LMH6551Q_15 Datasheet, PDF (18/26 Pages) Texas Instruments – Differential, High Speed Op Amp

LMH6551Q
SNOSB95E – NOVEMBER 2011 – REVISED MARCH 2013
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CAPACITIVE DRIVE
As noted in DRIVING ANALOG TO DIGITAL CONVERTERS, capacitive loads should be isolated from the
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 sufficient to isolate any subsequent capacitance. For other applications see Figure 6
and Figure 7 in Typical Performance Characteristics.
POWER DISSIPATION
The LMH6551Q is optimized for maximum speed and performance in the small form factor of the standard
VSSOP package, 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 importance to make sure that the T_JMAX
of 150°C is never exceeded due to the overall power dissipation.
Follow these steps to determine the Maximum power dissipation 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.)
(1)
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
(2)
3. Calculate the total RMS power:
P_T = P_AMP + P_D
(3)
The maximum power that the LMH6551Q package can dissipate at a given temperature can be derived with the
following equation:
P_MAX = (150° – T_AMB)/ θ_JA
where
• T_AMB = Ambient temperature (°C)
• θ_JA = Thermal resistance, from junction to ambient, for a given package (°C/W)
• θ_JA is 159 °C/W for the VSSOP-8 package
(4)
NOTE
If V_CM is not 0V then there will be quiescent current flowing in the feedback network. This
current should be included in the thermal calculations and added into the quiescent power
dissipation of the amplifier.
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