L6759D datasheet(35/51 Pages) STMICROELECTRONICS

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L6759D

Efficiency optimization

Doc ID 023240 Rev 1

35/51

9.2

Variable frequency diode emulation (VFDE)

As the current required by the load is reduced, the L6759D 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 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, therefore sinking current from

the output for a part of the off-time. This results in poor system efficiency.

The L6759D 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 for the necessary time until the 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, the “apparent” switching frequency reduction is

limited to 30 kHz.

9.3

Gate drive control (GDC)

Gate drive control (GDC) is a proprietary function which allows the L6759D to dynamically

control the Power MOSFETs driving voltage in order to further optimize the overall system

efficiency. According to the SVI power state commanded and the configuration received

through the PMBus, the device switches this pin (GDC) between the VCC5 or VDRV

(inputs). By connecting the power supply of external drivers directly to this pin, it is then

possible to control carefully the external MOSFET driving voltage.

Figure 12.

Output current vs. switching frequency in PSK mode

Iout = Ipp/2

Iout < Ipp/2

Tsw

Tvfde

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