PM6685 datasheet(22/52 Pages) STMICROELECTRONICS

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Device description

PM6685

22/52

The values in the table are measured with Vin =12V, operation mode = PWM and Iload = 2A.

The other output are unloaded.

7.2

Constant on time architecture

Figure 29 on page 23

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

A minimum off-time constrain (380ns typ) is introduced to allow inductor valley current

sensing on the synchronous switch. A minimum on-time(150ns typ) is also introduced to

assure the start-up switching sequence.

PM6685 has a one-shot generator for each power section that turns on the high side

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

is high, the synchronous rectifier current is below the current limit threshold, and the

minimum off-time has timed out. Once the on-time has timed out, the high side switch is

turned off, while the synchronous switch is turned on according to the anti-cross conduction

circuitry management. When the negative input voltage at the PWM comparator, which is a

scaled-down replica of the output voltage ripple (see the Rfb1/Rfb2 divider in Figure 29),

reaches the valley limit (determined by internal reference Vr=0.9V), the low-side MOSFET is

turned off according to the anti-cross conduction logic once again, and a new cycle begins.

Table 6.

FSEL pin selection

FSEL

SMPS 5V

SMPS 3.3V

Frequency

KON

Frequency

KON

SGND

212kHz

4,7us

297,6kHz

3.36us

VREF

323kHz

3us

400kHz

2.5us

LDO5

432kHz

2.31us

500kHz

2.0us

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