Date: 04/29/04

SPX1585

APPLICATION HINTS

The SPX1585 incorporates protection against over-current

faults, reversed load insertion, over temperature operation, and

positive and negative transient voltage. However, the use of

an output capacitor is required in order to insure the stability

and the performances.

Stability

The output capacitor is part of the regulator’s frequency

compensation system. Either a 22

µF aluminum electrolytic

capacitor or a 10

µF solid tantalum capacitor between the

output terminal and ground guarantees stable operation for all

operating conditions.

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 (220

µF for an aluminum electrolytic capacitor, or

47

µF for a solid tantalum capacitor). If the ADJ pin is not

bypass, the value of the output capacitor can be lowered to

10

µF for an electrolytic 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:

Where C

= value of the capacitor in Farads (select an

equal or larger standard value),

= ripple frequency in Hz,

If an ADJ-bypass capacitor is use, 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:

Where M

= multiplier for the ripple seen when the ADJ pin

is optimally bypassed.

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 insure the best thermal flow of the

component, a proper mounting is required. Note that the case

of the device is electrically connected to the output. In case

the case has to be electrically isolated, a thermally conductive

spacer can be used. However do not forget to consider its

contribution to thermal resistance.

Assuming:

°C/W,

Power dissipation under this condition

Junction Temperature

For the Control Section

°C

121.25

In both case reliable operation is insured by adequate junction

temperature.