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LM2485 Datasheet(PDF) 4 Page - National Semiconductor (TI) |
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LM2485 Datasheet(HTML) 4 Page - National Semiconductor (TI) |
4 / 8 page Theory of Operation The pinout and internal block diagram of the LM2485 is shown in Figure 1. The LM2485 is a triple channel bias clamp amplifier used to DC restore the AC coupled outputs of a DTV CRT driver and for cut-off adjustment. The clamp circuit amplifies the DC inputs, V IN, by the internally fixed gain of –25. Each DC clamp output, V OUT, will require a pull-up resistor to VCC. The clamp DC transfer function is shown in Figure 4. The LM2485 also has an integrated blanking amplifier that takes a positive-going 5V input pulse, VB IN, and outputs a negative-going pulse, VB OUT, with a selectable amplitude of either 20V P-P or 40VP-P via the Pin 13 (S40) option. If Pin 13 is floated, the VB OUT pulse amplitude is 20VP-P; if Pin 13 is grounded, the amplitude is 40V P-P (see Figure 5). With a simple clamp circuit, this inverted pulse can be used to drive the G1 grid of a CRT negative to blank the horizontal retrace lines during the vertical flyback period. The input pulse can be generated from a vertical flyback pulse or by a microcon- troller. The LM2485 can operate with a V CC supply of up to 220V and a selectable V BB supply of either 8V or 12V via the Pin 12 (S12) option. If Pin 12 is floated, then 8V is expected at the V BB pin; if Pin 12 is grounded, then 12V is expected. This V BB option ensures the LM2485 is compatible with any National Semiconductor DTV CRT driver sharing the same operating voltages in the application. Note that using a V BB supply of 12V will increase the input offset of the clamp circuit, which would have the effect of shifting the DC trans- fer function slightly to the right. Also, if the V BB option is incorrectly matched to the actual V BB supply voltage, an incorrect VB OUT amplitude will be produced. Therefore, it is important to ensure that the appropriate V BB option is used and the V BB supply is within the range stated in the Section Operating Ranges . Application Information 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. POWER SUPPLY BYPASS The LM2485 should have proper power supply bypassing for optimal arc protection and performance. A 0.1 µF capacitor should be connected from each of the supply pins, V CC and V BB, to ground, as close to the supply and ground pins as possible. Additionally,a1µF electrolytic capacitor should be connected from each supply to the LM2485 ground and placed reasonably close to both supply pins. ARC PROTECTION During normal CRT operation, internal arcing may occasion- ally occur. To protect the LM2485 against arcing, it is impor- tant that the recommended applications circuit is used and good layout practices are followed. Please refer to the Sec- tion NSC Demonstration Board for a detailed example on arc protection and PCB layout considerations. THERMAL CONSIDERATIONS Determining Maximum Power Dissipation and Ambient Temperature Power dissipation within an integrated circuit package is a very important parameter. An incorrect maximum power dis- sipation calculation may result in excessive thermal stress to the device, affecting its reliability and performance. To esti- mate the maximum power dissipation of the LM2485, the following system parameters should be determined first. • Maximum V CC and VBB supply voltages • Average CRT cut-off voltages • Maximum average beam currents • Highest duty cycle Once these parameters are known, the system designer can then calculate the maximum ambient temperature surround- ing the LM2485 without a heat sink. A small, low-cost heat sink may be used if the calculated maximum ambient tem- perature does not satisfy the worst-case operating tempera- ture inside the TV. The following example shows how to determine the maxi- mum power dissipation and maximum ambient temperature and assumes the recommended application circuit is used (see Figure 6). Example The LM2485 uses 1.0W of static power from the 220V and 8V supplies, assuming the worst-case supply currents. If the average CRT cut-off voltage of all channels is 180V and each LM2485 output sinks a maximum average beam cur- rent of 1.0 mA at 77% duty cycle (480p format) at the highest brightness setting, the LM2485 will dissipate: P DISS =PSTATIC +3x(VCUT-OFF xIAVGMAX)x δ,or P DISS =1.0W+3x (180V x 1.0 mA) x 0.77 = 1.42W. The maximum thermal resistance from junction-to-air, θ JA, and maximum junction temperature, T JMAX, are stated in the Section Absolute Maximum Ratings . Therefore, the maxi- mum ambient temperature surrounding the LM2485 can be calculated: T AMAX =TJMAX -(PDISS x θJA), or T AMAX = 150˚C - (1.42W x 68˚C/W) = 53˚C. TYPICAL APPLICATION Used in conjunction with NSC’s LM12XX Preamplifier and LM24XX CRT Driver, a complete analog video solution from the preamplifier input to the CRT cathode can be achieved. The LM2485 can support any AC coupled DTV application using formats up to 1080i with proper selection of a pream- plifier and CRT drivers. www.national.com 4 |
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