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TNY256P Datasheet(PDF) 6 Page - Power Integrations, Inc. |
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TNY256P Datasheet(HTML) 6 Page - Power Integrations, Inc. |
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6 / 20 page  C 7/01 TNY256 6 Figure 10. Normal Power-down Timing. Figure 11. Slow Power-down Timing with External (2 M Ω) Resistor Connected to EN/UV Pin. 0 .5 1 Time (s) 0 100 200 300 0 100 200 400 V DC-BUS V DRAIN 0 2.5 5 Time (s) 0 100 200 400 300 0 100 200 V DC-BUS V DRAIN During power-down, when an external resistor is used, the power MOSFET will switch for 32 ms after the output loses regulation. The power MOSFET will then remain off without any glitches since the under-voltage function prohibits restarts when the line voltage is low. Figure 10 illustrates a typical power-down timing waveform of TNY256. Figure 11 illustrates a very slow power-down timing waveform of TNY256 as in standby applications. The external resistor (2 M Ω) is connected to the EN/UV pin in this case to prevent restarts. The TNY256 does not require a bias winding 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 an extra bias winding and associated components. Secondly, for charger applications, the current-voltage characteristic often allows the output voltage to fall to low values while still delivering power. This type of application normally requires a forward-bias winding which has many more associated components, none of which are necessary with the TNY256. Current Limit Operation Each switching cycle is terminated when the DRAIN current reaches the current limit of the TNY256. For a given primary inductance and input voltage, the duty cycle is constant. However, the duty cycle does change inversely with the input voltage providing “voltage feed-forward” advantages: good line ripple rejection and relatively constant power delivery independent of the input voltage. BYPASS Pin Capacitor The BYPASS pin uses a small 0.1 µF ceramic capacitor for decoupling the internal power supply of the TNY256. Application Example The TNY256 is ideal for low cost, high efficiency power supplies in a wide range of applications such as PC standby, cellular phone chargers, AC adapters, motor control, appliance control and ISDN network termination. The 130kHz operation allows the use of a low cost EE16 core transformer while still providing good efficiency. The frequency jitter in TNY256 makes it possible to use a single inductor (or two small resistors if lower efficiency is acceptable) in conjunction with two input capacitors for input EMI filtering up to the 10W level. The auto-restart function allows the design to be optimized for maximum efficiency without consideration for short-circuit current on the secondary. For applications requiring under- voltage lockout (UVLO), the TNY256 eliminates several components and saves cost. As an example, Figure12 shows a 9V, 0.6A, AC adapter operating from a universal input range (85-265VAC). The AC input is rectified and filtered by D1-D4, C1 and C2 to create a high voltage DC bus which is connected to T1. Inductor L1 forms a pi-filter in conjunction with C1 and C2. The resistor R1 damps resonance in inductor L1. The frequency jitter in TNY256 allows it to meet worldwide conducted EMI standards using a simple pi-filter in combination with a small value Y1-capacitor C5 and a shield winding between primary and secondary windings inside transformer T1. Diode D5, capacitor C3 and resistor R3 form an RCD clamp circuit that limits the turn-off voltage spike to a safe value on the TNY256 DRAIN pin. The secondary winding is rectified and filtered by D6, C6 and C7 to provide the 9V output. Additional filtering is provided by L3 and C8. The output voltage is determined by the resistor network R7 and R8. Resistor R9 maintains a bias current |
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