L6711 datasheet(25/38 Pages) STMICROELECTRONICS

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L6711

13.1 LS MOSFET SENSE (CS_SEL=OPEN) OVERCURRENT

The device detects an Over Current condition for each phase when the current information IINFOx over-

comes the fixed threshold of IOCTH (35

µA Typ). When this happens, the device keeps the relative LS mos-

fet on, skipping clock cycles, until the threshold is crossed back and IINFOx results being lower than the

IOCTH threshold. This implies that the device limits the bottom of each inductor current ripple.

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

to increase. Considering now that the device, with this kind of current sense, has maximum on-time de-

pendence with the delivered current given by the following relationship:

Where IOUT is the output current (IOUT =

Σ·IPHASEx) 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

and results in two different over current behaviors of the device:

13.2 TON Limited Output Voltage.

This happens when the maximum ON time is reached before that the current in each phase reaches IOCPx

(IINFOx<IOCTH). Figure 18 shows the maximum output voltage that the device is able to regulate consider-

ing the TON limitation 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 the crossing.

In this case, the device doesn't perform constant current limitation but only limits the maximum ON time

following the previous relationship. The output voltage starts to decrease follows the resulting character-

istic (dotted in Figure 18) until UVP is detected or anyway until IDROOP =105

µA.

Figure 18. TON Limited Operation

13.2.1Constant Current Operation

This happens when the on-time limitation is reached after the valley current in each phase reaches IOCPx

(IINFOx > IOCTH).

The device enters in Quasi-Constant-Current operation: the low-side mosfets 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

ON,MAX

0.80

DROOP

–

3.8k

⋅

() T

0.80

SEN SE

---------------------- I

OU T

3.8 k

⋅⋅

–

⎝⎠

⎛⎞

SW =

0.80 T

DROOP

µA

⋅

0.40 T

DROOP

105

µA

⋅

⎩

⎪

⎨

⎪

⎧

⋅

0.80·VIN

0.40·VIN

VOUT

IOUT

IOCP=3·IOCPx

(IDROOP=105

µA)

TON Limited Output

characteristic

0.80·VIN

0.40·VIN

VOUT

IOUT

IOCP=3·IOCPx

(IDROOP=105

µA)

Desired Output

characteristic and

UVP threshold

Resulting Output

characteristic

Maximum output Voltage

b) TON Limited Output Voltage

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