PM6670 View Datasheet(PDF) - STMicroelectronics
Part Name
Description
MFG CO.
'PM6670' PDF : 53 Pages View PDF
PM6670
Device description
The duty-cycle of the buck converter is, in steady-state conditions, given by
Equation 2
D = V-----O----U---T--
VIN
The switching frequency is thus calculated as
Equation 3
where
VOUT
fSW
=
D
TON
=
VIN
K OSC
VSNS
VOSC
= αOSC ⋅ 1
αOUT KOSC
Equation 4a
αOSC
=
V-----O----S---C--
VIN
Equation 4b
αOUT
=
-V----S---N----S--
VOUT
Referring to the typical application schematic (figures on cover page and Figure 29), the
final expression is then:
Equation 5
fSW
=
α OSC
K OSC
= R2
R1 + R2
⋅1
K OSC
Even if the switching frequency is theoretically independent from battery and output
voltages, parasitic parameters involved in power path (like MOSFETs' on-resistance and
inductor's DCR) introduce voltage drops responsible for slight dependence on load current.
In addition, the internal delay is due to a small dependence on input voltage. The PM6670
switching frequency can be set by an external divider connected to the VOSC pin.
Figure 29. Switching frequency selection and VOSC pin
VIN
PM6670
R1
VOSC
R2
The voltage seen at this pin must be greater than 0.8V and lower than 2V in order to ensure
the system's linearity.
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