L6718 datasheet(64/71 Pages) STMICROELECTRONICS

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System control loop compensation

L6718

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DocID023399 Rev 3

limiting transition corresponds to the load-removal since the inductor results as being

discharged only by VOUT (while it is charged by VIN-VOUT during a load appliance).

Note:

The introduction of a capacitor (CI) in parallel to RFB significantly speeds up the transient

response by coupling the output voltage dV/dt on the FB pin, therefore using the error

amplifier as a comparator. The COMP pin suddenly reacts and, also thanks to the LTB

Technology control scheme, all the phases can be turned on together to immediately give

the output the required energy. A typical design considers starting from values in the range

of 100 pF, validating the effect through bench testing. An additional series resistor (RI) can

also be used.

12.2

LTB technology

LTB Technology further enhances the performances of the controller by reducing the system

latencies and immediately turning ON all the phases to provide the correct amount of energy

to the load optimizing the output capacitor count.

LTB Technology monitors the output voltage through a dedicated pin detecting load-

transients with selected dV/dt, it cancels the interleaved phase-shift, turning on

simultaneously all phases.

The LTB detector is able to detect output load transients by coupling the output voltage

through an RLTB - CLTB network. After detecting a load transient, all the phases are turned

on together and the EA latencies result as bypassed as well.

Sensitivity of the load transient detector can be programmed in order to control precisely

both the undershoot and the ring-back.

LTB Technology design tips.

–

Decrease RLTB to increase the system sensitivity, making the system sensitive to

smaller dVOUT.

–

Increase CLTB to increase the system sensitivity, making the system sensitive to

higher dV/dt.

–

Increase Ri to increase the width of the LTB pulse.

Increase Ci to increase the LTB sensitivity over frequency.

Short LTB pin to GND to disable the function on multi-phase rail. Since LTB technology is

embedded on single-phase rail, SVSEN pin needs to be filtered to disable this feature.

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