LMH6553 Datasheet, PDF (22/24 Pages) National Semiconductor (TI) – 900 MHz Fully Differential Amplifier With Output Limiting Clamp

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FIGURE 13. Split Supply Bypassing Capacitors
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FIGURE 14. Single Supply Bypassing Capacitors
POWER DISSIPATION
The LMH6553 is optimized for maximum speed and perfor-
mance in the small form factor of the standard LLP 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 of 150°C is never ex-
ceeded.
Follow these steps to determine the maximum power dissi-
pation for the LMH6553:
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.)
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 LMH6553 package can dissi-
pate at a given temperature can be derived with the following
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). For the PSOP package θ_JA is
59°C/W; LLP package θ_JA is 58°C/W.
NOTE: If V_CM is not mid-rail, then there will be quiescent cur-
rent flowing in the feedback network. This current should be
included in the thermal calculations and added into the qui-
escent power dissipation of the amplifier.
ESD PROTECTION
The LMH6553 is protected against electrostatic discharge
(ESD) on all pins. The LMH6553 will survive 4000V Human
Body model and 350V Machine model events. Under normal
operation the ESD diodes have no effect on circuit perfor-
mance. The current that flows through the ESD diodes will
either exit the chip through the supply pins or through the de-
vice, hence it is possible to power up a chip with a large signal
applied to the input pins.
BOARD LAYOUT
The LMH6553 is a very high performance amplifier. In order
to get maximum benefit from the differential circuit architec-
ture, board layout and component selection are very critical.
The circuit board should have a low inductance ground plane
and well bypassed wide supply lines. External components
should be leadless surface mount types. The feedback net-
work and output matching resistors should be composed of
short traces and precision resistors (0.1%). The output match-
ing resistors should be placed within 3 or 4 mm of the amplifier
as should the supply bypass capacitors. Refer to the section
titled Power Supply Bypassing for recommendations on by-
pass circuit layout. Evaluation boards are available free of
charge through the product folder on National’s web site.
By design, the LMH6553 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 R_F and R_G 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.
EVALUATION BOARD
National Semiconductor suggests the following evaluation
boards to be used with the LMH6553:
Device
LMH6553MR
LMH6553SD
Package
PSOP
LLP
Evaluation Board
Ordering ID
55160019
LMH6553SDEVAL
These evaluation boards can be shipped when a device sam-
ple request is placed with National Semiconductor.
www.national.com
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