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SPX3587AR

3A Low Dropout Voltage Regulator

Signal Processing Technologies 

Output Capacitor

To ensure the stability of the SPX1587, an output

capacitor of at least 10µF (ceramic or tantalum) or

22µF (aluminum) is required. The value may change

based on the application requirements of the output

load or temperature range. The value of ESR can vary

based on the type of capacitor used in the applica-

tions to guarantee stability. The recommended value

for ESR is 0.51 or less. A larger value of output

capacitance (up to 100µF) can improve the load

transient response.

Soldering Methods

The SPX1587 die is attached to the heatsink lead which

exits opposite the input, output, and ground pins.

Thermal Characteristics

The SPX1587 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 tempera-

ture does not exceed 125°C. Thermal protection is

activated at >179°C and deactiviated at <165 °C.

The thermal interaction from other components in

the application can effect the thermal resistance of

the SPX1587. The actual thermal resistance can be

determined with experimentation.

SPX1587 power dissipation is calculated as follows:

PD= (V IN- V OUT)(IOUT)

Maximum Junction Temperature range:

TJ=

T

A(max)

+

PD*

e

JA

(junction-to-ambient thermal resistance)

Maximum junction temperature must not exceed 125°C.

Ripple Rejection

Ripple rejection can be improved by adding a capaci-

tor 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 maxi-

mum. 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).

APPLICATION INFORMATION

However the value of the ADJ-bypass capacitor

should be chosen with respect to the following

equation:

Where

C = 1 / ( 6.28 * F * R )

R

1

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 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 = VOUT / VREF

Where M = multiplier for the ripple seen when the

ADJ pin is optimally bypassed.

VREF=1.25V

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

VOUT can be quickly approximated using the formula

V =1.25 *(R + R )/R

OUT

1

21

A small correction to this formula is required de-

pending on the values of resistors R and R , since the

1

2

adjustable pin current (approx 50µA) flows through

R . When I is taken into account, the formula

2

ADJ

becomes

VOUT= V REF(1+ (R2/R1)) + IADJ* R 2

where

VREF=1.25V

Layout Considerations

Parasitic line resistance can degrade load regulation.

In order to avoid this, connect R directly to V as

1

OUT

illustrated in Figure 13. For the same reason, R2

should be connected to the negative side of the load.

Jun8-07 Rev D

SPX1587 Low-Dropout Voltage Regulator



© 2007 Sipex Corporation

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