SC2441AEVB datasheet(33/37 Pages) SEMTECH

33 / 37 page

 2006 Semtech Corp.

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POWER MANAGEMENT

SC2441A

Applications Information

The inner current-loop is unstable (sub-harmonic

oscillation) unless the inductor current up-slope is steeper

than the inductor current down-slope. For stable

operation above 50% duty-cycle, a compensation ramp

is added to the sensed-current. In the SC2441A the

compensation ramp is made duty-ratio dependent. The

compensation ramp is approximately

0.734

SLOPE

230mV

(D)

D is the duty ratio.

The slope compensation voltage vs duty ratio is as shown

in Figure 22.

Figure 22. Slope Compensation Voltage Waveform

Illustrated as the picture above, as the duty ratio

increases, the slope compensation voltage added into

the control loop increases too. And the control loop

including the slope compensation is shown in Figure 23.

The voltage transconductance error amplifier (shown in

Figure 24) has a g

m of 315µA/V. C

, C

and R

construct

the compensation network for stable operation with

optimized load transient response.

The feedback gain h and the resistor values are

determined using the equations given in the “Setting the

Output Voltage” section with

Slope Compensation

Voltage

CS+

CS-

x 29

0.5V

BE+1.13V

+ -

Compensation

Network

Comp

PWM

Figure 23. Control Flow Chart with Slope

Compensation

Figure 24. A simple model of current-mode buck

converter

For the rated output current I

o, the first-order gain k is

determined as

k is the product of equivalent current sensing Rs and

current amplifier gain Gca=29. Furthermore the transfer

0.0

0.1

0.2

0.3

0.4

0.5

0.00.2

0.40.6

0.81.0

Duty Ratio (D)

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