
L6712A L6712
12/27
The transconductance resistor Rg can be designed in order to have current information of 25
μ
A per phase
at full nominal load; the over current intervention threshold is set at 140% of the nominal (I
INFOx
= 35
μ
A).
According to the above relationship, the over current threshold (I
OCPx
) for each phase, which has to be
placed at 1/2 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 I
OCPx
(I
INFOx
> 35
μ
A).
■
L6712 - Dynamic Maximum Duty Cycle Limitation
The maximum duty cycle is limited as a function of the measured current and, since the oscillator frequen-
cy is fixed once programmed, imply a maximum on-time limitation as follow (where T is the switching pe-
riod T=1/f
SW
and I
OUT
is the output current):
This linear dependence has a value at zero load of 0.80·T and at maximum current of 0.40·T typical and
results in two different behaviors of the device:
Figure 5. TON Limited Operation
1.TON Limited Output Voltage.
This happens when the maximum ON time is reached before the current in each phase reaches I
OCPx
(I
IN-
FOx
< 35
μ
A).
Figure 5a shows the maximum output voltage that the device is able to regulate considering the T
ON
lim-
itation imposed by the previous relationship. If the desired output characteristic crosses the T
ON
limited
maximum output voltage, the output resulting voltage will start to drop after crossing. In this case, the de-
vice doesn't perform constant current limitation but only limits the maximum duty cycle following the pre-
vious relationship. The output voltage follows the resulting characteristic (dotted in Figure 5b) until UVP is
detected or anyway until I
FB
= 70
μ
A.
I
OCPx
R
SENSE
---------------------------
Rg
I
R
35
μ
A
------------------------------------------
=
=
T
ON,MAX
0.80
I
FB
–
5.73k
(
)
T
0.80
R
Rg
----------------------
I
OUT
5.73k
–
T =
T
0.80 T I
FB
0.40 T I
FB
0
μ
A
70
μ
A
=
=
T
=
=
=
=
0.80·V
IN
0.40·V
IN
V
OUT
I
OUT
I
OCP
=2·I
OCPx
(I
DROOP
=70
A)
T
ON
Limited Output
characteristic
0.80·V
IN
0.40·V
IN
V
OUT
I
OUT
I
OCP
=2·I
OCPx
(I
DROOP
=70
μ
A)
Desired Output
characteristic and
UVP threshold
Resulting Output
characteristic
a
) Maximum output Voltage
b) TON Limited Output Voltage