CS5253−1

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APPLICATIONS NOTES

THEORY OF OPERATION

The CS5253−1 linear regulator provides adjustable

voltages from 1.26 V to 5.0 V at currents up to 3.0 A. The

regulator is protected against short circuits, and includes a

thermal shutdown circuit with hysteresis. The output, which

is current limited, consists of a PNP−NPN transistor pair and

requires an output capacitor for stability. A detailed procedure

for selecting this capacitor is included in the Stability

Considerations section.

The CS5253−1 utilizes a two supply approach to maximize

efficiency. The collector of the power device is brought out

circuitry and the drive for the output NPN transistor.

voltage. Special care has been taken to ensure that there are

no supply sequencing problems. The output voltage will not

turn on until both supplies are operating. If the control voltage

comes up first, the output current will be limited to about three

milliamperes until the power input voltage comes up. If the

power input voltage comes up first, the output will not turn on

at all until the control voltage comes up. The output can never

come up unregulated.

The CS5253−1 can also be used as a single supply device with

the control and power inputs tied together. In this mode, the

dropout will be determined by the minimum control voltage.

Output Voltage Sensing

The CS5253−1 five terminal linear regulator includes a

sensing of the output voltage. This feature can virtually

eliminate errors in the output voltage due to load regulation.

Regulation will be optimized at the point where the sense pin

is tied to the output.

DESIGN GUIDELINES

Adjustable Operation

This LDO adjustable regulator has an output voltage range

of 1.26 V to 5.0 V. An external resistor divider sets the output

voltage as shown in Figure 19. The regulator’s voltage

sensing error amplifier maintains a fixed 1.260 V reference

between the output pin and the adjust pin.

Figure 19. Typical Application Schematic.

1.260 V Reference Dropped Across R1.

CS5253−1

Adjust

R2

R1

5.0 V

3.3 V

2.5 V

@ 3.0 A

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 1.260 V across R1 and sets the overall output

voltage. The adjust pin current (typically 50

mA) also flows

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

output voltage is set according to the formula:

VOUT + 1.260 V

R1 ) R2

R1

) R2

IADJ

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. The divider resistors

should be placed physically as close to the load as possible.

While not required, a bypass capacitor connected between

the adjust pin and ground will improve transient response and

ripple rejection. A 0.1

mF tantalum capacitor is recommended

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

optimize performance vs. price.

Other Adjustable Operation Considerations

The CS5253−1 linear regulator has an absolute maximum

specification of 6.0 V for the voltage difference between

voltages in excess of 6.0 V. The two main considerations in

such a design are the sequencing of power supplies and short

circuit capability.

Power supply sequencing should be such that the

increases, the pass transistor will remain in dropout, and

Further increase in the supply voltage brings the pass

transistor out of dropout. In this manner, any output voltage

differential of 6.0 V is not exceeded.

There is a possibility of damaging the IC when

occurs. 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

than 6.0 V if fail safe operation is required. One possible

clamp circuit is illustrated in Figure 20; 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.