LMH6321_07 Datasheet, PDF (19/22 Pages) National Semiconductor (TI) – 300mA High Speed Buffer with Adjustable Current Limit

SINGLE SUPPLY OPERATION
If dual supplies are used, then the GND pin can be connected
to a hard ground (0V) (as shown in Figure 2). However, if only
a single supply is used, this pin must be set to a voltage of
one V_BE (∼0.7V) or greater, or more commonly, mid rail, by a
stiff, low impedance source. This precludes applying a resis-
tive voltage divider to the GND pin for this purpose. Figure 6
shows one way that this can be done.
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FIGURE 6. Using an Op Amp to Bias the GND Pin to ½ V
+ _{for Single Supply Operation}
In Figure 6, the op amp circuit pre-biases the GND pin of the
buffer for single supply operation.
The GND pin can be driven by an op amp configured as a
constant voltage source, with the output voltage set by the
resistor voltage divider, R₁ and R₂. It is recommended that
These resistors be chosen so as to set the GND pin to V⁺/2,
for maximum common mode range.
SLEW RATE
Slew rate is the rate of change of output voltage for large-
signal step input changes. For resistive load, slew rate is
limited by internal circuit capacitance and operating current
(in general, the higher the operating current for a given inter-
nal capacitance, the faster is the slew rate). Figure 7 shows
the slew capabilities of the LMH6321 under large signal input
conditions, using a resistive load.
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FIGURE 7. Slew Rate vs. Peak-to-Peak Input Voltage
However, when driving capacitive loads, the slew rate may be
limited by the available peak output current according to the
following expression.
dv/dt = I_PK/C_L
(10)
and rapidly changing output voltages will require large output
load currents. For example if the part is required to slew at
1000 V/μs with a load capacitance of 1 nF the current demand
from the LMH6321 would be 1A. Therefore, fast slew rate is
incompatible with large C_L. Also, since C_L is in parallel with
the load, the peak current available to the load decreases as
C_L increases.
Figure 8 illustrates the effect of the load capacitance on slew
rate. Slew rate tests are specified for resistive loads and/or
very small capacitive loads, otherwise the slew rate test would
be a measure of the available output current. For the highest
slew rate, it is obvious that stray load capacitance should be
minimized. Peak output current should be kept below 500 mA.
This translates to a maximum stray capacitance of 500 pF for
a slew rate of 1000 V/μs.
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FIGURE 8. Slew Rate vs. Load Capacitance
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