LTC3411 Linear Technology, LTC3411 Datasheet - Page 9

• Manufacturer Part Number: LTC3411
• Description: 1.25A/ 4MHz/ Synchronous Step-Down DC/DC Converter
• Manufacturer: Linear Technology

APPLICATIO S I FOR ATIO
typical surface mount inductors that work well in LTC3411
applications.
Table 1. Representative Surface Mount Inductors
MANU-
FACTURER PART NUMBER
Toko
Toko
Coilcraft
Coilcraft
Sumida
Sumida
Taiyo Yuden N06DB2R2M
Taiyo Yuden N05DB2R2M
Murata
Catch Diode Selection
Although unnecessary in most applications, a small im-
provement in efficiency can be obtained in a few applica-
tions by including the optional diode D1 shown in Figure 5,
which conducts when the synchronous switch is off.
When using Burst Mode operation or pulse skip mode, the
synchronous switch is turned off at a low current and the
remaining current will be carried by the optional diode. It
is important to adequately specify the diode peak current
and average power dissipation so as not to exceed the
diode ratings. The main problem with Schottky diodes is
that their parasitic capacitance reduces the efficiency,
usually negating the possible benefits for LTC3411 cir-
cuits. Another problem that a Schottky diode can intro-
duce is higher leakage current at high temperatures, which
could reduce the low current efficiency.
Remember to keep lead lengths short and observe proper
grounding (see Board Layout Considerations) to avoid
ringing and increased dissipation when using a catch
diode.
Input Capacitor (C
In continuous mode, the input current of the converter is
a square wave with a duty cycle of approximately V
V
IN
. To prevent large voltage transients, a low equivalent
A914BYW-2R2M-D52LC 2.2 H
A915AY-2ROM-D53LC
D01608C-222
LP01704-222M
CDRH4D282R2
CDC5D232R2
LQN6C2R2M04
IN
U
) Selection
U
VALUE CURRENT DCR HEIGHT
2.2 H
2.2 H
2.2 H
2.2 H
2.2 H
2.2 H
2.2 H
2 H
W
MAX DC
2.05A
2.04A
2.16A
3.3A
2.3A
2.4A
3.2A
2.9A
3.2A
120m
49m
22m
70m
23m
30m
29m
32m
24m
U
2.5mm
3.2mm
2.8mm
2mm
3mm
3mm
1mm
3mm
5mm
OUT
/
series resistance (ESR) input capacitor sized for the maxi-
mum RMS current must be used. The maximum RMS
capacitor current is given by:
where the maximum average output current I
the peak current minus half the peak-to-peak ripple cur-
rent, I
This formula has a maximum at V
I
to design because even significant deviations do not offer
much relief. Note that capacitor manufacturer’s ripple
current ratings are often based on only 2000 hours life-
time. This makes it advisable to further derate the capaci-
tor, or choose a capacitor rated at a higher temperature
than required. Several capacitors may also be paralleled to
meet the size or height requirements of the design. An
additional 0.1 F to 1 F ceramic capacitor is also recom-
mended on V
using an all ceramic capacitor solution.
Output Capacitor (C
The selection of C
minimize voltage ripple and load step transients. Typically,
once the ESR requirement is satisfied, the capacitance is
adequate for filtering. The output ripple ( V
mined by:
where f = operating frequency, C
and I
is highest at maximum input voltage since I
with input voltage. With I
will be less than 100mV at maximum V
with:
RMS
ESRC
I
RMS
V
= I
MAX
OUT
L
OUT
= ripple current in the inductor. The output ripple
OUT
= I
I
MAX
/2. This simple worst case is commonly used
< 150m
LIM
IN
I ESR
L
for high frequency decoupling, when not
– I
V
OUT
OUT IN
OUT
L
/2.
(
is driven by the required ESR to
V
) Selection
V
8
IN
f C
L
O OUT
= 0.3 • I
1
V
OUT
OUT
)
= output capacitance
LIM
IN
LTC3411
IN
= 2V
the output ripple
and f
OUT
OUT
L
MAX
) is deter-
O
increases
sn3411 3411fs
, where
= 1MHz
equals
9