LT1310_15 Datasheet, PDF (8/12 Pages) Linear Technology – 1.5A Boost DC/DC Converter with Phase-Locked Loop

LT1310
APPLICATIO S I FOR ATIO
Inductor Selection
Several inductors that work well with the LT1310 are listed
in Table 2. This table is not exclusive; there are many other
manufacturers and inductors that can be used. Consult
each manufacturer for more detailed information and for
their entire selection of related parts, as many different
sizes and shapes are available. Ferrite core inductors
should be used to obtain the best efficiency, as core losses
at high frequency are much lower for ferrite cores than for
the cheaper powdered-iron ones. Choose an inductor that
can handle at least 1.5A without saturating, and ensure
that the inductor has a low DCR (copper wire resistance)
to minimize I²R power losses. Note that in some applica-
tions, the current handling requirements of the inductor
can be lower, such as in the SEPIC topology where each
inductor only carries one-half of the total switch current.
Switching frequency will also affect inductor require-
ments with higher frequencies corresponding to lower
inductance values. A good starting point is to set the
inductor ripple current equal to one-third of the peak
switch current.
The inductors shown in Table 2 were chosen for small size.
For better efficiency, use similar valued inductors with a
larger volume.
Table 2. Recommended Inductors
PART
MAX
L DCR
(µH) (mΩ)
SIZE
L×W×H
(mm)
VENDOR
CDRH5D18-4R1
CDRH5D18-5R4
CDRH5D28-5R3
CDRH5D28-6R2
CDRH5D28-8R2
CR43-2R2
CR43-3R3
4.1 57 5.7 × 5.7 × 2 Sumida
5.4 76
[phone]
5.3 38 5.7 × 5.7 × 3 www.sumida.com
6.2 45
8.2 53
2.2 71 4.5 × 4 × 3.2
3.3 86
ELL6SH-4R7M
ELL6SH-5R6M
ELL6SH-6R8M
4.7 50
5.6 59
6.8 62
6.4 × 6 × 3 Panasonic
[phone]
www.panasonic.com
RLF5018T-4R7M1R4 4.7
RLF5018-1R5M2R1 1.5
RLF5018-2R7M1R8 2.7
RLF5018-4R7M1R4 4.7
RLF5018-100MR94 10
45 5.6 × 5.2 × 1.8 TDK
25 5.2 × 5.6 × 1.8 [phone]
33
www.tdk.com
45
67
LPO1704-122MC
LPO1704-222MC
1.2 80 5.5 × 6.6 × 1 Coilcraft
2.2 120
[phone]
www.coilcraft.com
Capacitor Selection
Low ESR (equivalent series resistance) capacitors should
be used at the output to minimize the output ripple voltage.
Multilayer ceramic capacitors are an excellent choice, as
they have an extremely low ESR and are available in very
small packages. X5R dielectrics are preferred, followed by
X7R, as these materials retain the capacitance over wide
voltage and temperature ranges. A 4.7µF to 20µF output
capacitor is sufficient for most applications, but systems
with very low output currents may need only a 1µF or 2.2µF
output capacitor. Solid tantalum or OS-CON capacitors
can be used, but they will occupy more board area than a
ceramic and will have a higher ESR. Always use a capacitor
with a sufficient voltage rating.
Ceramic capacitors also make a good choice for the input
decoupling capacitor, which should be placed as close as
possible to the LT1310. A 2.2µF to 4.7µF input capacitor is
sufficient for most applications. Table 3 shows a list of
several ceramic capacitor manufacturers. Consult the
manufacturers for detailed information on their entire
selection of ceramic parts.
Table 3. Ceramic Capacitor Manufacturers
Taiyo Yuden
[phone]
www.t-yuden.com
AVX
[phone]
www.avxcorp.com
Murata
[phone]
www.murata.com
Compensation—Adjustment
To compensate the feedback loop of the LT1310, a series
resistor-capacitor network should be connected from the
V_C pin to GND. For most applications, a capacitor in the
range of 220pF to 1500pF will suffice. With a switching
frequency of 1.6MHz, a good starting value for the com-
pensation capacitor, C_C, is 820pF. The compensation
resistor, R_C, is usually in the range of 5k to 30k. A good
technique to compensate a new application is to use a
30kΩ potentiometer in place of R_C, and use a 820pF
capacitor for C_C. By adjusting the potentiometer while
observing the transient response, the optimum value for
R_C can be found. Figures 3a to 3c illustrate this process
for the circuit of Figure 1 with a load current stepped from
8
sn1310 1310fs