TOP250 datasheet(9/54 Pages) POWERINT

9 / 54 page

Rev. R 10/20

TOP242-250

www.power.com

transients. This feed-forward operation is illustrated in

Figure 7 by the different values of I

L (Y, R or F package) or

M (P or G package). 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

MAX is also reduced from 78% (typical) at a voltage

slightly higher than the UV threshold to 30% (typical) at

the OV threshold (see Figure 11). Limiting DC

MAX at higher

line voltages helps prevent transformer saturation due to

large load transients in TOP248, TOP249 and TOP250

forward converter applications. DC

MAX of 38% at high

line was chosen to ensure that the power capability of

the TOPSwitch-GX is not restricted by this feature under

normal operation.

Remote ON/OFF and Synchronization

TOPSwitch-GX can be turned on or off by controlling

the current into the LINE-SENSE pin or out from the

EXTERNAL CURRENT LIMIT pin (Y, R or F package) and

into or out from the MULTI-FUNCTION pin (P or G package)

(see Figure 11). In addition, the LINE-SENSE pin has a 1 V

threshold comparator connected at its input. This voltage

threshold can also be used to perform remote ON/OFF

control. This allows easy implementation of remote

ON/OFF control of TOPSwitch-GX in several different ways.

A transistor or an optocoupler output connected between

the EXTERNAL CURRENT LIMIT or LINE-SENSE pins

(Y, R or F package) or the MULTI-FUNCTION pin (P or G

package) and the SOURCE pin implements this function

with “active-on” (Figures 22, 29 and 36) while a transistor

or an optocoupler output connected between the LINE-

SENSE pin (Y, R or F package) or the MULTI-FUNCTION

(P or G package) pin and the CONTROL pin implements

the function with “active-off” (Figures 23 and 37).

When a signal is received at the LINE-SENSE pin or the

EXTERNAL CURRENT LIMIT pin (Y, R or F package) or

the MULTI-FUNCTION pin (P or G package) to disable the

output through any of the pin functions such as OV, UV

and remote ON/OFF, TOPSwitch-GX always completes its

current switching cycle, as illustrated in Figure 10, 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 above pins changes state from disable to enable,

the internal oscillator starts the next switching cycle. This

approach allows the use of these pins to synchronize

TOPSwitch-GX to any external signal with a frequency

between its internal switching frequency and 20 kHz.

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

TOPSwitch-GX 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-GX and keep it in a very low power

consumption state for indefinitely long periods. If the

TOPSwitch-GX 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

µF 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 (160 mW typical at 230 VAC with M or X

pins open). When the TOPSwitch-GX is remotely turned

on after entering this mode, it will initiate a normal start-up

sequence with soft-start the next time the CONTROL pin

reaches 5.8 V. In the worst case, the delay from remote

on to start-up can be equal to the full discharge/charge

cycle time of the CONTROL pin, which is approximately

125 ms for a 47

µF CONTROL pin capacitor. This reduced

consumption remote off mode can eliminate expensive and

unreliable in-line mechanical switches. It also allows for

microprocessor controlled turn-on and turn-off sequences

that may be required in certain applications such as inkjet

and laser printers.

Soft-Start

Two on-chip soft-start functions are activated at start-up with

a duration of 10 ms (typical). Maximum duty cycle starts

from 0% and linearly increases to the default maximum

of 78% at the end of the 10 ms duration and the current

limit starts from about 85% and linearly increases to 100%

at the end of the 10 ms duration. In addition to start-up,

soft-start is also activated at each restart attempt during

auto-restart and when restarting after being in hysteretic

regulation of CONTROL pin voltage (V

C), due to remote OFF

or thermal shutdown conditions. This effectively minimizes

current and voltage stresses on the output MOSFET, the

clamp circuit and the output rectifier during start-up. This

feature also helps minimize output overshoot and prevents

saturation of the transformer during start-up.

Shutdown/Auto-Restart

To minimize TOPSwitch-GX power dissipation under fault

conditions, the shutdown/auto-restart circuit turns the

power supply on and off at an auto-restart duty cycle

of typically 4% if an out of regulation condition persists.

Loss of regulation interrupts the external current into the

CONTROL pin. V

C regulation changes from shunt mode to

the hysteretic auto-restart mode as described in CONTROL

pin operation section. When the fault condition is removed,

the power supply output becomes regulated, V

C regulation

returns to shunt mode, and normal operation of the power

supply resumes.

Hysteretic Over-Temperature Protection

Temperature protection is provided by a precision analog

circuit that turns the output MOSFET off when the junction

temperature exceeds the thermal shutdown temperature

(140

°C typical). When the junction temperature cools to

below the hysteretic temperature, normal operation

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