LMH6321 datasheet(15/21 Pages) NSC

15 / 21 page

Application Hints (Continued)

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 M

Ω) and low ESL.

LOAD IMPEDANCE

The LMH6321 is stable under any capacitive load when

driven by a 50

Ω 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 50

Ω load, this goes up to 200 nH.

However, a 100

Ω 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 R

1 in Figure 2).

Although the allowed current in the clamp can be as high as

5 mA, which would suggesta2k

Ω resistor from a 15V

source, it is recommended that the current be limited to

about 1 mA, hence the 10 k

Ω 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/(2

πR

1CIN) = 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

INCIN. Therefore,

1 = (5RIN/R1)(CIN)

(2)

from the Electrical Characteristics, R

IN is 250 k

Ω.

In the case of the example in Figure 2,R

INCIN produces a

time-constant of 870 ns, so C

1 should be chosen to be a

minimum of 4.4 µs, or 438 pF. The value of C

1 (1000 pF)

shown in Figure 2 gives 10 µs.

20138628

FIGURE 2. 50

Ω Coaxial Cable Driver with Dual Supplies

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