L6701 datasheet(32/44 Pages) STMICROELECTRONICS

32 / 44 page

13 Output voltage Monitor and Protections

L6701

32/44

inductor current, the ripple entity, when not negligible, impacts on the real OC threshold value

and must be considered.

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

IISENx overcomes the fixed threshold of IOCTH. When this happens, the device keeps the

relative LS MOSFET on, also skipping clock cycles, until the threshold is crossed back and

IISENx 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.

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 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 (see Section 10.5). When this happens, the device

works in Real Constant Current and the output voltage decrease as the load increase. Crossing

the UVP threshold causes the device to latch driving high the OSC pin.

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 MOSFETs are turned off, the FAULT pin is driven high and the device

stops working. Cycle the power supply or the EN pin to restart operation.

The maximum average current during the Constant-Current behavior results (see Figure 15):

in this particular situation, the switching frequency for each phase results reduced. The ON time

is the maximum allowed TON(max) while the OFF time depends on the application:

Figure 15. Constant Current Operation

PEAK

OC Px

OU T m in

()

–

------------------------------------------- TON max

()

⋅

OCPx

OUT min

()

–

------------------------------------------- 0.40 T

⋅

MAX tot

MAX

⋅

OC Px

PEAK

OCPx

–

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

⎝⎠

⎛⎞

⋅

OFF

PEAK

OC Px

–

OU T

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

⋅

ON max

()

OFF

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

VOUT

0.40 VIN

Constant Current (Exploded)

IOCP = N x IOCPx

(IDROOP = N x 35µA)

IOUT

UVP Threshold

Resulting Out. Char.

IMAX,tot

Limted-TON Char.

Droop Effect

TON(max)

IOCPx

IMAX

IPEAK

LS ON Skipping

Clock Cycles

TSW

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