TLC225xEP, TLC225xAEP

Advanced LinCMOS RAILTORAIL

VERY LOW POWER OPERATIONAL AMPLIFIERS

SGLS216 − NOVEMBER 2003

29

POST OFFICE BOX 655303

• DALLAS, TEXAS 75265

APPLICATION INFORMATION

driving large capacitive loads

The TLC225x is designed to drive larger capacitive loads than most CMOS operational amplifiers. Figure 56

and Figure 57 illustrate its ability to drive loads up to 1000 pF while maintaining good gain and phase margins

(Rnull = 0).

A smaller series resistor (Rnull) at the output of the device (see Figure 60) improves the gain and phase margins

when driving large capacitive loads. Figure 56 and Figure 57 show the effects of adding series resistances of

10

Ω, 50 Ω, 100 Ω, 200 Ω, and 500 Ω. The addition of this series resistor has two effects: the first is that it adds

a zero to the transfer function and the second is that it reduces the frequency of the pole associated with the

output load in the transfer function.

The zero introduced to the transfer function is equal to the series resistance times the load capacitance. To

calculate the improvement in phase margin, equation 1 can be used.

∆φ

m1 + tan

–1 2 × π× UGBW × R

null ×

C

L

Where :

(1)

∆φ

m1 + Improvement in phase margin

UGBW

+ Unity-gain bandwidth frequency

R

null + Output series resistance

C

L + Load capacitance

The unity-gain bandwidth (UGBW) frequency decreases as the capacitive load increases (see Figure 58). To

use equation 1, UGBW must be approximated from Figure 58.

Using equation 1 alone overestimates the improvement in phase margin, as illustrated in Figure 59. The

overestimation is caused by the decrease in the frequency of the pole associated with the load, thus providing

additional phase shift and reducing the overall improvement in phase margin.

Using Figure 60, with equation 1 enables the designer to choose the appropriate output series resistance to

optimize the design of circuits driving large capacitance loads.

50 k

Ω

50 k

Ω

VDD −/ GND

VDD +

Rnull

CL

VI

+

−

Figure 60. Series-Resistance Circuit