LTC3125 Linear Technology, LTC3125 Datasheet - Page 11

• Manufacturer Part Number: LTC3125
• Description: 1.2A Synchronous Step-Up DC/DC Converter
• Manufacturer: Linear Technology

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APPLICATIONS INFORMATION
(DC resistance of the windings) to reduce the I
losses, and must be able to support the peak inductor
current without saturating. Molded chokes and some chip
inductors usually do not have enough core area to support
the peak inductor currents of 1.8A seen on the LTC3125.
To minimize radiated noise, use a shielded inductor. See
Table 1 for suggested components and suppliers.
Table 1. Recommended Inductors
VENDOR
Coilcraft
[phone]
www.coilcraft.com
Coiltronics
www.cooperet.com
Murata
[phone]
www.murata.com
Sumida
[phone]
www.sumida
Taiyo-Yuden
www.t-yuden.com
TDK
[phone]
www.component.tdk.com
Wurth
[phone]
www.we-online.com
Output and Input Capacitor Selection
When selecting output capacitors for large pulsed loads,
the magnitude and duration of the pulsing current, together
with the ripple voltage specifi cation, determine the choice
of the output capacitor. Both the ESR of the capacitor and
the charge stored in the capacitor each cycle contribute
to the output voltage ripple. The ripple due to the charge
is approximately:
where I
during transmission burst and I
V
RIPPLE
PULSE
(
mV
and t
)
=
ON
(
I
PULSE
are the peak current and on time
–
PART/STYLE
LPO2506
LPS4012, LPS4018
MSS6122
MSS4020
MOS6020
DS1605, DO1608
SD52, SD53, SD3114, SD3118
LQH55D
CDH40D11
NP04SB
NR3015
NR4018
VLP , LTF
VLF , VLCF
WE-TPC Type S, M, MH, MS
I
C
STANDBY
OUT
STANDBY
)
•
is the current in
t
ON
2
R power
standby mode. The above is a worst-case approximation
assuming all the pulsing energy comes from the output
capacitor.
The ripple due to the capacitor ESR is:
Low ESR and high capacitance are critical to maintain low
output voltage ripple. Typically, two low profi le 2200μF
parallel Vishay TANTAMOUNT
tors are used. The capacitor has less than 40mΩ ESR.
These capacitors can be used in parallel for even larger
capacitance values. For applications requiring very high
capacitance, the GS, GS2 and GW series from Cap-XX,
the BestCap
Capacitors from Cooper all offer very high capacitance and
low ESR in various package options. Table 2 shows a list
of several reservoir capacitor manufacturers.
High capacitance values and low ESR can lead to instabil-
ity in some internally compensated boost converters. The
internal loop compensation of the LTC3125 is optimized to
be stable with output capacitor values greater than 500μF
with very low ESR.
Multilayer ceramic capacitors are an excellent choice for
input decoupling of the step-up converter as they have
extremely low ESR and are available in small footprints.
Input capacitors should be located as close as possible
to the device. While a 10μF input capacitor is suffi cient
for most applications, larger values may be used to re-
duce input current ripple without limitations. Consult the
manufacturers directly for detailed information on their
selection of ceramic capacitors. Although ceramic capaci-
tors are recommended, low ESR tantalum capacitors may
be used as well.
Table 2. Capacitor Vendor Information
SUPPLIER
Vishay
AVX
Cooper Bussman
Cap-XX
Panasonic
V
RIPPLE_ESR
TM
series from AVX and PowerStor
= (I
PHONE
[phone]
[phone]
[phone]
[phone]
[phone]
PULSE
– I
STANDBY
®
tantalum, low ESR capaci-
WEBSITE
www.vishay.com
www.avxcorp.com
www.cooperbussman.com
www.cap-xx-com
www.panasonic.com
) • ESR
LTC3125
®
Aerogel
11
3125f