L6718 datasheet(48/71 Pages) STMICROELECTRONICS

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L6718 power manager

L6718

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DocID023399 Rev 3

Delay in the intervention of DPM can be set using a filter capacitor on the IMON pin. Higher

capacitance can be used to increase the DPM intervention delay.

7.4

VFDE

In both rails, if the delivered current is low that the CCM/DCM boundary is reached, the

controller is able to enter variable frequency diode emulation. As a consequence, the

switching frequency decreases in order to reach high efficiency performance.

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). A further decrease of the load in this condition, maintaining CCM operation,

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

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

The L6718 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 into the audible range, the “apparent” switching frequency is reduced to

around 30 kHz by default, but this function can be disabled using the SMBus interface in

order to reach an even lower switching frequency.

Using the SMBus interface, VFDE (enable by default) can easily turn on/off on each rail

while, with SMBus OFF, it is possible to enable/disable VFDE by CONFIG3 for both rails.

When SWAP mode is enabled, the VFDE is disabled in the single-rail section and any

configuration command for this rail (by SMBus or pinstrapping) is ignored.

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

Iout = Ipp/2

Iout < Ipp/2

Tsw

Tvfde

AM12883v1

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