TOP252-262
Line Feed-Forward with DCMAX Reduction
Soft-Start
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. 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 DCMAX is also reduced from 78% (typical) at a voltage slightly
lower than the UV threshold to 36% (typical) at the OV threshold.
DCMAX of 36% at high line was chosen to ensure that the power
capability of the TOPSwitch-HX is not restricted by this feature
under normal operation. TOPSwitch-HX provides a better fit to the
ideal feed-forward by using two reduction slopes: -1% per μA for all
bus voltage less than 195 V (typical for 4 MΩ line impedance) and
-0.25% per μA for all bus voltage more than 195 V. This dual
slope line feed-forward improves the line ripple rejection
significantly compared with the TOPSwitch-GX.
The 17 ms soft-start sweeps the peak drain current and
switching frequency linearly from minimum to maximum value
by operating through the low frequency PWM mode and the
variable frequency mode before entering the full frequency
mode. 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.
Shutdown/Auto-Restart
To minimize TOPSwitch-HX 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 2% 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.
Remote ON/OFF
TOPSwitch-HX can be turned on or off by controlling the
current into the VOLTAGE-MONITOR pin or out from the
EXTERNAL CURRENT LIMIT pin (Y, E/L and M packages) and
into or out from the MULTI-FUNCTION pin (P and G package,
see Figure 12). In addition, the VOLTAGE-MONITOR pin has a
1 V threshold comparator connected at its input. This voltage
threshold can also be used to perform remote ON/OFF control.
Hysteretic Over-Temperature Protection
Temperature protection is provided by a precision analog circuit
that turns the output MOSFET off when the junction
When a signal is received at the VOLTAGE-MONITOR pin or the
EXTERNAL CURRENT LIMIT pin (Y, E/L and M packages) or the
MULTI-FUNCTION pin (P and G package) to disable the output
through any of the pin functions such as OV, UV and remote
ON/OFF, TOPSwitch-HX always completes its current switching
cycle before the output is forced off.
temperature exceeds the thermal shutdown temperature
(142 °C typical). When the junction temperature cools to below
the lower hysteretic temperature point, normal operation
resumes, thus providing automatic recovery. A large hysteresis
of 75 °C (typical) is provided to prevent overheating of the PC
board due to a continuous fault condition. VC is regulated in
hysteretic mode, and a 4.8 V to 5.8 V (typical) triangular waveform
is present on the CONTROL pin while in thermal shutdown.
As seen above, the remote ON/OFF feature can also be used as
a standby or power switch to turn off the TOPSwitch-HX and
keep it in a very low power consumption state for indefinitely
long periods. If the TOPSwitch-HX 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-HX 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
Bandgap Reference
All critical TOPSwitch-HX internal voltages are derived from a
temperature-compensated bandgap reference. This voltage
reference is used to generate all other internal current
references, which are trimmed to accurately set the switching
frequency, MOSFET gate drive current, current limit, and the
line OV/UV/OVP thresholds. TOPSwitch-HX has improved
circuitry to maintain all of the above critical parameters within
very tight absolute and temperature tolerances.
High-Voltage Bias Current Source
This high-voltage current source biases TOPSwitch-HX 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 approxi-
mately 35%. This duty cycle is determined by the ratio of
CONTROL pin charge (IC) and discharge currents (ICD1 and ICD2).
This current source is turned off during normal operation when
the output MOSFET is switching. The effect of the current
source switching will be seen on the DRAIN voltage waveform
as small disturbances and is normal.
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.
13
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Rev. F 01/09
POWERINT [ Power Integrations ]
