-60

-40

60

10

1

10

2

10

3

10

4

10

5

Frequency (Hz)

-20

0

20

40

10

6

0

-50

-100

-150

-180

-250

-200

BW = 16.7 kHz

PM = 61 degrees

CC2 =

50 x 10

3 + 900

2S (.159 x 10

3) (50 x 103) (900)

= 1.1 nF

1

2SCOUTRESR

250kHz

<

2S(20 x 10

3)(900)

3.16

1

d

d

CC1

d

d

CC1

28 nF

62 nF

2S(2.86 x 10

3)(900)

RC =

= 904:

(15.5)(0.001)(50 x 10

3)(0.508)(2.86 x 103) - (20 x 103)

(20 x 10

3)(50 x 103)

LM3477

SNVS141K – OCTOBER 2000 – REVISED MARCH 2013

www.ti.com

(57)

This sets the high frequency gain of the compensator such that a crossover frequency of fC is obtained. The

capacitor CC1 sets the compensator zero, fZ2. Set fZ2 between the power pole fP1 and the ½ decade before the

target crossover frequency fC:

(58)

Choosing CC1 = 62 x 10

−9 F will set f

Z2 = fP1, canceling out the power pole and insuring a −20dB/decade slope in

the low frequency magnitude response. In other words, the phase margin below the crossover frequency will

always be higher than the phase margin at the crossover frequency.

If better transient response times are desired, a second method is to set fZ2 between fP1 and ½ decade before fC,

the target crossover frequency. This trades more low frequency gain for less phase margin, which translates to

faster but more oscillatory step responses. We pick CC1 = 47nF.

If the esr zero of the output capacitor (fESR) is too low or if more phase margin is required, additional components

may be added to increase the flexibility of the compensator.

Use CC2 if fESR < ½ fS, that is if:

(59)

For this example, fESR = 159 kHz, so use CC2.

(60)

The equations used here for RC, CC1, and CC2 are approximations valid when CC2 << CC1. For exact equations,

see PLOTTING THE OPEN LOOP RESPONSE. In some cases, the desired inductance is several times higher

than the optimal inductance set by the internal slope compensation. This results in a Q lower than 0.15, in which

case additional methods of compensating are presented (see SAMPLING POLE QUALITY FACTOR).

Figure 34. Open Loop Frequency Response for LM3477 Compensation Design Example

26

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