VIN

RSO

VOUT

CL

CF = ¨

¨

©

§R

F + 2RIN

RF

2

CLROUT

RS = ROUTRIN

RF

16

LMV641

SNOSAW3D – SEPTEMBER 2007 – REVISED AUGUST 2016

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Device Functional Modes (continued)

The values for RS and CF are decided by ensuring that the zero attributed to CF lies at the same frequency as the

pole attributed to CL. This ensures that the effect of the second pole on the transfer function is compensated for

by the presence of the zero, and that the ROC is maintained at 20 dB/ decade. For the circuit shown in Figure 37

the values of RS and CF are given by Equation 1. Values of RS and CF required for maintaining stability for

different values of CL, as well as the phase margins obtained, are shown in Table 1. RF and RIN are 10 kΩ, RL is

2 k

Ω, while ROUT is 680Ω.

(1)

Table 1. Loop Compensation Stability

CL (nF)

RS (Ω)

CF (pF)

PHASE MARGIN (°)

0.5

680

10

17.4

1

680

20

12.4

1.5

680

30

10.1

The LMV641 is capable of driving heavy capacitive loads of up to 1 nF without oscillating, however it is

recommended to use compensation should the load exceed 1 nF. Using this methodology will reduce any

excessive ringing and help maintain the phase margin for stability. The values of the compensation network

tabulated above illustrate the phase margin degradation as a function of the capacitive load.

Although this methodology provides circuit stability for any load capacitance, it does so at the price of bandwidth.

The closed loop bandwidth of the circuit is now limited by RF and CF.

7.4.1.2 Compensation by External Resistor

In some applications it is essential to drive a capacitive load without sacrificing bandwidth. In such a case, in the

loop compensation is not viable. A simpler scheme for compensation is shown in Figure 38. A resistor, RISO, is

placed in series between the load capacitance and the output. This introduces a zero in the circuit transfer

function, which counteracts the effect of the pole formed by the load capacitance, and ensures stability. The

value of RISO to be used should be decided depending on the size of CL and the level of performance desired.

Values ranging from 5

Ω to 50Ω are usually sufficient to ensure stability. A larger value of RISO will result in a

system with less ringing and overshoot, but will also limit the output swing and the short circuit current of the

circuit.

Figure 38. Compensation by Isolation Resistor