PM6680ATR datasheet(20/48 Pages) STMICROELECTRONICS

20 / 48 page

Device description

PM6680A

20/48

7.2

Constant on time architecture

Figure 29 shows the simplified block diagram of a constant on time controller. A minimum

off-time constrain (350 ns typ.) is introduced to allow inductor valley current sensing on

synchronous switch. A minimum on-time (130 ns) is also introduced to assure the start-up

switching sequence.

PM6680A 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 (Figure 29), which is a scaled-

down replica of the output voltage (see the external R1/R2 divider in Figure 29), 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 29.

Constant on-time block diagram

In steady state the FB pin voltage is about Vr and the regulated output voltage depends on

the external divider:

Equation 4

OUT

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