LTC3125_15 Datasheet, PDF (11/18 Pages) Linear Technology – 1.2A Synchronous Step-Up DC/DC Converter with Input Current Limit

LTC3125
APPLICATIONS INFORMATION
power losses compared to cheaper powdered iron types,
improving efficiency. The inductor should have low DCR
(DC resistance of the windings) to reduce the I²R power
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
PART/STYLE
Coilcraft
[phone]
www.coilcraft.com
LPO2506
LPS4012, LPS4018
MSS6122
MSS4020
MOS6020
DS1605, DO1608
Coiltronics
www.cooperet.com
SD52, SD53, SD3114, SD3118
Murata
[phone]
www.murata.com
LQH55D
Sumida
[phone]
www.sumida
CDH40D11
Taiyo-Yuden
www.t-yuden.com
NP04SB
NR3015
NR4018
TDK
[phone]
www.component.tdk.com
VLP, LTF
VLF, VLCF
Wurth
[phone]
www.we-online.com
WE-TPC Type S, M, MH, MS
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 specification, 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:
( ) _VRIPPLE
(mV)=
I_PULSE –I_STANDBY
C_OUT
•t_ON
where I_PULSE and t_ON are the peak current and on time
during transmission burst and I_STANDBY is the current in
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:
V_{RIPPLE_ESR} = (I_PULSE – I_STANDBY) • ESR
Low ESR and high capacitance are critical to maintain low
output voltage ripple. Typically, two low profile 2200μF
parallel Vishay TANTAMOUNT^® tantalum, low ESR capaci-
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^TM series from AVX and PowerStor^® Aerogel
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.
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 sufficient
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
PHONE
WEBSITE
Vishay
[phone] www.vishay.com
AVX
[phone] www.avxcorp.com
Cooper Bussman [phone] www.cooperbussman.com
Cap-XX
[phone] www.cap-xx-com
Panasonic
[phone] www.panasonic.com
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