L6751B datasheet(42/58 Pages) STMICROELECTRONICS

42 / 58 page

Efficiency optimization

L6751B

42/58

Doc ID 024028 Rev 1

‘ILIM pin is lightly filtered in order to perform fast reaction of OC protection while IMON is

heavily filtered to perform correct averaging of the information.

While working continuously in DPM, the device compares the information of IMON and ILIM:

ILIM voltage is divided in N steps whose width is VOCP/(2*N) (where VOCP = 2.5 V and N the

number of stuffed phases).

If the DPM phase number resulting from IMON is not coherent with the step in which ILIM

stays, the phase number is increased accordingly.

The mechanism is active only to increase the phase number which is reduced again by

DPM.

9.2

Variable frequency diode emulation (VFDE)

As the current required by the load is reduced, the L6751B progressively reduces the

number of switching phases according to DPM settings on the multi-phase section. If single-

phase operation is configured, when the delivered current approaches the CCM/DCM

boundary, the controller enters VFDE operation. Single-phase section, being a single-

phase, enters VFDE operation always when the delivered current approaches the

CCM/DCM boundary.

In a common single-phase DC-DC converter, the boundary between CCM and DCM is

when the delivered current is perfectly equal to 1/2 of the peak-to-peak ripple into the

inductor (Iout = Ipp/2).

Further decreasing the load in this condition maintaining CCM operation would cause the

current into the inductor to reverse, so sinking current from the output for a part of the off-

time. This results in a poorly efficient system.

The L6751B is able (via the CSPx/CSNx pins) to detect the sign of the current across the

inductor (zero cross detection, ZCD), so it is able to recognize when the delivered current

approaches the CCM/DCM boundary. In VFDE operation, the controller fires the high-side

MOSFET for a TON and the low-side MOSFET for a TOFF (the same as when the controller

works in CCM mode) and waits the necessary time until next firing in high impedance (HiZ).

The consequence of this behavior is a linear reduction of the “apparent” switching frequency

that, in turn, results in an improvement of the efficiency of the converter when in very light

load conditions.

The “apparent” switching frequency reduction is limited to 30 kHz so as not to enter the

audible range.

L6751B Datasheet(PDF) 42 Page - STMicroelectronics