LNK353/354
pin through an external resistor. This facilitates powering of
LinkSwitch-HF externally through a bias winding to decrease
the no-load consumption to less than 50 mW.
LinkSwitch-HF Functional
Description
LinkSwitch-HF combines a high voltage power MOSFET
switch with a power supply controller in one device. Unlike
conventional PWM (pulse width modulator) controllers,
LinkSwitch-HF uses a simple ON/OFF control to regulate the
output voltage. The LinkSwitch-HF controller consists of an
oscillator, feedback (sense and logic) circuit, 5.8 V regulator,
BYPASSpinunder-voltagecircuit,over-temperatureprotection,
frequency jittering, current limit circuit, leading edge blanking
and a 700Vpower MOSFET. The LinkSwitch-HF incorporates
additional circuitry 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
The thermal shutdown circuitry senses the die temperature.
The threshold is set at 142 °C typical with a 75 °C hysteresis.
Whenthedietemperaturerisesabovethisthreshold(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 200 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-HF oscillator incorporates circuitry that
introduces a small amount of frequency jitter, typically 16 kHz
peak-to-peak,tominimizeEMIemission.Themodulationrateof
the frequency jitter is set to 1.5 kHz to optimize EMI reduction
for both average and quasi-peak emissions. The frequency
jitter 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-HF.
Auto-Restart
In the event of a fault condition such as output overload, output
short circuit, or an open loop condition, LinkSwitch-HF 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 30 ms, the power MOSFET
switching is disabled for 650 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
impedancesourcefolloweroutputsetat1.65V.Whenthecurrent
deliveredintothispinexceeds49µA,alowlogiclevel(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
thatcycle(enabled), otherwisethepowerMOSFETremainsoff
(disabled). Since the sampling is done only at the beginning of
each cycle, subsequent changes in the FB pin voltage or current
during the remainder of the cycle are ignored.
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-HF. When the MOSFET is on, the LinkSwitch-HF
runs off of the energy stored in the bypass capacitor. Extremely
low power consumption of the internal circuitry allows the
LinkSwitch-HF to operate continuously from the current drawn
fromtheDRAINpin.Abypasscapacitorvalueof0.1µFissufficient
for both high frequency decoupling and energy storage.
300
200
100
0
208 kHz
192 kHz
6.4
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.
F
2/05
3
POWERINT [ Power Integrations ]
