l6712a STMicroelectronics, l6712a Datasheet - Page 12

• Manufacturer Part Number: l6712a
• Description: Two-phase Interleaved Dc/dc Controller
• Manufacturer: STMicroelectronics

L6712A L6712
The transconductance resistor Rg can be designed in order to have current information of 25µA per phase
at full nominal load; the over current intervention threshold is set at 140% of the nominal (I
According to the above relationship, the over current threshold (I
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
I
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
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
FOx
Figure 5a shows the maximum output voltage that the device is able to regulate considering the T
itation imposed by the previous relationship. If the desired output characteristic crosses the T
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
12/27
T
ON,MAX
L6712 - Dynamic Maximum Duty Cycle Limitation
0.80·V
0.40·V
< 35µA).
V
OUT
IN
IN
SW
=
a ) Maximum output Voltage
(
and I
0.80 I
OUT
–
FB
T
is the output current):
ON
I
⋅
OCPx
5.73k
Limited Output
FB
characteristic
(I
= 70µA.
I
DROOP
OCP
=
) T
=2·I
⋅
35µA Rg
-------------------------- -
=70µA)
R
OCPx
=
SENSE
§
©
0.80
⋅
I
OUT
–
R
--------------------- - I
SENSE
Rg
⋅
Rg
OUT
0.80·V
0.40·V
=
V
OUT
⋅
I
------------------------------------------ -
OCPx
5.73k
IN
IN
b) TON Limited Output Voltage
OCPx
35µA
⋅
· T =
¹
R
⋅
) for each phase, which has to be
SENSE
­
°
®
°
¯
OCPx
T
T
=
=
(I
0.80 T I
0.40 T I
(I
INFOx
I
DROOP
OCP
=2·I
characteristic and
⋅
⋅
Resulting Output
=70µA)
> 35µA).
Desired Output
OCPx
UVP threshold
characteristic
INFOx
FB
FB
I
ON
OUT
=
=
= 35µA).
OCPx
0µA
70µA
ON
limited
(I
lim-
IN-