LM2435T datasheet(6/10 Pages) NSC

6 / 10 page

Application Hints (Continued)

board. Note that the heatsink is attached (soldered) to PCB

ground just to the left and just to the right of the device. The

LM2435 is attached to the heatsink using a screw, star

washer and nut. The star washer should be located on the

side of the heatsink opposite the device.

NSC DEMONSTRATION BOARD

Figure 11 shows routing and component placement on the

NSC LM1279/243X demonstration board. The schematic of

the board is shown in

Figure 10. This board provides a good

example of a layout that can be used as a guide for future

layouts. Note the location of the following components:

•

C54, C56 — V

CC bypass capacitor, located very close to

pin 4 and ground connections to the device.

•

C43, C44 — V

BB bypass capacitors, located close to pin

8 and ground.

•

C53, C55 — V

CC bypass capacitors, near LM2435 and

CC clamp diodes. This is very important for arc protec-

tion.

The routing of the LM2435 outputs to the CRT is very critical

to achieving optimum performance.

Figure 12 shows the

routing and component placement from pin 1 of the LM2435

to the red cathode. Note that the components are placed so

that they almost line up from the output pin of the LM2435 to

the red cathode pin of the CRT connector. This is done to

minimize the length of the video path between these two

components. Note also that the arc protection diodes, D16

and D17 are placed close to L1 and C14 in order to minimize

the size of the node connecting all these components. R21

and D9 are placed close to C14 and R32 for the same rea-

son. This minimizes parasitic capacitance in the video path

and also enhances the effectiveness of the protection di-

odes. The anode of protection diode D16 is connected di-

rectly to a section of the ground plane that has a short and

direct path to the LM2435 ground. The cathode of D16 is

connected to V

CC very close to decoupling capacitor C53

(see

Figure 12) which is connected to the same section of

the ground plane as D17. The diode placement and routing

is very important for minimizing the voltage stress on the

LM2435 during an arc over event. Lastly, notice that S1 is

placed very close to the red cathode and is tied directly to

CRT ground.

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