LMH6554_15 Datasheet, PDF (22/29 Pages) Texas Instruments – LMH6554 2.8-GHz Ultra Linear Fully Differential Amplifier

LMH6554
SNOSB30P – OCTOBER 2008 – REVISED JANUARY 2015
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11.3 Power Dissipation
The LMH6554 is optimized for maximum speed and performance in a small form factor 14 lead UQFN package.
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 is never exceeded due to the overall power dissipation.
Follow these steps to determine the maximum power dissipation for the LMH6554:
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_CM 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
• the current measured at the output pins of the differential amplifier as if they were single ended amplifiers
• 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 LMH6554 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 14 lead UQFN package, θ_JA is 60°C/W
(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.
11.4 ESD Protection
The LMH6554 is protected against electrostatic discharge (ESD) on all pins. The LMH6554 can survive 2000 V
Human Body model and 250 V Machine model events. Under normal operation the ESD diodes have no affect
on circuit performance. There are occasions, however, when the ESD diodes will be evident. If the LMH6554 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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