LNK623-626
converter annunciates this as an open-loop condition (top
resistor in potential divider is open or missing) and reduces the
Auto-restart time from 200 ms to approximately 6 clock cycles
(90 μs), whilst keeping the disable period of 2.1 seconds. This
effectively reduces the Auto-Restart duty cycle to less than 0.0±5.
LinkSwitch-CV Functional Description
The LinkSwitch-CV combines a high voltage power MOSFET
switch with a power supply controller in one device. Similar to
the LinkSwitch-LP and TinySwitch-III it uses ON/OFF control to
regulate the output voltage. The LinkSwitch-CV controller
consists of an oscillator, feedback (sense and logic) circuit, 6 V
regulator, over-temperature protection, frequency jittering,
current limit circuit, leading-edge blanking, and ON/OFF state
machine for CV control.
Over-Temperature Protection
The thermal shutdown circuitry senses the die temperature. The
threshold is set at ±42 °C typical with a 60 °C hysteresis. When
the die temperature rises above this threshold (±42 °C) the
power MOSFET is disabled and remains disabled until the die
temperature falls by 60 °C, at which point the MOSFET is
re-enabled.
Constant Voltage (CV) Operation
The controller regulates the feedback pin voltage to remain at
VFBth using an ON/OFF state-machine. The feedback pin
voltage is sampled 2.1 μs after the turn-off of the high voltage
switch. At light loads the current limit is also reduced to
decrease the transformer flux density.
Current Limit
The current limit circuit senses the current in the power
MOSFET. When this current exceeds the internal threshold
(ILIMIT), the power MOSFET is turned off for the remainder of that
cycle. The leading edge blanking circuit inhibits the current limit
comparator for a short time (tLEB) after the power MOSFET is
turned on. This leading edge blanking time has been set so that
current spikes caused by capacitance and rectifier reverse
recovery time will not cause premature termination of the
MOSFET conduction.
Auto-Restart and Open-Loop Protection
In the event of a fault condition such as an output short or an
open loop condition the LinkSwitch-CV enters into an
appropriate protection mode as described below.
In the event the feedback pin voltage during the Flyback period
falls below VFBth-0.3 V before the feedback pin sampling delay
(~2.1 μs) for a duration in excess of 200 ms (auto-restart on-
time (tAR-ON) the converter enters into Auto-restart, wherein the
power MOSFET is disabled for 2.1 seconds (~85 Auto-Restart
duty cycle). The auto-restart alternately enables and disables
the switching of the power MOSFET until the fault condition is
removed.
6.0 V Regulator
The 6 V regulator charges the bypass capacitor connected to the
BYPASS pin to 6 V by drawing a current from the voltage on the
DRAIN, whenever the MOSFET is off. The BYPASS pin is the
internal supply voltage node. When the MOSFET is on, the
device runs off of the energy stored in the bypass capacitor.
Extremely low power consumption of the internal circuitry allows
the LinkSwitch-CV to operate continuously from the current
drawn from the DRAIN pin. A bypass capacitor value of ± μF is
sufficient for both high frequency decoupling and energy storage.
In addition to the conditions for auto-restart described above, if
the sensed feedback pin current during the Forward period of
the conduction cycle (switch “on” time) falls below ±20 μA, the
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