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Designing with Shunt Regulators Fixed voltage regulators and opto isolation
Peter Abiodun A. Bode, Snr. Applications Engineer, Diodes Incorporated
An often used application of the 3-terminal shunt regulators or references is taking advantage of
their precision function to turn otherwise medium accuracy linear regulators into precision ones.
Figure 1 and Figure 2 show how this is done.
A major benefit is that the series regulator maintains all its in-built functions whilst behaving like
a precision device by virtue of the reference taking over the primary control function.
Improved accuracy and extended voltage range
The circuits are very simple and the output voltage, VOUT, is computed exactly as in other examples (see references at the end of this document).
(All features of the regulator such as output
current, short circuit protection, thermal
shutdown, etc, are maintained.)
R1 ⎞
VOUT = VREF ⎜1 +
R2 ⎠
Figure 1 Increasing output voltage of a fixed linear regulator
One such application of references is in using it to improve the accuracy and/or extend the range
and flexibility of fixed voltage regulators. In the circuit in Figure 1 above, both the output voltage
and its accuracy are entirely determined by the TLV431, R1 and R2. However the rest of the
features of the regulator (up to 5A output current, output current limiting and thermal shutdown)
are all still available thus combining the best of both devices.
AP1117 or AP1084
R3 =
R1 ⎞
VOUT = VREF ⎜1 +
R2 ⎠
0.1mA ≤ I B ≤ 18mA
(All features of the regulator such as short
circuit protection, thermal shutdown, etc, are
Figure 2 Adjustable linear voltage regulator
Issue 1 - September 2008
© Diodes Incorporated, 2008
Figure 2 is similar to Figure 1 with adjustability added. Note the addition of R3, which is only required for the AP1117 due to the fact that its ground or adjustment pin can only supply a few micro-Amps of current at best. R3 is therefore needed to provide sufficient bias current for the
Opto-isolated control
Regulated supply
Regulated supply
To controller
To controller
R1 ⎞
VOUT = VREF ⎜1 +
⎝ R2 ⎠
VOUT (min) − (V F + V KA(min) )
I F (min)
≥ R3 ≥
VOUT (max) − (V F + V KA(min) )
Figure 3 Using a reference as the regulating element in an isolated PSU
A frequent application of three-terminal references is in isolated power supplies where the
regulated output is galvanically isolated from the controller. In this case, an isolated feedback to
the controller is implemented as shown in Figure 3.
REF1 drives current, IF, through the optocoupler's LED which in turn drives the isolated transistor
which is connected to the controller on the primary side of the power supply. The opto-isolated
transistor is configured to suit the requirements of the controller on the primary side. An
equilibrium state is only achieved when REF1 has achieved VREF across its feedback pin.
This loop completes the feedback path that ensures that a stable isolated supply given by the
expression in Figure 3 above is maintained and the reference helps to "trim" out the non-linear
transfer function of the opto-isolator. In this way, the reference functions as a high gain amplifier,
and, as part of the feedback loop, ensures that variations in the opto-coupler over time and
temperature are not significant.
As an example, the Vishay SFH610 series opto-coupler requires an input current of between 1mA
and 15mA to achieve a useful current transfer ratio for control purposes. This range of current is
well within the capability of the TLV431. For the SFH610, VF is about 1.2V, and IFmin is taken to be
1mA. For the TLV431, VKAmin is taken to be 1.24V, though it can be slightly lower in practice. If for
example a 5V output is required, then suitable values of R1 and R2 are easily found to be:
R1 = 30.5k⍀
R2 = 10k⍀
The value of R3 determines the normal regulating input current IF of the opto-isolator. Assuming
VOUT (min) − (VF + VKA(min) )
5 − (1.2 + 1.24)
= 256 ohms
This value is not critical and R3 could be set to 240 or 270⍀.
Issue 1 - September 2008
© Diodes Incorporated, 2008
The above examples show how the superior parameters of the Diodes' 3-terminal references can
be used to improve COTS voltage regulators. It also shows how they can be used in an isolated
power supply application.
Recommended further reading
AN58 - Designing with Shunt Regulators - Shunt Regulation
AN59 - Designing with Shunt Regulators - Series Regulation
AN61 - Designing with Shunt Regulators - Extending the operating voltage range
AN62 - Designing with Shunt Regulators - Other Applications
AN63 - Designing with Shunt Regulators - ZXRE060 Low Voltage Regulator
Issue 1 - September 2008
© Diodes Incorporated, 2008
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© Diodes Incorporated, 2008