LTC3407AIDD Datasheet, PDF (8/16 Pages) Linear Integrated Systems – Dual Synchronous, 600mA, 1.5MHz Step-Down DC/DC Regulator

LTC3407
APPLICATIONS INFORMATION
load currents. This causes 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. Toroid
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 in-
ductor to use often depends more on the price vs size
requirements and any radiated field/EMI requirements
than on what the LTC3407 requires to operate. Table 1
shows some typical surface mount inductors that work
well in LTC3407 applications.
Input Capacitor (C_IN) Selection
In continuous mode, the input current of the converter is a
square wave with a duty cycle of approximately V_OUT/V_IN.
To prevent large voltage transients, a low equivalent series
resistance (ESR) input capacitor sized for the maximum
RMS current must be used. The maximum RMS capacitor
current is given by:
I_RMS ≈I_MAX
V_OUT (V_IN – V_OUT )
V_IN
where the maximum average output current I_MAX equals
the peak current minus half the peak-to-peak ripple cur-
rent, I_MAX = I_LIM – ΔI_L/2.
This formula has a maximum at V_IN = 2V_OUT, where I_RMS
= I_OUT/2. This simple worst-case is commonly used to
design because even significant deviations do not offer
much relief. Note that capacitor manufacturer’s ripple cur-
rent ratings are often based on only 2000 hours lifetime.
This makes it advisable to further derate the capacitor,
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
0.1μF to 1μF ceramic capacitor is also recommended on
V_IN for high frequency decoupling, when not using an all
ceramic capacitor solution.
Table 1. Representative Surface Mount Inductors
PART
NUMBER
VALUE DCR
MAX DC
SIZE
(μH) (Ω MAX) CURRENT (A) W × L × H (mm³)
Sumida
1.5 0.043
1.55
3.8 × 3.8 × 1.8
CDRH3D16 2.2 0.075
1.20
3.3 0.110
1.10
4.7 0.162
0.90
Sumida
CMD4D06
2.2 0.116
3.3 0.174
4.7 0.216
0.950
0.770
0.750
3.5 × 4.3 × 0.8
Panasonic
3.3
0.17
1.00
4.5 × 5.4 × 1.2
ELT5KT
4.7
0.20
0.95
Murata
1.0 0.060
1.00
2.5 × 3.2 × 2.0
LQH32CN
2.2 0.097
0.79
4.7 0.150
0.65
Output Capacitor (C_OUT) Selection
The selection of C_OUT is driven by the required ESR to
minimize voltage ripple and load step transients. Typically,
once the ESR requirement is satisfied, the capacitance
is adequate for filtering. The output ripple (ΔV_OUT) is
determined by:
V_OUT


I_L

ESR +

8f_O
1
C_OUT



where f = operating frequency, C_OUT = output capacitance
and ΔI_L = ripple current in the inductor. The output ripple
is highest at maximum input voltage since ΔI_L increases
with input voltage. With ΔI_L = 0.3 • I_LIM the output ripple
will be less than 100mV at maximum V_IN and f_O = 1.5MHz
with:
ESR_COUT < 150mΩ
Once the ESR requirements for C_OUT have been met, the
RMS current rating generally far exceeds the I_RIPPLE(P-P)
requirement, except for an all ceramic solution.
In surface mount applications, multiple capacitors may
have to be paralleled to meet the capacitance, ESR or RMS
current handling requirement of the application. Aluminum
electrolytic, special polymer, ceramic and dry tantulum
capacitors are all available in surface mount packages. The
OS-CON semiconductor dielectric capacitor available from
Sanyo has the lowest ESR(size) product of any aluminum
electrolytic at a somewhat higher price. Special polymer
3407fa
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