CS5208−1

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6

STABILITY CONSIDERATIONS

The output compensation capacitor helps determine three

main characteristics of a linear regulator: start−up delay,

load transient response, and loop stability.

The capacitor value and type is based on cost, availability,

size and temperature constraints. A tantalum or aluminum

electrolytic capacitor is best, since a film or ceramic

capacitor with almost zero ESR can cause instability. The

aluminum electrolytic capacitor is the least expensive

solution. However, when the circuit operates at low

temperatures, both the value and ESR of the capacitor will

vary considerably. The capacitor manufacturer’s data sheet

provides this information.

A 300 μF tantalum capacitor will work for most

applications, but with high current regulators such as the

CS5208−1 the transient response and stability improve with

higher values of capacitance. The majority of applications

for this regulator involve large changes in load current so the

output capacitor must supply the instantaneous load current.

The ESR of the output capacitor causes an immediate drop

in output voltage given by:

DV + DI

ESR

For microprocessor applications it is customary to use an

output capacitor network consisting of several tantalum and

ceramic capacitors in parallel. This reduces the overall ESR

and reduces the instantaneous output voltage drop under

transient load conditions. The output capacitor network

should be as close to the load as possible for the best results.

Protection Diodes

When large external capacitors are used with a linear

regulator it is sometimes necessary to add protection diodes.

If the input voltage of the regulator gets shorted, the output

capacitor will discharge into the output of the regulator. The

discharge current depends on the value of the capacitor, the

CS5208−1 regulator, the discharge path is through a large

junction and protection diodes are not usually needed. If the

regulator is used with large values of output capacitance and

the input voltage is instantaneously shorted to ground,

damage can occur. In this case, a diode connected as shown

in Figure 13 is recommended.

A rule of thumb useful in determining if a protection diode

is required is to solve for current

I +

C

V

T

where:

is shorted,

C is the value of the load capacitance,

V is the output voltage, and

high to being shorted.

If the calculated current is greater than or equal to the

typical short circuit current value provided in the

specifications, serious thought should be given to including

a protection diode.

Figure 13.

CS5208−1

Adj

Current Limit

The internal current limit circuit limits the output current

under excessive load conditions and protects the regulator.

Short Circuit Protection

The device includes foldback short circuit current limit

that clamps the output current at approximately two amperes

less than its current limit value.

Thermal Shutdown

The thermal shutdown circuitry is guaranteed by design to

become activated above a die junction temperature of 150°C

and to shut down the regulator output. This circuitry

includes a thermal hysteresis circuit with 25°C of typical

hysteresis, thereby allowing the regulator to recover from a

thermal fault automatically.

Calculating Power Dissipation and Heat Sink

Requirements

High power regulators such as the CS5208−1 usually

operate at high junction temperatures. Therefore, it is

important to calculate the power dissipation and junction

temperatures accurately to ensure that an adequate heat sink

CS5208−1, electrical isolation may be required for some

applications. Also, as with all high power packages, thermal

compound is necessary to ensure proper heat flow. For

added safety, this high current LDO includes an internal

thermal shutdown circuit

The thermal characteristics of an IC depend on the

following four factors. Junction temperature, ambient

temperature, die power dissipation, and the thermal

resistance from the die junction to ambient air. The

maximum junction temperature can be determined by:

TJ(max) + TA(max) ) PD(max)

RQJA