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PM6675AS Fiches technique(PDF) 34 Page - STMicroelectronics

No de pièce PM6675AS
Description  High efficiency step-down controller with embedded 2 A LDO regulator
PDF  48 Pages
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Fabricant  STMICROELECTRONICS [STMicroelectronics]
Site Internet  http://www.st.com
Logo STMICROELECTRONICS - STMicroelectronics

PM6675AS Fiches technique(HTML) 34 Page - STMicroelectronics

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Application information
PM6675AS
34/48
8
Application information
The purpose of this chapter is to show the design procedure of the switching section.
The design starts from three main specifications:
●
The input voltage range, provided by the battery or the AC adapter. 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 PM6675AS employes 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 0.8 V
and lower than 2 V in order to take advantage of the internal block 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 steady-state conditions, given by
Equation 26
The switching frequency is thus calculated as
Equation 27
τ
+
=
OSC
SNS
OSC
ON
V
V
K
T
IN
OUT
V
V
D
=
OSC
OUT
OSC
OSC
SNS
OSC
IN
OUT
ON
SW
K
1
V
V
K
V
V
T
D
f
⋅
α
α
=
⋅
=
=



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