MAX8550-MAX8551 MAXIM [Maxim Integrated Products], MAX8550-MAX8551 Datasheet - Page 19

• Manufacturer Part Number: MAX8550-MAX8551
• Description: Integrated DDR Power-Supply Solutions for Desktops, Notebooks, and Graphic Cards
• Manufacturer: MAXIM [Maxim Integrated Products]

Figure 6. Setting V OUT with a Resistive Voltage-Divider
divider from VTT to VTTS. The maximum value for VTT
is V
(max) at T
The termination reference voltage (VTTR) tracks 1/2
V
The switching frequency and inductor operating point
determine the inductor value as follows:
For example: I
2.5V, f
Find a low-loss inductor with the lowest possible DC
resistance that fits in the allotted dimensions. Ferrite
cores are often the best choice, although powdered
iron is inexpensive and can work well at frequencies up
to 200kHz. The core must be large enough not to satu-
rate at the peak inductor current (I
Most inductor manufacturers provide inductors in stan-
dard values, such as 1.0µH, 1.5µH, 2.2µH, 3.3µH, etc.
REFIN
VTTI
SW
.
L
- V
L
MAX8550/
MAX8551
= 600kHz, 30% ripple current or LIR = 0.3:
A
=
=
Integrated DDR Power-Supply Solutions for
DROPOUT
= +85°C.
I
12
PEAK
V
LOAD(MAX)
IN
V
Desktops, Notebooks, and Graphic Cards
×
PGND1
______________________________________________________________________________________
×
2 5
GND
OUT
=
.
600
DL
V
LX
FB
f
SW
V
OUT
where V
I
Inductor Selection (Buck)
LOAD MAX
(
kHz
12
×
V
(
= 12A, V
V
(
I
LOAD MAX
IN
×
- 2.5V)
Q2
DROPOUT
12
-
)
⎛
⎜
⎝
V
(
A
PEAK
1
OUT
L
+
×
IN
)
LIR
0 3
)
):
2
= 12V, V
.
×
= I
⎞
⎟
⎠
≈
LIR
R
R
V
C
D
VTT
1
OUT
µ
H
C
OUT
× 0.3Ω
OUT
=
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.
The input capacitor must meet the ripple current
requirement (I
I
V
(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 MAX8550/MAX8551 are
operated as the second stage of a two-stage power
conversion system, tantalum input capacitors are
acceptable. In either configuration, choose a capacitor
that has less than 10°C temperature rise at the RMS
input current for optimal reliability and lifetime.
The output filter capacitor must have low enough equiv-
alent series resistance (R
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
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
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
meet ripple specifications is:
RMS
OUT
ESR
has a maximum value of I
. For most applications, nontantalum capacitors
is needed to maintain an acceptable level of out-
ESR
ESR
I
. Therefore, the maximum R
RMS
Output Capacitor Selection (Buck)
is needed to prevent the output from dip-
Input Capacitor Selection (Buck)
R
RMS
ESR
=
R
) imposed by the switching currents:
ESR
I
LOAD
≤
I
≤
LOAD MAX
ESR
∆
V
I
OUT IN
LOAD MAX
V
(
) to meet output ripple and
RIPPLE
V
STEP
LOAD
(
V
(
)
V
IN
×
-
/ 2 when V
LIR
)
V
ESR
OUT
)
required to
IN
= 2 ×
19