ADP3210 Datasheet, PDF (31/38 Pages) ON Semiconductor – 7-Bit Programmable Multiphase Mobile CPU Synchronous

ADP3210
the design; some adjustments can be necessary to account for
PCB and component parasitic effects (see the Tuning Procedure
for ADP3210).
The first step is to compute the time constants for all of the
poles and zeros in the system
R_E
= n × R_O
+ A_D × R_DS
+
R_L × V_RT
V_ID
+
2 × L × (1 − n × D)×V_RT
n × C_X × R_O × V_VID
(29)
( ) T_A
= C_X
×
R_O
− R'
+
L_X
R_O
× ^{RO − R'}
R_X
(30)
T_B = (R_X + R'−R_O )× C _X
(31)
T_C
=
V_RT
× _{⎜⎜⎝⎛} L −
V_VID
A_D × R_DS
2 × f_SW
× R_E
⎟⎟⎠⎞
(32)
( ) ^TD
=
C_X
×
C_X ×C_Z
R_O − R'
× R_O²
+ C_Z
× R_O
(33)
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 mΩ (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:
C_A
=
n× R_O ×T_A
R_E ×R_B
(34)
R_A
=
T_C
C_A
(35)
C_B
=
T_B
R_B
(36)
C_FB
=
T_D
R_A
(37)
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.
C_IN SELECTION AND INPUT CURRENT D_I/D_T
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 × V_OUT/V_IN 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:
I _CRMS = D × I _O ×
1 _−1
n×D
(38)
I_CRMS = 0.164 × 44 A ×
1 _{− 1 = 10.3 A}
2 × 0.164
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 μF, 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 Ω to 10 Ω. 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.
R_Snubber
=
2×π ×
1
f _Ringing
× C_OSS
(i))
π ^{CSnubber =}
1
× f _Ringing × R_Snubber
(ii))
P = C ×V × f _Snubber
Snubber
2
Input
Swithing
(iii))
Where RSnubber is the snubber resistor.
CSnubber is the snubber capacitor.
fRininging 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.
Rev. 0.3 | Page 31 of 38