AS1358 / AS1359
Datasheet - Application Information
9 Application Information
9.1 Dropout Voltage
Dropout is the input to output voltage difference, below which the linear regulator ceases to regulate. At this point, the output voltage change
follows the input voltage change. Dropout voltage may be measured at different currents and, in particular at the regulator maximum one. From
this is obtained the MOSFET maximum series resistance over temperature etc. More generally:
VDROPOUT = ILOAD RSERIES
(EQ 1)
Dropout is probably the most important specification when the regulator is used in a battery application. The dropout performance of the
regulator defines the useful “end of life” of the battery before replacement or re-charge is required.
Figure 19. Graphical Representation of Dropout Voltage
VIN
VOUT
VIN = VOUT(TYP) + 0.5V
Dropout
Voltage
VOUT
100mV
VIN
VOUT
VIN
Figure 19 shows the variation of VOUT as VIN is varied for a certain load current. The practical value of dropout is the differential voltage (VOUT-
VIN) measured at the point where the LDO output voltage has fallen by 100mV below the nominal, fully regulated output value. The nominal
regulated output voltage of the LDO is that obtained when there is 500mV (or greater) input-output voltage differential.
9.2 Efficiency
Low quiescent current and low input-output voltage differential are important in battery applications amongst others, as the regulator efficiency is
directly related to quiescent current and dropout voltage. Efficiency is given by:
VLOAD ILOAD
--------------------------------------
Efficiency =
100 %
(EQ 2)
VINIQ + ILOAD
Where:
IQ = Quiescent current of LDO
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