L6714 View Datasheet(PDF) - STMicroelectronics
Part Name
Description
MFG CO.
L6714
STMicroelectronics
'L6714' PDF : 70 Pages View PDF
L6714
18.1
Over current protection
Low side MOSFET sense over current
The device detects an Over Current condition for each phase when the current information
IINFOx overcomes the fixed threshold of IOCTH (35µA Typ,). When this happens, the device
keeps the relative LS MOSFET on, also skipping clock cycles, until the threshold is crossed
back and IINFOx results being lower than the IOCTH threshold. After exiting the OC condition,
the LS MOSFET is turned off and the HS is turned on with a duty cycle driven by the PWM
comparator.
Keeping the LS on, skipping clock cycles, causes the on-time subsequent to the exit from
the OC condition, driven by the control loop, to increase. Considering now that the device
has a maximum on-time dependence with the delivered current given by the following
relationship:
⎧0.80 ⋅
TON(max)
=
⎨
⎩
0.40
⋅
TSW
TSW
IDROOP = 0µA
IDROOP = 140µA
Where IOUT is the output current ( IOUT = ΣIPHASEx ) and TSW is the switching period
(TSW=1/FSW). This linear dependence has a value at zero load of 0.80·TSW and at maximum
current of 0.40·TSW typical.
When the current information IINFOx overcomes the fixed threshold of IOCTH (35µA Typ), the
device enters in Quasi-Constant-Current operation: the low-side MOSFET stays ON until
the current read becomes lower than IOCPx (IINFOx < IOCTH) skipping clock cycles. The high
side MOSFET can be then turned ON with a TON imposed by the control loop after the LS
turn-off and the device works in the usual way until another OCP event is detected.
This means that the average current delivered can slightly increase in Quasi-Constant-
Current operation since the current ripple increases. In fact, the ON time increases due to
the OFF time rise because of the current has to reach the IOCPx bottom. The worst-case
condition is when the ON time reaches its maximum value.
When this happens, the device works in Constant Current and the output voltage decrease
as the load increase. Crossing the UVP threshold causes the device to latch (Figure 20
shows this working condition).
It can be observed that the peak current (IPEAK) is greater than IOCPx but it can be
determined as follow:
IPEAK
=
IOCPx
+
V-----I-N-----–-----V----O----U----T---(--m----i-n---)
L
⋅
TON(max)
=
IOCPx
+
V-----I-N-----–-----V----O----U----T---(--m----i-n---)
L
⋅
0.40 ⋅
TSW
Where VoutMIN is the UVP threshold, (inductor saturation must be considered). When that
threshold is crossed, all MOSFET are turned OFF and the device stops working. Cycle the
power supply or the OUTEN pin to restart operation.
The maximum average current during the Constant-Current behavior results:
IMAX, tot = N ⋅
IMAX = N ⋅
⎛
⎝
IO
C
P
x
+
-I-P----E---A----K----2–-----I--O----C----P---x-⎠⎞
51/70
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