8
LTC3405A-1.5/LTC3405A-1.8
3405a1518fa
APPLICATIO S I FOR ATIO
WUUU
Table 1. Representative Surface Mount Inductors
MAX DC
MANUFACTURER PART NUMBER VALUE CURRENT DCR HEIGHT
Taiyo Yuden LB2016T2R2M 2.2µH 315mA 0.13Ω 1.6mm
LB2012T2R2M 2.2µH 240mA 0.23Ω 1.25mm
LB2016T3R3M 3.3µH 280mA 0.2Ω 1.6mm
Panasonic ELT5KT4R7M 4.7µH 950mA 0.2Ω 1.2mm
Murata LQH32CN2R2M33 4.7µH 450mA 0.2Ω 2mm
Taiyo Yuden LB2016T4R7M 4.7µH 210mA 0.25Ω 1.6mm
Panasonic ELT5KT6R8M 6.8µH 760mA 0.3Ω 1.2mm
Panasonic ELT5KT100M 10µH 680mA 0.36Ω 1.2mm
Sumida CMD4D116R8MC 6.8µH 620mA 0.23Ω 1.2mm
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
IN
and C
OUT
.
Inductor Selection
For most applications, the inductor value will fall in the
range of 2.2µH to 10µH. 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
IN
or V
OUT
also increases the ripple
current as shown in equation 1. A reasonable starting point
for setting ripple current is ∆I
L
= 120mA (40% of 300mA).
∆ =
()( )
−
⎛
⎝
⎜
⎞
⎠
⎟
I
fL
V
V
V
L OUT
OUT
IN
1
1
(1)
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
L
) will cause this
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.
C
IN
and C
OUT
Selection
In continuous mode, the source current of the top MOSFET
is a square wave of duty cycle V
OUT
/V
IN
. To prevent large
voltage transients, a low ESR input capacitor sized for the
maximum RMS current must be used. The maximum
RMS capacitor current is given by:
CI
VVV
V
IN OMAX
OUT IN OUT
IN
required I
RMS
≅
−
()
[]
12/
This formula has a maximum at V
IN
= 2V
OUT
, where
I
RMS
= I
OUT
/2. This simple worst-case condition is com-
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
OUT
is driven by the required effective
series resistance (ESR). Typically, once the ESR require-
ment for C
OUT
has been met, the RMS current rating
generally far exceeds the I
RIPPLE(P-P)
requirement. The
output ripple ∆V
OUT
is determined by:
∆≅∆ +
⎛
⎝
⎜
⎞
⎠
⎟
V I ESR
fC
OUT L
OUT
1
8
where f = operating frequency, C
OUT
= output capacitance
and ∆I
L
= ripple current in the inductor. For a fixed output
voltage, the output ripple is highest at maximum input
voltage since ∆I
L
increases with input voltage.
