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MC33065DW-H Folha de dados(PDF) 9 Page - ON Semiconductor |
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MC33065DW-H Folha de dados(HTML) 9 Page - ON Semiconductor |
9 / 18 page MC34065−H, L http://onsemi.com 9 When designing a high power switching regulator it may be desirable to reduce the internal clamp voltage in order to keep the power dissipation of RS to a reasonable level. A simple method to adjust this voltage is shown in Figure 19. The two external diodes are used to compensate the internal diodes, yielding a constant clamp voltage over temperature. Erratic operation due to noise pickup can result if there is an excessive reduction of the Ipk(max) clamp voltage. A narrow spike on the leading edge of the current waveform can usually be observed and may cause the power supply to exhibit an instability when the output is lightly loaded. This spike is due to the power transformer interwinding capacitance and output rectifier recovery time. The addition of an RC filter on the Current Sense input with a time constant that approximates the spike duration will usually eliminate the instability, refer to Figure 24. Undervoltage Lockout Two Undervoltage Lockout comparators have been incorporated to guarantee that the IC is fully functional before the output stages are enabled. The positive power supply terminal (VCC) and the reference output (Vref) are each monitored by separate comparators. Each has built−in hysteresis to prevent erratic output behavior as their respective thresholds are crossed. The VCC comparator upper and lower thresholds are 14 V/10 V for −H suffix, and 8.4 V/7.6 V for −L suffix. The Vref comparator upper and lower thresholds are 3.6 V/3.4 V respectively. The large hysteresis and low startup current of the −H suffix version makes it ideally suited in off−line converter applications where efficient bootstrap startup techniques are required (Figure 28). The −L suffix version is intended for lower voltage dc−to−dc converter applications. The minimum operating voltage for the −H suffix is 11 V and 8.2 V for the −L suffix. Drive Outputs and Drive Ground Each section contains a single totem−pole output stage that is specifically designed for direct drive of power MOSFETs. The Drive Outputs are capable of up to ±400 mA peak current with a typical rise and fall time of 50 ns with a 1.0 nF load. Additional internal circuitry has been added to keep the outputs in a sinking mode whenever an Undervoltage Lockout is active. This characteristic eliminates the need for an external pull−down resistor. The totem−pole output has been optimized to minimize cross−conduction current in high speed operation. The addition of two 10 Ω resistors, one in series with the source output transistor and one in series with the sink output transistor, reduces the cross−conduction current to minimal levels, as shown in Figure 13. Although the Drive Outputs were optimized for MOSFETs, they can easily supply the negative base current required by bipolar NPN transistors for enhanced turn−off (Figure 25). |
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