TNY263-268
(disable) is generated at the output of the enable circuit. This
enable circuit output is sampled at the beginning of each
cycle on the rising edge of the clock signal. If high, the power
MOSFETisturnedonforthatcycle(enabled). Iflow, thepower
MOSFET remains off (disabled). Since the sampling is done
only at the beginning of each cycle, subsequent changes in the
EN/UV pin voltage or current during the remainder of the
cycle are ignored.
TinySwitch-II Functional Description
TinySwitch-II combines a high voltage power MOSFET switch
withapowersupplycontrollerinonedevice.Unlikeconventional
PWM (pulse width modulator) controllers, TinySwitch-II uses
a simple ON/OFF control to regulate the output voltage.
The TinySwitch-II controller consists of an oscillator,
enable circuit (sense and logic), current limit state machine,
5.8 V regulator, BYPASS pin under-voltage circuit, over-
temperature protection, current limit circuit, leading edge
blanking and a 700 V power MOSFET. TinySwitch-II
incorporates additional circuitry for line under-voltage sense,
auto-restart 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-II 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
EN/UV pin voltage levels 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
maximum duty cycle signal (DCMAX) and the clock signal that
indicates the beginning of each cycle.
Undermostoperatingconditions(exceptwhenclosetono-load),
the low impedance of the source follower keeps the voltage on
the EN/UV pin from going much below 1.0 V in the disabled
state. This improves the response time of the optocoupler that
is usually connected to this pin.
The TinySwitch-II 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
forbothaverageandquasi-peakemissions.Thefrequencyjitter
shouldbemeasuredwiththeoscilloscopetriggeredatthefalling
edge of the DRAIN waveform. The waveform in Figure 4
illustrates the frequency jitter of the TinySwitch-II.
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
ontheDRAINpinwhenevertheMOSFETisoff. TheBYPASS
pin is the internal supply voltage node for the TinySwitch-II.
When the MOSFET is on, the TinySwitch-II operates from the
energy stored in the bypass capacitor. Extremely low power
consumption of the internal circuitry allows TinySwitch-II 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.
Enable Input and Current Limit State Machine
The enable input circuit at the EN/UV pin consists of a low
impedance source follower output set at 1.0 V. The current
through the source follower is limited to 240 µA. When the
current out of this pin exceeds 240 µA, a low logic level
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
pin through an external resistor. This facilitates powering of
TinySwitch-II externally through a bias winding to decrease the
no-load consumption to about 50 mW.
600
500
VDRAIN
400
BYPASS Pin Under-Voltage
300
The BYPASS pin under-voltage circuitry disables the power
MOSFET when the BYPASS pin voltage drops below 4.8 V.
Once the BYPASS pin voltage drops below 4.8 V, it must rise
back to 5.8 V to enable (turn-on) the power MOSFET.
200
100
0
Over Temperature Protection
136 kHz
Thethermalshutdowncircuitrysensesthedietemperature. The
threshold is typically set at 135 °C with 70 °C hysteresis. When
thedietemperaturerisesabovethisthresholdthepowerMOSFET
is disabled and remains disabled until the die temperature falls
by 70 °C, at which point it is re-enabled. A large hysteresis of
128 kHz
0
5
10
Time (µs)
Figure 4. Frequency Jitter.
G
4/05
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