L6928 View Datasheet(PDF) - STMicroelectronics
Part Name
Description
MFG CO.
'L6928' PDF : 16 Pages View PDF
L6928
5.1.2
5.1.3
5.2
Detailed description
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 switching. In this phase, the internal circuitries are off, so to reduce the
device consumption down to a typical value of 25 μA.
Low noise mode
If the 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 and 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.
Synchronization
The device can also be synchronized by an external signal from 1 MHz up to 2 MHz. In this
case the low noise mode is automatically selected. The device skips some cycles in very
light load conditions. The internal synchronization circuit is inhibited in short-circuit and
overvoltage conditions in order to keep the protection effective.
Short-circuit protection
During the device operation, the inductor current increases during the high-side turn-on
phase and decreases during the high-side turn-off phase based on the following equations:
Equation 1:
ΔION
=
(---V----I-N-----–----V----O----U---T---)-
L
.
TON
Equation 2:
ΔIO F F=
(---V----O-----U-----T- ).
L
TO
FF
In strong overcurrent or short-circuit conditions, the VOUT can be very close to zero. In this
case ΔION increases and ΔIOFF decreases. When the inductor peak current reaches the
current limit, the high-side MOSFET turns off and so the TON is reduced down to the
minimum value (250 ns typ.) in order to reduce as much as possible ΔION. Anyway, if VOUT
is low enough, the inductor peak current increases furtherly because during the TOFF the
current decays very slowly. Due to this reason a second protection, fixing 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 than a specified
threshold (valley current limit). Basically the TOFF increases as much as required to bring
the inductor current down to this threshold. So, the maximum peak current in worst case
conditions is:
Equation 3:
. IPEA K= IV ALL EY
+
V-----I-N--
L
TON _MIN
Where IPEAK is the valley current limit (1.4 A typ.) and TON_MIN is the minimum TON of the
high-side MOSFET.
DocID11051 Rev 9
11/21
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