LMH6321 Datasheet, PDF (14/21 Pages) National Semiconductor (TI) – 300 mA High Speed Buffer with Adjustable Current Limit

Application Hints
BUFFERS
Buffers are often called voltage followers because they have
largely unity voltage gain, thus the name has generally come
to mean a device that supplies current gain but no voltage
gain. Buffers serve in applications requiring isolation of
source and load, i.e., high input impedance, low output
impedance (high output current drive). In addition, they offer
gain flatness and wide bandwidth.
Most operational amplifiers, that meet the other given re-
quirements in a particular application, can be configured as
buffers, though they are generally more complex and are, by
and large, not optimized for unity gain operation. The com-
mercial buffer is a cost effective substitute for an op amp.
Buffers serve several useful functions, either in tandem with
op amps or in standalone applications. As mentioned, their
primary function is to isolate a high impedance source from
a low impedance load, since a high Z source can’t supply the
needed current to the load. For example, in the case where
the signal source to an analog to digital converter is a
sensor, it is recommended that the sensor be isolated from
the A/D converter. The use of a buffer ensures a low output
impedance and delivery of a stable output to the converter.
In A/D converter applications buffers need to drive varying
and complex reactive loads.
Buffers come in two flavors: Open Loop and Closed Loop.
While sacrificing the precision of some DC characteristics,
and generally displaying poorer gain linearity, open loop
buffers offer lower cost and increased bandwidth, along with
less phase shift and propagation delay than do closed loop
buffers. The LMH6321 is of the open loop variety.
Figure 1 shows a simplified diagram of the LMH6321 topol-
ogy, revealing the open loop complementary follower design
approach. Figure 2 shows the LMH6321 in a typical applica-
tion, in this case, a 50Ω coaxial cable driver.
FIGURE 1. Simplified Schematic
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SUPPLY BYPASSING
The method of supply bypassing is not critical for frequency
stability of the buffer, and, for light loads, capacitor values in
the neighborhood of 1 nF to 10 nF are adequate. However,
under fast slewing and large loads, large transient currents
are demanded of the power supplies, and when combined
with any significant wiring inductance, these currents can
produce voltage transients. For example, the LMH6321 can
slew typically at 1000 V/µs. Therefore, under a 50Ω load
condition the load can demand current at a rate, di/dt, of
20 A/µs. This current flowing in an inductance of 50 nH
(approximately 1.5” of 22 gage wire) will produce a 1V
transient. Thus, it is recommended that solid tantalum ca-
pacitors of 5 µF to 10 µF, in parallel with a ceramic 0.1 µF
capacitor be added as close as possible to the device supply
pins.
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