PM6686 datasheet(26/50 Pages) STMICROELECTRONICS

26 / 50 page

Device description

PM6686

26/50

Doc ID 15281 Rev 4

8.1.2

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 (TON) generator triggered on the

output voltage valley: when VOUT reaches the regulation value a new TON starts. The TON

duration is given by the following equation:

Equation 2

Where TON is the on time duration, K is a constant, VOUT is the sensed output voltage and

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.

Frequency configurations

TON

SMPS 1

SMPS 2

Frequency

VCC

200 kHz

5 µs

300 kHz

3.33 µs

VREF2 or open

400 kHz

2.5 µs

300 kHz

3.33 µs

GND

400 kHz

2.5 µs

500 kHz

2 µs

OUT

⋅

OUT

⋅

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