PM6685 datasheet(25/52 Pages) STMICROELECTRONICS

25 / 52 page

PM6685

Device description

25/52

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

33kHz 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.

Figure 32. Frequency clamp skip mode

The low side switch is turned on until the output voltage crosses about Vreg+1%. Then the

high side MOSFET is turned on for a fixed on time period. Afterwards the low side switch is

enabled until the inductor current reaches the zero-crossing threshold. This keeps the

switching frequency higher than 33kHz. As a consequence of the control, the regulated

voltage can be slightly higher than Vreg (up to 1%).

Time

Ton1 Toff

Inductor current

PWM mode

Pulse skip mode

Ton2

Low side off

Toff

Low side on

Load current

Ton1=Ton2

Time

Inductor current

No-audible skip mode

Low side

∼30us

PM6685 Datasheet(PDF) 25 Page - STMicroelectronics