.:2004)

1.5A Low Dropout Voltage Regulator

Signal Processing Technologies 

APPLICATION INFORMATION: Continued

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 22µF for an

electrolytic aluminum capacitor or 10µF 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 * R )

R

1

Where

C = value of the capacitor in Farads

(select an equal or larger standard value),

FR = ripple frequency in Hz,

R = value of resistor R1 in Ohms.

1

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 = VOUT / VREF

Where M = multiplier for the ripple seen when

the ADJ pin is optimally bypassed.

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 can be quickly approxi-

OUT

mated using the formula

VOUT=1.25 *(R1 + R2)/R1

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

R.

2

When I is taken

ADJ

into account, the formula becomes

VOUT = VREF(1+ (R2/R1)) + IADJ * R2

where

VREF=1.25V

Layout Considerations

Parasitic line resistance can degrade load

regulation. In order to avoid this, connect R1

directly to VOUT as illustrated in Figure 13.

For the same reason, R2 should be connected

to the negative side of the load.

VREF =1.25V

0

-10

-20

-30

-40

-50

-60

-70

-80

-90

10

100

1000

10000

100000 1000000

Frequency, Hz

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

Iload=200mA

V IN

IN

4.7µF + C 1

SPX2815

ADJ

IOUT =

VREF

R1

OUT

VOUT

C2

R1

IOUT

LOAD

Figure 6. Current Source

Date: 5/17/04

SPX2815 1.5A Low Dropout Voltage Regulator

4

© Copyright 2004 Sipex Corporation

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