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TEA1552T Datasheet(PDF) 9 Page - NXP Semiconductors |
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TEA1552T Datasheet(HTML) 9 Page - NXP Semiconductors |
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9 / 24 page  2002 Aug 27 9 Philips Semiconductors Product specification GreenChip ™II SMPS control IC TEA1552 As soon as the oscillator voltage is high again and the secondary stroke has ended, the circuit waits for the lowest drain voltage before starting a new primary stroke. This method is called valley detection. Figure 8 shows the drain voltage together with the valley signal, the signal indicating the secondary stroke and the oscillator signal. In an optimum design, the reflected secondary voltage on the primary side will force the drain voltage to zero. Thus, zero voltage switching is very possible, preventing large capacitive switching losses , and allowing high frequency operation, which results in small and cost effective inductors. OverCurrent Protection (OCP) The cycle-by-cycle peak drain current limit circuit uses the external source resistor to measure the current accurately. This allows optimum size determination of the transformer core (cost issue). The circuit is activated after the leading edge blanking time tleb. The OCP protection circuit limits the ‘sense’ voltage to an internal level. OverPower Protection (OPP) During the primary stroke, the rectified mains input voltage is measured by sensing the current drawn from pin DEM. This current is dependent on the mains voltage, according to the following formula: where: The current information is used to adjust the peak drain current, which is measured via pin Isense. The internal compensation is such that an almost mains independent maximum output power can be realized. The OPP curve is given in Fig.9. Minimum and maximum ‘on-time’ The minimum ‘on-time’ of the SMPS is determined by the Leading Edge Blanking (LEB) time. The IC limits the ‘on-time’ to 50 µs. When the system desires an ‘on-time’ longer than 50 µs, a fault condition is assumed, and the IC will stop switching and enter the safe restart mode. Short winding protection After the leading edge blanking time, the short winding protection circuit is also activated. If the ‘sense’ voltage exceeds the short winding protection voltage Vswp, the converter will stop switching. Once VCC drops below the UVLO level, capacitor CVCC will be recharged and the supply will restart again. This cycle will be repeated until the short-circuit is removed (safe restart mode). The short winding protection will also protect in case of a secondary diode short-circuit. P 1 2 --- CV 2 f × × × = I DEM V aux R DEM --------------- NV mains × R DEM -------------------------- ≈≈ N N aux N p ------------ = handbook, halfpage MGU236 0.52 V (typ) 0.3 V (typ) IDEM Vsense(max) −24 µA (typ) −100 µA (typ) Fig.9 OPP correction curve. |
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