L6758ATR datasheet(33/42 Pages) STMICROELECTRONICS

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L6758A

Efficiency optimization

Delay in the intervention of DPM can be adjusted by properly sizing the filter across the

IMON pin. Increasing the capacitance results in increased delay in the DPM intervention.

9.2

Variable frequency diode emulation (VFDE)

As the current required by the load is reduced, the L6758A 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. The 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 in the inductor

(Iout = Ipp/2). Further decreasing the load in this condition maintaining CCM operation

would cause the current in the inductor to reverse, therefore sinking current from the output

for a part of the OFF-time. This results in a poorly efficient system.

The L6758A is able (via 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.

To prevent entering the audible range, “apparent” switching frequency is reduced until

30 kHz.

Figure 9. Output current vs. switching frequency in PSK mode

Iout = Ipp/2

Iout < Ipp/2

Tsw

Tvfde

AM11142v

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