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CS5231-3

500 mA, 3.3 V Linear Regulator with Auxiliary Control

ON Semiconductor 

CS5231−3

APPLICATION INFORMATION

THEORY OF OPERATION

The CS5231−3 is a fixed 3.3 V linear regulator that

contains an auxiliary drive control feature. When VIN is

greater than the typical 4.5 V threshold, the IC functions as

a linear regulator. It provides up to 500 mA of current to a

load through a composite PNP−NPN pass transistor. An

output capacitor greater than 10 mF with equivalent series

resistance less than 1.0 W is required for compensation.

More information is provided in the Stability Considerations

section.

The CS5231−3 provides an auxiliary drive feature that

allows a load to remain powered even if the VIN supply for

the IC is absent. An external p−channel FET is the only

additional component required to implement this function if

an auxiliary power supply is available. The PFET gate is

connected to the AuxDrv lead. The PFET drain is connected

to the auxiliary power supply, and the PFET source is

connected to the load. The polarity of this connection is very

important, since the PFET body diode will be connected

between the load and the auxiliary supply. If the PFET is

connected with its drain to the load and its source to the

supply, the body diode will be forward−biased if the

auxiliary supply is turned off. This will result in the linear

regulator providing current to everything on the auxiliary

supply rail.

The AuxDrv lead is internally connected to a 10 kW

resistor and to a saturating NPN transistor that acts as a

switch. If the VIN supply is off, the AuxDrv output will

connect the PFET gate to ground through the 10 kW resistor,

and the PFET will conduct current to the load.

As the VIN supply begins to rise, the AuxDrv lead will also

rise until it reaches a typical voltage of about 650 mV. The

NPN transistor connected to the AuxDrv lead will saturate

at this point, and the gate of the PFET will be pulled down

to a typical voltage of about 100 mV. The PFET will

continue to conduct current to the load.

The VIN supply voltage will continue to rise, but the linear

regulator output is disabled until VIN reaches a typical

threshold of 4.5 V. During this time, the load continues to be

powered by the auxiliary driver. Once the 4.5 V VIN

threshold is reached, the saturating NPN connected to the

AuxDrv lead turns off. The on−chip 10 kW pull−up resistor

will pull the PFET gate up to VIN, thus turning the PFET off.

The linear regulator turns on at the same time. An external

compensation capacitor is required for the linear regulator

to be stable, and this capacitance also serves as a charge

reservoir to minimize any “glitching” that might result

during the supply changeover. Hysteresis is present in the

AuxDrv circuitry, requiring VIN to drop by 100 mV (typical)

after the linear regulator is providing power to the load

before the AuxDrv circuitry can be re−enabled.

VIN

VOUT

VAUXDRV

IOUT = STARTUP 375 mA

Figure 13. Initial Power−Up, VAUX Not

Present ROUT = 8.8 W

VIN

VOUT

VAUXDRV

IOUT = 375 mA VAUX = 3.30 V

Figure 14. Power−Up, VAUX = 3.3 V. Note the

“Oscillatory Performance” as the Linear Regulator

Changes the VOUT Node. IOUT y RDS(ON) 9 130 mV

VIN

VOUT

VAUXDRV

IOUT = 375 mA VAUX = 3.30

Figure 15. Power−Down, VAUX = 3.3 V. Again,

Note DV = I RDS(ON) 9 130 mV

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