SPX1585AT-LSLASHTR datasheet(3/7 Pages) EXAR

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SPX1585

APPLICATION HINTS

The SPX1585 incorporates protection against over-current

faults, reversed load insertion, over temperature operation, and

positive and negative transient voltage

s. However, the use of an

output capacitor is required in order to insure the stability and

the performance

of the device.

Stability

The output capacitor is part of the regulator’s frequency

compensation system. Either a 22µF aluminum electrolytic

capacitor or a 10µF solid tantalum capacitor between the output

terminal and ground guarantees stable operation for all operat-

ing conditions. The recommended value for ESR is 0.5Ohms or

less.

However, in order to minimize overshoot and undershoot, and

therefore optimize the design, please refer to the section ‘Ripple

Rejection’.

Ripple Rejection

Ripple rejection can be improved by adding a capacitor between

the ADJ pin and ground. When ADJ pin bypassing is used, the

value of the output capacitor required increases to its maximum

(220µF for an aluminum electrolytic capacitor, or 47µF for a

solid tantalum capacitor). If the ADJ pin is not bypassed, the

value of the output capacitor can be lowered to 10µF for an elec-

trolytic aluminum capacitor or 4.7µF for a solid tantalum

capacitor.

However the value of the ADJ-bypass capacitor should be

chosen with respect to the following equation:

C = 1 / ( 6.28 * FR * R1 )

Where C

= value of the capacitor in Farads

(select an equal or larger standard value),

FR = ripple frequency in Hz,

R1 = value of resistor R1 in Ohms.

If an ADJ-bypass capacitor is use

d, 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 = Reference Voltage

Reducing parasitic resistance and inductance

One solution to minimize parasitic resistance and inductance is

to connect in parallel capacitors. This arrangement will

improve the transient response of the power supply if your

system requires rapidly changing current load condition.

Thermal Consideration

Although the SPX1585 offers some limiting circuitry for

overload conditions, it is necessary not to exceed the maximum

junction temperature, and therefore to be careful about thermal

resistance. The heat flow will follow the lowest resistance path,

which is the Junction-to-case thermal resistance. In order to in-

sure the best thermal flow of the component, a proper mounting

is required. Note that the case of the device is electrically con-

nected to the output. If the case must be electrically isolated, a

thermally conductive spacer can be used. However do not for-

get to consider its contribution to thermal resistance.

Assuming:

VIN = 10V, VOUT = 5V, IOUT = 1.5A, TA = 50°C/W,

θHeatsink Case= 6°C/W, θHeatsink Case = 0.5°C/W, θ JC = 3°C/W

Power dissipation under this condition

PD = (VIN – VOUT) * IOUT = 7.5W

Junction Temperature

TJ = TA + PD * (θCase – HS + θ HS θ JC)

For the Control Section

TJ = 50 + 7.5*(0.5 + 6=3) = 121.25°C

121.25°C < TJ (max) for the Control & Power Sections.

In both

conditions, reliable operation is insured by adequate

junction temperature.

3/7

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