TOP242-250
feature, the maximum duty cycle DCMAX 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 DCMAX at higher line voltages helps prevent
transformer saturation due to large load transients in forward
converter applications. DCMAX of 38% at the OV threshold
was chosen to ensure that the power capability of the
TOPSwitch-GX is not restricted by this feature under normal
operation.
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
CONTROLpin 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.
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,RorFpackage)ortheMULTI-FUNCTION(PorGpackage)
pin and the CONTROL pin implements the function with
“active-off” (Figures 23 and 37).
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 (VC), 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.
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.
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. VC
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, VC regulation returns to shunt mode, and
normal operation of the power supply resumes.
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 under-voltage 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
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 resumes
providing automatic recovery. A large hysteresis of 70 °C
(typical)isprovidedtopreventoverheatingofthePC boarddue
toacontinuousfaultcondition.VC isregulatedinhystereticmode
and a 4.8 V to 5.8 V (typical) sawtooth waveform is present on
the CONTROL pin while in thermal shutdown.
M
12/04
9
POWERINT [ Power Integrations ]
