PM6675 datasheet(15/47 Pages) STMICROELECTRONICS

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PM6675

Device description

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6.1

Switching section - constant on-time PWM controller

The PM6675 employees a pseudo-fixed frequency, Constant On-Time (COT) controller as

the core of the switching section. It is well known that the COT controller uses a relatively

simple algorithm and uses the ripple voltage derived across the output capacitor's ESR to

trigger the On-Time one-shot generator. In this way, the output capacitor's ESR acts as a

current sense resistor providing the appropriate ramp signal to the PWM comparator.

Nearly constant switching frequency is achieved by the system's loop in steady-state

operating conditions by varying the On-Time duration, avoiding thus the need for a clock

generator. The On-Time one shot duration is directly proportional to the output voltage,

detected by theVSNS pin, and inversely proportional to the input voltage, detected by the

the VOSC pin, as follows:

Equation 1

where KOSC is a constant value (130ns typ.) and τ is the internal propagation delay

(40ns typ.). The one-shot generator directly drives the high-side MOSFET at the beginning

of each switching cycle allowing the inductor current to increase; after the On-Time has

expired, an Off-Time phase, in which the low-side MOSFET is turned on, follows. The Off-

Time duration is solely determined by the output voltage: when lower than the set value (i.e.

the voltage at VSNS pin is lower than the internal reference VR = 0.6V), the synchronous

rectifier is turned off and a new cycle begins (Figure 4).

Figure 4.

Inductor current and output voltage in steady state conditions

OSC

SNS

OSC

Ton

Toff

Inductor

current

Output

voltage

Vreg

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