Datasheet

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 * R
DS(on)
* I
LS
) 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 T
ON
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.
R2
R1
Vout sense
FB
-
+
PWM
COMPARATOR
0.9V
0.725
VREF
ToffMIN
S
R
Q
Q
Ton generator
PHASE - GND
+
-
320mV
VIN
GND
PHASE
Vout sense
Α*Rdson*ILS
ILS
Α*Rdson*ILS
+
AM11793v1