LMH6321 Datasheet, PDF (15/21 Pages) National Semiconductor (TI) – 300 mA High Speed Buffer with Adjustable Current Limit

Application Hints (Continued)
FIGURE 2. 50Ω Coaxial Cable Driver with Dual Supplies
20138628
For values of capacitors in the 10 µF to 100 µF range,
ceramics are usually larger and more costly than tantalums
but give superior AC performance for bypassing high fre-
quency noise because of their very low ESR (typically less
than 10 MΩ) and low ESL.
LOAD IMPEDANCE
The LMH6321 is stable under any capacitive load when
driven by a 50Ω source. As shown by the Overshoot vs.
Capacitive Load graph in the Typical Performance Charac-
teristics, worst case overshoot is for a purely capacitive load
of about 1 nF. Shunting the load capacitance with a resistor
will reduce the overshoot.
SOURCE INDUCTANCE
Like any high frequency buffer, the LMH6321 can oscillate
with high values of source inductance. The worst case con-
dition occurs with no input resistor, and a purely capacitive
load of 50 pF, where up to 100 nH of source inductance can
be tolerated. With a 50Ω load, this goes up to 200 nH.
However, a 100Ω resistor placed in series with the buffer
input will ensure stability with a source inductances up to 400
nH with any load.
OVERVOLTAGE PROTECTION
(Refer to the simplified schematic in Figure 1).
If the input-to-output differential voltage were allowed to
exceed the Absolute Maximum Rating of 5V, an internal
diode clamp would turn on and divert the current around the
compound emitter followers of Q1/Q3 (D1 – D11 for positive
input), or around Q2/Q4 (D2 – D12 for negative inputs).
Without this clamp, the input transistors Q1 – Q4 would
zener, thereby damaging the buffer.
To limit the current through this clamp, a series resistor
should be added to the buffer input (see R₁ in Figure 2).
Although the allowed current in the clamp can be as high as
5 mA, which would suggest a 2 kΩ resistor from a 15V
source, it is recommended that the current be limited to
about 1 mA, hence the 10 kΩ shown.
The reason for this larger resistor is explained in the follow-
ing: One way that the input/output voltage differential can
exceed the Abs Max value is under a short circuit condition
to ground while driving the input with up to ±15V. However,
in the LMH6321 the maximum output current is set by the
programmable Current Limit pin (C_L). The value set by this
pin is guaranteed to be accurate to 5 mA ±5%. If the input/
output differential exceeds 5V while the output is trying to
supply the maximum set current to a shorted condition or to
a very low resistance load, a portion of that current will flow
through the clamp diodes, thus creating an error in the total
load current. If the input resistor is too low, the error current
can exceed the 5 mA ±5% budget.
BANDWIDTH AND STABILITY
As can be seen in the schematic of Figure 2, a small capaci-
tor is inserted in parallel with the series input resistors. The
reason for this is to compensate for the natural band-limiting
effect of the 1st order filter formed by this resistor and the
input capacitance of the buffer. With a typical C_IN of 3.5 pF
(Figure 2), a pole is created at
fp2 = 1/(2πR₁C_IN) = 4.5 MHz
(1)
This will band-limit the buffer and produce further phase lag.
If used in an op amp-loop application with an amplifier that
has the same order of magnitude of unity gain crossing as
fp2, this additional phase lag will produce oscillation.
The solution is to add a small feed-forward capacitor (phase
lead) around the input resistor, as shown in Figure 2. The
value of this capacitor is not critical but should be such that
the time constant formed by it and the input resistor that it is
in parallel with (R_IN) be at least five times the time constant
of R_INC_IN. Therefore,
C₁ = (5R_IN/R₁)(C_IN)
(2)
from the Electrical Characteristics, R_IN is 250 kΩ.
In the case of the example in Figure 2, R_INC_IN produces a
time-constant of 870 ns, so C₁ should be chosen to be a
minimum of 4.4 µs, or 438 pF. The value of C₁ (1000 pF)
shown in Figure 2 gives 10 µs.
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