Rev. F 05/15

8

TOP264-271

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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 M

W 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

section above) and runs entirely off the high-voltage DC input,

but with very low power consumption (<100 mW typical at

230 VAC with X pin open). When the TOP264-271 is remotely

Voltage Monitor and External Current Limit Pin Table*

Figure Number

13

14

15

16

17

18

19

20

21

22

23

24

Three Terminal Operation

3

Line Undervoltage (UV)

3

3

3

3

3

Line Overvoltage (OV)

3

3

3

3

3

Line Feed-Forward (DC

MAX)

3

3

3

3

Output Overvoltage Protection (OVP)

3

Overload Power Limiting (OPP)

3

External Current Limit

3

3

3

3

3

3

Remote-ON/OFF

3

3

3

Device Reset

3

3

3

Fast AC Reset

3

AC Brown-Out

3

*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.