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CS5257A-1GDP5

7A LDO 5-Pin Adjustable Linear Regulator

Cherry semiconductor 

Typical Performance Characteristics: continued

Adjust Pin Current vs Output Current

77.00

76.00

VPOWER=2.05

VCONTROL=2.75V

75.00

74.00

73.00

72.00

0.00 1.00 2.00 3.00 4.00 5.00 6.00 7.00 8.00

Output Current (A)

Application Notes

Theory of Operation

Design Guidelines

The CS5257A-1 linear regulator provides adjustable volt-

ages from 1.25V to 5V at currents up to 7A. The regulator

is protected against short circuits, and includes a thermal

shutdown circuit with hysteresis. The output, which is cur-

rent limited, consists of a PNP-NPN transistor pair and

requires an output capacitor for stability. A detailed pro-

cedure for selecting this capacitor is included in the

Stability Considerations section.

VPOWER Function

The CS5257A-1 utilizes a two supply approach to maxi-

mize efficiency. The collector of the power device is

brought out to the VPOWER pin to minimize internal power

dissipation under high current loads. VCONTROL provides

power for the control circuitry and the drive for the output

NPN transistor. VCONTROL should be at least 1V greater

than the output 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 typically limited to about 3mA 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 CS5257A-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 CS5257A-1 five terminal linear regulator includes a

dedicated VSENSE function. This allows for true Kelvin

sensing of the output voltage. This feature can virtually

eliminate errors in the output voltage due to load regula-

tion. Regulation will be optimized at the point where the

sense pin is tied to the output.

Adjustable Operation

This LDO adjustable regulator has an output voltage range

of 1.25V to 5V. An external resistor divider sets the output

voltage as shown in Figure 1. The regulatorÕs voltage sens-

ing error amplifier maintains a fixed 1.253V 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 1.253V across R1 and sets the overall out-

put 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 neces-

sary. The output voltage is set according to the formula:

VOUT = 1.253V ´

R1+R2

R1

+ 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 a least 10mA. R1 and R2 should be of the same composi-

tion for best tracking over temperature. The divider resis-

tors should be placed physically as close to the load as pos-

sible.

VCONTROL VOUT

CS5257A-1

VPOWER VSENSE

Adjust

R1

R2

Figure 1: An external resistor divider sets the value of VOUT. The 1.253V

reference voltage drops across R1.

6

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