CS5231-3GDPR5 datasheet(6/11 Pages) CHERRY

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Stability Considerations

The output capacitor helps determine three main charac-

teristics of a linear regulator: startup, transient response

and stability.

Startup is affected because the output capacitor must be

charged. At initial startup, the VIN supply may not be pre-

sent, and the output capacitor will be charged through the

PFET. The PFET will initially provide current to the load

through its body diode. The diode will act as a voltage fol-

lower until sufficient voltage is present to turn the FET on.

Since most commercial power supplies have a fairly low

ramp rate, charging through the body diode should effec-

tively limit in-rush current to the capacitor.

During normal operation, transient load current require-

ments will be satisfied from the charge stored in the output

capacitor until either the linear regulator or the auxiliary

supply can respond. Larger values of capacitance will

improve transient response, but will also cost more. A lin-

ear regulator will respond within microseconds, where an

external power supply may take milliseconds to react. The

output capacitance will provide the difference in current

until this occurs. The result will be an instantaneous volt-

age change at the output. This change is the product of the

current change and the capacitor ESR:

Application Information: continued

VIN

VOUT

VAUXDRV

IOUT = 375mA VAUX = 3.30

Figure 3b. Power-down, VAUX = 3.135V. The difference in voltage is

now IOUT × RDS(ON) plus the difference in supply voltages (3.30 -VAUX).

VIN

VOUT

VAUXDRV

IOUT = 375mA VAUX = 3.465

VIN

VOUT

VAUXDRV

IOUT = 375mA VAUX = 3.135V

VIN

VOUT

VAUXDRV

IOUT = 375mA VAUX = 3.135

Figure 4a. Power-up, VAUX = 3.465V. IOUT × RDS(ON) is compensated by

the higher value of VAUX

VIN

VOUT

VAUXDRV

IOUT = 375mA VAUX = 3.465

Figure 2b. Power-down, VAUX = 3.30V. Again, note ∆V= IOUT RDS(ON) ≈

130mV.

Figure 3a. Power-up, VAUX = 3.135V. The “oscillatory performance”

mode lasts longer because the difference between VAUX and 3.30 is

greater.

Figure 4b. Power-down, VAUX = 3.465V.

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