LMH6551Q Datasheet, PDF (16/20 Pages) Texas Instruments – Differential, High Speed Op Amp

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FIGURE 11. Transformer Out Low Impedance Load
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FIGURE 12. Driving 50Ω Test Equipment
CAPACITIVE DRIVE
As noted in the Driving ADC section, capacitive loads should
be isolated from the amplifier output with small valued resis-
tors. This is particularly the case when the load has a resistive
component that is 500Ω or higher. A typical ADC has capac-
itive components of around 10 pF and the resistive compo-
nent could be 1000Ω or higher. If driving a transmission line,
such as 50Ω coaxial or 100Ω twisted pair, using matching re-
sistors will be sufficient to isolate any subsequent capaci-
tance. For other applications 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_JMAXof 150°C is never exceeded
due to the overall power dissipation.
Follow these steps to determine the Maximum power dissi-
pation for the LMH6551Q:
1. Calculate the quiescent (no-load) power: P_AMP = I_CC*
(V_S), where V_S = V⁺ - V^−. (Be sure to include any current
through the feedback network if V_OCM is not mid rail.)
2. Calculate the RMS power dissipated in each of the output
stages: P_D (rms) = rms ((V_S - V^+OUT) * I^+OUT) + rms ((V_S
− V^−OUT) * I^−OUT) , where V_OUT and I_OUT are the voltage
and the current measured at the output pins of the
differential amplifier as if they were single ended
amplifiers and V_S is the total supply voltage.
3. Calculate the total RMS power: P_T = P_AMP + P_D.
The maximum power that the LMH6551Q package can dis-
sipate at a given temperature can be derived with the follow-
ing equation:
P_MAX = (150° – T_AMB)/ θ_JA, where T_AMB = Ambient temperature
(°C) and θ_JA = Thermal resistance, from junction to ambient,
for a given package (°C/W). θ_JA is 159 °C/W for the MSOP-8
package.
NOTE: If V_CM is not 0V then there will be quiescent current
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 13 shows the maximum power dissipation vs. ambient
temperature for the MSOP-8 package when mounted on a 4
layer JEDEC board.
1.8
MSOP
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
-40 -20 0 20 40 60 80 100 120 140
TA (°C)
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FIGURE 13. Maximum Power Dissipation vs. Ambient
Temperature
At high ambient temperatures, the LMH6551Q's quiescent
power dissipation approaches the maximum power shown in
Figure 13, when operated close to the maximum operating
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_JA by increasing heatsinking possibly by either in-
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
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