■ FIGURE 4
As mentioned previously,
switchmode supplies respond
sluggishly to sudden load
changes, which would be the
norm for this project. Severe overshoot and ringing usually occur as
the control loop attempts to
stabilize, which could (in the
worst case) cause permanent
damage to the unit. There is a
control scheme however, known
as the hysteretic — or ripple —
regulator, which responds the
fastest for sudden load changes.
As its name implies, it self-oscillates between two user-set
ripple boundaries. Using this
search string plus the LED driver, I
found a very exciting device from
Supertex, Inc. The HV9930
feature set not only satisfied the
hysteretic current control scheme,
but it also employs the Cuk
(buck-boost) powertrain topology,
meaning that the output voltage
may be higher or lower than the
input voltage for increased
versatility. It also drives an
external MOSFET, and thus its
output drive may be as large as
required, depending on the
external device ratings.
The device is available in
both eight pin thru hole and SMT
packages. It is available through
Mouser — important since all the
wonderful specs are meaningless
to a hobbyist, if the device is only
sold through a distributor network
which caters to small orders.
Better still, the device has an
available plug-and-play evaluation
board (HV9930DB2), which is a
major plus if one doesn’t have the
time or inclination to design a
switchmode supply from scratch.
The board has a quick responding
enable input which can be used
for either PWM dimming, or in
this case, for pulsing.
One last consideration when
employing this module is that it
has a minimum eight volts DC
input requirement, with 12 volts
DC preferred. This is okay, since
the high power requirements
34
January 2008
