AN1882 Datasheet, PDF (7/30 Pages) STMicroelectronics – This application note details the main features

AN1882
3
Functional description
Functional description
The main loop uses 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 increases with a slope fixed by the operating conditions. When the sensed current
(a part of the high side current) reaches the output value of the error amplifier E/A, COMP
pin, the internal logic turns off the high side MOSFET and turns on the low side one until the
next clock cycle begins or the current flowing through it goes down to zero (ZERO
CROSSING comparator). During the load transients, the voltage control loop keeps the
output voltage in regulation changing the COMP pin value, fixing a new turn off threshold.
Moreover, during these dynamic conditions the choke must not saturate and the inductor
peak current must never exceed the maximum value. This value is function of the internal
slope compensation (see related section).
3.1
3.1.1
3.1.2
Modes of operation
Low consumption mode
At light load, the device operates in burst mode in order to keep the efficiency very high also
in these conditions.
While the device is not switching the load discharges the output capacitor and the output
voltage goes down. The COMP pin, due to the feedback loop, increases and when a fixed
internal threshold is reached, the device starts to switch again. In this condition the peak
current limit is set approximately in the range of 200 mA-400 mA, depending on the slope
compensation (see related section). Once the device starts to switch the output capacitor is
recharged. The repetition time of the bursts depend on parameters like input and output
voltages, load, inductor and output capacitors.
Between two bursts, most of the internal circuitries are off, so reducing the device
consumption down to a typical value of 25 µA. During the burst, the frequency of the pulses
is equal to the internal frequency.
Low noise mode
In case the very low frequencies generated by the low consumption mode are undesirable,
the low noise mode can be selected. The efficiency is a little bit lower compared with the low
consumption mode conditions when working close to zero loads, while the trend is to reach
the efficiency of low consumption mode for intermediate light loads.
The device could skip some cycles in order to keep the output voltage in regulation. In the
Figure 5 and 6 the LCM and LNM typical waveforms are shown.
Doc ID 10209 Rev 4
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