LTC3407-4 Datasheet, PDF (8/16 Pages) Linear Technology – Dual Synchronous, 800mA, 2.25MHz Step-Down DC/DC Regulator

LTC3407-4
APPLICATIO S I FOR ATIO
begins when the peak inductor current falls below a level
set by the burst clamp. Lower inductor values result in
higher ripple current which causes this to occur at lower
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 mate-
rials are small and don’t radiate much energy, but gener-
ally cost more than powdered iron core inductors with
similar electrical characterisitics. The choice of which
style inductor to use often depends more on the price vs
size requirements and any radiated field/EMI require-
ments than on what the LTC3407-4 requires to operate.
Table 1 shows some typical surface mount inductors that
work well in LTC3407-4 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 maxi-
mum RMS current must be used. The maximum RMS
capacitor current is given by:
( ) _{IRMS ≈IMAX}
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 – ∆I_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
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 0.1µF to 1µF ceramic capacitor is also recom-
mended 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
CDRH3D16
3.3
4.7
0.075
0.110
0.162
1.20
3.8 × 3.8 × 1.8
1.10
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
LQH32CN
1.0
0.060
2.2
0.097
1.00
2.5 × 3.2 × 2.0
0.79
Toko
D312F
2.2
0.060
3.3
0.260
1.08
2.5 × 3.2 × 2.0
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 (∆V_OUT) is deter-
mined by:
∆V_OUT
≈
⎛
∆I_{L ⎝⎜}ESR
+
8f_O
1
C_OUT
⎞
⎠⎟
where f = operating frequency, C_OUT = output capacitance
and ∆I_L = ripple current in the inductor. The output ripple
is highest at maximum input voltage since ∆I_L increases
with input voltage. With ∆I_L = 0.3 • I_LIM 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. Alumi-
num 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
34074f