5

Applications Information: continued

Figure 1. Resistor divider scheme.

The CS52015-1 linear regulator has an absolute maximum

ages in excess of 7V. The main considerations in such a

design are power-up and short circuit capability.

is fairly slow, typically on the order of several tens of mil-

liseconds, while the regulator responds in less than one

microsecond. In this case, the linear regulator begins

large enough that the pass transistor conducts current. The

load at this point is essentially at ground, and the supply

voltage is on the order of several hundred millivolts, with

the result that the pass transistor is in dropout. As the sup-

reaches the point at which the IC is in regulation. Further

increase in the supply voltage brings the pass transistor

out of dropout. The result is that the output voltage fol-

lows the power supply ramp-up, staying in dropout until

the regulation point is reached. In this manner, any output

voltage may be regulated. There is no theoretical limit to

tial of 7V is not exceeded.

However, the possibility of destroying the IC in a short

circuit condition is very real for this type of design. Short

circuit conditions will result in the immediate operation of

the pass transistor outside of its safe operating area. Over-

voltage stresses will then cause destruction of the pass

transistor before overcurrent or thermal shutdown circuit-

ry can become active. Additional circuitry may be required

safe operation is required. One possible clamp circuit is

illustrated in figure 2; however, the design of clamp cir-

cuitry must be done on an application by application basis.

Care must be taken to ensure the clamp actually protects

the design. Components used in the clamp design must be

able to withstand the short circuit condition indefinitely

while protecting the IC.

Figure 2: Short Circuit Protection Circuit for High Voltage Application.

The output or 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 capaci-

tor with almost zero ESR can cause instability. The alu-

minum electrolytic capacitor is the least expensive solu-

tion. However, when the circuit operates at low tempera-

tures, both the value and ESR of the capacitor will vary

considerably. The capacitor manufacturersÕ data sheet pro-

vides this information.

A 22µF tantalum capacitor will work for most applications,

but with high current regulators such as the CS52015-1 the

transient response and stability improve with higher val-

ues of capacitance. The majority of applications for this

regulator involve large changes in load current so the out-

put capacitor must supply the instantaneous load current.

The ESR of the output capacitor causes an immediate drop

in output voltage given by:

ÆV = ÆI

´ 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 load transient conditions. The output capacitor net-

work should be as close as possible to the load for the best

results.

When large external capacitors are used with a linear regu-

lator 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 capaci-

the CS52015-1 linear regulator, the discharge path is

through a large junction and protection diodes are not usu-

ally needed. If the regulator is used with large values of

output capacitance and the input voltage is instantaneous-

ly shorted to ground, damage can occur. In this case, a

diode connected as shown in Figure 2 is recommended.

Protection Diodes

Stability Considerations

EXTERNAL SUPPLY

CS52015-1

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