LTC3458_15 Datasheet, PDF (11/16 Pages) Linear Technology – 1.4A, 1.5MHz Synchronous Step-Up DC/DC Converter with Output Disconnect

LTC3458
APPLICATIO S I FOR ATIO
COMPONENT SELECTION
Inductor Selection
The high frequency operation of the LTC3458 allows for
the use of small surface mount inductors. Since the
internal slope compensation circuit relies on the inductor’s
current slope and frequency, Table 1 should be used to
select an inductor value for a given frequency of operation
(± 25%). The recommended value will yield optimal tran-
sient performance while maintaining stable operation.
Inductor values larger than listed in Table 1 are permis-
sible to reduce the current ripple.
Table 1. Recommended Inductor Values
Frequency
Inductor Value(μH)
1.5MHz
3.3 to 4.7
1.25MHz
4.7 to 6.8
1MHz
6.8 to 10
750Hz
10 to 15
500kHz
15 to 22
For high efficiency, choose an inductor with high fre-
quency core material, such as ferrite, to reduce core
losses. The inductor should have low ESR (equivalent
series resistance) to reduce the I²R losses, and must be
able to handle the peak inductor current without saturat-
ing. Molded chokes or chip inductors usually do not have
enough core to support peak inductor currents in the
1A to 3A region. To minimize radiated noise, use a
toroidal or shielded inductor. (Note that the inductance of
shielded types will drop more as current increases, and
will saturate more easily). See Table 2 for a list of inductor
manufacturers.
Table 2. Inductor Vendor Information
Supplier Phone
Website
Coilcraft
[phone] www.coilcraft.com
TDK
[phone] www.component.tdk.com
Murata
USA: [phone]
[phone]
www.murata.com
Sumida
USA: [phone]
Japan: 81-3-3607-5111
www.japanlink.com/sumida
COEV
[phone] www.coev.net
Toko
Wu..rth
[phone]
[phone]
www.tokoam.com
www.we-online.com
Some example inductor part types are:
Coilcraft: DO1608 and MSS5131 Series
TDK: RLF5018T and SLF7045 Series
Murata: LQH4C and LQN6C Series
Sumida: CDRH4D28 and CDRH6D28 Series
COEV: DQ7545 Series
TO_..KO: D62CB and D63LCB Series
WURTH: WE-PD2 Series
Output Capacitor Selection
The output voltage ripple has three components to it. The
bulk value of the capacitor is set to reduce the ripple due
to charge into the capacitor each cycle. The max ripple due
to charge is given by:
V_RBULK
=
I_P • V_IN
C_OUT • V_OUT
•
f
where I_P = peak inductor current and f = switching
frequency.
The ESR (equivalent series resistance) is usually the most
dominant factor for ripple in most power converters. The
ripple due to capacitor ESR is given by:
V_RCESR = I_P • C_ESR
where C_ESR = Capacitor Series Resistance.
The ESL (equivalent series inductance) is also an impor-
tant factor for high frequency converters. Using small,
surface mount ceramic capacitors, placed as close as
possible to the V_OUT pins, will minimize ESL.
Low ESR/ESL capacitors should be used to minimize
output voltage ripple. For surface mount applications, AVX
TPS Series tantalum capacitors, Sanyo POSCAP, or Taiyo
Yuden X5R type ceramic capacitors are recommended.
For through-hole applications, Sanyo OS-CON capacitors
offer low ESR in a small package size.
In all applications, a minimum of 4.7μF (generally 22μF is
recommended), low ESR ceramic capacitor should be
placed as close to the V_OUT pin as possible, and grounded
to a local ground plane.
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