CS5207A-1 datasheet(5/8 Pages) ONSEMI

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CS5207A−1

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APPLICATION NOTES

THEORY OF OPERATION

The CS5207A−1 linear regulator has a composite

PNP−NPN output stage that requires an output capacitor for

stability. A detailed procedure for selecting this capacitor is

included in the Stability Considerations section.

ADJUSTABLE OPERATION

Design Guidelines

This LDO adjustable regulator has an output voltage

range of 1.25 V to 5.0 V. An external resistor divider sets the

output voltage as shown in Figure 11. The regulator’s

voltage sensing error amplifier maintains a fixed 1.25 V

reference between the output pin and the adjust pin.

A resistor divider network R1 and R2 causes a fixed current

to flow to ground. This current creates a voltage across R2 that

adds to the 1.25 V across R1 and sets the overall output

voltage. The adjust pin current (typically 50 μA) also flows

through R2 and adds a small error that should be taken into

account if precise adjustment of VOUT is necessary. The

output voltage is set according to the formula:

VOUT + VREF

R1 ) R2

) R2

IAdj

The term IAdj × R2 represents the error added by the adjust

pin current.

R1 is chosen so that the minimum load current is at least

10 mA. R1 and R2 should be of the same composition for best

tracking over temperature. For best results, the divider

resistor should be placed near the regulator with a seperate

metal trace connecting them to output..

Figure 11.

VIN

CS5207A−1

VOUT

Adj

While not required, a bypass capacitor connected between

the adjust pin and ground will improve transient response

and ripple rejection. A 0.1 μF tantalum capacitor is

recommended for “first cut” design. Value and type may be

varied to optimize performance vs price.

OTHER ADJUSTABLE OPERATION CONSIDERATIONS

The CS5207A−1 linear regulator has an absolute

maximum specification of 6.0 V for the voltage difference

between VIN and VOUT. However, the IC may be used to

regulate voltages in excess of 6.0 V. The main considerations

in such a design are power−up and short circuit capability.

In most applications, ramp−up of the power supply to VIN

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

charging the output capacitor as soon as the VIN to VOUT

differential is large enough that the pass transistor conducts

current. VOUT is essentially at ground, and VIN is on the

order of several hundred millivolts, so the pass transistor is

in dropout. As VIN increases, the pass transistor will remain

in dropout, and current is passed to the load until VOUT is in

regulation. Further increase in VIN 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 VIN to VOUT differential of

6.0 V is not exceeded.

However, maximum ratings of the IC will be exceeded in

a short circuit condition. 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 circuitry can become active. Additional

circuitry may be required to clamp VIN to VOUT differential

to less than 6.0 V if failsafe operation is required. One

possible clamp circuit is illustrated in Figure 12; 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 conditions

indefinitely while protecting the IC.

Figure 12.

VIN

VOUT

VAdj

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