Datasheet

to 200kHz. The core must be large enough not to satu-
rate at the peak inductor current (I
PEAK
):
Most inductor manufacturers provide inductors in stan-
dard values, such as 1.0µH, 1.5µH, 2.2µH, 3.3µH, etc.
Also look for nonstandard values, which can provide a
better compromise in LIR across the input voltage range.
If using a swinging inductor (where the no-load induc-
tance decreases linearly with increasing current), evalu-
ate the LIR with properly scaled inductance values.
Input Capacitor Selection (Buck)
The input capacitor must meet the ripple current
requirement (I
RMS
) imposed by the switching currents:
I
RMS
has a maximum value of I
LOAD
/ 2 when V
IN
= 2 ×
V
OUT
. For most applications, nontantalum capacitors
(ceramic, aluminum, POS, or OSCON) are preferred
due to their resistance to power-up surge currents typi-
cal of systems with a mechanical switch or connector in
series with the input. If the MAX8632 is operated as the
second stage of a two-stage power conversion system,
tantalum input capacitors are acceptable. In either con-
figuration, choose a capacitor that has less than 10°C
temperature rise at the RMS input current for optimal
reliability and lifetime.
Output Capacitor Selection (Buck)
The output filter capacitor must have low enough equiv-
alent series resistance (R
ESR
) to meet output ripple and
load-transient requirements, yet have high enough ESR
to satisfy stability requirements.
For processor core voltage converters and other appli-
cations in which the output is subject to violent load
transients, the output capacitor’s size depends on how
much R
ESR
is needed to prevent the output from dip-
ping too low under a load transient. Ignoring the sag
due to finite capacitance:
In applications without large and fast load transients,
the output capacitor’s size often depends on how much
R
ESR
is needed to maintain an acceptable level of out-
put voltage ripple. The output ripple voltage of a step-
down controller is approximately equal to the total
inductor ripple current multiplied by the output capaci-
tor’s R
ESR
. Therefore, the maximum R
ESR
required to
meet ripple specifications is:
The actual capacitance value required relates to the
physical size needed to achieve low ESR, as well as to
the chemistry of the capacitor technology. Thus, the
capacitor is usually selected by ESR and voltage rating
rather than by capacitance value (this is true of tanta-
lums, OSCONs, polymers, and other electrolytics).
When using low-capacity filter capacitors, such as
ceramic capacitors, size is usually determined by the
capacity needed to prevent V
SAG
and V
SOAR
from
causing problems during load transients. Generally,
once enough capacitance is added to meet the over-
shoot requirement, undershoot at the rising load edge
is no longer a problem (see the V
SAG
and V
SOAR
equa-
tions in the Transient Response (Buck) section).
However, low-capacity filter capacitors typically have
high-ESR zeros that can affect the overall stability (see
the Stability Requirements section).
R
V
I LIR
ESR
RIPPLE
LOAD MAX
()
≤
×
R
V
I
ESR
STEP
LOAD MAX
()
≤
∆
II
VVV
V
RMS LOAD
OUT IN OUT
IN
=
()
-
II
LIR
PEAK LOAD MAX ()
=+






1
2
MAX8632
Integrated DDR Power-Supply Solution for
Desktops, Notebooks, and Graphic Cards
______________________________________________________________________________________ 19
Figure 6. Setting V
OUT
with a Resistive Voltage-Divider
MAX8632
DL
PGND1
GND
LX
L
FB
R
D
R
C
C
OUT
OUT
Q2
V
OUT