Shunt Regulator

How does a shunt regulator work?

A shunt regulator detects output voltage variation via external resistors by using an error amplifier and controls a transistor connected in parallel to the load to keep the output voltage constant. In other words, the characteristic of a shunt regulator is to minimize current variation of power supply (operation in the direction of minimizing variations) even if the load fluctuates. However, as a basic limitation of shunt regulators, they can not tolerate more fluctuations of current flowing in the load than the difference between the min. level and the max. level of the current flowing in the transistor.
The basic operation of a shunt regulator is described below using a simplified circuit.




When the output voltage drops due to a variation of the load, the voltage applied to resistors R₁ and R₂ drops. This then lowers the voltage of V₁, which is the output voltage divided by R₁ and R₂. In other words, the input voltage of the error amplifier's non-inverted pin (+) is also lowered (below the internal reference voltage). As a result, the error amplifier causes the voltage applied to the base of transistor TR to drop, which suppresses the current flowing to the TR collector. This in turn raises the output voltage, stabilizing it.

Conversely, when the output voltage rises due to a variation of the load, V₁ also rises, causing the error amplifier to raise the voltage of the TR base. This in turn increases the current flowing to the TR collector, which lowers the output voltage, stabilizing it. In other words, the shunt regulator operates to ensure that V₁ is always equivalent to the internal reference voltage.

(2007/11)

What is V_REF (standard connection) listed in the uPC1093 data sheet, and how is V_OUT calculated with V_REF?

V_REF is the input voltage of the REF pin, and is the reference voltage. The output voltage is led by V_REF and two external resistors.



The following relationships exist as a result.
Here, I_REF being a current of the µA order, it is possible to posit the following equation by omitting I_REF × R₁:

This can also be easily visualized using law of dividing voltage.

V_REF becomes constant at 2.495 V (typ.), so that the output voltage (V_OUT) can be changed (recommended operation condition: Min. V_REF to Max. 36 V) depending on the ratio of R₁ and R₂.

When using the uPC1093 as a 2.5 V constant power supply, the following connection with R₁ = 0, R₂ = ∞ (open) is used.

(2007/11)

What should one be careful about when selecting R₁ and R₂, R₀ shown for the standard connection of the uPC1093?

Regarding R₁ and R₂, set the resistance value so that the reference current flowing to the REF pin can be fully secured. However, if these resistances are set to low values, wasteful power is consumed (V_IN → R₀ → R₁ → R₂ → GND).
On the other hand, if these resistances are set to values that are too large, the I_REF current cannot be secured.
Select appropriate values taking these two points into consideration.

R₀ is determined by the voltage difference between V_IN and V_OUT and the value of the current flowing from K to A.

(2007/11)

Shunt regulator: The (uPC1093's) circuit operating current is not included in the electrical specifications of the uPC1093. What points should I be careful about regarding application circuit design using the uPC1093?

Owing to its circuit configuration, in the uPC1093 (from the viewpoint of the anode), two currents, the cathode current and the reference current, flow to the anode. Thus the cathode current and the reference current of the uPC1093 can be thought of as equivalent to the circuit current.
Therefore, when designing application circuits, care must be taken so that that cathode current and reference current do not exceed the absolute maximum ratings. Also, as described in the recommended operating conditions in the data sheet, the recommended minimum value of the cathode current is 1 mA.
Note that in the case of a design that does not secure a cathode current of 1 mA or higher, the current falls short of that required for uPC1093 startup, thereby making startup impossible.

[uPC1093 Equivalent Circuit]

(2007/02)

What are the materials and weight of the uPC1093?

The package is made of epoxy resin and the weight is approximately 0.29 g.

It seems that this product has variations such as -1 and -2. What are they?

These are products with special specification that differ from general-purpose products with regard to Vref.
At present, Vref is as follows.
The contents of the special specification are subject to change. For details, refer to the shipping documents.

Vref (unit:V)