ZLDO300 Datasheet, PDF (6/9 Pages) Zetex Semiconductors – 3.0 VOLT ULTRA LOW DROPOUT REGULATOR

ZLDO300
APPLICATIONS
1). Simple 3.0V Supply
+5V In
IC1
ZLDO300
LBF Spg
SC Gnd
Vin D/C
N/C Vout
C3
100nF
0V In
+5V Out
C1
10pF
+3.0V Out
C2
1µF
0V Out
Figure 1
Using a circuit such as Figure 1, the ZLDO300
can easily provide a 3.0V logic supply from an
available 5V rail where most standard
regulators could not guarantee correct
operation. Although this approach is not
particularly energy efficient, if the load taken at
3.0V is not too large, then the added complexity
and cost of a 3.0V switching converter may not
be justifiable and so this linear solution can be
preferable. This circuit will also give far less
noise than a switching regulator which can be
important when handling low level analogue
signals or low voltage measurements.
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 changes 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 voltage of the
ZLDO300 allows this circuit technique to be
implemented very effectively, giving a highly
stable and accurate low noise supply.
Voltage
Feedback
D1
+5V Out
TR1
Switching
Regulator
C4
220µF
D2
ZLDO300
LBF Spg
SC Gnd
Vin D/C
N/C Vout
C1
10pF
+3.0V Out
Figure 2
C5
220µF
C2
1µF
0V Out