LMP7718MASLASHNOPB datasheet(16/42 Pages) TI2

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AOL

Gmin

fGBWP

UNITY-GAIN STABLE OP AMP

DECOMPENSATED OP AMP

100k

100M

-20

100

10M

10k

FREQUENCY (Hz)

GAIN

PHASE

-20

100

100k

100M

-20

100

10M

10k

FREQUENCY (Hz)

-20

100

LMP7717, LMP7718

SNOSAY7H – MARCH 2007 – REVISED MARCH 2013

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Figure 46. LMP7717 AVOL vs. Frequency

Figure 47. LMP7715 AVOL vs. Frequency

Figure 46 shows the much larger 88 MHz bandwidth of the LMP7717 as compared to the 17 MHz bandwidth of

the LMP7715 shown in Figure 47. The decompensated LMP7717 has five times the bandwidth of the LMP7715.

What is a Decompensated Op Amp?

The differences between the unity gain stable op amp and the decompensated op amp are shown in Figure 48.

This Bode plot assumes an ideal two pole system. The dominant pole of the decompensated op amp is at a

higher frequency, f1, as compared to the unity gain stable op amp which is at fd as shown in Figure 48. This is

done in order to increase the speed capability of the op amp while maintaining the same power dissipation of the

unity gain stable op amp. The LMP7717/LMP7718 have a dominant pole at 1.6 kHz. The unity gain stable

LMP7715/LMP7716 have their dominant pole at 300 Hz.

Figure 48. Open Loop Gain for Unity Gain Stable Op Amp and Decompensated Op Amp

Having a higher frequency for the dominate pole will result in:

1. The DC open loop gain (AVOL) extending to a higher frequency.

2. A wider closed loop bandwidth.

3. Better slew rate due to reduced compensation capacitance within the op amp.

The second open loop pole (f2) for the LMP7717/LMP7718 occurs at 45 MHz. The unity gain (fu’) occurs after the

second pole at 51 MHz. An ideal two pole system would give a phase margin of 45° at the location of the second

pole. The LMP7717/LMP7718 have parasitic poles close to the second pole, giving a phase margin closer to 0°.

Therefore it is necessary to operate the LMP7717/LMP7718 at a closed loop gain of 10 or higher, or to add

external compensation in order to assure stability.

For the LMP7715, the gain bandwidth product occurs at 17 MHz. The curve is constant from fd to fu which occurs

before the second pole.

For the LMP7717/LMP7718 the GBW = 88 MHz and is constant between f1 and f2. The second pole at f2 occurs

before AVOL =1. Therefore fu’ occurs at 51 MHz, well before the GBW frequency of 88 MHz. For decompensated

op amps the unity gain frequency and the GBW are no longer equal. Gmin is the minimum gain for stability and

for the LMP7717/LMP7718 this is a gain of 18 to 20 dB.

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LMP7718MA/NOPB Datasheet(PDF) 16 Page - Texas Instruments

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