AN2115

Operation description

Doc ID 11165 Rev 5

9/33

4

Operation description

The main loop uses a constant frequency peak current mode architecture. Each cycle, the

high side MOSFET is turned on, triggered by the oscillator, so that the current flowing

through it (which is the same as the inductor current) increases.

When this current reaches the threshold (set by the output of the error amplifier E/A, the

peak current limit comparator, PEAK_CL, turns off the high side MOSFET and turns on the

low side MOSFET until the next clock cycle begins, or if the current flowing through it

decreases to zero (ZERO CROSSING comparator).

In particular, the error amplifier output is dependent on the FB pin voltage. When the output

current increases, the output capacitor is discharged and so the FB pin voltage decreases.

This produces an increase in the error amplifier output, allowing a higher value for the peak

inductor current. For the same reason, when the output current decreases due to a load

transient, the error amplifier output goes low, thus reducing the peak inductor current to

meet the new load requirements.

The system includes a slope compensation signal, added to the sensed high side ramp

current, which provides loop stability even in high duty cycle conditions (see related

section).

4.1

Light load modes of operation

Depending on the SYNC pin value, the device can operate in LCM (low consumption mode)

or LNM (low noise mode). If the SYNC pin is high (greater than 1.3 V) low consumption

mode is selected while low noise mode is selected if the SYNC pin is low (less than 0.5 V).

4.2

Low consumption mode

In this mode of operation, the device operates discontinuously based on the COMP pin

voltage in order to maintain very high efficiency even in light load conditions. When the

device is not switching, the load discharges the output capacitor and the output voltage

decreases. When the feedback voltage goes below the internal reference, the COMP pin

voltage increases, and when an internal threshold is reached the device begins to switch. In

this condition the peak current limit is set in the range of approximately 200 mA - 400 mA,

depending on the slope compensation (see related section).

Once the device starts to switch, the output capacitor is recharged. The feedback pin

voltage increases and, when it reaches a value slightly higher than the reference voltage,

the output of the error amplifier decreases until a clamp is activated.

At this point the device stops switching. In this phase, most of the internal circuitry is off,

reducing the device’s consumption down to a typical value of 25 µA.

4.2.1

Low noise mode

If for noise considerations, the very low frequencies of low consumption mode are

undesirable, low noise mode can be selected. In low noise mode, efficiency is slightly lower

compared to low consumption mode in very light load conditions, but for medium-high load

currents the efficiency values are very similar.