LMP7709MTSLASHNOPB datasheet(27/41 Pages) TI1

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C =

2SfRC

fP =

= 240 kHz

2SRCC

RP + R1//RF = 2 RC

= (1 +

)(1 +

)

f =

RP + R1//RF

= 6 V/V

LMP7707, LMP7708, LMP7709

www.ti.com

SNOSAW5B – JUNE 2007 – REVISED MARCH 2013

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:

(18)

Now substitute RF/R1 = 1 into the equation above since this is a unity gain, inverting amplifier, then

(19)

According to Step 2 use Equation 14:

(20)

which leads to:

(21)

Choose a value of RF that is below 2 kΩ, in order to minimize the possibility of shunt capacitance across high

value resistors producing a negative effect on high frequency operation. If RF = R1 = 1 kΩ, then RF // R1 = 500 Ω.

For simplicity, choose RP = 0 Ω . The value of RC is derived from Equation 19 and has a value of RC = 250 Ω.

This is not a standard value. A value of RC = 330 Ω is a first choice (using 10% tolerance components).

The value of capacitor C is 2.2 nF. This value is significantly higher than the parasitic capacitances associated

with passive components and board layout, and is therefore a good solution.

Bench results:

For bench evaluation the LMP7707 in an inverting configuration has been verified under three different

conditions:

•

Uncompensated

•

Lead-lag compensation resulting in a phase margin of 45°

•

Lead lag overcompensation resulting in a phase margin larger than 45°

The calculated components for these three conditions are

Condition

Uncompensated

Compensated

330

Ω

2.2 nF

Overcompensated

240

Ω

3.3 nF

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