LNK304-306
pin through an external resistor. This facilitates powering of
LinkSwitch-TN externally through a bias winding to decrease
the no-load consumption to about 50 mW.
LinkSwitch-TN Functional
Description
LinkSwitch-TNcombinesahighvoltagepowerMOSFETswitch
withapowersupplycontrollerinonedevice.Unlikeconventional
PWM(pulsewidthmodulator)controllers,LinkSwitch-TNuses
a simple ON/OFF control to regulate the output voltage. The
LinkSwitch-TN controller consists of an oscillator, feedback
(sense and logic) circuit, 5.8 V regulator, BYPASS pin under-
voltagecircuit,over-temperatureprotection,frequencyjittering,
current limit circuit, leading edge blanking and a 700 V power
MOSFET.TheLinkSwitch-TNincorporatesadditionalcircuitry
for auto-restart.
BYPASS Pin Under-Voltage
The BYPASS pin under-voltage 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 5.8 V to enable (turn-on) the power MOSFET.
Over-Temperature Protection
Thethermalshutdowncircuitrysensesthedietemperature. 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 MOSFET is disabled and remains disabled until the die
temperature falls by 75 °C, at which point it is re-enabled.
Oscillator
The typical oscillator frequency is internally set to an average
of 66 kHz. Two signals are generated from the oscillator: the
maximum duty cycle signal (DCMAX) and the clock signal that
indicates the beginning of each cycle.
Current Limit
ThecurrentlimitcircuitsensesthecurrentinthepowerMOSFET.
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
switching pulse.
The LinkSwitch-TN oscillator incorporates circuitry that
introduces a small amount of frequency jitter, typically 4 kHz
peak-to-peak, to minimize EMI emission. The modulation rate
of the frequency jitter is set to 1 kHz to optimize EMI reduction
forbothaverageandquasi-peakemissions. Thefrequencyjitter
should be measured with the oscilloscope triggered at the
falling edge of the DRAIN waveform. The waveform in
Figure 4 illustrates the frequency jitter of the LinkSwitch-TN.
Auto-Restart
In the event of a fault condition such as output overload, output
short, or an open loop condition, LinkSwitch-TN enters into
auto-restart operation. An internal counter clocked by the
oscillator gets reset every time the FB pin is pulled high. If the
FB pin is not pulled high for 50 ms, the power MOSFET
switching is disabled for 800 ms. The auto-restart alternately
enables and disables the switching of the power MOSFET until
the fault condition is removed.
Feedback Input Circuit
The feedback input circuit at the FB pin consists of a low
impedance source follower output set at 1.65 V. When the
current delivered into this pin exceeds 49 µA, a low logic level
(disable) is generated at the output of the feedback circuit. This
output is sampled at the beginning of each cycle on the rising
edge of the clock signal. 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 FB pin
voltageorcurrentduringtheremainderofthecycleareignored.
600
500
V
DRAIN
400
5.8 V Regulator and 6.3 V Shunt Voltage Clamp
The 5.8 V regulator charges the bypass capacitor connected to
the BYPASS pin to 5.8 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 for the LinkSwitch-TN.
When the MOSFET is on, the LinkSwitch-TN runs off of the
energy stored in the bypass capacitor. Extremely low power
consumption of the internal circuitry allows the LinkSwitch-TN
tooperatecontinuouslyfromthecurrentdrawnfromtheDRAIN
pin. A bypass capacitor value of 0.1 µF is sufficient for both
high frequency decoupling and energy storage.
300
200
100
0
68 kHz
64 kHz
20
0
In addition, there is a 6.3 V shunt regulator clamping the
BYPASS pin at 6.3 V when current is provided to the BYPASS
Time (µs)
Figure 4. Frequency Jitter.
D
1/04
3
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