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L6919E
Figure 4. Current Reading Timing (Left) and Circuit (Right)
Where RSENSE is an external sense resistor or the rds,on of the low side mosfet and Rg is the transconductance
resistor used between ISENx and PGNDSx pins toward the reading points; IPHASE is the current carried by each
phase and, in particular, the current measured in the middle of the oscillator period
The current information reproduced internally is represented by the second term of the previous equation as
follow:
Since the current is read in differential mode, also negative current information is kept; this allow the device to
check for dangerous returning current between the two phases assuring the complete equalization between the
phase's currents. From the current information of each phase, information about the total current delivered (IFB
=IINFO1 +IINFO2) and the average current for each phase (IAVG =(IINFO1 +IINFO2)/2 ) is taken. IINFOX is then com-
pared to IAVG to give the correction to the PWM output in order to equalize the current carried by the two phases.
The transconductance resistor Rg can be designed in order to have current information of 25
A per phase at
full nominal load; the over current intervention threshold is set at 140% of the nominal (IINFOx = 35A). According
to the above relationship, the over current threshold (IOCPx) for each phase, which has to be placed at one half
of the total delivered maximum current, results:
Since the device senses the output current across the low-side mosfets (or across a sense resistors in series
with them) the device limits the bottom of the inductor current triangular waveform: an over current is detected
when the current flowing into the sense element is greater than IOCPx (IINFOx > 35A).
Introducing now the maximum ON time dependence with the delivered current (where T is the switching period
T=1/fSW):
This linear dependence has a value at zero load of 0.80T and at maximum current of 0.40T typical and results
in two different behaviors of the device:
R
SE
N
S
E
Rg
50
A
IISENx
I
PH
A
S
E
Rg
LGATEX
ISENX
PGNDSX
ILS1
ILS2
Track & Hold
Total current
information
I
IN FO x
R
SENSE
I
PHASE
R
g
----------------------------------------------
=
I
OCPx
35
ARg
R
SEN SE
---------------------------
=
Rg
I
OC P x
R
SE NSE
35
A
-------------------------------------------
=
T
ON,MAX
0.80
I
FB
5.73 k
() T
–
0.80
R
SENSE
Rg
---------------------- I
OUT
5.73k
T
0.80 T I
FB
0
A
=
0.40 T I
FB
70
A
=
–
==