PM6685 datasheet(21/52 Pages) STMICROELECTRONICS

Part #	PM6685

Description	Dual step-down main supply controller with auxilary voltages for notebook system power

PDF	52 Pages

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Manufacturer	STMICROELECTRONICS [STMicroelectronics]

Direct Link	http://www.st.com

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21 / 52 page

PM6685

Device description

21/52

Figure 28. Constant on time PWM control

The duty cycle D 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 pin FSEL as shown in the following table:

Ton

Toff

Inductor

current

Output

voltage

Vreg

DC error

OUT

PM6685 Datasheet(PDF) 21 Page - STMicroelectronics