LTC3407-2_15 Datasheet, PDF (8/16 Pages) Linear Technology – Dual Synchronous, 800mA, 2.25MHz Step-Down DC/DC Regulator

LTC3407-2
APPLICATIONS INFORMATION
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 do not radiate much energy, but generally cost
more than powdered iron core inductors with similar elec-
trical characteristics. The choice of which style inductor
to use often depends more on the price vs size require-
ments and any radiated field/EMI requirements than on
what the LTC3407-2 requires to operate. Table 1 shows
some typical surface mount inductors that work well in
LTC3407-2 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.
8
Table 1. Representative Surface Mount Inductors
PART
NUMBER
VALUE DCR
MAX DC
SIZE
(μH) (Ω MAX) CURRENT (A) W × L × H (mm³)
Sumida
2.2
0.075
1.20
3.8 × 3.8 × 1.8
CDRH3D16 3.3
0.110
1.10
4.7
0.162
0.90
Sumida
1.5
CDRH2D11 2.2
0.068
0.170
0.900
0.780
3.2 × 3.2 × 1.2
Sumida
2.2
0.116
CMD4D11
3.3
0.174
0.950
0.770
4.4 × 5.8 × 1.2
Murata
1.0
0.060
1.00
2.5 × 3.2 × 2.0
LQH32CN
2.2
0.097
0.79
Toko
D312F
2.2
0.060
1.08
2.5 × 3.2 × 2.0
3.3
0.260
0.92
Panasonic
3.3
0.17
ELT5KT
4.7
0.20
1.00
4.5 × 5.4 × 1.2
0.95
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 = 0.4 • I_OUT(MAX), the output
ripple will be less than 100mV at maximum V_IN and f_O =
2.25MHz 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
tantalum 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 capacitors, such as Sanyo POSCAP,
Panasonic Special Polymer (SP), and Kemet A700, of-
34072fc