PM6675AS datasheet(18/48 Pages) STMICROELECTRONICS

18 / 48 page

Device description

PM6675AS

18/48

Referring to the typical application schematic (fig. 1 and 23), the final expression is then:

Figure 23.

Switching frequency selection and VOSC pin

Equation 5

Even if the switching frequency is theoretically independent from input and output voltages,

parasitic parameters involved in power path (like MOSFET on-resistance and inductor DCR)

introduce voltage drops responsible of a slight dependence on load current.

In addition, the internal delay is cause of a light dependence from input voltage.

The PM6675AS switching frequency can be set by an external divider connected to the

VOSC pin.

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

ensure system's linearity.

7.1.1

Constant-on-time architecture

Figure 24

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

The switching regulator of the PM6675AS owns 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 constrain (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=0.6 V), the low-side MOSFET is turned off according to the anti-cross conduction logic

once again, and a new cycle begins.

PM6675AS

VOSC

VIN

OSC

⋅

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