L6751 datasheet(49/59 Pages) STMICROELECTRONICS

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L6751

System control loop compensation

Doc ID 023992 Rev 1

49/59

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, so 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 required

energy to the output. Typical design considers starting from values in the range of 100 pF,

and validating the effect by bench testing. Additional series resistor (RI) can also be used.

11.2

LTB Technology

LTB Technology further enhances the performance 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 also result as bypassed.

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.

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