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CS5231-3

500mA, 3.3V Linear Regulator with Auxiliary Control

Cherry semiconductor 

Application Circuit

+5V PCI

C1

33µF

VIN

VOUT

CS5231-3

Gnd AuxDrv

+3.3V VAUX

C1

33µF

M1

C3

33µF

ASIC

VDD

*indicates PFET body diode

Application Information

Theory of Operation

The CS5231-3 is a fixed 3.3V linear regulator that contains

an auxiliary drive control feature. When VIN is greater than

the typical 4.5V threshold, the IC functions as a linear regu-

lator. It provides up to 500mA of current to a load through

a composite PNP-NPN pass transistor. An output capacitor

greater than 10µF with equivalent series resistance less

than 1Ω 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 connect-

ed 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 connect-

ed 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 auxil-

iary supply is turned off. This will result in the linear regu-

lator providing current to everything on the auxiliary sup-

ply rail.

The AuxDrv lead is internally connected to a 10kΩ 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 10kΩ 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 650mV. 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 100mV. The PFET will contin-

ues 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 4.5V. During this time, the load continues to

be powered by the auxiliary driver. Once the 4.5V VIN

threshold is reached, the saturating NPN connected to the

AuxDrv lead turns off. The on-chip 10kΩ 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 100mV

(typical) after the linear regulator is providing power to the

load before the AuxDrv circuitry can be re-enabled.

VIN

VOUT

VAUXDRV

IOUT = STARTUP 375mA

Figure 1. Initial power-up, VAUX not present ROUT = 8.8Ω.

VIN

VOUT

VAUXDRV

IOUT = 375mA VAUX = 3.30V

Figure 2a. Power-up, VAUX = 3.30V. Note the “oscillatory performance”

as the linear regulator charges the VOUT node. IOUT × RDS(ON) ≈ 130mV

5

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