TH

134 C

70 C

R

16.8 C / W

3.8W

° -

°

=

=

°

OBSOLETE

LM2459

www.ti.com

SNOSA66B – MAY 2004 – REVISED APRIL 2013

THERMAL CONSIDERATIONS

Figure 9 shows the performance of the LM2459 in the test circuit shown in Figure 3 as a function of case

temperature. The figure shows that the rise and fall times of the LM2459 increase by approximately 18% and

29%, respectively, as the case temperature increases from 50°C to 90°C. This corresponds to a speed

degradation of 5% and 7% for every 10°C rise in case temperature.

Figure 10 shows the maximum power dissipation of the LM2459 vs. frequency when the device is driving a 10pF

operating at the specified frequency), which is typical in a TV application. The other 23% of the time the device is

assumed to be sitting at the black level (165V in this case). Table 1 also shows the typical power dissipation of

the LM2459 for various video patterns in the 480i and 480p video formats.

Figure 10, Figure 11, and Table 1 give the designer the information needed to determine the heatsink

requirement for the LM2459. For example, if an HDTV application uses the 480p format and "Vertical Lines 1 On

1 Off" is assumed to be the worst-case pattern to be displayed, then the power dissipated will be 3.8W (from

Table 1). Figure 11 shows that the maximum allowed case temperature is 134°C when 3.8W is dissipated. If the

maximum expected ambient temperature is 70°C, then a maximum heatsink thermal resistance can be

calculated:

(1)

This example assumes a capacitive load of 10pF and no resistive load. The designer should note that if the load

capacitance is increased, then the AC component of the total power dissipation will also increase.

NOTE

An LM126X preamplifier, with rise and fall times of about 2 ns, was used to drive the

LM2459 for these power measurements. Using a preamplifier with rise and fall times

slower than the LM126X will cause the LM2459 to dissipate less power than shown in

Table 1.

OPTIMIZING TRANSIENT RESPONSE

Referring to Figure 13, 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. Ferrite core

inductors from J.W. Miller Magnetics (part # 78FR_ _k) were used for optimizing the performance of the device in

the TI application board. The values shown in Figure 14 can be used as a good starting point for the evaluation

of the LM2459. Using a variable resistor for R1 will simplify finding the value needed for optimum performance in

a given application. Once the optimum value is determined, the variable resistor can be replaced with a fixed

value.

preamplifier.

The transient response can also be improved by adding a capacitor from pin 6 to the ground plane used by the

LM2459. A small capacitor, such as a 22pF ceramic, will notably improve the fall time and only increase the

overshoots and settling times slightly. Note that increasing the capacitance beyond 22pF will only improve the fall

time marginally, but will increase the settling times significantly. This option allows for better matching between

the rise and fall time.

Copyright © 2004–2013, Texas Instruments Incorporated

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