L6714 datasheet(51/70 Pages) STMICROELECTRONICS

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L6714

Over current protection

51/70

18.1

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:

Where IOUT is the output current (

) 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:

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:

ON max

()

0.80

DROOP

µA

⋅

0.40

DROOP

140

µA

⋅

⎩

⎨

⎧

OUT

ΣI

PHASEx

PEAK

OCPx

OUT min

()

–

------------------------------------------

ON max

()

⋅

OCPx

OUT min

()

–

------------------------------------------

0.40

⋅

MAX tot

MAX

⋅

OCPx

PEAK

OCPx

–

------------------------------------

⎝⎠

⎛⎞

⋅

L6714 Datasheet(PDF) 51 Page - STMicroelectronics

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