l6928 STMicroelectronics, l6928 Datasheet - Page 8

• Manufacturer Part Number: l6928
• Description: High Efficiency Monolithic Synchronous Step Down Regulator
• Manufacturer: STMicroelectronics

Operation description
4.1
4.1.1
4.1.2
4.1.3
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Modes of operation
Depending on the SYNC pin value the device can operate in low consumption or low noise
mode. If the SYNC pin is high (higher than 1.3V) the low consumption mode is selected
while the low noise mode is selected if the SYNC pin is low (lower than 0.5V).
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 200mA-
400mA, 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 25µA.
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 1.4MHz. 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 with an external signal from 1MHz up to 2MHz.
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 shortcircuit and overvoltage conditions in
order to keep the protections effective (see relative sections).
L6928