LMP7709MT/NOPB National Semiconductor, LMP7709MT/NOPB Datasheet - Page 21

• Manufacturer Part Number: LMP7709MT/NOPB
• Description: IC AMP PREC RRIO QUAD 14-TSSOP
• Manufacturer: National Semiconductor
• Series: LMP®, PowerWise®r

Specifications of LMP7709MT/NOPB

• Amplifier Type: General Purpose
• Number Of Circuits: 4
• Output Type: Rail-to-Rail
• Slew Rate: 5.9 V/µs
• Gain Bandwidth Product: 15MHz
• Current - Input Bias: 0.2pA
• Voltage - Input Offset: 37µV
• Current - Supply: 3.2mA
• Current - Output / Channel: 86mA
• Voltage - Supply, Single/dual (±): 2.7 V ~ 12 V, ±1.35 V ~ 6 V
• Operating Temperature: -40°C ~ 125°C
• Mounting Type: Surface Mount
• Package / Case: 14-TSSOP
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• -3db Bandwidth: -
• Other names: LMP7709MT

Procedure:
The compensation circuit shown in Figure 14 is implemented.
The inverse feedback function is shaped by the solid line in
Figure 15. The 1/F plot is 6 dB at low frequencies. At higher
frequencies, it is made to intersect the loop gain G at fre-
quency f
a magnitude of six times. This follows the recommendations
in Rule 1. The 1/F pole f
section point (f
a frequency f
calculate the values of the compensation components:
Step 1) Set 1/F equal to G
Step 2) Set the 1/F pole one decade below the intersection
This method uses bode plot approximation. Some fine-tuning
may be needed to get the best results.
Calculations:
As described in Step 1, use Equation 17.
Now substitute R
is a unity gain, inverting amplifier, then
According to Step 2 use Equation 14
which leads to:
Choose a value of R
the possibility of shunt capacitance across high value resis-
tors producing a negative effect on high frequency operation.
If R
R
has a value of R
value of R
components).
The value of capacitor C is 2.2 nF. This value is significantly
higher than the parasitic capacitances associated with pas-
sive components and board layout, and is therefore a good
solution.
Bench results:
For bench evaluation the LMP7707 in an inverting configura-
tion has been verified under three different conditions:
•
•
•
The calculated components for these three conditions are
P
Uncompensated.
Lead-lag compensation resulting in a phase margin of 45°.
Lead lag overcompensation resulting in a phase margin
larger than 45°.
F
= 0 Ω . The value of R
Overcompensated
Uncompensated
= R
Compensated
Condition
1
2
a value for resistor R
point using Equation 14. This gives a value for ca-
pacitor C.
= 1 kΩ, then R
with gain amplitude of 16 dB (G
C
= 330 Ω is a first choice (using 10% tolerance
p
2
= 240 kHz. The next steps should be taken to
= 2.4 MHz) as given in Rule 2, and results in
C
F
/R
= 250 Ω. This is not a standard value. A
F
1
that is below 2 kΩ, in order to minimize
= 1 into the equation above since this
F
p
// R
is set one decade below the inter-
C
MIN
is derived from Equation 19 and
1
= 500 Ω. For simplicity, choose
C
using Equation 17. This gives
330 Ω
240 Ω
.
NA
R
C
MIN
), which equals
2.2 nF
3.3 nF
NA
C
(18)
(19)
(20)
(21)
21
Figure 16 shows the results of the compensation of the
LMP7707.
The top waveform shows the output response of a uncom-
pensated LMP7707 using no external compensation compo-
nents. This trace shows ringing and is unstable (as expected).
The middle waveform is the response of a compensated
LMP7707 using the compensation components calculated
with the described procedure. The response is reasonably
well behaved. The bottom waveform shows the response of
an overcompensated LMP7707.
Finally, Figure 17 compares the step response of the com-
pensated LMP7707 to that of the unity gain stable LMP7701.
The increase in dynamic performance is clear.
The application of input lead-lag compensation to a decom-
pensated op amp enables the realization of circuit gains of
less than the minimum specified by the manufacturer. This is
accomplished while retaining the advantageous speed versus
power characteristic of decompensated op amps.
FIGURE 16. Bench Results for Lead- Lag Compensation
FIGURE 17. Bench Results for Comparison of LMP7701
and LMP7707
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