TinySwitch Functional Description
TinySwitch combines a high voltage power MOSFET switch
with a power supply controller in one device. Unlike conventional
PWM (Pulse Width Modulator) controllers, TinySwitch uses a
simple ON/OFF control to regulate the output voltage.
The TNY256 controller consists of an Oscillator, Enable (Sense
and Logic) circuit, 5.8 V Regulator, Bypass pin Under-Voltage
circuit, Over Temperature Protection, Current Limit circuit,
Leading Edge Blanking and a 700 V power MOSFET. The
TNY256 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 typical oscillator frequency is internally set to an average of
130 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.
The TNY256 oscillator incorporates circuitry that introduces a
small amount of frequency jitter, typically 5 kHz peak-to-peak,
to minimize EMI emission. The modulation rate of the frequency
jitter (1 kHz) is set 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 Figure4 illustrates the
frequency jitter of the TNY256.
Enable Input Circuit
The enable input circuit at the EN/UV pin consists of a low
impedance source follower output set at 1.5 V. The current
through the source follower is limited to 50 µA with 10 µA of
hysteresis. When the current drawn out of the this pin exceeds
Figure 4. Frequency Jitter.
50 µA, a low logic level (disable) is generated at the output of
the enable 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 EN/UV pin voltage or current during the
remainder of the cycle are ignored.
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 1.5 V, in the
disabled state. This improves the response time of the
optocoupler that is usually connected to this pin.
5.8 V Regulator
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 TinySwitch.
When the MOSFET is on, the TinySwitch runs off of the energy
stored in the bypass capacitor. Extremely low power
consumption of the internal circuitry allows the TinySwitch to
operate continuously from the current drawn from the DRAIN
pin. A bypass capacitor value of 0.1 µF is sufficient for both
high frequency de-coupling and energy storage.
BYPASS Pin Under-Voltage
The BYPASS pin under-voltage circuitry disables the power
MOSFET when the BYPASS pin voltage drops below 5.1 V.
Once the BYPASS pin voltage drops below 5.1 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 135 oC with 70 oC hysteresis. When the
die temperature rises above this threshold (135 oC) the power
MOSFET is disabled and remains disabled until the die
temperature falls by 70 oC, at which point it is re-enabled.
The current limit circuit senses the current in the power
MOSFET. When this current exceeds the internal threshold
(I ), the power MOSFET is turned off for the remainder of
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 primary-side capacitance and
secondary-side rectifier reverse recovery time will not cause
premature termination of the switching pulse.
In the event of a fault condition such as output overload, output