PM6681A STMicroelectronics, PM6681A Datasheet - Page 20

• Manufacturer Part Number: PM6681A
• Description: Dual synchronous step down controller with adjustable LDO
• Manufacturer: STMicroelectronics

Device description
7.2
7.3
20/47
Constant on time architecture
Figure 31
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.
PM6681A 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 (
down replica of the output voltage (see the external R1/R2 divider in
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 31. Constant on-time block diagram
Output ripple compensation and loop stability
In a classic constant on time control, the system regulates the valley value of the output
voltage and not the average value, as shown in
voltage ripple is source of a DC static error.
To compensate this error, an integrator network can be introduced in the control loop, by
connecting the output voltage to the COMP1/COMP2 (for the OUT1 and OUT2 sections
respectively) pin through a capacitor CINT as in
shows the simplified block diagram of a constant on time controller. A minimum
Figure 30
Figure 32
. In this condition, the output
.
Figure 31
), which is a scaled-
Figure 32
), reaches the
PM6681A