Datasheet

Operation description L6928
8/16 Doc ID 11051 Rev 7
4.1.1 Low consumption mode
In this mode of operation, at light load, the device operates discontinuously based on the
COMP pin voltage, 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. When the feedback voltage goes lower than the internal reference, the COMP
pin voltage increases and when an internal threshold is reached, the device starts to switch.
In these conditions 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 feedback pin
increases and, when it reaches a value slightly higher than the reference voltage, the output
of the error amplifier goes down until a clamp is activated. At this point, the device stops to
switch. In this phase, most of the internal circuitries are off, so reducing the device
consumption down to a typical value of 25 µA.
4.1.2 Low noise mode
If for noise reasons, the very low frequencies of the low consumption mode are undesirable,
the low noise mode can be selected. In low noise mode, the efficiency is a little bit lower
compared with the low consumption mode in very light load conditions but for medium-high
load currents the efficiency values are very similar.
Basically, the device switches with its internal free running frequency of 1.4 MHz. Obviously,
in very light load conditions, the device could skip some cycles in order to keep the output
voltage in regulation.
4.1.3 Synchronization
The device can also be synchronized with an external signal from 1 MHz up to 2 MHz.
In this case the low noise mode is automatically selected. The device will eventually skip
some cycles in very light load conditions. The internal synchronization circuit is inhibited in
short-circuit and overvoltage conditions in order to keep the protections effective (see
relative sections).
4.2 Short circuit protection
During the device operation, the inductor current increases during the high side turn ON
phase and decrease during the high side turn off phase based on the following equations:
Equation 1
Equation 2
In strong overcurrent or short-circuit conditions the V
OUT
can be very close to zero. In this
case ΔI
ON
increases and ΔI
OFF
decreases. When the inductor peak current reaches the
ΔI
ON
V
IN
V
OUT
–()
L
----------------------------------
T
ON
⋅=
ΔI
OFF
V
OUT
()
L
-------------------
T
OFF
⋅=