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CS5205A-1

5.0 A Adjustable Linear Regulator

ON Semiconductor 

CS5205A−1

APPLICATIONS INFORMATION

The CS5205A−1 linear regulator provides an adjustable

voltage at currents up to 5.0 A. The regulator is protected

against short circuit, and include thermal shutdown and safe

area protection (SOA) circuitry. The SOA protection

circuitry decreases the maximum available output current as

the input−output differential voltage increases.

The CS5205A−1 has a composite PNP−NPN output

transistor and requires an output capacitor for stability. A

detailed procedure for selecting this capacitor is included in

the Stability Considerations section.

Adjustable Operation

The adjustable regulator (CS5205A−1) has an output

voltage range of 1.25 V to 13 V. An external resistor divider

sets the output voltage as shown in Figure 7. The regulator

maintains a fixed 1.25 V (typical) 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:

ǒ Ǔ VOUT + VREF

R1 ) R2

R1

) IAdj

R2

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 the same type, e.g. metal film

for best tracking over temperature. The adjust pin is

bypassed to improve the transient response and ripple

rejection of the regulator.

VIN

C1

VIN

VOUT

CS5205A−1

Adj

VREF

R1

VOUT

C2

IAdj

R2

CAdj

Stability Considerations

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

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 μF tantalum capacitor will work for most

applications, but with high current regulators such as the

CS5205A−1 the transient response and stability improve

with higher values of capacitor. 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.

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

output voltage and the rate at which VIN drops. In the

CS5205A−1 linear 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 8 is recommended.

Figure 7. Resistor Divider Scheme for the Adjustable

Version

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