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CS5207-1

7A Adjustable Linear Regulator

Cherry semiconductor 

Applications Information

The CS5207-1 linear regulator provides adjustable volt-

ages at currents up to 7A. 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 CS5207-1 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 has an output voltage range of

1.25V to 13V. An external resistor divider sets the output

voltage as shown in Figure 1. The regulator maintains a

fixed 1.25V (typical) reference between the output pin and

the adjust pin.

A resistor divider network R1 and R2 causes a fixed cur-

rent to flow to ground. This current creates a voltage

across R2 that adds to the 1.25V across R1 and sets the

overall output voltage. The adjust pin current (typically

50µA) 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 + IAdj × R2

R1

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

10mA. 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.

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 manufacturers data sheet pro-

vides this information.

A 22µF tantalum capacitor will work for most applications,

but with high current regulators such as the CS5207-1 the

transient response and stability improve with higher val-

ues of capacitor. The majority of applications for this regu-

lator 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:

∆V = ∆I × 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.

Protection Diodes

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-

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

the CS5207-1 regulator, the discharge path is through a

large junction and protection diodes are not usually need-

ed. If the regulator is used with large values of output

capacitance and the input voltage is instantaneously short-

ed to ground, damage can occur. In this case, a diode con-

nected as shown in Figure 2 is recommended.

VIN

C1

VIN

VOUT

CS5207-1

Adj

VREF

R1

IAdj

R2

CAdj

VOUT

C2

Figure 1. Resistor divider scheme for the adjustable version.

VIN

C1

IN4002 (optional)

VIN

VOUT

CS5207-1

R1

Adj

VOUT

C2

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 capaci-

Figure 2. Protection diode scheme for adjustable output regulator.

Output Voltage Sensing

Since the CS5207-1 is a three terminal regulator, it is not

possible to provide true remote load sensing. Load regula-

tion is limited by the resistance of the conductors connect-

ing the regulator to the load.

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