LTC3878 Linear Technology Corporation, LTC3878 Datasheet - Page 13

• Manufacturer Part Number: LTC3878
• Description: Wide Operating Range No RSENSE Step-Down Controller
• Manufacturer: Linear Technology Corporation

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A reasonable starting point is to choose a ripple current
that is about 40% of I
occurs at the highest V
does not exceed a specifi ed maximum, the inductance
should be chosen according to:
Once the value for L is known, the type of inductor must
be selected. High effi ciency converters generally cannot
tolerate the core loss of low cost powdered iron cores,
forcing the use of more expensive ferrite materials such as
molypermalloy or Kool Mμ
designed for high current, low voltage applications are
available from manufacturers such as Sumida, Panasonic,
Coiltronics, Coilcraft, Toko, Vishay, Pulse and Wurth.
Inductor Core Selection
Once the inductance value is determined, the type of in-
ductor must be selected. Core loss is independent of core
size for a fi xed inductor value, but it is very dependent
on inductance selected. As inductance increases, core
losses go down. Unfortunately, increased inductance
requires more turns of wire and therefore copper losses
will increase.
Ferrite designs have very low core loss and are preferred
at high switching frequencies, so design goals can con-
centrate on copper loss and preventing saturation. Ferrite
core material saturates “hard,” which means that induc-
tance collapses abruptly when the peak design current is
exceeded. This results in an abrupt increase in inductor
ripple current and consequent output voltage ripple. Do
not allow the core to saturate!
C
The input capacitance C
wave current at the drain of the top MOSFET. Use a low ESR
capacitor sized to handle the maximum RMS current.
APPLICATIONS INFORMATION
IN
L
I
RMS
and C
=
⎛
⎜
⎝
≅
f
OP
OUT
I
OUT MAX
•
V
Δ
Selection
(
OUT
I
IL MAX
(
)
•
OUT(MAX)
V
IN
)
V
⎞
⎟
⎠
OUT
IN
. To guarantee that ripple current
IN
⎛
⎜
⎝
1
®
is required to fi lter the square
–
•
cores. A variety of inductors
V
. The largest ripple current
IN MAX
V
V
V
OUT
OUT
(
IN
– 1
)
⎞
⎟
⎠ ⎠
This formula has a maximum at V
= I
monly used for design because even signifi cant deviations
do not offer much relief. Note that ripple current ratings
from capacitor manufacturers are often based on only
2000 hours of life, which makes it advisable to de-rate
the capacitor.
The selection of C
required to minimize voltage ripple and load step transients.
The ΔV
Since ΔI
is highest at maximum input voltage. Typically, once the
ESR requirement is satisfi ed, the capacitance is adequate
for fi ltering and has the necessary RMS current rating.
Multiple capacitors placed in parallel may be needed to
meet the ESR and RMS current handling requirements.
Dry tantalum, specialty polymer, aluminum electrolytic
and ceramic capacitors are all available in surface mount
packages. Specialty polymer capacitors offer very low
ESR but have lower specifi c capacitance than other types.
Tantalum capacitors have the highest specifi c capacitance
but it is important to only use types that have been surge
tested for use in switching power supplies. Aluminum
electrolytic capacitors have signifi cantly higher ESR,
but can be used in cost-sensitive applications providing
that consideration is given to ripple current ratings and
long-term reliability. Ceramic capacitors have excellent
low ESR characteristics but can have a high voltage co-
effi cient and audible piezoelectric effects. The high Q of
ceramic capacitors with trace inductance can also lead to
signifi cant ringing. When used as input capacitors, care
must be taken to ensure that ringing from inrush currents
and switching does not pose an overvoltage hazard to the
power switches and controller. To dampen input voltage
transients, add a small 5μF to 40μF aluminum electrolytic
capacitor with an ESR in the range of 0.5Ω to 2Ω. High
performance though-hole capacitors may also be used,
but an additional ceramic capacitor in parallel is recom-
mended to reduce the effect of lead inductance.
OUT(MAX)
Δ
V
OUT
OUT
L
increases with input voltage, the output ripple
≤
/2. This simple worst-case condition is com-
is approximately bounded by:
Δ
I
L
⎛
⎜
⎝
OUT
ESR
is primarily determined by the ESR
+
8 •
f
OP
1
•
C
IN
OUT
= 2V
⎞
⎟
⎠
LTC3878
OUT
, where I
13
RMS
3878f