PM6670ASTR STMicroelectronics, PM6670ASTR Datasheet - Page 38

• Manufacturer Part Number: PM6670ASTR
• Description: IC CTLR DDR2/3 MEM PS 24VFQFPN
• Manufacturer: STMicroelectronics

Specifications of PM6670ASTR

• Applications: Memory, DDR2/DDR3 Regulator
• Current - Supply: 800µA
• Voltage - Supply: 4.5 V ~ 36 V
• Operating Temperature: -40°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 24-VFQFN, 24-VFQFPN
• For Use With: 497-8412 - BOARD EVAL PM6670AS DDR2/3497-8411 - BOARD EVAL PM6670S DDR2/3
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Other names: 497-6305-2; PM6670ATR

Application information
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8.1
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Application information
The purpose of this chapter is to show the design procedure of the switching section.
The design starts from three main specifications:
●
●
●
It's also possible that specific designs should involve other specifications.
The following paragraphs will guide the user into a step-by-step design.
External components selection
The PM6670AS 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 0.8 V and lower
than 2 V 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 24
where K
(40 ns typ.).
The duty cycle of the buck converter is, under steady state conditions, given by
Equation 25
The switching frequency is thus calculated as
Equation 26
The input voltage range, provided by the battery or the AC adapter. The two extreme
values (V
The maximum load current, indicated by I
The maximum allowed output voltage ripple V
OSC
is a constant value (130 ns typ.) and τ is the internal propagation delay
INMAX
and V
f
SW
INmin
=
Doc ID 14436 Rev 2
) are important for the design.
T
D
ON
T
=
ON
K
=
OSC
D =
K
V
V
OSC
OUT
⋅
IN
V
V
V
LOAD,MAX
V
OUT
OSC
SNS
IN
V
V
OSC
SNS
RIPPLE,MAX
=
+
α
α
τ
OSC
OUT
.
⋅
K
.
OSC
1
PM6670AS