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3. Dynamic Range Limit

The simplest and most economical feedback approach is to divide the output voltage with a simple resistor divider

as shown in Figure 3.

VIN

The Power

Stage and Filter

VOUT

Rs

VSENSE

Si8250

Vs

Rx

Figure 3. Resistor Divider

Thus, the output voltage is proportional to the inverted resistor divider ratio, Equation 2, times the sense voltage as

shown in Equation 1. For simplicity, the inverted resistor divider ratio from this point forward will be referred to as

the α ratio.

Vo = αVs

Equation 1. Output Voltage/Sense Voltage Relation

α = R-----s--R--+---x--R----x-

Equation 2. Inverted Resistor Divider Ratio

Similarly, it is easily shown that the range of the output voltage is proportional by the α ratio to the range of the

sense voltage, and this is where fundamental limits can be seen. The range of the sense voltage is bounded by the

common mode input specification, ∆Vs = 1.2–0.6 V = 0.6 V.

Voh – Vol = αVsh– αVsl → ∆Vo = α∆Vs

Equation 3. Output / Sense Dynamic Range Relation

For example, the operating range for a particular design is 2.2 to 5.5 V with 200 mV of operating margin on either

side. Thus, the absolute range of the supply is defined to be 2.0 to 5.7 V. However, starting with the minimum

sense voltage at the minimum output voltage, the maximum achievable output voltage is 2.0 V + α∆Vs = 4.0 V. At

an output of 4.0 V, the sense voltage is at its maximum limit of 1.2 V. This graphical relationship is shown in

Figure 4. Thus the desired 5.7 V maximum range is not achievable with the feedback circuit shown in Figure 3.

5.7 V

4.0 V

VOUT

2.0 V

Out of Range

Within the Common

Mode Range

1.2 V

VS

0.6 V

0V 0V

Figure 4. Output Voltage Limit

2 Rev. 0.1