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CS5203-1

3.0 A Adjustable Linear Regulator

ON Semiconductor 

CS5203−1

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 manufacturer’s data sheet

provides this information.

A 22 mF tantalum capacitor will work for most

applications, but with high current regulators such as the

CS5203−1 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

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

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

CS5203−1 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 14 is recommended.

IN4002 (Optional)

VIN

VIN

VOUT

VOUT

CS5203−1

C1

Adj

C2

R1

R2

CAdj

Figure 14. Protection Diode Scheme for Large

Output Capacitors

Output Voltage Sensing

Since the CS5203−1 is a three terminal regulator, it is not

possible to provide true remote load sensing. Load

regulation is limited by the resistance of the conductors

connecting the regulator to the load.

For the adjustable regulator, the best load regulation

occurs when R1 is connected directly to the output pin of the

regulator as shown in Figure 15. If R1 is connected to the

load, RC is multiplied by the divider ratio and the effective

resistance between the regulator and the load becomes.

ǒ Ǔ RC

R1 ) R2

R1

where RC = conductor parasitic resistance.

VIN

VIN

VOUT

Conductor Parasitic

RC

Resistance

CS5203−1

R1

Adj

RLOAD

R2

Figure 15. Grounding Scheme for Adjustable Output

Regulator to Minimize Parasitic Resistance Effects

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