PM6644 datasheet(17/35 Pages) STMICROELECTRONICS

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PM6644

Device description

Doc ID 023203 Rev 1

17/35

Figure 20.

Constant-on-time controller architecture

The PM6644 has a one-shot generator that turns on the high-side MOSFET when the

following conditions are satisfied simultaneously:

●

The PWM comparator is high

●

The inductor valley current is below the current limit threshold

●

The minimum OFF-time has timed out

A slope proportional to the low-side MOSFET current (A * RDS(on) * ILS) is added at the input

of the PWM comparator in order to ensure stability. The slope determines a load line on the

output voltage of about 0.16

Ω when the controller works in PWM mode.

2.1.4

Skip mode management

To improve efficiency at light load, the PM6644 implements pulse skip operation mode. The

inductor current is sensed and, if it is equal to zero, the synchronous MOSFET is turned off.

As a consequence, the output capacitor is left floating and the discharge depends only on

the current sourced by the load. The new TON starts when the output reaches the voltage

regulation. As a consequence, at light load conditions the switching frequency decreases,

improving the total efficiency of the converter. Working in discontinuous current mode, the

switching and the conduction losses are reduced by skipping some cycles.

If the output load is high enough to make the system work in CCM (continuous conduction

mode), Skip mode is automatically changed into PWM mode.

Vout sense

PWM

COMPARATOR

0.9V

0.725

VREF

ToffMIN

Ton generator

PHASE - GND

320mV

VIN

GND

PHASE

Vout sense

Α*Rdson*ILS

ILS

Α*Rdson*ILS

AM11793v1

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