PM6675S datasheet(28/53 Pages) STMICROELECTRONICS

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Device description

PM6675S

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7.1.3

Pulse-skip and no-audible pulse-skip modes

High efficiency at light load conditions is achieved by PM6675S by entering the Pulse-Skip

Mode (if enabled). At light load conditions the zero-crossing comparator truncates the low-

side switch On-Time as soon as the inductor current becomes negative; in this way the

comparator determines the On-Time duration instead of the output ripple.

(see Figure 37).

Figure 37.

Inductor current and output voltage at light load with Pulse-Skip

As a consequence, the output capacitor is left floating and its discharge depends solely on

the current drained from the load. When the output ripple on the pin COMP falls under the

reference, a new shot is triggered and the next cycle begins. The Pulse-Skip mode is

naturally obtained enabling the zero-crossing comparator and automatically takes part in the

COT algorithm when the inductor current is about half the ripple current amount, i.e.

migrating from continuous conduction mode (C.C.M.) to discontinuous conduction mode

(D.C.M.).

The output current threshold related to the transition between PWM Mode and Pulse-Skip

Mode can be approximately calculated as:

Equation 19

At higher loads, the inductor current never crosses the zero and the device works in pure

PWM mode with a switching frequency around the nominal value.

A physiological consequence of Pulse-Skip Mode is a more noisy and asynchronous (than

normal conditions) output, mainly due to very low load. If the Pulse-Skip is not compatible

with the application, the PM6675S allows the user to choose between forced-PWM and No-

Audible Pulse-Skip alternative modes (see Section 7.1.4: Mode-of-operation selection on

page 30

for details).

Output

voltage

Inductor

current

Vreg

TON TOFF

TIDLE

OUT

LOAD

)

Skip

PWM

(

⋅

−

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