OBSOLETE

LM2402

SNOS445B – AUGUST 1999 – REVISED APRIL 2013

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over current that is seen by the diodes while R1 limits the current into the LM2402 as well as the voltage stress

at the outputs of the device. R2 should be a ½W solid carbon type resistor. R1 can be a ¼W metal or carbon film

type resistor. Inductor L1 is critical to reduce the initial high frequency voltage levels that the LM2402 would be

subjected to during an arc-over. Having large value resistors for R1 and R2 would be desirable, but this has the

effect of increasing rise and fall times. The inductor will not only help protect the device but it will also help

optimize 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 10. The values of L1 and R1 may need to be adjusted for a

particular application. The recommended minimum value for R1 is 43

Ω, with L1 = .049 µH.

Figure 10. One Channel of the LM2402 with the Recommended Arc Protection Circuit

Optimizing Transient Response

Referring to Figure 10, there are three components (R1, R2 and L1) that can be adjusted to optimize the

transient response of the application circuit. Increasing the values of R1 and R2 will slow the circuit down while

decreasing overshoot. Increasing the value of L1 will speed up the circuit as well as increase overshoot. It is very

important to use inductors with very high self-resonant frequencies, preferably above 300 MHz. Air core inductors

from J.W. Miller Magnetics (part #75F518MPC) were used for optimizing the performance of the device in the TI

application board. The values shown in Figure 10 can be used as a good starting point for the evaluation of the

LM2402.

Effect of Load Capacitance

The output rise and fall times as well as overshoot will vary as the load capacitance varies. The values of the

output circuit (R1, R2 and L1 in Figure 10) should be chosen based on the nominal load capacitance. Once this

is done the performance of the design can be checked by varying the load based on what the expected variation

will be during production.

Effect of Offset

Figure 8 shows the variation in rise and fall times when the output offset of the device is varied from 40 to 50

center point of 45V, making this the optimum operating point since there is little increase in the rise time.

Thermal Considerations

Figure 5 shows the performance of the LM2402 in the test circuit shown in Figure 3 as a function of case

temperature. Figure 5 shows that both the rise and fall times of the LM2402 become slightly slower as the case

temperature increases from 40°C to 125°C. In addition to exceeding the safe operating temperature, the rise and

fall times will typically exceed 3 nsec. Please note that the LM2402 is never to be operated over a case

temperature of 100°C.

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