TOP232-234
voltage reaches the under-voltage threshold. At power down,
UV prevents auto-restart attempts after the output goes out of
regulation. This eliminates power down glitches caused by the
slowdischargeofinputstoragecapacitorpresentinapplications
such as standby supplies. A single resistor connected from the
MULTI-FUNCTION pin to the rectified DC high voltage bus
sets UV threshold during power up. Once the power supply is
successfully turned on, UV is disabled to allow extended input
voltage operating range. Input voltage is not checked again
until the power supply loses regulation and attempts another
turn-on. This is accomplished by enabling the UV comparator
only when the divide-by-8 counter used in auto-restart reaches
full count (S7) which is also the state that the counter is reset to
at power up (see Figure 5). The UV feature can be disabled
independent of OV feature as shown in Figure 16.
feed forward operation is illustrated in Figure 4 by the different
values of IM. Note that for the same CONTROL pin current,
higher line voltage results in smaller operating duty cycle. As
an added safety measure, the maximum duty cycle DCMAX is
alsoreducedfrom78%(typical)atavoltageslightlyhigherthan
the UV threshold to 38% (typical) at the OV threshold (see
Figures 4, 8). DCMAX of 38% at the OV threshold was chosen
to ensure that the power capability of the TOPSwitch-FX is not
restricted by this feature under normal operation.
Remote ON/OFF and Synchronization
TOPSwitch-FX can be turned on or off by controlling the
current into or out from the MULTI-FUNCTION pin (see
Figure8). ThisallowseasyimplementationofremoteON/OFF
controlofTOPSwitch-FXinseveraldifferentways. Atransistor
or an optocoupler output connected between the MULTI-
FUNCTIONpinandtheSOURCEpinimplementsthisfunction
with“active-on”(Figure19)whileatransistororanoptocoupler
outputconnectedbetweentheMULTI-FUNCTIONpinandthe
CONTROL pin implements the function with “active-off”
(Figure 20).
Line Overvoltage Shutdown (OV)
ThesameresistorusedforUValsosetsanovervoltagethreshold
which, once exceeded, will force TOPSwitch-FX output into
off-state. The ratio of OV and UV thresholds is preset at 4.5
as can be seen in Figure 8. This feature turns off the
TOPSwitch-FX power MOSFET when the rectified DC high
voltage exceeds the OV threshold. When the MOSFET is off,
therectifiedDChighvoltagesurgecapabilityisincreasedtothe
voltage rating of the MOSFET (700 V), due to the absence of
the reflected voltage and leakage spikes on the drain. Small
amountofhysteresisisprovidedontheOVthresholdtoprevent
noise triggering. The OV feature can be disabled independent
of UV feature as shown in Figure 15.
When a signal is received at the MULTI-FUNCTION pin to
disable the output through any of the MULTI-FUNCTION pin
functions such as OV, UV and remote ON/OFF,
TOPSwitch-FX always completes its current switching cycle as
illustrated in Figure 7 before the output is forced off. The
internal oscillator is stopped slightly before the end of the
current cycle and stays there as long as the disable signal exists.
When the signal at the MULTI-FUNCTION pin changes state
from disable to enable, the internal oscillator starts the next
switching cycle. This approach allows the use of this pin to
synchronize TOPSwitch-FX to any external signal with a
frequency lower than its internal switching frequency.
Line Feed Forward with DCMAX Reduction
The same resistor used for UV and OV also implements line
voltage feed forward which minimizes output line ripple and
reduces power supply output sensitivity to line transients. This
Oscillator
(SAW)
D
MAX
Enable from
M Pin (STOP)
Time
PI-2558-092999
Figure 7. Synchronization Timing Diagram.
B
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PAM [ POWER ANALOG MICOELECTRONICS ]
