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ADP3207D Datasheet(PDF) 28 Page - ON Semiconductor

Part No. ADP3207D
Description  7-Bit Programmable, Multi-Phase Mobile, CPU Synchronous Buck
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Maker  ONSEMI [ON Semiconductor]
Homepage  http://www.onsemi.com
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ADP3207D Datasheet(HTML) 28 Page - ON Semiconductor

 
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ADP3207D
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28
impedance that is entirely resistive over the widest possible
frequency range, including dc, and equal to the droop
resistance (RO). With the resistive output impedance, the
output voltage droops in proportion with the load current at any
load current slew rate. This ensures the optimal positioning and
minimizes the output decoupling.
With the multi−mode feedback structure of the ADP3207D,
users need to set the feedback compensation to make the
converter output impedance work in parallel with the output
decoupling. Several poles and zeros are created by the output
inductor and decoupling capacitors (output filter) that need to
be compensated for.
A type−three compensator on the voltage feedback is
adequate for proper compensation of the output filter.
Equations 28 to 36 is intended to yield an optimal starting
point for the design; some adjustments can be necessary to
account for PCB and component parasitic effects (refer to the
Tuning Procedure for ADP3207D section).
The first step is to compute the time constants for all of the
poles and zeros in the system:
RE + n RO ) AD RDS )
RL VRT
VID
(eq. 28)
)
2
L
(1 * n
D)
VRT
n
CX RO VVID
TA + CX (RO * R ) )
LX
RO
RO * R
RX
(eq. 29)
TB + (RX ) R * RO) CX
(eq. 30)
TC +
VRT
L *
AD RDS
2 fSW
VVID RE
(eq. 31)
TD +
CX CZ RO 2
CX (RO * R ) ) CZ RO
(eq. 32)
Where:
R’ is the PCB resistance from the bulk capacitors to the
ceramics. RDS is the total low−side MOSFET on−resistance
per phase.
For this example, AD is 5, VRT = 1.5 V, R’ is approximately
0.4 m
W (assuming an 8−layer motherboard), and LX is
250 pH for the four Panasonic SP capacitors.
The compensation values can be solved using the
following:
CA +
n
RO TA
RE RB
(eq. 33)
RA +
TC
CA
(eq. 34)
CB +
TB
RB
(eq. 35)
CFB +
TD
RA
(eq. 36)
The standard values for these components are subject to
the tuning procedure, as introduced in the CIN Selection and
Input Current DI/DT Reduction section.
CIN Selection and Input Current DI/DT Reduction
In continuous inductor current mode, the source current of
the high−side MOSFET is approximately a square wave
with a duty ratio equal to n x VOUT/VIN and an amplitude of
1−nth the maximum output current. To prevent large voltage
transients, a low ESR input capacitor sized for the maximum
rms current must be used. The maximum rms capacitor
current happens at the lowest input voltage, and is given by:
ICRMS + D IO
1
n
D
* 1
(eq. 37)
ICRMS + 0.164 44 A
1
2
0.164
* 1 + 10.3 A
In a typical notebook system, the battery rail decouplings
are MLCC capacitors or a mixture of MLCC capacitors and
bulk capacitors. In this example, the input capacitor bank is
formed by eight pieces of 10
mF, and 25 V MLCC capacitors
with a ripple current rating of about 1.5 A each.
RC Snubber
It is important in any buck topology to use a resistor
capacitor snubber across the low side power MOSFET. The
RC snubber dampens ringing on the switch node when the
high side MOSFET turns on. The switch node ringing could
cause EMI system failures and increased stress on the power
components and controller. The RC snubber should be
placed as close as possible to the low side MOSFET. Typical
values for the resistor range from 1
W to 10 W. Typical values
for the capacitor range from 330 pF to 4.7 nF. The exact
value of the RC snubber depends on the PCB layout and
MOSFET selection. Some fine tuning must be done to find
the best values. The equation below is used to find the
starting values for the RC subber.
RSnubber +
1
2
p
fRinging COSS
(eq. 38)
CSnubber +
1
p
fRinging RSnubber
(eq. 39)
PSnubber + CSnubber VInput 2 fSwithing
(eq. 40)
Where RSnubber is the snubber resistor.
CSnubber is the snubber capacitor.
fRinging is the frequency of the ringing on the switch node
when the high side MOSFET turns on.
COSS is the low side MOSFET output capacitance at VInput.
This is taken from the low side MOSFET data sheet.
Vinput is the input voltage.
fSwitching is the switching frequency.
PSnubber is the power dissipated in RSnubber.


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