TOP264-271
If the power supply is operating under heavy load or low input
line conditions when an open loop occurs, the output voltage
may not rise significantly. Under these conditions, a latching
shutdown will not occur until load or line conditions change.
Nevertheless, the operation provides the desired protection by
preventing significant rise in the output voltage when the line or
load conditions do change. Primary side OVP protection with
the TOP264-271 in a typical application will prevent a nominal
12 V output from rising above approximately 20 V under open
loop conditions. If greater accuracy is required, a secondary
sensed OVP circuit is recommended.
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. Note that for the
same CONTROL pin current, higher line voltage results in smaller
operating duty cycle. As an added feature, the maximum duty
cycle DC
MAX
is also reduced from 78% (typical) at a voltage slightly
lower than the UV threshold to 36% (typical) at the OV threshold.
DC
MAX
of 36% at high line was chosen to ensure that the power
capability of the TOP264-271 is not restricted by this feature under
normal operation. TOP264-271 provides a better fit to the ideal
feed-forward by using two reduction slopes: -1% per
mA
for all bus
voltage less than 195 V (typical for 4 MW line impedance) and
-0.25% per
mA
for all bus voltage more than 195 V.
Remote ON/OFF
TOP264-271 can be turned on or off by controlling the current
into the VOLTAGE-MONITOR pin or out from the EXTERNAL
CURRENT LIMIT pin. In addition, the VOLTAGE-MONITOR pin
has a 1 V threshold comparator connected at its input. This
voltage threshold can also be used to perform remote ON/OFF
control.
When a signal is received at the VOLTAGE-MONITOR pin or the
EXTERNAL CURRENT LIMIT pin to disable the output through
any of the pin functions such as OV, UV and remote ON/OFF,
TOP264-271 always completes its current switching cycle before
the output is forced off.
As seen above, the remote ON/OFF feature can also be used as
a standby or power switch to turn off the TOP264-271 and keep
it in a very low power consumption state for indefinitely long
periods. If the TOP264-271 is held in remote off state for long
enough time to allow the CONTROL pin to discharge to the
internal supply undervoltage threshold of 4.8 V (approximately
32 ms for a 47
mF
CONTROL pin capacitance), the CONTROL
pin goes into the hysteretic mode of regulation. In this mode,
the CONTROL pin goes through alternate charge and discharge
cycles between 4.8 V and 5.8 V (see CONTROL pin operation
Voltage Monitor and External Current Limit Pin Table*
Figure Number
Three Terminal Operation
Line Undervoltage (UV)
Line Overvoltage (OV)
Line Feed-Forward (DC
MAX
)
Output Overvoltage Protection (OVP)
Overload Power Limiting (OPP)
External Current Limit
Remote ON/OFF
Device Reset
Fast AC Reset
AC Brownout
*
This table is only a partial list of many VOLTAGE MONITOR and EXTERNAL CURRENT LIMIT Pin Configurations that are possible.
Table 2.
VOLTAGE MONITOR (V) Pin and EXTERNAL CURRENT LIMIT (X) Pin Configuration Options.
13
14
15
16
17
18
19
20
21
22
23
24
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
8
Rev. E 10/11
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