LTC3405A-1.5 LINER [Linear Technology], LTC3405A-1.5 Datasheet - Page 8

• Manufacturer Part Number: LTC3405A-1.5
• Description: 1.5V, 1.8V, 1.5MHz, 300mA Synchronous Step-Down Regulators in ThinSOT
• Manufacturer: LINER [Linear Technology]

LTC3405A-1.5/LTC3405A-1.8
APPLICATIO S I FOR ATIO
The basic LTC3405A series parts application circuit is
shown in Figure 1. External component selection is driven
by the load requirement and begins with the selection of L
followed by C
Inductor Selection
For most applications, the inductor value will fall in the
range of 2.2 H to 10 H. Its value is determined by the
desired ripple current. Large value inductors lower ripple
current and small value inductors result in higher ripple
currents. Higher V
current as shown in equation 1. A reasonable starting point
for setting ripple current is I
The DC current rating of the inductor should be at least
equal to the maximum load current plus half the ripple
current to prevent core saturation. Thus, a 360mA rated
inductor should be enough for most applications (300mA
+ 60mA). For better efficiency, choose a low DC-resistance
inductor.
The inductor value also has an effect on Burst Mode
operation. The transition to low current operation begins
when the inductor current peaks fall to approximately
100mA. Lower inductor values (higher I
to occur at lower load currents, which can cause a dip in
efficiency in the upper range of low current operation. In
Burst Mode operation, lower inductance values will cause
the burst frequency to increase.
Inductor Core Selection
Different core materials and shapes will change the size/
current and price/current relationship of an inductor. Tor-
oid or shielded pot cores in ferrite or permalloy materials
are small and don’t radiate much energy, but generally cost
more than powdered iron core inductors with similar
electrical characteristics. The choice of which style induc-
tor to use often depends more on the price vs size require-
ments and any radiated field/EMI requirements than on
what the LTC3405A series parts require to operate. Table
1 shows some typical surface mount inductors that work
well in LTC3405A series parts applications.
8
I
L
f L
1
IN
and C
V
OUT
IN
U
OUT
or V
1
.
U
V
OUT
V
OUT
IN
L
= 120mA (40% of 300mA).
also increases the ripple
W
L
) will cause this
U
(1)
Table 1. Representative Surface Mount Inductors
MANUFACTURER PART NUMBER
Taiyo Yuden
Panasonic
Murata
Taiyo Yuden
Panasonic
Panasonic
Sumida
C
In continuous mode, the source current of the top MOSFET
is a square wave of duty cycle V
voltage transients, a low ESR input capacitor sized for the
maximum RMS current must be used. The maximum
RMS capacitor current is given by:
This formula has a maximum at V
I
monly used for design because even significant deviations
do not offer much relief. Note that the capacitor
manufacturer’s ripple current ratings are often based on
2000 hours of life. This makes it advisable to further derate
the capacitor, or choose a capacitor rated at a higher
temperature than required. Always consult the manufac-
turer if there is any question.
The selection of C
series resistance (ESR). Typically, once the ESR require-
ment for C
generally far exceeds the I
output ripple V
where f = operating frequency, C
and I
voltage, the output ripple is highest at maximum input
voltage since I
RMS
IN
C
and C
IN
V
= I
OUT
L
required I
OUT
= ripple current in the inductor. For a fixed output
OUT
/2. This simple worst-case condition is com-
OUT
Selection
I ESR
LB2016T2R2M
LB2012T2R2M
LB2016T3R3M
ELT5KT4R7M
LQH32CN2R2M33 4.7 H
LB2016T4R7M
ELT5KT6R8M
ELT5KT100M
CMD4D116R8MC 6.8 H
L
has been met, the RMS current rating
L
OUT
RMS
increases with input voltage.
OUT
is determined by:
is driven by the required effective
I
OMAX
8
fC
RIPPLE(P-P)
1
OUT
VALUE CURRENT DCR HEIGHT
2.2 H
2.2 H
3.3 H
4.7 H
4.7 H
6.8 H
10 H
V
OUT
OUT
OUT
/V
MAX DC
= output capacitance
315mA
240mA
280mA
950mA
450mA
210mA
760mA
680mA
620mA
IN
IN
V
IN
requirement. The
. To prevent large
= 2V
V
sn3405a1518 3405a1518fs
IN
V
0.13
0.23
0.25
0.36
0.23
OUT
0.2
0.2
0.2
0.3
OUT
, where
1.25mm
1 2 /
1.6mm
1.6mm
1.2mm
1.6mm
1.2mm
1.2mm
1.2mm
2mm