LM6172AMWG-QML National Semiconductor, LM6172AMWG-QML Datasheet - Page 11

• Manufacturer Part Number: LM6172AMWG-QML
• Description: Dual High Speed/ Low Power/ Low Distortion/ Voltage Feedback Amplifiers
• Manufacturer: National Semiconductor

1
Application Notes
LM6172 Performance Discussion
The LM6172 is a dual high-speed, low power, voltage feed-
back amplifier. It is unity-gain stable and offers outstanding
performance with only 2.3 mA of supply current per channel.
The combination of 100 MHz unity-gain bandwidth,
3000V/µs slew rate, 50 mA per channel output current and
other attractive features makes it easy to implement the
LM6172 in various applications. Quiescent power of the
LM6172 is 138 mW operating at
LM6172 Circuit Operation
The class AB input stage in LM6172 is fully symmetrical and
has a similar slewing characteristic to the current feedback
amplifiers. In the LM6172 Simplified Schematic, Q1 through
Q4 form the equivalent of the current feedback input buffer,
R
ers the inverting input. The triple-buffered output stage iso-
lates the gain stage from the load to provide low output im-
pedance.
LM6172 Slew Rate Characteristic
The slew rate of LM6172 is determined by the current avail-
able to charge and discharge an internal high impedance
node capacitor. This current is the differential input voltage
divided by the total degeneration resistor R
slew rate is proportional to the input voltage level, and the
higher slew rates are achievable in the lower gain configura-
tions.
When a very fast large signal pulse is applied to the input of
an amplifier, some overshoot or undershoot occurs. By plac-
ing an external series resistor such as 1 k
LM6172, the slew rate is reduced to help lower the over-
shoot, which reduces settling time.
±
⁄
5V supply.
E
2
the equivalent of the feedback resistor, and stage A buff-
LM6172 Simplified Schematic
±
15V supply and 46 mW at
E
. Therefore, the
to the input of
11
Reducing Settling Time
The LM6172 has a very fast slew rate that causes overshoot
and undershoot. To reduce settling time on LM6172, a 1 k
resistor can be placed in series with the input signal to de-
crease slew rate. A feedback capacitor can also be used to
reduce overshoot and undershoot. This feedback capacitor
serves as a zero to increase the stability of the amplifier cir-
cuit. A 2 pF feedback capacitor is recommended for initial
evaluation. When the LM6172 is configured as a buffer, a
feedback resistor of 1 k
feedback capacitor.
Another possible source of overshoot and undershoot
comes from capacitive load at the output. Please see the
section “Driving Capacitive Loads” for more detail.
Driving Capacitive Loads
Amplifiers driving capacitive loads can oscillate or have ring-
ing at the output. To eliminate oscillation or reduce ringing,
an isolation resistor can be placed as shown in Figure 1 . The
combination of the isolation resistor and the load capacitor
forms a pole to increase stability by adding more phase mar-
gin to the overall system. The desired performance depends
on the value of the isolation resistor; the bigger the isolation
resistor, the more damped (slow) the pulse response be-
comes. For LM6172, a 50
mended for initial evaluation.
DS012581-55
must be added in parallel to the
isolation resistor is recom-
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