LNK584-586
feedback circuit. This output is sampled at the beginning of
each cycle. If high, the power MOSFET is turned on for that
cycle (enabled), otherwise the power MOSFET remains off
(disabled). Since the sampling is done only at the beginning of
each cycle, subsequent changes in the FEEDBACK pin voltage
during the remainder of the cycle are ignored.
LinkZero-AX Functional Description
LinkZero-AX comprises a 700 V power MOSFET switch with a
power supply controller on the same die. Unlike conventional
PWM (pulse width modulation) controllers, it uses a simple
ON/OFF control to regulate the output voltage. The controller
consists of an oscillator, feedback (sense and logic) controller,
5.85 V regulator, BYPASS pin undervoltage protection, over-
temperature protection, frequency jittering, current limit protection,
and leading edge blanking. The controller includes a proprietary
Power-Down mode that automatically reduces standby consump-
tion to levels that are immeasurable on most power meters.
Output Power Limiting
When the FEEDBACK pin voltage at full load falls below 1.70 V,
the oscillator frequency linearly reduces to typically 60% at the
auto-restart threshold voltage of 0.9 V. This function limits the
power supply output current and power.
5.85 V Regulator
Power-Down Mode
The BYPASS pin voltage is regulated by drawing a current from
the DRAIN whenever the MOSFET is off if needed to charge up
the BYPASS pin to a typical voltage of 5.85 V. When the
MOSFET is on, LinkZero-AX runs off of the energy stored in the
bypass capacitor. Extremely low power consumption of the
internal circuitry allows LinkZero-AX to operate continuously
from the current drawn from the DRAIN pin. A bypass
capacitor value of 0.1 mF is sufficient for both high frequency
decoupling and energy storage.
The internal controller will go into Power-Down mode when 160
switching cycles are skipped. This can occur due to the
FEEDBACK pin being pulled high using an external Power-Down
pulse signal or due to a light load condition where the total
loading on the transformer (output plus feedback circuit loads)
has reduced to ~0.6% of full load. The device then operates in
an ultra low consumption Power-Down mode where switching
is disabled completely. The controller wakes up (or is reset)
when the BYPASS pin is pulled below 1.5 V and then released
to be recharged through the internal drain connected 5.85 V
regulator block (see Figure 2). When the BYPASS capacitor
recharges to the VBP BYPASS pin threshold, the device starts
switching and operates normally. If the FEEDBACK pin is pulled
high such that 160 cycles are again skipped, the device returns
to Power-Down mode operation as described above. In
applications with dynamic loads it may not be desirable to go
into Power-Down mode under light or no-load conditions.
Techniques to ensure this is avoided are discussed below in the
LinkZero-AX Power-Down Mode Design Considerations section.
6.5 V Shunt Regulator and 8.5 V Clamp
In addition, there is a shunt regulator that helps maintain the
BYPASS pin at 6.5 V when current is provided to the BYPASS
pin externally. This facilitates powering the device externally
through a resistor from the bias winding or power supply output
in non-isolated designs, to decrease device dissipation and
increase power supply efficiency.
The 6.5 V shunt regulator is only active in normal operation, and
when in Power-Down mode a clamp at a higher voltage (typical
8.5 V) will clamp the BYPASS pin.
Oscillator
The typical oscillator frequency is internally set to an average of
100 kHz. An internal circuit senses the duty cycle of the MOSFET
switch conduction-time and adjusts the oscillator frequency so
that during long conduction intervals (low line voltage) the
frequency is about 100 kHz and at short conduction intervals
(high line voltage) the oscillator frequency is about 78 kHz. This
internal frequency adjustment is used to make the peak power
point constant over line voltage. Two signals are generated from
the oscillator: the maximum duty cycle signal (DCMAX) and the
clock signal that indicates the beginning of a switching cycle.
BYPASS Pin Undervoltage Protection
The BYPASS pin undervoltage circuitry disables the power
MOSFET when the BYPASS pin voltage drops below 4.85 V.
Once the BYPASS pin voltage drops below 4.85 V, it must rise
back to 5.85 V to enable (turn on) the power MOSFET.
BYPASS Pin Overvoltage Protection
If the BYPASS pin gets pulled above 6.5 V and the current into
the shunt exceeds 6.5 mA a latch will be set and the power
MOSFET will stop switching. To reset the latch the BYPASS pin
has to be pulled down to below 1.5 V.
The oscillator incorporates circuitry that introduces a small
amount of frequency jitter, typically 6% of the switching
frequency, to minimize EMI. The modulation rate of the
frequency jitter is set to 1 kHz to optimize EMI reduction for
both average and quasi-peak measurements. The frequency
jitter, which is proportional to the oscillator frequency, should be
measured with the oscilloscope triggered at the falling edge of
the DRAIN voltage waveform. The oscillator frequency is
gradually reduced when the FEEDBACK pin voltage is lowered
below 1.70 V.
Over-Temperature Protection
The thermal shutdown circuit senses the die temperature. The
threshold is set at 142 °C typical with a 70 °C hysteresis. When
the die temperature rises above this threshold (142 °C) the
power MOSFET is disabled and remains disabled until the die
temperature falls by 70 °C, at which point the MOSFET is
re-enabled.
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
Feedback Input Circuit CV Mode
The feedback input circuit reference is set at 1.70 V. When the
FEEDBACK pin voltage reaches a VFB reference voltage (1.70 V),
a low logic level (disable) is generated at the output of the
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