Diodes ZLDO330 - Manual (Page 6)

ZLDO330
1). Simple 3.3V Supply
Using a circuit such as Figure 1, the ZLDO330
can easily provide a 3.3V 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 3.3V is not too large, then the added
complexity and cost of a 3.3V 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 ZLDO330 allows this circuit
technique to be implemented very effectively,
giving a highly stable and accurate low noise
supply.
APPLICATIONS
Spg
D/C
Vout
LBF
SC
Vin
N/C
Gnd
IC1
ZLDO330
C2
1uF
C1
10pF
C3
100nF
+5V In +5V Out
+3.3V Out
0V Out 0V In
Figure 1
Spg
D/C
Vout
LBF
SC
Vin
N/C
Gnd
ZLDO330
C2
1uF
C1
10pF
+ 5V Out
0V Out
C4
220uF
C5
220uF
+ 3.3V Out
TR1
Feedback
Switching
Regulator
Voltage
D1
D2
Figure 2
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