PM6675STR datasheet(22/53 Pages) STMICROELECTRONICS

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Device description

PM6675S

22/53

Figure 33.

Switching frequency selection and VOSC pin

The voltage seen at this pin must be greater than 0.8 V and lower than 2 V in order to

ensure the system linearity.

7.1.1

Constant-On-Time architecture

Figure 34

shows the simplified block diagram of the Constant-On-Time controller.

The switching regulator of the PM6675S controls a one-shot generator that turns on the

high-side MOSFET when the following conditions are simultaneously satisfied: the PWM

comparator is high (i.e. output voltage is lower than Vr = 0.6 V), the synchronous rectifier

current is below the current limit threshold and the minimum off-time has expired.

A minimum Off-Time contraint (300ns typ.) is introduced to assure the boot capacitor charge

and allow inductor valley current sensing on low-side MOSFET. A minimum On-Time is also

introduced to assure the start-up switching sequence.

Once the On-Time has timed out, the high side switch is turned off, while the synchronous

rectifier is ignited according to the anti-cross conduction management circuitry.

When the output voltage reaches the valley limit (determined by internal reference

Vr = 0.6 V), the low-side MOSFET is turned off according to the anti-cross conduction logic

once again, and a new cycle begins.

PM6675

VOSC

VIN

PM6675

VOSC

VIN

PM6675S

PM6675STR Datenblatt(PDF) 22 Page - STMicroelectronics