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71128 Datasheet(PDF) 4 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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AN731
Vishay Siliconix
www.vishay.com
S FaxBack 408-970-5600
4
Document Number: 71128
28-Jan-00
DYNAMIC RESPONSE PERFORMANCE
Figures 7 and 8 show the dynamic response of the output
voltage with different dynamic control voltages. The VO
response and settling time depend on the following factors:
1. the bandwidth of the converter control loop: the larger the
bandwidth, the faster VO response time
2. the large signal response slew rate of the converter: the
faster the slew rate, the faster the response
3. the slew rate of the converter control loop error amplifier
4. the bandwidth of the difference amplifier.
The bandwidth of the converter control loop is the most
important parameter. The maximum attainable bandwidth of a
converter control loop is 1/(2*pi) times the switching frequency.
For a converter switching at 100 kHz, the control bandwidth is
limited to 16 kHz. Vishay Siliconix high-frequency switching
regulators (including the Si9165, Si9169, and Si9170) offer
switching frequencies up to 2 MHz and have a theoretical
bandwidth limit of 318 kHz. Thus they would
be good
candidates for this kind of application. Figure 5 shows a
complete example of a dc-to-dc converter using the Si9165
controller IC and the op-amp circuit. The output voltage can be
programmed from 0.4 V to 3.4 V with a control voltage from
0.2 V to 2.7 V. Figure 6 shows a similar example using the
Si9166 controller.
The signal response slew rate of the converter dictates how
fast the converter can slew its output voltage up or down given
an infinitely large control loop bandwidth. The converter slew
rate depends on the converter output averaging LC filter. The
smaller the LC, the faster the slew rate.
With respect to the slew rate of the converter control loop error
amplifier, most of the converter control loop is figured as an
integrator. The error amplifier often drives a fairly large
integrator capacitor. The slew rate of the op-amp should be
sized adequately.
The bandwidth of the difference amplifier requires the
difference amplifier to have its unity gain bandwidth a decade
or more larger than the control loop bandwidth.
CONCLUSION
A simple op-amp circuit is used to program the output voltage
of a typical dc-to-dc converter. The circuit offers flexible voltage
scaling using a linear control voltage. The control voltage can
be hard-wired for fixed-voltage operation. When used together
with a wide bandwidth dc-to-dc converter, the circuit offers a
simple solution for controlling the output voltage dynamically.




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