TOP232 datasheet(7/38 Pages) POWERINT

7 / 38 page

Rev. D 10/20

TOP232-234

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Figure 7. Synchronization Timing Diagram.

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.

As seen above, the remote ON/OFF feature allows the

TOPSwitch-FX to be turned on and off instantly, on a cycle-by-

cycle basis, with very little delay. However, remote ON/OFF

can also be used as a standby or power switch to turn off the

TOPSwitch-FX and keep it in a very low power consumption

state for indefinitely long periods. If the TOPSwitch-FX is held

in remote off state for long enough time to allow the CONTROL

pin to dishcharge to the internal supply undervoltage threshold

of 4.8 V (approximately 32 ms for a 47 µF CONTROL pin

capacitance), the CONTROL pin goes into the hysteretic mode

of regulation. In this mode, the CONTROL pin goes through

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