B

8/99

TNY256

3

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.

Oscillator

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 (DC

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

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.

MOSFET is disabled and remains disabled until the die

Current Limit

The current limit circuit senses the current in the power

MOSFET. When this current exceeds the internal threshold

(I

that cycle.

The leading edge blanking circuit inhibits the current limit

comparator for a short time (t

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.

Auto-Restart

In the event of a fault condition such as output overload, output

Figure 4. Frequency Jitter.

0

.5

1

Time (

µs)

0

100

200

400

500

600

300

V

DRAIN

132.5 kHz

127.5 kHz