LMH6554 2.8-GHz Ultra Linear Fully Differential Amplifier (Rev. P)
SNOSB30P – October2008 – revisedJanuary 2015
11.1 Layout Guidelines
The LMH6554 is a high speed, high performance amplifier. In order to get maximum benefit from the differential circuit architecture board layout and component selection is very critical. The circuit board should have a low inductance ground plane and well bypassed broad supply lines. External components should be leadless surface mount types. The feedback network and output matching resistors should be composed of short traces and precision resistors (0.1%). The output matching resistors should be placed within 3 or 4 mm of the amplifier as should the supply bypass capacitors. Refer to Power Supply Bypassing for recommendations on bypass circuit layout. Evaluation boards are available through the product folder on ti.com.
By design, the LMH6554 is relatively insensitive to parasitic capacitance at its inputs. Nonetheless, ground and power plane metal should be removed from beneath the amplifier and from beneath RF and RG for best performance at high frequency.
With any differential signal path, symmetry is very important. Even small amounts of asymmetry can contribute to distortion and balance errors.
11.2 Layout Example
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 TJMAX is never exceeded due to the overall power dissipation.
Follow these steps to determine the maximum power dissipation for the LMH6554:
- Calculate the quiescent (no-load) power:
- VS = V+ − V-. (Be sure to include any current through the feedback network if VCM is not mid-rail)
- Calculate the RMS power dissipated in each of the output stages:
- VOUT and IOUT are the voltage
- the current measured at the output pins of the differential amplifier as if they were single ended amplifiers
- VS is the total supply voltage
- Calculate the total RMS power:
The maximum power that the LMH6554 package can dissipate at a given temperature can be derived with the following equation:
- TAMB = Ambient temperature (°C)
- θJA = Thermal resistance, from junction to ambient, for a given package (°C/W)
- For the 14 lead UQFN package, θJA is 60°C/W
If VCM 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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