PM6685 View Datasheet(PDF) - STMicroelectronics
Part Name
Description
MFG CO.
PM6685
STMicroelectronics
'PM6685' PDF : 52 Pages View PDF
PM6685
Figure 28. Constant on time PWM control
Inductor
current
Device description
Output
voltage
Vreg
Ton
Toff
The duty cycle D of the buck converter in steady state is:
Equation 2
D = VOUT
VIN
DC error
t
The PWM control works at a nearly fixed frequency fSW:
Equation 3
VOUT
fsw
=
D
TON
=
VIN
K ON
×
VOUT
VIN
=
1
K ON
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:
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