DN40

Design Note 40
Issue 1 June 1996
Applications of the ZMR250 Miniature SOT23
Regulator
Adjustable Regulator
There are many occasions when a power
supply at a non-standard voltage is
required, for which there are no
off-the-shelf three terminal regulators
available. A common technique of
dealing with such requirements, is to
use a standard regulator selected with
an output voltage lower than needed
and to add a potential divider connected
to its ground pin to raise the output to
the desired level. The power supply
circuit shown in Figure1 is an example of
this technique.
IC1
ZMR250
+ve
Vin Vout
Gnd
+ve
R1
2k4
Input
(4.7V - 20V)
Output
(3.0V)
R2
470
-ve
A serious problem with this circuit,
particularly for battery powered
equipment, is the current drain of the
output potential divider. The resistors
selected for this potential divider must
be chosen so as to pass around 50 times
the quiescent current of the regulator if
output accuracy is not to be degraded.
Industry standard regulators such as the
78L series usually have quiescent
currents in the region of 2mA typical, so
to maintain accuracy a divider current of
100mA would be required. However, the
quiescent current of the Zetex ZMR250
SOT23 miniature voltage regulator, is
only 40µA maximum - even less than
that of purpose designed adjustable
regulators. Use of the ZMR250 in the
above circuit allows the divider string
current to be kept down to only 1mA
without significantly compromising
accuracy, making the power supply
viable for even battery powered circuits
where efficiency is important.
-ve
Figure 1
Power Supply with Potential Divider;
Designed to Deliver Non-Standard
Output.
DN 40 - 1
Design Note 40
Issue 1 June 1996
IC1
ZMR250
The output voltage given by the power
supply circuit can be calculated using
the following formula:Vout=Iq x R2+Vreg
+ve
(R1+R2)
R1
Input Range
(4.2 - 20V)
Where Iq is the quiescent current of the
ZMR250 (30µA typical) and Vreg is the
normal output voltage of the ZMR250
(2.5V).
Vin
Vout
Gnd
R1
1k
Output Current
(2.5mA)
-ve
The power supply will operate with
inputs from 4.7 to 20V and supply loads
up to 50mA (maximum continuous load
current may be restricted by package
power dissipation).
Figure 2
Using the ZMR250 as a Current Source.
The output current is set by resistor R1,
following the relationship:Iout = Iq +
Current Generator
The low quiescent current rating of the
ZMR250 also makes the device very
useful for current source applications.
Figure 2 shows all that is needed to use
the ZMR250 as a current source. This
circuit behaves as a two terminal device
which passes a current independent of
the voltage across it as long as this
voltage is in the range of 4.2V to 20V.
Vreg
R1
here Iq is the quiescent current of the
ZMR250 (30µA typical) and Vreg is the
normal output voltage of the ZMR250
(2.5V).
The output impedance of the current
source is typically 7.75MΩ a n d b y
adjusting the value of resistor R1, the
circuit can be configured to source
currents in the range of 50µA to 50mA.
DN40 - 2
AN 1 - 4
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