AN2115

Operation description

Doc ID 11165 Rev 5

11/33

4.3

System stability

Since the device is designed with a current mode architecture, loop stability is rarely a

significant issue. For most applications a 220 pF capacitor connected between the COMP

pin and ground is sufficient to guarantee stability. If very low ESR capacitors are used for the

output filter, such as multilayer ceramic capacitors, the zero introduced by the capacitor itself

can shift at very high frequency and the transient loop response could be affected. Adding a

series resistor to the 220 pF capacitor may resolve this problem. The appropriate value for

the resistor (in the range of 50 k

Ω) can be determined by checking the load transient

response of the device. Basically, the output voltage should be checked with an oscilloscope

after the load steps required by the application.

If there are stability problems, the output voltage could oscillate before reaching the

regulated value after a load step. The current mode stability can be observed in two

consecutive steps: first, the inner (current) loop is closed, then the second (voltage) loop

stability is considered.

4.3.1

Current loop compensation

The constant frequency, peak current mode control architecture offers numerous

advantages: easy compensation with ceramic output capacitors, fast transient response and

intrinsic peak current measurement which simplifies the current limit protection.

One 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 an external ramp to the one coming from the sensed current (or, by subtracting it

from the control value, which is the E/A output voltage).

This additional ramp is called "slope compensation". In the L6928 the slope compensation is

implemented from a duty cycle of around 25% - 30%, as shown in Figure 8.

Figure 8.

Slope compensation

The figure above shows that the current limit value will depend on the duty factor, so

changing the output voltage will also change the maximum output load.

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.