PM6686 STMicroelectronics, PM6686 Datasheet - Page 26

• Manufacturer Part Number: PM6686
• Description: Dual Step-down Controller With Adjustable Voltages, Adjustable Ldo And Auxiliary Charge Pump Controller For Notebook
• Manufacturer: STMicroelectronics

Device description
8.1.2
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Constant on time control (COT)
PM6686 implements a pseudo-fixed frequency algorithm using the COT architecture. The
two sections are completely independent with separated switching controllers (SMPS).
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
output voltage valley: when VOUT reaches the regulation value a new T
duration is given by the following equation:
Equation 2
Where T
VIN is the input voltage.
The duty cycle in a buck converter is:
Equation 3
The switching frequency in continuous current mode (CCM) is:
Equation 4
The switching frequency is theoretically constant, but in a real application it depends on
parasitic voltage drops that occur during the charging path (high-side switch resistance,
inductor resistance (DCR)) and discharging path (low-side switch resistance, DCR). As a
result the switching frequency increases as a function of the load current. The following
table shows the switching frequencies that can be selected through TON pin:
Table 8.
VREF2 or open
GND
TON
VCC
ON
is the on time duration, K is a constant, V
Frequency configurations
Frequency
200 kHz
400 kHz
400 kHz
Doc ID 15281 Rev 4
f
SMPS 1
SW
=
T
T
T
SW
T
ON
ON
D
ON
=
=
2.5 µs
2.5 µs
=
5 µs
D
K
K
K
⋅
=
V
⋅
V
V
OUT
V
V
V
OUT
IN
V
V
IN
OUT
OUT
IN
IN
OUT
=
K
1
ON
is the sensed output voltage and
Frequency
) generator triggered on the
300 kHz
300 kHz
500 kHz
SMPS 2
ON
starts. The T
3.33 µs
3.33 µs
2 µs
K
PM6686
ON