CS5233−3

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7

APPLICATIONS INFORMATION

INPUT AND OUTPUT VOLTAGE MATRIX

Input

Outputs

AuxDrv/5.0 V Detect

0 V

0 V

0 V

On (low)

0 V

0 V

0 V

0 V

3.3 V

On (low)

0 V

5.0 V

0 V

Off (high)

0 V

5.0 V

3.3 V

Off (high)

5.0 V

0 V

0 V

Off (high)

5.0 V

0 V

3.3 V

Off (high)

5.0 V

5.0 V

0 V

Off (high)

5.0 V

5.0 V

3.3 V

Off (high)

THEORY OF OPERATION

Linear Regulator

The CS5233−3 is a dual input fixed 3.3 V linear regulator

output at currents of up to 500 mA. The linear regulator is

composed of a composite PNP−NPN pass transistor to

provide low−voltage dropout capability. An output capacitor

greater than 10

mF with equivalent series resistance (ESR)

less than 1.0

W is required for compensation. More

information is provided in the Stability Considerations

section.

Auxiliary Drive Feature

The CS5233−3 provides an auxiliary drive feature that

allows a load to remain powered even if both supplies to the

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

additional component required to implement this function

when the auxiliary power supply is available. The PFET gate

is connected to the IC’s AuxDrv output, 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

could

be

forward−biased if the auxiliary supply is not present. This

would result in the linear regulator providing current to

everything on the auxiliary supply rail.

The AuxDrv (5.0 V detect) output is pulled up to the input

below 4.4 V, the AuxDrv output goes low, turning on an

external PFET that connects the 3.3 V auxiliary supply to the

available.

There is 100 mV of hysteresis (typical) in the circuitry that

STABILITY CONSIDERATIONS

The output capacitor helps determine three main

characteristics of a linear regulator: loop stability, load

transient response, and start−up delay. The CS5233−3 is

designed to be stable with an output capacitor that has a

minimum value of 10

mF and an equivalent series resistance

less than 1.0

W. To guarantee loop stability, the output

capacitor should be located close to the regulator output and

ground pins. The load transient response, during the time it

takes the regulator to respond, is also determined by the

output capacitor. For large changes in load current, the ESR

of the output capacitor causes an immediate drop in output

voltage given by:

DV + DI

ESR

There is then an additional drop in output voltage given

by:

DV + DI

T C

where T is the time for the regulation loop to begin to

respond, (typically 4.0

ms for the CS5233−3). If tight output

regulation is required with fast changing loads, a capacitor

network of tantalum and low ESR ceramic capacitors can be

added as close to the load as possible, with enough

capacitance and a reduced ESR to minimize the voltage

change, as determined by the formulas above.

Input Capacitors and the Vin Thresholds

transient response. During a load transient, the input

capacitor serves as a charge “reservoir,” providing the

needed extra current until the external power supply can