PM6680 datasheet(25/49 Pages) STMICROELECTRONICS

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PM6680

Device description

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7.4

Pulse skip mode

If the SKIP pin is tied to ground, the device works in skip mode.

At light loads a zero-crossing comparator truncates the low-side switch on-time when the

inductor current becomes negative. In this condition the section works in discontinuous

conduction mode. The threshold between continuous and discontinuous conduction mode

is:

Equation 5

For higher loads the inductor current doesn't cross the zero and the device works in the

same way as in PWM mode and the frequency is fixed to the nominal value.

Figure 31.

PWM and pulse skip mode inductor current

Figure 31 shows inductor current waveforms in PWM and SKIP mode. In order to keep

average inductor current equal to load current, in SKIP mode some switching cycles are

skipped. When the output ripple reaches the regulated voltage Vreg, a new cycle begins.

The off cycle duration and the switching frequency depend on the load condition.

As a result of the control technique, losses are reduced at light loads, improving the system

efficiency.

7.5

No-audible skip mode

If SKIP pin is tied to VREF, a no-audible skip mode with a minimum switching frequency of

33 KHz is enabled. At light load condition, If there is not a new switching cycle within a 30us

(typ.) period, a no-audible skip mode cycle begins.

OUT

)

SKIP

(

ILOAD

−

Time

Ton1 Toff

Inductor current

PWM mode

Pulse skip mode

Ton2

Low side off

Toff

Low side on

Load current

Ton1=Ton2

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