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MC34262 bảng dữ liệu(PDF) 8 Page - ON Semiconductor |
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MC34262 bảng dữ liệu(HTML) 8 Page - ON Semiconductor |
8 / 16 page MC34262 MC33262 8 MOTOROLA ANALOG IC DEVICE DATA Figure 18. Inductor Current and MOSFET Gate Voltage Waveforms Inductor Current Average MOSFET Q1 On Off 0 Peak Current Sense Comparator and RS Latch The Current Sense Comparator RS Latch configuration used ensures that only a single pulse appears at the Drive Output during a given cycle. The inductor current is converted to a voltage by inserting a ground–referenced sense resistor R7 in series with the source of output switch Q1. This voltage is monitored by the Current Sense Input and compared to a level derived from the Multiplier output. The peak inductor current under normal operating conditions is controlled by the threshold voltage of Pin 4 where: Pin 4 Threshold R7 IL(pk ) = Abnormal operating conditions occur during preconverter startup at extremely high line or if output voltage sensing is lost. Under these conditions, the Multiplier output and Current Sense threshold will be internally clamped to 1.5 V. Therefore, the maximum peak switch current is limited to: 1.5 V R7 Ipk(max) = An internal RC filter has been included to attenuate any high frequency noise that may be present on the current waveform. This filter helps reduce the ac line current distortion especially near the zero crossings. With the component values shown in Figure 20, the Current Sense Comparator threshold, at the peak of the haversine varies from 1.1 V at 90 Vac to 100 mV at 268 Vac. The Current Sense Input to Drive Output turn–off propagation delay is typically less than 200 ns. Timer A watchdog timer function was added to the IC to eliminate the need for an external oscillator when used in stand–alone applications. The Timer provides a means to automatically start or restart the preconverter if the Drive Output has been off for more than 620 µs after the inductor current reaches zero. The restart time delay versus temperature is shown in Figure 8. Undervoltage Lockout and Quickstart An Undervoltage Lockout comparator has been incorporated to guarantee that the IC is fully functional before enabling the output stage. The positive power supply terminal (VCC) is monitored by the UVLO comparator with the upper threshold set at 13 V and the lower threshold at 8.0 V. In the stand–by mode, with VCC at 7.0 V, the required supply current is less than 0.4 mA. This large hysteresis and low startup current allow the implementation of efficient bootstrap startup techniques, making these devices ideally suited for wide input range off–line preconverter applications. An internal 36 V clamp has been added from VCC to ground to protect the IC and capacitor C4 from an overvoltage condition. This feature is desirable if external circuitry is used to delay the startup of the preconverter. The supply current, startup, and operating voltage characteristics are shown in Figures 13 and 14. A Quickstart circuit has been incorporated to optimize converter startup. During initial startup, compensation capacitor C1 will be discharged, holding the error amp output below the Multiplier threshold. This will prevent Drive Output switching and delay bootstrapping of capacitor C4 by diode D6. If Pin 2 does not reach the multiplier threshold before C4 discharges below the lower UVLO threshold, the converter will “hiccup” and experience a significant startup delay. The Quickstart circuit is designed to precharge C1 to 1.7 V, Figure 7. This level is slightly below the Pin 2 Multiplier threshold, allowing immediate Drive Output switching and bootstrap operation when C4 crosses the upper UVLO threshold. Drive Output The MC34262/MC33262 contain a single totem–pole output stage specifically designed for direct drive of power MOSFETs. The Drive Output is capable of up to ± 500 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 Drive Output in a sinking mode whenever the Undervoltage Lockout is active. This characteristic eliminates the need for an external gate pull–down resistor. The totem–pole output has been optimized to minimize cross–conduction current during 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, helps to reduce the cross–conduction current and radiated noise by limiting the output rise and fall time. A 16 V clamp has been incorporated into the output stage to limit the high state VOH. This prevents rupture of the MOSFET gate when VCC exceeds 20 V. |
Số phần tương tự - MC34262 |
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Mô tả tương tự - MC34262 |
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