PM6681ATR datasheet(19/47 Pages) STMICROELECTRONICS

19 / 47 page

PM6681A

Device description

19/47

Figure 30.

Constant on time PWM control

The duty cycle of the buck converter in steady state is:

Equation 2

The PWM control works at a nearly fixed frequency fSW:

Equation 3

As mentioned the steady state switching frequency is theoretically independent from battery

voltage and from output voltage.

Actually the frequency depends on parasitic voltage drops that are present 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.

Standard switching frequency values can be selected for both sections by connecting pin

FSEL to SGND, VREF or LDO5 pin. The following table shows the typical switching

frequencies that can be obtained as a function of the programmed output voltage. The

measures are referred to switching sections with 2 A load, 12 V input voltage and working in

continuous conduction mode.

Table 7.

FSEL pin selection: typical switching frequency

Fsw @ OUT1 = 1.5 V (kHz)

Fsw @ OUT2 = 1.05 V (kHz)

FSEL = GND

200

325

FSEL = VREF

290

425

FSEL = LDO5

390

590

Vin

Vout

Vin

Vout

Vin

Vout

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