S R FB
Figure 7. Remote ON/OFF VOLTAGE MONITOR Pin Control.
5.9 V Regulator/Shunt Voltage Clamp
The internal 5.9 V regulator charges the bypass capacitor
connected to the BYPASS pin to 5.9 V by drawing a current
from the voltage on the DRAIN pin whenever the power FET is
off. The BYPASS pin is the internal supply voltage node. When
the power FET is on, the device operates from the energy stored
in the bypass capacitor. Extremely low power consumption of the
internal circuitry allows LYTSwitch-4 to operate continuously from
current it takes from the DRAIN pin. A bypass capacitor value
of 47 or 4.7 mF is sufficient for both high frequency decoupling
and energy storage. In addition, there is a 6.4 V shunt regulator
clamping the BYPASS pin at 6.4 V when current is provided to
the BYPASS pin through an external resistor. This facilitates
powering of LYTSwitch-4 externally through a bias winding to
increase operating efficiency. It is recommended that the
BYPASS pin is supplied current from the bias winding for
In the event of an open-loop fault (open FEEDBACK pin resistor
or broken path to feedback winding), output short-circuits or an
overload condition the controller enters into the auto-restart
mode. The controller annunciates both short-circuit and
open-loop conditions once the FEEDBACK pin current falls
auto-restart circuit turns the power supply on (same as the
soft-start period) and off at an auto-restart duty cycle of
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supply will remain in auto-restart until the full off-time count is
completed. Special consideration must be made to appropriately
size the output capacitor to ensure that after the soft-start
period (tSOFT) the FEEDBACK pin current is above the IFB(AR)
threshold to ensure successful power-supply start-up. After the
soft-start time period, auto-restart is activated only when the
FEEDBACK pin current falls below IFB(AR).
The current limit circuit senses the current in the power FET.
When this current exceeds the internal threshold (ILIMIT), the power
FET is turned off for the remainder of that cycle. A leading edge
blanking circuit inhibits the current limit comparator for a short
been set so
is turned on.
This leading edge
spikes caused by
capacitance and rectifier reverse recovery will not cause
premature termination of the power FET conduction.
Line Overvoltage Protection
This device includes overvoltage detection to limit the maximum
operating voltage detected through the VOLTAGE MONITOR pin.
An external peak detector consisting of a diode and capacitor is
required to provide input line peak voltage to the VOLTAGE
MONITOR pin through a resistor.
The resistor sets line overvoltage (OV) shutdown threshold which,
once exceeded, forces the LYTSwitch-4 to stop switching. Once
the line voltage returns to normal, the device resumes normal
operation. A small amount of hysteresis is provided on the OV
threshold to prevent noise-generated toggling. When the power
FET is off, the rectified DC high voltage surge capability is
increased to the voltage rating of the power FET (725 V), due to the
absence of the reflected voltage and leakage spikes on the drain.
Hysteretic Thermal Shutdown
The thermal shutdown circuitry senses the controller die
temperature. The threshold is set at 142 °C typical with a 75 °C
hysteresis. When the die temperature rises above this threshold
(142 °C) the power FET is disabled and remains disabled until
the die temperature falls by 75 °C, at which point the power FET
Safe Operating Area (SOA) Protection
The device also features a safe operating area (SOA) protection
mode which disables FET switching for 40 cycles in the event
the soft-start period when auto-restart protection is inhibited.
The SOA protection mode remains active in normal operation.
Rev. C 11/14