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TNY375-380 Datasheet(PDF) 5 Page - Power Integrations, Inc. |
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TNY375-380 Datasheet(HTML) 5 Page - Power Integrations, Inc. |
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5 / 20 page  Rev. A 05/07 5 TNY375-380 www.powerint.com Enable Function TinySwitch-PK senses the EN/UV pin to determine whether or not to proceed with the next switching cycle. The sequence of cycles is used to determine the current limit. Once a cycle is started, it always completes the cycle (even when the EN/UV pin changes state halfway through the cycle). This operation results in a power supply in which the output voltage ripple is determined by the output capacitor, amount of energy per switch cycle, and the delay of the feedback. The EN/UV pin signal is generated on the secondary by comparing the power supply output voltage with a reference voltage. The EN/UV pin signal is high when the power supply output voltage is less than the reference voltage. In a typical implementation, the EN/UV pin is driven by an optocoupler. The collector of the optocoupler transistor is connected to the EN/UV pin, and the emitter is connected to the SOURCE pin. The optocoupler LED is connected in series with a Zener diode across the DC output voltage to be regulated. When the output voltage exceeds the target regulation voltage level (optocoupler LED voltage drop plus Zener voltage), the optocoupler LED will start to conduct, pulling the EN/UV pin low. The Zener diode can be replaced by a TL431 reference circuit for improved accuracy. ON/OFF Operation with Current Limit State Machine The internal clock of the TinySwitch-PK runs at all times. At the beginning of each clock cycle, it samples the EN/UV pin to decide whether or not to implement a switch cycle, and based on the sequence of samples over multiple cycles, it determines the appropriate current limit. At high loads, the state machine sets the current limit to its highest value. With TinySwitch-PK, when the state machine sets the current limit to its highest value, the oscillator frequency is also doubled, providing the unique peak mode operation. At lighter loads, the state machine sets the current limit to reduced values. At these lower current limit levels, the oscillator frequency returns to the standard value. At near maximum load, TinySwitch-PK will conduct during nearly all of its clock cycles (Figure 6). At slightly lower load, it will “skip” additional cycles in order to maintain voltage regulation at the power supply output (Figure 7). At medium loads, more cycles will be skipped, the current limit will be V DRAIN V EN CLOCK DC DRAIN I MAX PI-2749-082305 V DRAIN V EN CLOCK DC DRAIN I MAX PI-2667-082305 Figure 6. Operation at Near Maximum Loading (f OSC 264 kHz). Figure 7. Operation at Moderately Heavy Loading (f OSC 264 kHz). PI-4540-050407 V DRAIN V EN CLOCK DC DRAIN I MAX Figure 8. Operation at Medium Loading (f OSC 132 kHz). |
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