SC2446AEVB datasheet(19/27 Pages) SEMTECH

19 / 27 page

 2005 Semtech Corp.

www.semtech.com

POWER MANAGEMENT

SC2446A

Application Information (Cont.)

use R

2 = 770kΩ.

With K = 1, it is further calculated that

oesr

≈

use C

3 = 10pF. The Bode plot of the loop transfer function

(magnitude and phase) is shown in Figure 13

100

1.10

100

69.241

20.73

−

20 log G vc f() C f()

()

⋅

310

100

1.10

88.78

−

92.702

−

arg G vc f() C f()

⋅

()

180

⋅

310

Figure 13. The loop transfer function Bode plot of the

example.

It is clear that the resulted crossover frequency is about

27.1 kHz with phase margin 91o.

It is noted that the current sensing gain k was first

estimated using the DC value in order to quickly get the

compensation parameter value. When the circuit is

operational and stable, one can further improve the

compensation parameter value using AC current sensing

gain. One simple and practical method is to effectively

measure the output current at two points, e.g. I

o1 and Io2

and the corresponding error amplifier output voltage V

and V

c2. Then, the first order AC gain is

−

∆

With this k value, one can further calculate the improved

compensator parameter value using the previous

equations.

For example, if one measured that I

o1=1A, Io2=15A and

c1=2.139V, Vc2=2.457V. k is then calculated as 44.

Substituting this parameter to the equations before, one

can derive that

2 ≈ 2.024nF. Select C2 = 2.2nF.

2 ≈ 127.3kΩ. Select R2 = 127kΩ.

3 ≈ 61.78pF. Select C3 = 47pF

In some initial prototypes, if the circuit noise makes the

control loop jittering, it is suggested to use a bigger C

value than the calculated one here. Effectively, the

converter bandwidth is reduced in order to reject some

high frequency noises. In the final working circuit, the

loop transfer function should be measured using network

analyzer and compared with the design to ensure circuit

stability under different line and load conditions. The load

transient response behavior is further tested and

measured to meet the specification.

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