TOP232-234
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-FXis held
inremoteoffstateforlongenoughtimetoallowtheCONTROL
pin to dishcharge 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 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 pin open).
When the TOPSwitch-FX 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
eliminateexpensiveandunreliablein-linemechanicalswitches.
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. See Figure 27 under application
examples for more information.
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 described above. 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.
Hysteretic Over-Temperature Protection
Temperature protection is provided by a precision analog
circuit that turns the output MOSFET off when the junction
temperatureexceedsthethermalshutdowntemperature(135˚C
typical). When the junction temperature cools to below the
hysteretic temperature, normal operation resumes. A large
hysteresis of 70 ˚C (typical) is provided to prevent overheating
of the PC board due to a repeating fault condition. VC is
regulated in hysteretic mode and a 4.8 V to 5.8 V (typical)
sawtooth waveform is present on the CONTROL pin when the
power supply is turned off.
Bandgap Reference
All critical TOPSwitch-FX internal voltages are derived from a
temperature-compensatedbandgapreference. Thisreferenceis
also used to generate a temperature-compensated current
reference which is trimmed to accurately set the switching
frequency, MOSFET gate drive current, current limit, and the
lineOV/UVthresholds. TOPSwitch-FXhasimprovedcircuitry
to maintain all of the above critical parameters within very tight
absolute and temperature tolerances.
Soft-Start
An on-chip soft-start function is activated at start-up with a
duration of 10 ms (typical). Maximum duty cycle starts from
zero and linearly increases to the default maximum of 78% 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
whenrestartingafterbeinginhystereticregulationofCONTROL
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
outputrectifier,duringstart-up. Thisfeaturealsohelpsminimize
output overshoot and prevents saturation of the transformer
during start-up.
High-Voltage Bias Current Source
This current source biases TOPSwitch-FX from the DRAIN pin
and charges the CONTROL pin external capacitance during
start-up or hysteretic operation. Hysteretic operation occurs
duringauto-restart,remoteoffandover-temperatureshutdown.
In this mode of operation, the current source is switched on and
off with an effective duty cycle of approximately 35%. This
duty cycle is determined by the ratio of CONTROL pin charge
(IC) and discharge currents (ICD1 and ICD2). This current source
isturnedoffduringnormaloperationwhentheoutputMOSFET
is switching.
Shutdown/Auto-Restart
To minimize TOPSwitch-FX power dissipation under fault
conditions, the shutdown/auto-restart circuit turns the power
B
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PAM [ POWER ANALOG MICOELECTRONICS ]
