Datasheet

|
R
1
10 k:
INPUT
C
1
1 nF
C
IN
C
3
0.1 PF
R
4
50:
50: COAXIAL
CABLE
R
6
50:
OUTPUT
R
3
10 k: 1%
V
CL
TP1
EF
V
+
C
2
0.1 PF
R
2
10 k:1%
V
-
GND
C
L
LMH6321
EF
V
+
V
-
V
OUT
V
IN
LMH6321
SNOSAL8C –APRIL 2006–REVISED MARCH 2013 www.ti.com
Figure 52. 50Ω Coaxial Cable Driver with Dual Supplies
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 frequency 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 Figure 3 in
TYPICAL PERFORMANCE CHARACTERISTICS, 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
condition 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 51).
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 52).
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 following: 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 specified 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.
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Product Folder Links: LMH6321