13/29
L6712A L6712
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).
■ 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/fSW and IOUT is the output current):
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:
Figure 9. TON Limited Operation
TON Limited Output Voltage.
This happens when the maximum ON time is reached before the current in each phase reaches IOCPx (IIN-
FOx < 35A).
Figure 9a shows the maximum output voltage that the device is able to regulate considering the TON lim- itation imposed by the previous relationship. If the desired output characteristic crosses the TON 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 9b) until UVP is
detected or anyway until IFB = 70A.
Constant Current Operation
This happens when ON time limitation is reached after the current in each phase reaches IOCPx (IINFOx >
35
A).
The device enters in Quasi-Constant-Current operation: the low-side mosfets stays ON until the current
read becomes lower than IOCPx (IINFOx < 35A) skipping clock cycles. The high side mosfets can be turned
ON with a TON imposed by the control loop at the next available clock cycle and the device works in the
I
OCPx
35
ARg
R
SENSE
---------------------------
Rg
I
OCPx RSENSE
35
A
-------------------------------------------
=
T
ON,MAX
0.80
I
FB
–
5.73k
() T0.80
R
SENSE
Rg
---------------------- I
OUT
5.73k
–
T =
T
0.80 T I
FB
0
A
=
=
T
0.40 T I
FB
70
A
=
=
=
=
0.80VIN
0.40VIN
VOUT
IOUT
IOCP=2IOCPx
(IDROOP=70A)
TON Limited Output
characteristic
0.80VIN
0.40VIN
VOUT
IOUT
IOCP=2IOCPx
(IDROOP=70A)
Desired Output
characteristic and
UVP threshold
Resulting Output
characteristic
a
) Maximum output Voltage
b) TON Limited Output Voltage