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MC33065DW Folha de dados(PDF) 9 Page - ON Semiconductor |
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MC33065DW Folha de dados(HTML) 9 Page - ON Semiconductor |
9 / 15 page MC34065 MC33065 9 MOTOROLA ANALOG IC DEVICE DATA 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 and 10 V respectively. The hysteresis and low startup current makes these devices ideally suited to off–line converter applications where efficient bootstrap startup techniques are required (Figure 28). The Vref comparator disables the Drive Outputs until the internal circuitry is functional. This comparator has upper and lower thresholds of 3.6 V and 3.4 V. A 17 V zener is connected as a shunt regulator from VCC to ground. Its purpose is to protect the IC and power MOSFET gate from excessive voltage that can occur during system startup. The guaranteed minimum operating voltage after turn–on is 11 V. Drive Outputs and Drive Ground Each channel 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 ±1.0 A peak current with a typical rise and fall time of 28 ns with a 1.0 nF load. 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. Cross–conduction current in the totem–pole output stage has been minimized for high speed operation, as shown in Figure 13. The average added power due to cross–conduction with VCC = 15 V is only 60 mW at 500 kHz. 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). The outputs do not contain internal current limiting, therefore an external series resistor may be required to prevent the peak output current from exceeding the 1.0 A maximum rating. The sink saturation (VOL) is less than 0.4 V at 100 mA. A separate Drive Ground pin is provided and, with proper implementation, will significantly reduce the level of switching transient noise imposed on the control circuitry. This becomes particularly useful when reducing the Ipk(max) clamp level. Figure 23 shows the proper ground connections required for current sensing power MOSFET applications. Drive Output 2 Enable Pin This input is used to enable Drive Output 2. Drive Output 1 can be used to control circuitry that must run continuously such as volatile memory and the system clock, or a remote controlled receiver, while Drive Output 2 controls the high power circuitry that is occasionally turned off. Reference The 5.0 V bandgap reference is trimmed to ±2.0% tolerance at TJ = 25°C. The reference has short circuit protection and is capable of providing in excess of 30 mA for powering any additional control system circuitry. Design Considerations Do not attempt to construct the converter on wire–wrap or plug–in prototype boards. High frequency circuit layout techniques are imperative to prevent pulse–width jitter. This is usually caused by excessive noise pick–up imposed on the Current Sense or Voltage Feedback inputs. Noise immunity can be improved by lowering circuit impedances at these points. The printed circuit layout should contain a ground plane with low current signal and high current switch and output grounds returning on separate paths back to the input filter capacitor. Ceramic bypass capacitors (0.1 µF) connected directly to VCC and Vref may be required depending upon circuit layout. This provides a low impedance path for filtering the high frequency noise. All high current loops should be kept as short as possible using heavy copper runs to minimize radiated EMI. The Error Amp compensation circuitry and the converter output voltage–divider should be located close to the IC and as far as possible from the power switch and other noise generating components. |
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