NCP1086

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Figure 15. Resistor Divider Scheme

Adj

NCP1086

The adjustable output linear regulator has an absolute

maximum specification of 7.0 V for the voltage difference

regulate voltages in excess of 7.0 V.

The

main

considerations in such a design are powerup and short circuit

capability.

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

milliseconds, 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 mV, with the result

increases, the pass transistor will remain in dropout, and

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 follows 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 the regulated voltage as long as the

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.

Overvoltage stresses will then cause destruction of the pass

transistor before overcurrent or thermal shutdown circuitry

can become active. Additional circuitry may be required to

fail−safe operation is required. One possible clamp circuit is

illustrated in Figure 16; however, the design of clamp

circuitry 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 16. Short Circuit Protection Circuit for High

Voltage Application

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NCP1086

EXTERNAL

SUPPLY

Stability Considerations

The output or compensation capacitor helps determine

three main characteristics of a linear regulator: startup 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 manufacturers’ data sheet

provides this information.

A 22

mF tantalum capacitor will work for most

applications, but with high current regulators such as the

NCP1086 series 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

load transient conditions. The output capacitor network

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