LM2633 datasheet(33/40 Pages) NSC

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Control Loop Design (Continued)

TABLE 5. R1 and R2 Values vs. VID

VID4:0

DAC (V)

R2/(R1+R2)

00000

2.00

25k

17.1k

0.41

00001

1.95

25k

18.4k

0.42

00010

1.90

25k

17.4k

0.41

00011

1.85

25k

21.4k

0.46

00100

1.80

25k

19.3k

0.43

00101

1.75

25k

22.0k

0.47

00110

1.70

25k

22.1k

0.47

00111

1.65

25k

30.0k

0.55

01000

1.60

25k

24.5k

0.49

01001

1.55

25k

27.3k

0.52

01010

1.50

25k

26.0k

0.51

01011

1.45

25k

34.6k

0.58

01100

1.40

25k

29.3k

0.54

01101

1.35

25k

36.0k

0.59

01110

1.30

25k

36.4k

0.59

01111

NO CPU

25k

64.3k

0.72

10000

1.275

12.5k

23.2k

0.65

10001

1.250

12.5k

25.7k

0.67

10010

1.225

12.5k

24.5k

0.66

10011

1.200

12.5k

32.1k

0.72

10100

1.175

12.5k

27.5k

0.69

10101

1.150

12.5k

33.3k

0.73

10110

1.125

12.5k

33.6k

0.73

10111

1.100

12.5k

56.2k

0.82

11000

1. 075

12.5k

39.6k

0.76

11001

1.050

12.5k

47.4k

0.79

11010

1.025

12.5k

43.4k

0.78

11011

1.000

12.5k

75.0k

0.86

11100

0.975

12.5k

53.7k

0.81

11101

0.95

12.5k

81.8k

0.87

11110

0.925

12.5k

83.7k

0.87

11111

0.900

12.5k

∞

The signal path from output voltage to control voltage is the

feedback path. It typically contains a voltage divider, an error

amplifier and a compensation network. Those are shown In

Figure 7 as R

1,R2, the gm amplifier, and Zc. For Channel 1

of the LM2633, since an R-2R ladder network is used, R

and R

2 values change with the VID setting. For information

regarding their values and ratios, refer to

Table 5. For Chan-

nel 2, R

1 and R2 are simply the external voltage divider

resistors.

To achieve the gain shape in

Figure 10,Z

c in Figure 7 should

take the form of two RC branches in parallel, as shown in

Figure 11. In the scheme, C1 and R3 form the first zero f

z1,

C2 and R3 form the second pole f

p2, and C2 and R4 form the

second zero f

z2.

The gain of the compensation network can be calculated as

the following. If the ESR zero frequency f

z is higher than the

low frequency pole f

p, then there should be a −20dB/decade

section from f

p (310 Hz) to fz (8.8 kHz) in the plant gain plot,

such as shown in

Figure 9. Find the frequency where this

section (or the extension of this section) crosses 0dB by

using the following equation:

c_o =M •fp

(41)

If the desired loop transfer function cross-over frequency is

c_c, then the gain of the compensation network at fp should

be:

(42)

To determine the component values in

Figure 11, the follow-

ing equations can be used:

(43)

where B is the desired gain at f

z1, and gm is the transcon-

ductance of the error amplifier.

(44)

20000864

FIGURE 10. 2-Pole 2-Zero (lag-lag) Network Asymptotic

Gain Plot

20000865

FIGURE 11. Compensation Network

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