TNY256  
					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  
					powerMOSFETisturnedonforthatcycle(enabled),otherwise  
					thepowerMOSFETremainsoff(disabled). Sincethesampling  
					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 Functional Description  
					TinySwitch combines a high voltage power MOSFET switch  
					withapowersupplycontrollerinonedevice. Unlikeconventional  
					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  
					TNY256incorporatesadditionalcircuitryforLineUnder-Voltage  
					Sense, Auto-Restart 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  
					voltageontheEN/UVpinfromgoingmuchbelow1.5V,inthe  
					disabled state. This improves the response time of the  
					optocoupler that is usually connected to this pin.  
					5.8 V Regulator  
					Oscillator  
					The 5.8 V regulator charges the bypass capacitor connected to  
					theBYPASSpinto5.8Vbydrawingacurrentfromthevoltage  
					on the DRAIN, whenever the MOSFET is off. The BYPASS  
					pin is the internal supply voltage node for the TinySwitch.  
					WhentheMOSFETison,theTinySwitchrunsoffoftheenergy  
					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.  
					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,  
					tominimizeEMIemission. Themodulationrateofthefrequency  
					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  
					theDRAINwaveform. ThewaveforminFigure4illustratesthe  
					frequency jitter of the TNY256.  
					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.  
					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  
					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.  
					600  
					500  
					V
					DRAIN  
					Current Limit  
					400  
					The current limit circuit senses the current in the power  
					MOSFET. When this current exceeds the internal threshold  
					(ILIMIT), the power MOSFET is turned off for the remainder of  
					that cycle.  
					300  
					200  
					100  
					The leading edge blanking circuit inhibits the current limit  
					comparator for a short time (tLEB) after the power MOSFET is  
					turnedon. Thisleadingedgeblankingtimehasbeensetsothat  
					current spikes caused by primary-side capacitance and  
					secondary-side rectifier reverse recovery time will not cause  
					premature termination of the switching pulse.  
					0
					132.5 kHz  
					127.5 kHz  
					.5  
					1
					0
					Time (µs)  
					Auto-Restart  
					Intheeventofafaultconditionsuchasoutputoverload, output  
					Figure 4. Frequency Jitter.  
					B
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