SPX3940T-L-3-3 Просмотр технического описания (PDF) - Exar Corporation

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SPX3940T-L-3-3

1A Low Dropout Voltage Regulator

Exar Corporation 

THEORY OF OPERATION

The SPX3 940 incorporates protection against

over - current faults, reversed load insertion,

over temperature operation, and positive and

negative transient voltage.

THERMAL CONSIDERATIONS

Although the SPX3940 o ffers limiting circuitry

for overload conditions, it is still necessary to

insure that the maximum junction

temperature is not exceeded in the

application. Heat will flow through the lowest

resistance path, the junction - to - case path. In

order to insure th e best thermal flow of the

component, proper mount - ing is required.

Consult heatsink manufacturer for thermal

resistance and design of heatsink.

TO - 220 Design Example:

Assume that V IN = 10V, V OUT = 5V,

I OUT = 1.5A, T A  ƒ&H:A=1°CLJ/W,

LJCH ƒ&:JC D= 3Q°CG/W.LJ

Where T A = ambient temperature

LJHA = heatsink to ambient thermal resistance

LJCH = case to heatsink thermal resistance

LJJC = junction to case thermal resistance

The power calculated under these condition

s

is:

PD = (V IN ±VOUT) * I OUT = 7.5W.

And the junction temperature is calculated as

SPX3 9 4 0

1 A Low Dr opou t Volt age Regu lat or

TJ = T A + P D HALJCLJH JLJC) or

TJ = 50 + 7.5 * (1 + 2 + 3) = 95°C

Reliable operation is insured.

CAPACITOR REQUIREMENTS

The output capacitor is needed to insure

stabi lity and minimize the output noise. The

value of the capacitor varies with the load.

However, a minimum value of 10 µF aluminum

capacitor will guarantee stability over all load

conditions. A tantalum capacitor is

recommended if a faster load transient

resp onse is needed.

If the power source has a high AC impedance,

a 0.1 µF ceramic capacitor between input &

ground is recommended.

M INIMUM LOAD CURRENT

To ensure a proper behavior of the regulator

under light load, a minimum load of 5mA for

SPX3940 is required.

Fig. 20 : Fixed Output Linear Regulator

© 2012 Exar Corporation

8/ 11

Rev. 1.0.0

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