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SPX2940

1A Low Dropout Voltage Regulator

Exar Corporation 

SPX2940

1A Low Dropout Voltage Regulator

THEORY OF OPERATION

The SPX2940 incorporates protection against

over-current faults, reversed load insertion,

over temperature operation, and positive and

negative transient voltage.

recommended if a faster load transient

response is needed.

If the power source has a high AC impedance,

a 0.1µF ceramic capacitor between input &

ground is recommended.

THERMAL CONSIDERATIONS

Although the SPX2940 offers 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 the best thermal flow of the

component, proper mounting is required.

Since the case of the device is electrically

connected to the output, the case must be

electrically isolated using a thermally

conductive spacer, which itself contributes

some small additional thermal resistance.

Consult heatsink manufacturer for thermal

resistance and design of heatsink.

TO-220 Design Example:

Assume that VIN = 10V, VOUT = 5V,

IOUT = 1.5A, TA = 50°C/W, θHA= 1°C/W,

θCH = 2°C/W, and θJC = 3°C/W.

Where TA = ambient temperature

θHA = heatsink to ambient thermal resistance

θCH = case to heatsink thermal resistance

θJC = junction to case thermal resistance

The power calculated under these conditions

is:

PD = (VIN – VOUT) * IOUT = 7.5W.

And the junction temperature is calculated as

TJ = TA + PD * (θHA + θCH + θJC) or

TJ = 50 + 7.5 * (1 + 2 + 3) = 95°C

Reliable operation is insured.

MINIMUM LOAD CURRENT

To ensure a proper behavior of the regulator

under light load, a minimum load of 5mA for

SPX2940 is required.

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Fig. 20: Fixed Output Linear Regulator

Fig. 21: Ouput Cap ESR vs IOUT

CAPACITOR REQUIREMENTS

The output capacitor is needed to insure

stability and minimize the output noise. The

value of the capacitor varies with the load.

However, a minimum value of 10µF aluminum

capacitor will guarantee stability over all load

conditions. A tantalum capacitor is

© 2009 Exar Corporation

7/10

Fig. 22: 5.0VOUT PSSR VIN=10V, VOUT=5V

COUT=22F, IOUT=10mA

Rev. 2.0.0

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