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
slow discharge of input storage capacitor present in applications
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.
Line Overvoltage Shutdown (OV)
The same resistor used for UV also sets an overvoltage threshold
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,
the rectified DC high voltage surge capability is increased to the
voltage rating of the MOSFET (700 V), due to the absence of
the reflected voltage and leakage spikes on the drain. Small
amount of hysteresis is provided on the OV threshold to prevent
noise triggering. The OV feature can be disabled independent
of UV feature as shown in Figure 15.
Line Feed Forward with DC
MAX
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
feed forward operation is illustrated in Figure 4 by the different
values of I
M
. 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 DC
MAX
is
also reduced from 78% (typical) at a voltage slightly higher than
the UV threshold to 38% (typical) at the OV threshold (see
Figures 4, 8). DC
MAX
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
Figure 8). This allows easy implementation of remote ON/OFF
control of
TOPSwitch-FX
in several different ways. A transistor
or an optocoupler output connected between the MULTI-
FUNCTION pin and the SOURCE pin implements this function
with “active-on” (Figure 19) while a transistor or an optocoupler
output connected between the MULTI-FUNCTION pin and the
CONTROL pin implements the function with “active-off”
(Figure 20).
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.
Oscillator
(SAW)
DMAX
Enable from
M Pin (STOP)
Time
PI-2558-092999
Figure 7. Synchronization Timing Diagram.
B
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PAM [ POWER ANALOG MICOELECTRONICS ]
