L6740L datasheet(35/44 Pages) STMICROELECTRONICS

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L6740L

Output voltage monitoring and protections

35/44

rent 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 IOCP_NB bottom. The worst-case con-

dition is when the ON time reaches its maximum value (see Section 6.8). When this hap-

pens, the Section works in Real Constant Current and the output voltage decrease as the

load increase. Crossing the UV threshold causes the device to latch accordingly.

It can be observed that the peak current (IPEAK_NB) is greater than IOCP_NB but it can be

determined as follow:

Where VOUT(min) is the UV threshold, (inductor saturation must be considered). When that

threshold is crossed, UV is detected. Cycle the power supply or the EN pin to restart opera-

tion.

The maximum average current during the Constant-Current behavior results (see

Figure 14):

in this particular situation, the switching frequency for the NB Section results reduced. The

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

The transconductance resistor RISEN can be designed considering that the Section limits

the inductor current ripple valley. Moreover the additional current due to the output filter

charge during on-the-Fly VID transitions must be considered.

where IOCP_NB is defined above.

Figure 14.

NB Section - Constant current operation.

PEAK_NB

OCP_NB

OUT(min)

–

--------------------------------------- T

ON(max)

⋅

OCP_NB

OUT(min)

–

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

⋅

MAX_NB

OCP_NB

PEAK

OCP_NB

–

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

OFF

PEAK

OCP_NB

–

OUT

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

⋅

ON(max)

OFF

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

ISEN

OCP_NB(max)

dsON(max)

⋅

OCTH_NB(min)

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

VOUT

0.40 VIN

Constant Current (Exploded)

IOCP_NB

(IDROOP_NB = 35µA)

IOUT

UVP Threshold

Voltage Positioning

IMAX_NB

Droop Effect

TON(max)

IOCP_NB

IMAX_NB

IPEAK_NB

LS ON Skipping

Clock Cycles

TSW

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