LMH6586

LMH6586

LMH6586

LMH6586

OUTPUT

(0-15)

INPUT

(0-31)

INPUT

(32-63)

INPUT

(64-95)

INPUT

(96-127)

75:

75:

75:

75:

16

16

16

16

32

32

32

32

LMH6586

SNCS105D – JULY 2008 – REVISED MARCH 2013

www.ti.com

Figure 53. 128 x 16 Crosspoint Array

DRIVING CAPACITIVE LOAD

When many outputs are wired together, as in the case of expansion, each output buffer sees the normal load

impedance as well as the impedance the other shutdown outputs. This impedance has a resistive and a

capacitive component. The resistive components reduce the total effective load for the driving output. Total

capacitance is the sum of the capacitance of all the outputs and depends on the size of the matrix. As the size of

the matrix increases, the length of the PC board traces also increases, adding more capacitance. The output

buffers have been designed to drive more than 30 pF of capacitance while still maintaining a good AC response.

If the output capacitance exceeds this amount then the AC response will be degraded. To prevent this, one

option is to reduce the number of output wired-or together by using more LMH6586 device. Another option is to

put a resistor in series with the output before the capacitive load to limit excessive ringing and oscillations.

A low pass filter is created from the series resistor (R) and parasitic capacitance (C) to ground. A single R-C

does not affect the performance at video frequencies, however, in large system, there may be many such R-Cs

cascaded in series. This may result in high frequency roll-off resulting in “softening of the picture”. There are two

solutions to improve performance in this case. One way is to design the PC board traces with some inductance

between the R and C elements. By routing the traces in a repeating “S” configuration, the traces that are nearest

each other will exhibit a mutual inductance increasing the total inductance. This series inductance causes the

amplitude response to increase or peak at higher frequencies, offsetting the roll-off from the parasitic

capacitance. Another solution is to add a small-value inductor between the R and C elements to add peaking to

the frequency response.

THERMAL MANAGEMENT

The LMH6586 operates on a 5V supply and draws a load current of approximately 300 mA. Thus it dissipates

approximately 1.75W of power. In addition, each equivalent video load (150

Ω) connected to the outputs should

be budgeted 30 mW of power consumption.

The following calculations show the thermal resistance,

θJA, required, to ensure safe operation and to prevent

exceeding the maximum junction temperature, given the maximum power dissipation.

PDMAX = (TJMAX – TAMAX)/θJA

where

•

TJMAX = Maximum junction temperature = 150°C

•

TAMAX = Maximum ambient temperature = +85°C

•

θJA = Thermal resistance of the package

(1)

22

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