PM6680ATR datasheet(19/48 Pages) STMICROELECTRONICS

Part #	PM6680ATR

Description	Dual synchronous step-down controller with adjustable output voltages plus LDO

PDF	48 Pages

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Manufacturer	STMICROELECTRONICS [STMicroelectronics]

Direct Link	http://www.st.com

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19 / 48 page

PM6680A

Device description

19/48

Figure 28.

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 input

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 pin FSEL as

shown in the following table:

Table 6.

FSEL pin selection: typical switching frequency

Fsw@OUT1 = 3.3 V (kHz)

Fsw@OUT2 = 1.8 V (kHz)

FSEL = GND

195

335

FSEL = VREF

295

440

FSEL = LDO5

390

600

OUT

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OUT

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OUT

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⁄

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