APPLICATION HINTS

The Calogic Semiconductor CLM2815 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 220

µF aluminum electrolytic

capacitor or a 47

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

100

µF for an electrolytic aluminum capacitor or 15µF 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),

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:

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 CLM2815 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:

(no external heat sink, no wind)

Power dissipation under these conditions

Junction Temperature

For the Control Section

114

For the Power Section

109.5

In both cases reliable operation is insured by adequate

junction temperature.

CALOGIC CORPORATION, 237 Whitney Place, Fremont, California 94539, Telephone: 510-656-2900, FAX: 510-651-3025

CLM2815

CORPORATION