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Robot Builder Articles
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Electronics
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DC-DC Converter using a PIC16C620A
Posted by
fsmyth
on
Tuesday, April 19, 2005 (GST)
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In order to eliminate a second (system) battery,
I am investigating using a DC-DC converter.
This is a first attempt to use PIC microcontrollers
(a PIC16C620) in an SMPS controller function. This
implementation is a step-down (buck-mode) DC-DC
converter for use with a 12v battery input, which
may vary from 10v (discharged) to 16v (max charge),
and produce a stable rail voltage for other
imbedded microcontrollers, regardless of the main
battery charge state.
The PIC is used for: set point generation, error
amplifier, modulator, and control algorithm for a
pulse-skipping modulation scheme (as opposed to
pulse-width-modulation). The theoretical limit to
V(out) is determined by the input voltage and the
maximum duty cycle (minus switching losses).
The output voltage is determined by sampling the
output through a divider (R5/R6) and comparing it
to the internal reference (Vref) in the PIC.
The current control function is implemented by
comparing the feedback voltage from a low-side
current sense resistor to the load with the same
reference.
The 16C620 allows the reference set-point to be
adjusted in firmware, and is set at Vdd/2.
According to AN216, the line regulation is less
than 5 mV/V, the load regulation is -0.04 mV/mA,
and output ripple less than 12 mV rms. Efficiency
varies from 45%(14.8v in) to 72% (8.8v in).
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