L6928 View Datasheet(PDF) - STMicroelectronics
Part Name
Description
MFG CO.
'L6928' PDF : 16 Pages View PDF
Detailed description
L6928
5
Detailed description
The main loop uses slope compensated PWM current mode architecture. For each cycle,
the high-side MOSFET is turned on, triggered by the oscillator, so that the current, flowing
through it (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 one until the next
clock cycle begins or the current, flowing through it, goes down to zero (zero-crossing
comparator). The peak inductor current required to trigger PEAK_CL depends on the slope
compensation signal and on the output of the error amplifier. In particular, the error
amplifier output depends on the VFB pin voltage. When the output current increases, the
output capacitor is discharged and the VFB pin decreases. This produces the error
amplifier output rise, so to allow 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, so to reduce the peak inductor current to meet the new load requirements.
The slope compensation signal allows the loop stability in high duty cycle conditions.
Figure 3: Device block diagram
SYNC
RUN
VCC
COM P
LOLOWW
NNOOISISEE/
CCOONNSSUUMMPPTTIOIONN
OOSSCCIILLLATTOORR
GND SENSE
PMOS
SLOPE
P O WER
PMOS
FB
E/A
VVREF
LLOOOPP
CCOONTTRROOLL
PEAK
CL
GND
DDRRIVIVEERR
LX
0.6 V
OOVVPP
PG OOD
VREF
PGOOD
0.9 V
Vcc
Zero-crossing
GND
SENSE Vcc
NMOS
VALLEY
CL
POWER
NMOS
GN D GIPG140420151256LM
5.1
5.1.1
10/21
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 1.3 V) the low consumption mode is selected
while the low noise mode is selected if the SYNC pin is low (lower than 0.5 V).
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 in these conditions. While the
device doesn't switch, 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 switching
and the output capacitor is recharged. In these conditions, the peak current limit is set
approximately in the range of 200 mA to 400 mA, depending on the slope compensation.
DocID11051 Rev 9
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