ADP3210 datasheet(31/38 Pages) ONSEMI

31 / 38 page

ADP3210

Rev. 0.3 | Page 31 of 38

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

()

VID

−

(29)

()

−

(30)

(

)

−

(31)

VID

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

−

(32)

()

−

(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:

(34)

R =

(35)

C =

(36)

(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.

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 × 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:

−

CRMS

(38)

0.1

164

−

CRMS

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.

OSS

Ringing

Snubber

(i))

Snubber

Ringing

Snubber

(ii))

Swithing

Input

Snubber

(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.

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