8
DEMO MANUAL DC236
DESIGN-READY SWITCHERS
Refer to the LTC1628 data sheet for further information on
the internal operation and functionality descriptions of
the IC.
Overcurrent and Overvoltage Protection
The RUN/SS capacitor, C
SS
, is used initially to turn on and
limit the inrush current of the controller. After the control-
ler has been started and given adequate time to charge the
output capacitor and provide full load current, C
SS
is used
as a short-circuit time-out circuit. If the output voltage falls
to less than 70% of its nominal output voltage, C
SS
begins
discharging on the assumption that the output is in an
overcurrent and/or short-circuit condition. If the condition
lasts for a long enough period as determined by the size of
C
SS
, the controller will be shut down until the RUN/SS pin
voltage is recycled. This built-in latch-off can be overrid-
den by providing >5
µ
A pull-up at a compliance of 4V to the
RUN/SS pin. This current shortens the soft start period but
also prevents net discharge of the RUN/SS capacitor
during an overcurrent and/or short-circuit condition. Fold-
back current limiting is activated when the output voltage
falls below 70% of its nominal level, whether or not the
short-circuit latch-off circuit is enabled.
The output is protected from overvoltage by a “soft latch.”
When the output voltage exceeds the regulation value by
more than 7.5%, the synchronous MOSFET turns on and
remains on for as long as the overvoltage condition is
present. If the output voltage returns to a safe level, normal
operation resumes. This self-resetting action prevents
“nuisance trips” due to momentary transients and elimi-
nates the need for the Schottky diode that is necessary
with conventional OVP to prevent V
OUT
reversal.
DC236 Physical Design
The demonstration board is manufactured using a typical
4-layer copper PC board. The outside layers are 2 oz
copper and the inside layers are 1oz copper. The board is
designed to use the minimum number of external compo-
nents but has a few components added to facilitate
optional IC configurations. These added components will
not be required in a final design. These components
include R9, R10, R12 and C19 to C21. Other components
that may not be necessary depending upon the particular
design include C9, C10, C13, C14, C18 and R11. Certain
components may be larger than specific applications will
require. The output capacitance and the inductance values
selected are larger than may be required in order to
accommodate the very wide operating frequency range
(130kHz to 300kHz) capability of the demonstration board.
Output capacitance as low as 47
µ
F and inductance values
as low as several microhenries will work well at the higher
frequencies. The 2-phase controller technique signifi-
cantly reduces the capacity and ESR requirements of the
input capacitor when compared to a 1-phase approach.
The dual output MOSFETs used in the design reduce the
overall size of the design and take advantage of an
extended copper foil trace to help dissipate power on the
board. The Schottky diodes, D1 and D2, can also be
removed to reduce system cost but will decrease effi-
ciency slightly.
Active Loads—Beware!
Beware of Active Loads. They are convenient but problem-
atic. Some active loads do not turn on until the applied
voltage rises above 0.1V to 0.8V. The turn-on may be
delayed as well. Under these conditions, a switching
regulator with soft start may appear to start up and then
shut down before eventually reaching the correct output
voltage. What actually happens is as follows: at switching
regulator turn-on, the output voltage is below the active
load’s turn-on requirements. The switching regulator’s
output rises to the correct output voltage level due to the
inherent delay in the active load. The active load turns on
after its internal delay and now pulls down the switching
regulator’s output because the switcher is in its soft start
interval. The switching regulator’s output may come up at
some later time when the soft start interval has passed.
A switching regulator with foldback current limit will also
have difficulty with the unrealistic I-V characteristic of the
active load. Foldback current limiting will reduce the
output current available as the output voltage drops below
a threshold level (this level is 70% of nominal V
OUT
for the
LTC1628). This reduction in available output current will
result in the active load immediately pulling down the
output because the active load’s current demand remains
constant as the output voltage decreases. Most actual
OPERATIO
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