PM6670STR datasheet(22/53 Pages) STMICROELECTRONICS

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Device description

PM6670S

22/53

7.1.1

Constant-on-time architecture

Figure 30

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

The switching regulator of the PM6670S owns a one-shot generator that ignites the high-

side MOSFET when the following conditions are simultaneously satisfied: the PWM

comparator is high (i.e. output voltage is lower than Vr = 0.9 V), the synchronous rectifier

current is below the current limit threshold and the minimum off-time has expired.

A minimum Off-Time constraint (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 = 0.9 V), the low-side MOSFET is turned off according to the anti-cross conduction logic

once again, and a new cycle begins.

Figure 30.

Switching section simplified block diagram

driver

Level

shifter

driver

Ton

Ton--min

min

Anti cross-

conduction

circuitry

Toff

Toff--min

min

PWM Comparator

Positive Current Limit comparator

0.9V

Min

Min fsw

fsw

counter

Zero

Zero--crossing

crossing

Comparator

bandgap

1.236V

PHASE

PGND

LGATE

BOOT

CSNS

COMP

Integrator

VSNS

VBG

PULSE

PULSE -- SKIP

SKIP

MODE

MODE<4V

1-Shot generator

Ton

VSNS

VOSC

DDRSEL

0.9V

100uA

500mV

2.5V

HGATE

VREF

SGND

VOSC

VCC

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