Datasheet

PM6644 Device description
Doc ID 023203 Rev 1 15/35
The COT architecture bases its algorithm on the output ripple derived across the output
capacitor’s ESR. The controller has an internal on-time (T
ON
) generator triggered on the
output voltage valley: when V
OUT
reaches the regulation value a new T
ON
starts. The T
ON
duration is given by the following equation:
Equation 2
where T
ON
is the on-time duration, C is an integrated capacitance (9.3 pF typ.), R
TON
is the
resistor between the VIN and TON pins, V
OUT
is the sensed output voltage and V
IN
is the
input voltage (sensed at the VIN pin). Figure 19 shows how the on-time is generated.
Figure 19. On-time generator
The duty cycle in a buck converter is:
Equation 3:
The switching frequency in continuous current mode (CCM) is:
Equation 4
In order to reduce noise in T
ON
generation, a further capacitance C
1
may be added between
the TON pin and the GND pin. In this case the switching frequency is:
T
ON
0.9V R
TON
C⋅⋅
V
IN
----------------------------------------------=
TON
VIN
C=9.3pF
BYP
α=0.263
FB
+
-
On time start
TON
t
TON
t
R
TON
AM11779v1
T
ON
T
SW
----------- D
V
OUT
V
IN
--------------==
f
sw
D
T
ON
-----------
V
OUT
V
IN
0.9V R
TON
C⋅⋅
V
IN
----------------------------------------------
----------------------------------------------
-----------------------------------------------
V
OUT
0.9V R
TON
C⋅⋅
----------------------------------------------===