PM6670AS datasheet(22/54 Pages) STMICROELECTRONICS

22 / 54 page

Device description

PM6670AS

22/54

Figure 27. 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's linearity.

7.1.1

Constant-On-Time architecture

Figure 28

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

The switching regulator of the PM6670AS owns a one-shot generator that ignites the high-

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

comparator is high (i.e. output voltage is lower than Vr = 0.9 V), the synchronous rectifier

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

A minimum Off-Time constraint (300 ns 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 = 0.9 V), the low-side MOSFET is turned off according to the anti-cross conduction logic

once again, and a new cycle begins.

PM6670

VOSC

VIN

PM6670AS

PM6670AS Datenblatt(PDF) 22 Page - STMicroelectronics