Datasheet

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MAXIMUM POWER DISSIPATION (W)
TEMPERATURE (°C)
225 LFPM FORCED AIR
STILL AIR
LMH6739
www.ti.com SNOSAD2G –MAY 2004–REVISED MARCH 2013
DRIVING CAPACITIVE LOADS
Capacitive output loading applications will benefit from the use of a series output resistor R
OUT
. Figure 28 shows
the use of a series output resistor, R
OUT
, to stabilize the amplifier output under capacitive loading. Capacitive
loads of 5 to 120 pF are the most critical, causing ringing, frequency response peaking and possible oscillation.
The charts “Suggested R
OUT
vs. Cap Load” give a recommended value for selecting a series output resistor for
mitigating capacitive loads. The values suggested in the charts are selected for .5 dB or less of peaking in the
frequency response. This gives a good compromise between settling time and bandwidth. For applications where
maximum frequency response is needed and some peaking is tolerable, the value of R
OUT
can be reduced
slightly from the recommended values.
LAYOUT CONSIDERATIONS
Whenever questions about layout arise, use the evaluation board as a guide. The LMH730275 is the evaluation
board for the LMH6739.
To reduce parasitic capacitances ground and power planes should be removed near the input and output pins.
Components in the feedback loop should be placed as close to the device as possible. For long signal paths
controlled impedance lines should be used, along with impedance matching elements at both ends.
Bypass capacitors should be placed as close to the device as possible. Bypass capacitors from each rail to
ground are applied in pairs. The larger electrolytic bypass capacitors can be located farther from the device, the
smaller ceramic capacitors should be placed as close to the device as possible. The LMH6739 has multiple
power and ground pins for enhanced supply bypassing. Every pin should ideally have a separate bypass
capacitor. Sharing bypass capacitors may slightly degrade second order harmonic performance, especially if the
supply traces are thin and /or long. In Figure 21 and Figure 22 C
SS
is optional, but is recommended for best
second harmonic distortion. Another option to using C
SS
is to use pairs of .01 μF and 0.1 μF ceramic capacitors
for each supply bypass.
VIDEO PERFORMANCE
The LMH6739 has been designed to provide excellent performance with production quality video signals in a
wide variety of formats such as HDTV and High Resolution VGA. NTSC and PAL performance is nearly flawless.
Best performance will be obtained with back terminated loads. The back termination reduces reflections from the
transmission line and effectively masks transmission line and other parasitic capacitances from the amplifier
output stage. Figure 24 shows a typical configuration for driving a 75Ω Cable. The amplifier is configured for a
gain of two to make up for the 6 dB of loss in R
OUT
.
Figure 29. Maximum Power Dissipation
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