Application Information: continued
V
IN
V
IN
V
OUT
V
OUT
V
AUXDRV
V
AUXDRV
I
= 375mA V
= 3.465
AUX
OUT
I
= 375mA V
= 3.30
AUX
OUT
Figure 4a. Power-up, VAUX = 3.465V. IOUT × RDS(ON) is compensated by
the higher value of VAUX
Figure 2b. Power-down, VAUX = 3.30V. Again, note ∆V = IOUT RDS(ON)
130mV.
≈
V
IN
V
IN
V
OUT
V
OUT
V
AUXDRV
V
AUXDRV
I
= 375mA V
= 3.465
AUX
I
= 375mA V
= 3.135V
AUX
OUT
OUT
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.
Stability Considerations
The output capacitor helps determine three main charac-
teristics of a linear regulator: startup, transient response
and stability.
V
IN
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.
V
OUT
V
AUXDRV
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:
I
= 375mA V
= 3.135
AUX
OUT
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).
6
CHERRY [ CHERRY SEMICONDUCTOR CORPORATION ]
