AN1882

Functional description

Doc ID 10209 Rev 4

9/30

The current mode stability can be studied in two consecutive steps; first the inner loop is

closed (current loop) and then the second loop stability is considered (voltage loop).

3.2.1

Current loop compensation

The selected control architecture brings many advantages: easy compensation with ceramic

capacitors, fast transient response and intrinsic peak current measurement that simplify the

current limit protection. A known drawback, however, is that the current loop becomes

unstable, when the duty cycle exceeds 50%.

This phenomenon is known as "sub-harmonic oscillation" and can be avoided by adding a

slope compensation signal. Due to this fact, the current limit of the device decreases when

the slope compensation signal is applied. The slope compensation is internally implemented

from a duty around 30% and

Figure 7 shows how the slope compensation affects the device

current limit.

Figure 7.

Slope compensation

The amount of slope compensation depends on the inductor current slope during the OFF

time. This slope, for a given duty cycle, is inversely proportional to the inductor value. Since

the device can be synchronized at higher frequency, it is reasonable to calculate the inductor

value in terms of it. Finally, the input voltage affects the OFF time slope as well. This is

obvious because, for a given duty cycle, the output voltage (and so the OFF time inductor

current slope) is directly proportional to the input one. In order to better manage these

issues, the amount of slope compensation does not depend only on the duty cycle but also

on the switching frequency and the input voltage.

Table 2.

Suggested inductor values for different switching frequencies, at

Minimum inductor value [µH]

600

6.8

1000

3.6

1400

2.7