LM6172IMX National Semiconductor, LM6172IMX Datasheet - Page 14

• Manufacturer Part Number: LM6172IMX
• Description: IC, OP-AMP, 100MHZ, 3000V/µs, SOIC-8
• Manufacturer: National Semiconductor

Specifications of LM6172IMX

• Op Amp Type: Low Power
• No. Of Amplifiers: 2
• Bandwidth: 100MHz
• Slew Rate: 3000V/µs
• Supply Voltage Range: 5.5V To 36V
• Amplifier Case Style: SOIC
• No. Of Pins: 8
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant

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LM6172 Simplified Schematic (Each Amplifier)
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 2.3mA of supply current per channel.
The combination of 100MHz unity-gain bandwidth, 3000V/µs
slew rate, 50mA per channel output current and other attrac-
tive features makes it easy to implement the LM6172 in
various applications. Quiescent power of the LM6172 is
138mW 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
buffers the inverting input. The triple-buffered output stage
isolates the gain stage from the load to provide low output
impedance.
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 configu-
rations.
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 1kΩ to the input of
LM6172, the slew rate is reduced to help lower the over-
shoot, which reduces settling time.
REDUCING SETTLING TIME
The LM6172 has a very fast slew rate that causes overshoot
and undershoot. To reduce settling time on LM6172, a 1kΩ
resistor can be placed in series with the input signal to
E
the equivalent of the feedback resistor, and stage A
±
15V supply and 46mW at
E
. Therefore, the
±
5V supply.
14
decrease slew rate. A feedback capacitor can also be used
to reduce overshoot and undershoot. This feedback capaci-
tor serves as a zero to increase the stability of the amplifier
circuit. A 2pF feedback capacitor is recommended for initial
evaluation. When the LM6172 is configured as a buffer, a
feedback resistor of 1kΩ must be added in parallel to the
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
margin to the overall system. The desired performance de-
pends on the value of the isolation resistor; the bigger the
isolation resistor, the more damped (slow) the pulse re-
sponse becomes. For LM6172, a 50Ω isolation resistor is
recommended for initial evaluation.
FIGURE 1. Isolation Resistor Used
to Drive Capacitive Load
01258155
01258145