TNY274-280
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 71 μA and ꢀꢀ1 μA.
measured with the oscilloscope triggered at the falling edge of
the DRAIN waveform. The waveform in Figure 4 illustrates the
frequency jitter.
Enable Input and Current Limit State Machine
The EN/UV pin also senses line undervoltage conditions through
an external resistor connected to the DC line voltage. If there is
no external resistor connected to this pin, TinySwitch-III detects
its absence and disables the line undervoltage function.
The enable input circuit at the EN/UV pin consists of a low
impedance source follower output set at ꢀ.2 V. The current
through the source follower is limited to ꢀꢀ1 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,
SOURCE (S) Pin:
This pin is internally connected to the output MOSFET source for until the current out of this pin is reduced to less than the
high voltage power return and control circuit common.
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/UV pin voltage or current during
the remainder of the cycle are ignored.
TinySwitch-III Functional Description
TinySwitch-III combines a high voltage power MOSFET switch
with a power supply controller in one device. Unlike conventional
PWM (pulse width modulator) controllers, it uses a simple
ON/OFF control to regulate the output voltage.
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
and reduces the transformer flux density, including the
associated 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.
The controller consists of an oscillator, enable circuit (sense and
logic), current limit state machine, 1.81 V regulator, BYPASS/
MULTI-FUNCTION pin undervoltage, overvoltage circuit, 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
undervoltage sense, auto-restart, adaptive switching cycle on-
time extension, and frequency jitter. Figure 2 shows the
functional block diagram with the most important features.
Under most operating conditions (except when close to no-
load), the low impedance of the source follower keeps the
voltage on the EN/UV pin from going much below ꢀ.2 V in the
disabled state. This improves the response time of the
optocoupler that is usually connected to this pin.
Oscillator
The typical oscillator frequency is internally set to an average of
ꢀ32 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.
5.85 V Regulator and 6.4 V Shunt Voltage Clamp
The 1.81 V regulator charges the bypass capacitor connected
to the BYPASS pin to 1.81 V by drawing a current from the
voltage on the DRAIN pin whenever the MOSFET is off. The
BYPASS/MULTI-FUNCTION pin is the internal supply voltage
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.ꢀ μF is sufficient for both high
frequency decoupling and energy storage.
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 ꢀ kHz to optimize EMI reduction for both average
and quasi-peak emissions. The frequency jitter should be
600
500
VDRAIN
400
In addition, there is a 6.4 V shunt regulator clamping the
BYPASS/MULTI-FUNCTION pin at 6.4 V when current is
provided to the BYPASS/MULTI-FUNCTION pin through an
external resistor. This facilitates powering of TinySwitch-III
externally through a bias winding to decrease the no-load
consumption to well below 10 mW.
300
200
100
0
BYPASS/MULTI-FUNCTION Pin Undervoltage
136 kHz
The BYPASS/MULTI-FUNCTION pin undervoltage circuitry
disables the power MOSFET when the BYPASS/MULTI-
FUNCTION pin voltage drops below 4.9 V in steady state
operation. Once the BYPASS/MULTI-FUNCTION pin voltage
drops below 4.9 V in steady state operation, it must rise back to
1.81 V to enable (turn-on) the power MOSFET.
128 kHz
0
5
10
Time (μs)
Figure 4. Frequency Jitter.
3
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Rev. I 01/09
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