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Switched Regulator All of the digital circuitry on the VOCON is supplied 5 volt regulated
dc by a switched mode regulator (refer to Figure 3 on page 5 of Chapter
4). The fundamental parts of the regulator are U409, L402, C470,
CR403, C463, and U407. Module U409 is a pulse width modulating
(PWM) switched regulator controller. Coil L402 is an energy storage
element, C470 is an output ripple filter, and CR403 is a Schottky diode
switch. Capacitor C463 is added for UNSW_B+ ripple filter and is
necessary for stability of the regulator. Module U407 is a supply
supervisory IC, which provides a system reset function when the
output of the regulator falls out of regulation, typically around 4.6Vdc.
This switched mode regulator works by supplying just sufficient
energy to the storage element to maintain the output power of the
regulator at 5Vdc. It can be related to a flywheel in the sense that just
enough energy can be added to a spinning flywheel to keep it spinning
at a constant speed. In contrast to a typical linear type regulator, which
basically shunts unused current to ground through an active resistive
divider. The switched mode regulator is much more energy efficient. It
can be noted that input current to the regulator is less than the load
current. In fact, as input voltage to the regulator goes up, current
supplied to the regulator actually goes down for a constant load.
Module U409 works off of a clock with a nominal operating frequency
of 160kHz (kit number NTN7749E), or 260kHz (kit numbers
NTN7749F and NTN7749G). This may vary a little based on the load
and input voltage. It maintains regulation by varying the duty cycle of
a clock output driving L402. This signal is referred to as Lx on U409
(refer to Waveform W1). As long as the clock output is high, current
flows from the supply into L402 allowing energy to be stored. When
the clock output goes low, the diode CR403 conducts, allowing current
to continue to flow from ground through L402. A pulse width on the
Lx signal can be obtained, which provides the correct amount of
energy to keep the output in regulation. Capacitor C470 is an output
filter to reduce ripple on the output from the clock transitions.
Module U409 is supplied directly from the unswitched battery supply.
It is turned on and off through the control line connected to SHDN*/
ON/OFF. This is the same control line from the MCU, which controls
the series pass element Q207, which switches SW_B+. A voltage level
of approximately 2Vdc is required to turn the regulator on.
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