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71128 Datasheet(PDF) 1 Page - Vishay Siliconix

Part No. 71128
Description  Simple Solution for Dynamically Programming the Output Voltage of DC-DC Converters
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Maker  VISHAY [Vishay Siliconix]
Homepage  http://www.vishay.com
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 1 page
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AN731
Vishay Siliconix
Document Number: 71128
28-Jan-00
www.vishay.com
S FaxBack 408-970-5600
1
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, Vr. 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 VC 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, Vr
Control voltage: VC = VC1 to VC2
Output voltage: VO = VO1 at VC = VC1, VO = VO2 at VC = VC2,
and VO is linear for any value of VC 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 = (VC1, VO1) and B =
(VC2, VO2).
R1
Power Stage
PWM
VOUT
R2
+
FB
Vr
VIN
PWM Controller
EA
+
B
Vr2
LM358
VX
R3
R4
VC
A
Op-Amp Circuit
FIGURE 1. Typical DC/DC Converter Configuration
R1
Power Stage
PWM
VOUT
R2
+
FB
Vr
VIN
PWM Controller
EA
FIGURE 2. Output Voltage vs Control Voltage Requirement
VO (V)
B (VC2, VO2)
VC (V)
A (VC1, VO1)
FIGURE 3. Op-Amp Circuit Offers Programmable Output Function




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