lmh6551qmmx National Semiconductor Corporation, lmh6551qmmx Datasheet - Page 16

• Manufacturer Part Number: lmh6551qmmx
• Description: Differential, High Speed Op Amp
• Manufacturer: National Semiconductor Corporation

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CAPACITIVE DRIVE
As noted in the
ERS
amplifier output with small valued resistors. This is particularly
the case when the load has a resistive component that is
500Ω or higher. A typical ADC has capacitive components of
around 10 pF and the resistive component could be 1000Ω
or higher. If driving a transmission line, such as 50Ω coaxial
or 100Ω twisted pair, using matching resistors will be suffi-
cient to isolate any subsequent capacitance. For other appli-
cations see the “Suggested Rout vs. Cap Load” charts in the
Typical Performance Characteristics section.
POWER DISSIPATION
The LMH6551Q is optimized for maximum speed and perfor-
mance in the small form factor of the standard MSOP pack-
age, and is essentially a dual channel amplifier. To ensure
maximum output drive and highest performance, thermal
shutdown is not provided. Therefore, it is of utmost impor-
tance to make sure that the T
due to the overall power dissipation.
Follow these steps to determine the Maximum power dissi-
pation for the LMH6551Q:
1.
2.
3.
The maximum power that the LMH6551Q package can dis-
sipate at a given temperature can be derived with the follow-
ing equation:
P
(°C) and θ
for a given package (°C/W). θ
package.
NOTE: If V
flowing in the feedback network. This current should be in-
cluded in the thermal calculations and added into the quies-
cent power dissipation of the amplifier.
Figure 12
temperature for the MSOP-8 package when mounted on a 4
layer JEDEC board.
MAX
FIGURE 12. Maximum Power Dissipation vs. Ambient
Calculate the quiescent (no-load) power: P
(V
through the feedback network if V
Calculate the RMS power dissipated in each of the output
stages: P
− V
and the current measured at the output pins of the
differential amplifier as if they were single ended
amplifiers and V
Calculate the total RMS power: P
= (150° – T
section, capacitive loads should be isolated from the
S
), where V
−
OUT
shows the maximum power dissipation vs. ambient
JA
CM
) * I
= Thermal resistance, from junction to ambient,
D
is not 0V then there will be quiescent current
DRIVING ANALOG TO DIGITAL CONVERT-
(rms) = rms ((V
−
AMB
1.8
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
OUT
S
-40 -20 0
= V
)/ θ
) , where V
S
is the total supply voltage.
JA
+
Temperature
- V
, where T
−
20 40 60 80 100 120 140
JMAX
. (Be sure to include any current
JA
T A (°C)
S
OUT
is 159 °C/W for the MSOP-8
- V
of 150°C is never exceeded
AMB
and I
+
OUT
= Ambient temperature
T
OCM
) * I
= P
OUT
MSOP
is not mid rail.)
AMP
+
are the voltage
OUT
301579100
AMP
+ P
) + rms ((V
D
= I
.
CC
*
S
16
At high ambient temperatures, the LMH6551Q's quiescent
power dissipation approaches the maximum power shown in
Figure
supply voltage of 11V. This leaves little room for additional
load power dissipation. In such applications, any of the fol-
lowing steps can be taken to alleviate any junction tempera-
ture concerns:
• Reduce the total supply voltage
• Reduce θ
creasing the PC board area devoted to heatsinking or forced
air cooling or both
• Reduce maximum ambient temperature
ESD PROTECTION
The LMH6551Q is protected against electrostatic discharge
(ESD) on all pins. The LMH6551Q will survive 2000V Human
Body model and 200V Machine model events. Under normal
operation the ESD diodes have no effect on circuit perfor-
mance. There are occasions, however, when the ESD diodes
will be evident. If the LMH6551Q is driven by a large signal
while the device is powered down the ESD diodes will con-
duct . The current that flows through the ESD diodes will either
exit the chip through the supply pins or will flow through the
device, hence it is possible to power up a chip with a large
signal applied to the input pins.
BOARD LAYOUT
The LMH6551Q is a very high performance amplifier. In order
to get maximum benefit from the differential circuit architec-
ture board layout and component selection is very critical. The
circuit board should have low a inductance ground plane and
well bypassed broad supply lines. External components
should be leadless surface mount types. The feedback net-
work and output matching resistors should be composed of
short traces and precision resistors (0.1%). The output match-
ing resistors should be placed within 3-4 mm of the amplifier
as should the supply bypass capacitors.
The LMH6551Q is sensitive to parasitic capacitances on the
amplifier inputs and to a lesser extent on the outputs as well.
Ground and power plane metal should be removed from be-
neath the amplifier and from beneath R
With any differential signal path symmetry is very important.
Even small amounts of asymmetry will contribute to distortion
and balance errors.
12, when operated close to the maximum operating
JA
by increasing heatsinking possibly by either in-
F
and R
G
.