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LM2432 Datasheet(PDF) 6 Page - National Semiconductor (TI) |
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LM2432 Datasheet(HTML) 6 Page - National Semiconductor (TI) |
6 / 13 page Theory of Operation The LM2432 is a high voltage monolithic single channel CRT driver suitable for HDTV applications. The LM2432 typically operates with 220V and 12V power supplies. The part is housed in a staggered 7-lead TO-220 molded plastic power package. The circuit diagram of the LM2432 is shown in Figure 2. The PNP emitter follower, Q5, provides input buffering. Q1 and Q2 form a fixed gain cascode amplifier with resistors R1 and R2 setting the gain at −53. Emitter followers Q3 and Q4 isolate the high output impedance of the cascode stage from the capacitance of the CRT cathode which decreases the sensitivity of the device to load capacitance. Q6 provides biasing to the output emitter follower stage to reduce cross- over distortion at low signal levels. The LM2432 has an IK current output (pin 5) that produces a replica of the actual cathode current into V OUT (pin 2). The IK output pin is internally connected to the collector of Q4. If IK feedback is not used in the application, the IK pin should be connected to the same ground as pin 3 (GND). Otherwise, the IK output can interface with a RGB video processor with IK feedback for automatic cathode calibration. Note: During the non-blanking period, video current levels can be as high as several mA, which is much higher than the reference currents (in µA range) produced during the IK measurement interval. These high currents have the potential to produce large voltages at the IK output pin. To avoid damage to Q4, the IK output voltage, V IK, must not exceed +16V (VIKMAX). Please see the section “Cathode Current Output for IK Feed- back Systems” for more information on the usage and pro- tection of the IK output. Application Hints INTRODUCTION National Semiconductor (NSC) is committed to provide ap- plication information that assists our customers in obtaining the best performance possible from our products. The fol- lowing information is provided in order to support this com- mitment. The reader should be aware that the optimization of performance was done using a specific printed circuit board designed at NSC. Variations in performance can be realized due to physical changes in the printed circuit board and the application. Therefore, the designer should know that com- ponent value changes may be required in order to optimize performance in a given application. The values shown in this document can be used as a starting point for evaluation purposes. When working with high bandwidth circuits, good layout practices are also critical to achieving maximum per- formance. IMPORTANT INFORMATION The LM2432 performance is targeted for the HDTV market. The application circuits shown in this document to optimize performance and to protect against damage from CRT ar- cover are designed specifically for the LM2432. If another member of the NSC DTV CRT Driver family is used, please refer to its data sheet. POWER SUPPLY BYPASS Since the LM2432 is a wide bandwidth amplifier, proper power supply bypassing is critical for optimum performance and for robustness against arcover. Improper power supply bypassing can result in large overshoot, ringing or oscilla- tion, and even arcover failure. 0.1 µF capacitors should be connected from the supply pins, V CC and VBB, to ground using very short traces. Additionally, a 10 µF or larger elec- trolytic capacitor should be connected from both supply pins to ground reasonably close to the LM2432. ARC PROTECTION During normal CRT operation, internal arcing may occasion- ally occur. This fast, high voltage, high energy pulse can damage the LM2432 output stage since it is DC coupled to the cathode. In a DC coupled application, an external spark gap with an arcover voltage rating of 200 to 300V DC on the cathode is NOT recommended. The internal CRT socket spark gap (1 to 2 kV DC rating) can sufficiently reduce the initial arcover voltage seen at the cathode. The output circuit shown in Figure 13 is designed to help clamp the voltage at the output of the LM2432 to a safe level during an arcover. External arc protection clamp diodes, D1 and D2, should have a fast transient response, high peak current rating, low series impedance and low shunt capacitance. 1SS83 or equivalent diodes like BAV21 are recommended. D1 and D2 should have short, low impedance connections to V CC and ground respectively. The cathode of D1 should have a very short connection to a separate V CC bypass capacitor, C3. The ground connection of D2 and the C3 should have a short, direct path to ground. This will significantly reduce the high frequency voltage transients that the LM2432 would be subjected to during an arcover. Resistor R2, which limits the arcover current that is seen by the diodes, should be a 1⁄2W solid carbon type resistor. R1 limits the current into the LM2432 as well as the voltage stress at the outputs of the device and can be a 1⁄4W metal or carbon film type resistor. Having large value resistors for R1 and R2 would be desirable, but this has the effect of increas- ing rise and fall times. Inductor L1 is critical to reduce the initial high frequency voltage levels that the LM2432 would be subjected to. The inductor will not only help protect the device but it will also help minimize rise and fall times as well as minimize EMI. For proper arc protection, it is important to not omit any of the arc protection components shown in Figure 13. www.national.com 6 |
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