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PM6675 Fiches technique(PDF) 32 Page - STMicroelectronics

No de pièce PM6675
Description  Very fast load transient response using constant on-time control loop
PDF  47 Pages
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Fabricant  STMICROELECTRONICS [STMicroelectronics]
Site Internet  http://www.st.com
Logo STMICROELECTRONICS - STMicroelectronics

PM6675 Fiches technique(HTML) 32 Page - STMicroelectronics

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Application information
PM6675
32/47
7
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.
7.1
External components selection
The PM6675 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 2V in order to ensure system's linearity.
Nearly constant switching frequency is achieved by the system's 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 (130ns typ.) and τ is the internal propagation delay
(40ns 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
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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