PM6670AS datasheet(20/54 Pages) STMICROELECTRONICS

20 / 54 page

Device description

PM6670AS

20/54

7.1

VDDQ section - constant On-Time PWM controller

The PM6670AS uses 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, sensed at

VSNS pin, and inversely proportional to the input voltage, sensed at the VOSC pin, as

follows:

Equation 1

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

(40 ns 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 = 0.9 V), the synchronous

rectifier is turned off and a new cycle begins (Figure 26).

Figure 26. Inductor current and output voltage in steady state conditions

OSC

SNS

OSC

Ton

Toff

Inductor

current

Output

voltage

Vreg