PM6670AS datasheet(40/54 Pages) STMICROELECTRONICS

40 / 54 page

Application information

PM6670AS

40/54

where

Equation 27a

Equation 27b

Referring to the typical application schematic (figure in cover page and Figure 27), the final

expression is then:

Equation 28

The switching frequency directly affects two parameters:

●

Inductor size: greater frequencies mean smaller inductances. In most of the

applications, real estate solutions (i.e. low-profile power inductors) are mandatory also

with high saturation and r.m.s. currents.

●

Efficiency: switching losses are proportional to the frequency. Generally, higher

frequencies imply lower efficiency.

Even if the switching frequency is theoretically independent from input and output voltages,

parasitic parameters involved in power path (like MOSFETs' on-resistance and inductor's

DCR) introduce voltage drops responsible for a slight dependence on load current.

In addition, the internal delay is due to a light dependence on input voltage.

Table 5. Typical values for switching frequency selection

R1 (k

Ω)R2 (kΩ)

Approx switching frequency (kHz)

330

250

330

300

330

350

330

400

330

450

330

500

OSC

OUT

SNS

OUT

OSC

⋅

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