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Question

How should I calculate the external divider resistors’ value when I set the output voltage of an output voltage of adjustable type LDO?

Answer

LDO Linear RegulatorsHow to use

Power management ICs with externally adjustable output voltage are definitely equipped with a feedback pin (named VFB or ADJ pin, the pin name is different depending on our part#). Here, the feedback pin which determines the reference voltage of the LDO is described as "VFB pin".

Power management ICs operate to maintain the VFB pin voltage, so the output voltage is adjustable by using divider resistors as shown in the circuit below.

In the fixed output type LDOs, Ricoh's some parts have both VOUT pin and VFB pin. Technically, those kinds can be adjustable by this method mentioned below, however, external components recommendation value might be different from the fixed output voltage type to secure phase compensation.

Circuit structure

I1 = IIC + I2   ...   (1)
I2 = VFB / R2   ...   (2)

By the formula (1) and (2),

I1 = IIC + VFB / R2   ...   (3)

Here,

VOUT = VFB + R1 x I1   ...   (4)

As a result of applying (3) to (4),

VOUT = VFB + R1x(IIC + VFB / R2)
= VFB x (1 + R1 / R2) + R1 x IIC   ...   (5)
IIC = VFB / RIC   ...   (6)

Then error factor, R1 x IIC can be calculated by next formula.

R1 x IIC = R1 x VFB / RIC
= VFB x R1 x RIC   ...   (7)

To select the R1 as R1 << RIC , the error can be ignored.
RIC value is different depending on the part#.

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