LT1933
8
1933fe
APPLICATIONS INFORMATION
Table 1.Inductor Vendors
Vendor URL Part Series Inductance Range (μH) Size (mm)
Coilcraft www.coilcraft.com D01608C 10 to 22 2.9 × 4.5 × 6.6
MSS5131 10 to 22 3.1 × 5.1 × 5.1
MSS6122 10 to 33 2.2 × 6.1 × 6.1
Sumida www.sumida.com CR43 10 to 22 3.5 × 4.3 × 4.8
CDRH4D28 10 to 33 3.0 × 5.0 × 5.0
CDRH5D28 22 to 47 3.0 × 5.7 × 5.7
Toko www.toko.com D52LC 10 to 22 2.0 × 5.0 × 5.0
D53LC 22 to 47 3.0 × 5.0 × 5.0
Würth Elektronik www.we-online.com WE-TPC MH 10 to 22 2.8 × 4.8 × 4.8
WE-PD4 S 10 to 22 2.9 × 4.5 × 6.6
WE-PD2 S 10 to 47 3.2 × 4.0 × 4.5
saturation current should be about 30% higher. For robust
operation in fault conditions the saturation current should
be ~1A. To keep effi ciency high, the series resistance (DCR)
should be less than 0.2. Table 1 lists several vendors
and types that are suitable.
Of course, such a simple design guide will not always re-
sult in the optimum inductor for your application. A larger
value provides a slightly higher maximum load current,
and will reduce the output voltage ripple. If your load is
lower than 500mA, then you can decrease the value of
the inductor and operate with higher ripple current. This
allows you to use a physically smaller inductor, or one
with a lower DCR resulting in higher effi ciency. There are
several graphs in the Typical Performance Characteristics
section of this data sheet that show the maximum load
current as a function of input voltage and inductor value
for several popular output voltages. Low inductance may
result in discontinuous mode operation, which is OK, but
further reduces maximum load current. For details of
maximum output current and discontinuous mode opera-
tion, see Linear Technology Application Note 44. Finally,
for duty cycles greater than 50% (V
OUT
/V
IN
> 0.5), there
is a minimum inductance required to avoid subharmonic
oscillations. Choosing L greater than 3(V
OUT
+ V
D
) µH
prevents subharmonic oscillations at all duty cycles.
Catch Diode
A 0.5A or 1A Schottky diode is recommended for the catch
diode, D1. The diode must have a reverse voltage rating
equal to or greater than the maximum input voltage. The
ON Semiconductor MBR0540 is a good choice; it is rated
for 0.5A forward current and a maximum reverse voltage
of 40V. The MBRM140 provides better effi ciency, and will
handle extended overload conditions.
Input Capacitor
Bypass the input of the LT1933 circuit with a 2.2µF or
higher value ceramic capacitor of X7R or X5R type. Y5V
types have poor performance over temperature and ap-
plied voltage, and should not be used. A 2.2µF ceramic
is adequate to bypass the LT1933 and will easily handle
the ripple current. However, if the input power source has
high impedance, or there is signifi cant inductance due to
long wires or cables, additional bulk capacitance may be
necessary. This can be provided with a low performance
electrolytic capacitor.
Step-down regulators draw current from the input sup-
ply in pulses with very fast rise and fall times. The input
capacitor is required to reduce the resulting voltage
ripple at the LT1933 and to force this very high frequency
