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TNY256P Datasheet(PDF) 4 Page - Power Integrations, Inc. |
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TNY256P Datasheet(HTML) 4 Page - Power Integrations, Inc. |
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4 / 20 page  B 8/99 TNY256 4 short, or an open loop condition, TNY256 enters into auto- restart operation. An internal counter clocked by the oscillator gets reset every time the EN/UV pin is pulled low. If the EN/ UV pin is not pulled low for 32 ms, the power MOSFET switching is disabled for 128 ms (except in the case of line under-voltage condition). The auto-restart alternately enables and disables the switching of the power MOSFET until the fault condition is removed. Figure 5 illustrates auto-restart circuit operation in the presence of a temporary output short. In the event of line under-voltage condition, the switching of the power MOSFET is disabled beyond its normal 128 ms time until the line under-voltage condition goes away. Line Under-Voltage (UVLO) Sense Circuit The DC line voltage can be monitored by connecting an external resistor from the DC line to the EN/UV pin. During power-up or when the switching of the power MOSFET is disabled in auto-restart, the current into the EN/UV pin must exceed 50 µA to initiate switching of the power MOSFET. During power-up, this is implemented by holding the BYPASS pin to 5.1 V while the line under-voltage condition exists. The BYPASS pin then rises from 5.1 V to 5.8V when the line under- voltage condition goes away. When the switching of the power MOSFET is disabled in auto-restart mode and the line under- voltage condition exists, the counter is stopped. This stretches the disable time beyond its normal 128ms until the line under- voltage condition goes away. The line under-voltage circuit also detects when there is no external resistor connected to the EN/UV pin. In this case the line under-voltage function is disabled. TinySwitch Operation TinySwitch devices operate in the current limit mode. When enabled, the oscillator turns the power MOSFET on at the beginning of each cycle. The MOSFET is turned off when the current ramps up to the current limit. The maximum on-time of the MOSFET is limited to DC MAX by the oscillator. Since the current limit and frequency of the TNY256 is constant, the power delivered is proportional to the primary inductance of the transformer and is relatively independent of the input voltage. Therefore, the design of the power supply involves calculating the primary inductance of the transformer for the maximum power required. If the TNY256 is appropriately chosen for the PI-2377-120998 V DRAIN V EN CLOCK D DRAIN I MAX Figure 5. TNY256 Auto-Restart Operation. Figure 6. TNY256 Operation at Heavy Load. Figure 7. TNY256 Operation at Light Load. V DRAIN V EN CLOCK D DRAIN I MAX PI-2373-120998 0 250 500 Time (ms) 0 10 0 20 100 200 300 DRAIN VOLTAGE OUTPUT VOLTAGE |
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