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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.
Stage and Filter
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.
Out of Range
Within the Common
Figure 4. Output Voltage Limit
2 Rev. 0.1