Datasheet

5
LT1937
1937f
Inductor Selection
A 22µH inductor is recommended for most LT1937 appli-
cations. Although small size and high efficiency are major
concerns, the inductor should have low core losses at
1.2MHz and low DCR (copper wire resistance). Some
inductors in this category with small size are listed in
Table␣ 1. The efficiency comparison of different inductors
is shown in Figure 4.
Table 1. Recommended Inductors
CURRENT
DCR RATING
PART NUMBER (Ω) (mA) MANUFACTURER
LQH3C220 0.71 250 Murata
814-237-1431
www.murata.com
ELJPC220KF 4.0 160 Panasonic
714-373-7334
www.panasonic.com
CDRH3D16-220 0.53 350 Sumida
847-956-0666
www.Sumida.com
LB2012B220M 1.7 75 Taiyo Yuden
408-573-4150
www.t-yuden.com
LEM2520-220 5.5 125 Taiyo Yuden
408-573-4150
www.t-yuden.com
APPLICATIO S I FOR ATIO
WUUU
Capacitor Selection
The small size of ceramic capacitors makes them ideal for
LT1937 applications. X5R and X7R types are recom-
mended because they retain their capacitance over wider
voltage and temperature ranges than other types such as
Y5V or Z5U. A 1µF input capacitor and a 0.22µF output
capacitor are sufficient for most LT1937 applications.
Table 2. Recommended Ceramic Capacitor Manufacturers
MANUFACTURER PHONE URL
Taiyo Yuden 408-573-4150 www.t-yuden.com
AVX 843-448-9411 www.avxcorp.com
Murata 814-237-1431 www.murata.com
Kemet 408-986-0424 www.kemet.com
Diode Selection
Schottky diodes, with their low forward voltage drop and
fast reverse recovery, are the ideal choices for LT1937
applications. The forward voltage drop of a Schottky diode
represents the conduction losses in the diode, while the
diode capacitance (C
T
or C
D
) represents the switching
losses. For diode selection, both forward voltage drop and
diode capacitance need to be considered. Schottky diodes
with higher current ratings usually have lower forward
voltage drop and larger diode capacitance, which can
cause significant switching losses at the 1.2MHz switch-
ing frequency of the LT1937. A Schottky diode rated at
100mA to 200mA is sufficient for most LT1937 applica-
tions. Some recommended Schottky diodes are listed in
Table 3.
Table 3. Recommended Schottky Diodes
FORWARD VOLTAGE DIODE
PART CURRENT DROP CAPACITANCE
NUMBER (mA) (V) (pF) MANUFACTURER
CMDSH-3 100 0.58 at 7.0 at Central
100mA 10V 631-435-1110
www.centralsemi.com
CMDSH2-3 200 0.49 at 15 at Central
200mA 10V 631-435-1110
www.centralsemi.com
BAT54 200 0.53 at 10 at Zetex
100mA 25V 631-543-7100
www.zetex.com
Figure 4. Efficiency Comparison of Different Inductors
LOAD CURRENT (mA)
0
EFFICIENCY (%)
90
85
80
75
70
65
60
55
50
16
1937 F04
4 8 12 20142 6 10 18
MURATA LQH3C-220
PANASONIC ELJPC220KF
SUMIDA CDRH3D16-220
TAIYO YUDEN LB2012B220M
TAIYO YUDEN LEM2520-220
V
IN
= 3.6V
3LEDs