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-FX
is held
in remote off state for long enough time to allow the CONTROL
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
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. See Figure 27 under application
examples for more information.
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
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-FX
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 described above. 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 (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. V
C
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-compensated bandgap reference. This reference is
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
line OV/UV thresholds.
TOPSwitch-FX
has improved circuitry
to maintain all of the above critical parameters within very tight
absolute and temperature tolerances.
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
during auto-restart, remote off and over-temperature shutdown.
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
(I
C
) and discharge currents (I
CD1
and I
CD2
). This current source
is turned off during normal operation when the output MOSFET
is switching.
8
B
7/01
PAM [ POWER ANALOG MICOELECTRONICS ]
