LT1933 Linear Technology, LT1933 Datasheet - Page 7

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LT1933

Manufacturer Part Number
LT1933
Description
600mA 500kHz Step-Down Switching Regulator in SOT-23
Manufacturer
Linear Technology
Datasheet

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APPLICATIO S I FOR ATIO
be ~1A. To keep efficiency high, the series resistance
(DCR) should be less than 0.2Ω. Table 1 lists several
vendors and types that are suitable.
Of course, such a simple design guide will not always
result 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 efficiency. 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
a minimum inductance required to avoid subharmonic
oscillations. Choosing L greater than 3(V
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
Table 1. Inductor Vendors
Vendor
Coilcraft
Sumida
Toko
Würth Elektronik
U
U
URL
www.coi1craft.com
www.sumida.com
www.toko.com
www.we-online.com
OUT
W
/V
IN
< 0.5), there is
OUT
U
+ V
D
) µH
Part Series
DO1608C
MSS5131
MSS6122
CR43
CDRH4D28
CDRH5D28
D52LC
D53LC
WE-TPC MH
WE-PD4 S
WE-PD2 S
for 0.5A forward current and a maximum reverse voltage
of 40V. The MBRM140 provides better efficiency, 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 significant inductance due to long
wires or cables, additional bulk capacitance may be nec-
essary. This can be provided with a low performance
electrolytic capacitor.
Step-down regulators draw current from the input supply
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
switching current into a tight local loop, minimizing EMI.
A 2.2µF capacitor is capable of this task, but only if it is
placed close to the LT1933 and the catch diode; see the
PCB Layout section. A second precaution regarding the
ceramic input capacitor concerns the maximum input
voltage rating of the LT1933. A ceramic input capacitor
combined with trace or cable inductance forms a high
quality (under damped) tank circuit. If the LT1933 circuit
is plugged into a live supply, the input voltage can ring to
twice its nominal value, possibly exceeding the LT1933’s
Inductance Range (µH)
10 to 22
10 to 22
10 to 33
10 to 22
10 to 33
22 to 47
10 to 22
22 to 47
10 to 22
10 to 22
10 to 47
Size (mm)
2.9 × 4.5 × 6.6
3.1 × 5.1 × 5.1
2.2 × 6.1 × 6.1
3.5 × 4.3 × 4.8
3.0 × 5.0 × 5.0
3.0 × 5.7 × 5.7
2.0 × 5.0 × 5.0
3.0 × 5.0 × 5.0
2.8 × 4.8 × 4.8
2.9 × 4.5 × 6.6
3.2 × 4.0 × 4.5
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LT1933
7
1933f

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