Diodes ZLDO500 - Manual (Page 6)

1). Operation From A Low Voltage
Battery Pack
Fig.1 shows the ZLDO500 regulator being
used to stabilise the output of a 6V battery
pack. The ultra low dropout voltage of only
100mV at full load (300mA) given by the
regulator allows the minimum number of
cells to be used in the pack and also
maximises the energy that can be removed
from the battery before the output of the
regulator starts to fail.
At a load current of 100mA the dropout
voltage falls to around 30mV. The endurance
of the battery pack is not only dependent on
dropout voltage. When operating, some low
dropout regulators can consume high
quiescent currents, sometimes approaching
as much as a tenth of their maximum load
current specification when approaching
dropout conditions. Despite its 300mA output
rating, when enabled the ZLDO500 consumes
typically only 630uA regulating normally and
3mA when the input falls too low for
regulation.
2). Post Converter Regulation
A common problem with multiple output
switch mode converters is that only one
output can be used in the feedback control
loop of the switching regulator. Thus only
one output is fully regulated. All other
outputs are prone to tracking errors that
occur if the load on any output change
significantly. By ensuring close coupling of all
transformer windings and minimising the
impedance of all outputs, these errors can be
reduced but never eliminated. A simple way
round this problem is to wind the switching
regulator transformer to give a slightly higher
voltage than required and regulate down
from this to the desired voltage with a linear
regulator. This is indicated in Figure 2. To
keep losses low and so maintain the
advantages of a switch mode supply, it is
important that the voltage drop across this
regulator is kept as low as possible, i.e. just
high enough to compensate for the poor
output impedance of the switching power
supply but no higher. The low dropout
Spg
D/C
Vout
LBF
SC
Vin
N/C
Gnd
ZLDO500
6V C2
1uF
C1
10pF
C3
100nF Output
+5V
0V
Spg
D/C
Vout
LBF
SC
Vin
N/C
Gnd
ZLDO500
C2
1uF
C1
10pF
+ 12V Out
0V Out
C4
220uF
C5
220uF
+ 5.0V Out
TR1
Feedback
Switching
Regulator
Voltage
D1
D2
Figure 2
ZLDO500
APPLICATIONS
Figure 1
4-85