PM6675STR datasheet(38/53 Pages) STMICROELECTRONICS

38 / 53 page

Application information

PM6675S

38/53

Application information

The purpose of this chapter is showing the design procedure of the switching section.

The design starts from three main specifications:

●

The input voltage range, provided by the battery or the external supply. The two

extreme values (VINMAX and VINmin ) are important for the design.

●

The maximum load current, indicated with ILOAD,MAX.

●

The maximum allowed output voltage ripple VRIPPLE,MAX.

It's also possible that specific designs should involve other specifications.

The following paragraphs will guide the user into a step-by-step design.

8.1

External components selection

The PM6675S uses a pseudo-fixed frequency, Constant On-Time (COT) controller as the

core of the switching section. The switching frequency can be set by connecting an external

divider to the VOSC pin. The voltage seen at this pin must be greater than 0.8V and lower

than 2 V in order to ensure system linearity.

Nearly constant switching frequency is achieved by the system loop in steady-state

operating conditions by varying the On-Time duration, avoiding thus the need for a clock

generator. The On-Time one shot duration is directly proportional to the output voltage,

sensed at VSNS pin, and inversely proportional to the input voltage, sensed at the VOSC

pin, as follows:

Equation 25

where KOSC is a constant value (130 ns typ.) and τ is the internal propagation delay

(40 ns typ.).

The duty cycle of the buck converter is, in under steady state conditions, given by

Equation 26

The switching frequency is thus calculated as

Equation 27

OSC

SNS

OSC

OUT

OSC

OUT

OSC

SNS

OSC

OUT

⋅

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