LTC3532_15 Datasheet, PDF (11/16 Pages) Linear Technology – Micropower Synchronous Buck-Boost DC/DC Converter

LTC3532
APPLICATIO S I FOR ATIO
1 RT
2 BURST
3 SW1
4 SW2
5 GND
LTC3532
V_C 10
FB 9
SHDN/SS 8
V_IN 7
V_OUT 6
GND
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Figure 6. Recommended Component Placement. Traces Carrying
High Current are Direct. Trace area at FB and V_C Pins are Kept
Low. Lead Length to Battery Should be Kept Short
Inductor Selection
The high frequency operation of the LTC3532 allows the
use of small surface mount inductors. The inductor ripple
current is typically set to 20% to 40% of the maximum
inductor current. For a given ripple the inductance terms
are given as follows:
L_BOOST
>
V_IN(MIN) • (V_OUT – V_IN(MIN))
f • ΔI_L • V_OUT
H
L_BUCK
>
V_OUT • (V_IN(MAX) – V_OUT )
f • ΔI_L • V_IN(MAX)
H
where f = Operating frequency, Hz
ΔIL = Maximum allowable inductor ripple current, A
V_IN(MIN) = Minimum input voltage
V_IN(MAX) = Maximum input voltage
V_OUT = Output voltage
I_OUT(MAX) = Maximum output load current
For high efficiency, choose a ferrite inductor with a high
frequency core material to reduce core losses. The induc-
tor should have low ESR (equivalent series resistance) to
reduce the I2R losses, and must be able to handle the peak
inductor current without saturating. Molded chokes or chip
inductors usually do not have enough core to support the
peak inductor currents in the 1A to 2A region. To minimize
radiated noise, use a shielded inductor. See Table 1 for a
suggested list of inductor suppliers.
Table 1. Inductor Vendor Information
SUPPLIER
Coilcraft
Murata
Sumida
TDK
TOKO
WEB SITE
www.coilcraft.com
www.murata.com
www.sumida.com
www.component.tdk.com
www.tokoam.com
Output Capacitor Selection
The bulk value of the output filter capacitor is set to reduce
the ripple due to charge into the capacitor each cycle. The
steady state ripple due to charge is given by:
% RIPPLE_BOOST =
I_OUT(MAX) • (V_OUT – V_IN(MIN)) • 100 _%
C_OUT
•
V_OU
2
T
•
f
% RIPPLE_BUCK =
1 _• ^(V_IN(MAX) ^{– V}_OUT ^{) • 100%}
8LCf²
V_{IN(M A X )}
where C_OUT = output filter capacitor in Farads and
f = switching frequency in Hz.
The output capacitance is usually many times larger than
the minimum value in order to handle the transient response
requirements of the converter. As a rule of thumb, the ratio
of the operating frequency to the unity-gain bandwidth of
the converter is the amount the output capacitance will
have to increase from the above calculations in order to
maintain the desired transient response.
The other component of ripple is due to the ESR (equiva-
lent series resistance) of the output capacitor. Low ESR
capacitors should be used to minimize output voltage
ripple. For surface mount applications, Taiyo Yuden or
TDK ceramic capacitors, AVX TPS series tantalum capaci-
tors or Sanyo POSCAP are recommended. See Table 2 for
contact information.
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