PM6670AS datasheet(20/54 Pages) STMICROELECTRONICS

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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

Ton

Toff

Inductor

current

Output

voltage

Vreg

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