LMH6550 Datasheet, PDF (24/33 Pages) National Semiconductor (TI) – Differential, High Speed Op Amp

LMH6550
SNOSAK0I – DECEMBER 2004 – REVISED JANUARY 2015
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11.3 Power Dissipation
The LMH6550 is optimized for maximum speed and performance in the small form factor of the standard SOIC
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 LMH6550:
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 LMH6550 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)
• For the SOIC package θ_JA is 150°C/W
• For the VSSOP package θ_JA is 235°C/W
(1)
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.
11.4 ESD Protection
The LMH6550 is protected against electrostatic discharge (ESD) on all pins. The LMH6550 will survive 2000 V
Human Body model and 200 V Machine model events. Under normal operation the ESD diodes have no effect
on circuit performance. There are occasions, however, when the ESD diodes will be evident. If the LMH6550 is
driven by a large signal while the device is powered down the ESD diodes will conduct. 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. Using the shutdown mode is one way to
conserve power and still prevent unexpected operation.
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