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How is the VREF used and what is the meaning of this voltage?

Latest Updated:04/01/2007

Question:

A reference voltage (VREF) is defined in the electrical specifications of the three-terminal positive adjustable regulator (uPC317). What is the meaning of this voltage?
How is the reference voltage (VREF) used?

Answer:

Although the uPC317 is a three-terminal voltage regulator IC, any output voltage can be set externally. Therefore, the reference voltage (VREF) is defined and publicized.

The desired output voltage can be easily calculated by using R1 and R2, based on the reference voltage (VREF). The following diagram and description below explains the operation.


[Block diagram]


The operating circuit current necessary for each block is output from the OUTPUT pin and flows through R1 and R2 into external GND.
The current (IADJ) flowing out of the ADJ pin is 100 uA at the most, but the current ratio of IADJ to VREF/R1 must be sufficiently large in order to obtain a stable output voltage.
The recommended value of R1 is 240 Ω and, therefore, VREF / R1 = 1.25 V /240 Ω = 5.2 mA. In this case, its current ratio to IADJ is about 52. Although the details are explained later, a stable output voltage can be set with only the external resistance ratio if a current ratio as much as this is obtained.

When the output voltage is stabilized, the voltage between both ends of R1 is equal to VREF (when imaginary short-circuiting of the error amplifier is established).
If the voltage of OUTPUT drops for some reason in this status, the voltage between both ends of R1 falls below VREF. Consequently, a positive voltage is applied to the non-inverted pin of the error amplifier, raising the bias voltage of Tr1 and forcing the voltage of OUTPUT to rise. Conversely, if OUTPUT increases, the voltage between both ends of R1 rises above VREF, lowering the bias voltage of Tr1 and forcing the voltage of OUTPUT to lower.

In this way, VREF serves as a reference in the above control action to keep the voltage of OUTPUT constant.

In addition, any output voltage can be set by the external circuit, by using the operation that keeps the output voltage constant. An example of a standard connection for this is given below.



The voltage between the ADJ pin and OUTPUT pin is kept the same as the reference voltage (VREF) if the ADJ pin is used as a common point. Since the current flowing between these pins (1.25 V / 240 Ω = 5.2 mA) ends up to be kept constant, by changing the resistance of R2, the voltage between the ADJ pin and ground can be changed.
The output voltage is determined by the following expression.
VO = R2 (IADJ + VREF / R1) + VREF
≈ R2 × VREF / R1 + VREF (IADJ: Current flowing out of ADJ pin is ignored.)
= (1 + R2 / R1) × VREF
From this expression, it can be learned that the output voltage can be changed by the rate of R1 and R2 while VREF serves as a reference voltage.

Related FAQ: How should the resistors (R1 and R2) for the output voltage setting circuit of the three-terminal positive adjustable regulator (uPC317) be set?

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