lt3493edcb-3 Linear Technology Corporation, lt3493edcb-3 Datasheet - Page 10

• Manufacturer Part Number: lt3493edcb-3
• Description: 1.2a, 750khz Step-down Switching Regulator In 2mm X 3mm Dfn
• Manufacturer: Linear Technology Corporation

APPLICATIO S I FOR ATIO
LT3493-3
larger value provides a higher maximum load current and
reduces output voltage ripple at the expense of slower
transient response. If your load is lower than 1.2A, 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 okay, but further
reduces maximum load current. For details of the maxi-
mum output current and discontinuous mode operation,
see Linear Technology Application Note 44.
Catch Diode
Depending on load current, a 1A to 2A 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 MBRM140
is a good choice; it is rated for 1A continuous forward
current and a maximum reverse voltage of 40V.
Input Capacitor
Bypass the input of the LT3493-3 circuit with a 1µF 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 1µF ceramic is
adequate to bypass the LT3493-3 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
10
Table 1. Inductor Vendors
Vendor
Sumida
Toko
Würth Elektronik
U
U
URL
www.sumida.com
www.we-online.com
www.toko.com
W
U
Part Series
CDRH4D28
CDRH5D28
CDRH8D28
A916CY
D585LC
WE-TPC(M)
WE-PD2(M)
WE-PD(S)
necessary. 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 LT3493-3 and to force this very high frequency
switching current into a tight local loop, minimizing EMI.
A 1µF capacitor is capable of this task, but only if it is
placed close to the LT3493-3 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 LT3493-3. A ceramic input capacitor
combined with trace or cable inductance forms a high
quality (underdamped) tank circuit. If the LT3493-3 circuit
is plugged into a live supply, the input voltage can ring to
twice its nominal value, possibly exceeding the LT3493-
3’s voltage rating. This situation is easily avoided; see the
Hot Plugging Safely section.
Output Capacitor
The output capacitor has two essential functions. Along
with the inductor, it filters the square wave generated by
the LT3493-3 to produce the DC output. In this role it
determines the output ripple so low impedance at the
switching frequency is important. The second function is
to store energy in order to satisfy transient loads and
stabilize the LT3493-3’s control loop.
Ceramic capacitors have very low equivalent series resis-
tance (ESR) and provide the best ripple performance. A
good value is:
C
OUT
Inductance Range (µH)
1.2 to 4.7
2.5 to 10
2.5 to 33
2 to 12
1.1 to 39
1 to 10
2.2 to 22
1 to 27
= 65/V
OUT
Size (mm)
4.5 × 4.5
5.5 × 5.5
8.3 × 8.3
6.3 × 6.2
8.1 × 8.0
4.8 × 4.8
5.2 × 5.8
7.3 × 7.3
3493-3f