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TNY275GN-TL Datasheet(PDF) 7 Page - Power Integrations, Inc. |
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TNY275GN-TL Datasheet(HTML) 7 Page - Power Integrations, Inc. |
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7 / 24 page  7 TNY274-280 E 2/06 Power Up/Down The TinySwitch-III requires only a 0.1 µF capacitor on the BYPASS/MULTI-FUNCTION pin to operate with standard current limit. Because of its small size, the time to charge this capacitor is kept to an absolute minimum, typically 0.6 ms. The time to charge will vary in proportion to the BYPASS/MULTI- FUNCTIONpincapacitorvaluewhenselectingdifferentcurrent limits. Due to the high bandwidth of the ON/OFF feedback, there is no overshoot at the power supply output. When an external resistor (4 MΩ) is connected from the positive DC input to the EN/UV pin, the power MOSFET switching will be delayed during power-up until the DC line voltage exceeds the threshold (100 V). Figures 10 and 11 show the power-up timing waveform in applications with and without an external resistor (4 MΩ) connected to the EN/UV pin. Understartupandoverloadconditions,whentheconductiontime is less than 400 ns, the device reduces the switching frequency to maintain control of the peak drain current. During power-down, when an external resistor is used, the power MOSFET will switch for 64 ms after the output loses regulation. The power MOSFET will then remain off without any glitches since the under-voltage function prohibits restart when the line voltage is low. Figure 12 illustrates a typical power-down timing waveform. Figure 13 illustrates a very slow power-down timing waveform as in standby applications. The external resistor (4 MΩ) is connected to the EN/UV pin in this case to prevent unwanted restarts. No bias winding is needed to provide power to the chip because it draws the power directly from the DRAIN pin (see Functional Description above). This has two main benefits. First, for a nominal application, this eliminates the cost of a bias winding and associated components. Secondly, for battery charger applications, the current-voltage characteristic often allows the output voltage to fall close to zero volts while still delivering power. TinySwitch-III accomplishes this without a forward bias winding and its many associated components. For applications that require very low no-load power consumption (50 mW), a resistor from a bias winding to the BYPASS/ MULTI-FUNCTION pin can provide the power to the chip. The minimum recommended current supplied is 1 mA. The BYPASS/MULTI-FUNCTION pin in this case will be clamped at 6.4 V. This method will eliminate the power draw from the DRAIN pin, thereby reducing the no-load power consumption and improving full-load efficiency. Current Limit Operation Each switching cycle is terminated when the DRAIN current reaches the current limit of the device. Current limit operation providesgoodlineripplerejectionandrelativelyconstantpower delivery independent of input voltage. BYPASS/MULTI-FUNCTION Pin Capacitor The BYPASS/MULTI-FUNCTION pin can use a ceramic capacitor as small as 0.1 µF for decoupling the internal power supplyofthedevice.Alargercapacitorsizecanbeusedtoadjust the current limit. For TNY275-280, a 1 µF BP/M pin capacitor will select a lower current limit equal to the standard current limit of the next smaller device and a 10 µF BP/M pin capacitor will select a higher current limit equal to the standard current limit of the next larger device. The higher current limit level of the TNY280 is set to 850 mA typical. The TNY274 MOSFET does not have the capability for increased current limit so this feature is not available in this device. |
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