TNY274-280
MOSFET is controlled by this pin. MOSFET switching is
terminated when a current greater than a threshold current is
drawn from this pin. Switching resumes when the current being
pulled from the pin drops to less than a threshold current. A
modulation of the threshold current reduces group pulsing. The
threshold current is between 60 µA and 115 µA.
maximum duty cycle signal (DCMAX) and the clock signal that
indicates the beginning of each cycle.
The oscillator incorporates circuitry that introduces a small
amount of frequency jitter, typically 8 kHz peak-to-peak, to
minimize EMI emission. The modulation rate of the frequency
jitter is set to 1 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.
TheEN/UVpinalsosenseslineunder-voltageconditionsthrough
an external resistor connected to the DC line voltage. If there is
noexternalresistorconnectedtothispin, TinySwitch-IIIdetects
its absence and disables the line under-voltage function.
SOURCE (S) Pin:
Enable Input and Current Limit State Machine
This pin is internally connected to the output MOSFET source
for high voltage power return and control circuit common.
The enable input circuit at the EN/UV pin consists of a low
impedance source follower output set at 1.2 V. The current
through the source follower is limited to 115 µA. When the
current out of this pin exceeds the threshold current, a low
logic level (disable) is generated at the output of the enable
circuit, until the current out of this pin is reduced to less than
the threshold current. This enable circuit 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). If low, the power MOSFET remains off (disabled).
Since the sampling is done only at the beginning of each cycle,
subsequent changes in the EN/UVpin voltage or current during
the remainder of the cycle are ignored.
TinySwitch-III Functional
Description
TinySwitch-IIIcombinesahighvoltagepowerMOSFETswitch
withapowersupplycontrollerinonedevice.Unlikeconventional
PWM (pulse width modulator) controllers, it uses a simple
ON/OFF control to regulate the output voltage.
Thecontrollerconsistsofanoscillator,enablecircuit(senseand
logic), current limit state machine, 5.85 V regulator, BYPASS/
MULTI-FUNCTIONpinunder-voltage,overvoltagecircuit,and
current limit selection circuitry, over- temperature protection,
current limit circuit, leading edge blanking, and a 700 V power
MOSFET. TinySwitch-III incorporates additional circuitry for
line under-voltage sense, auto-restart, adaptive switching cycle
on-time extension, and frequency jitter. Figure 2 shows the
functional block diagram with the most important features.
The current limit state machine reduces the current limit by
discrete amounts at light loads when TinySwitch-III is likely to
switch in the audible frequency range. The lower current limit
raises the effective switching frequency above the audio range
andreducesthetransformerfluxdensity,includingtheassociated
audible noise. The state machine monitors the sequence of
enable events to determine the load condition and adjusts the
current limit level accordingly in discrete amounts.
Oscillator
The typical oscillator frequency is internally set to an average
of 132 kHz. Two signals are generated from the oscillator: the
Undermostoperatingconditions(exceptwhenclosetono-load),
the low impedance of the source follower keeps the voltage on
the EN/UV pin from going much below 1.2 V in the disabled
state. This improves the response time of the optocoupler that
is usually connected to this pin.
600
500
VDRAIN
400
5.85 V Regulator and 6.4 V Shunt Voltage Clamp
The 5.85 V regulator charges the bypass capacitor connected
to the BYPASS pin to 5.85 V by drawing a current from the
voltage on the DRAIN pin whenever the MOSFET is off. The
BYPASS/MULTI-FUNCTIONpinistheinternalsupplyvoltage
node. When the MOSFET is on, the device operates from the
energy stored in the bypass capacitor. Extremely low power
consumption of the internal circuitry allows TinySwitch-III to
operate continuously from current it takes from the DRAIN
pin. A bypass capacitor value of 0.1 µF is sufficient for both
high frequency decoupling and energy storage.
300
200
100
0
136 kHz
128 kHz
0
5
10
Time (µs)
Figure 4. Frequency Jitter.
E
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POWERINT [ Power Integrations ]
