L6928

Operation description

Doc ID 11051 Rev 6

9/16

minimum value (250 ns typ.) in order to reduce as much as possible

ΔI

Due to this reason a second protection that fixes the maximum inductor valley current has

been introduced. This protection doesn't allow the high side MOSFET to turn on if the

current flowing through the inductor is higher that a specified threshold (valley current limit).

this threshold. So, the maximum peak current in worst case conditions will be:

Equation 3

high side MOSFET.

4.3

Slope compensation

In current mode architectures, when the duty cycle of the application is higher than

approximately 50%, a pulse-by-pulse instability (the so called sub harmonic oscillation) can

occur. To allow loop stability also in these conditions a slope compensation is present. This

is realized by reducing the current flowing through the inductor necessary to trigger the

COMP comparator (with a fixed value for the COMP pin voltage). With a given duty cycle

higher than 50%, the stability problem is particularly present with an higher input voltage

(due to the increased current ripple across the inductor), so the slope compensation effect

increases as the input voltage increases. From an application point of view, the final effect is

that the peak current limit depends both on the duty cycle (if higher than approximately 40%)

and on the input voltage.

4.4

Loop stability

Since the device is realized with a current mode architecture, the loop stability is usually not

a big issue. For most of the application a 220 pF connected between the COMP pin and

ground is enough to guarantee the stability. In case 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 can solve this problem.

The right value for the resistor (in the range of 50 K) can be determined by checking the load

transient response of the device. Basically, the output voltage has to be checked at the

scope after the load steps required by the application. In case of stability problems, the

output voltage could oscillates before to reach the regulated value after a load step.

I

PEAK

I

VALLEY

V

IN

L

--------- T

ON_MIN

⋅

+

=