71128 Datasheet, PDF (1/7 Pages) Vishay Siliconix – Simple Solution for Dynamically Programming the Output Voltage of DC-DC Converters

AN731
Vishay Siliconix
Simple Solution for Dynamically Programming the
Output Voltage of DC-DC Converters
INTRODUCTION
Figure 1 shows a typical dc-to-dc converter configuration. As in
many PWM controllers, the non-inverting input of the voltage
feedback error amplifier is internally connected to the
reference voltage, V_r. The output voltage of the converter is set
by a resistor divider network R1 and R2. This configuration
enables a fixed output voltage that is equal to or greater than
the reference voltage. But in many power conversion designs,
it is useful to vary the output to a value lower than the reference
voltage and to dynamically adjust the output voltage. The
op-amp circuit shown in Figure 3 gives designers a simple way
to do this. Scaling the output voltage with respect to a control
voltage V_C is totally flexible. This application note describes
how to use this simple solution and provides a step-by-step
design procedure to assist designers in calculating the specific
parameters required by their circuits.
DESIGN REQUIREMENT
Reference voltage, V_r
Control voltage: V_C = V_C1 to V_C2
Output voltage: V_O = V_O1 at V_C = V_C1, V_O = V_O2 at V_C = V_C2,
and V_O is linear for any value of V_C in its range.
These design requirements are shown in Figure 2. The output
voltage is a linear function with respect to the control voltage.
The function crosses two end points A = (V_C1, V_O1) and B =
(V_C2, V_O2).
V_IN
Power Stage
V_OUT
R1
PWM
FB
–
EA
+
V_r
PWM Controller
R2
FIGURE 1. Typical DC/DC Converter Configuration
V_O (V)
B (V_C2, V_O2)
A (V_C1, V_O1)
V_C (V)
FIGURE 2. Output Voltage vs Control Voltage Requirement
V_IN
Document Number: 71128
28-Jan-00
Power Stage
V_OUT
PWM
FB
–
EA
+
V_r
PWM Controller
R1
A
R2
V_X
R3
B
R4
–
V_C
LM358
+
V_r2
Op-Amp Circuit
FIGURE 3. Op-Amp Circuit Offers Programmable Output Function
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