LTC3445_15 Datasheet, PDF (17/24 Pages) Linear Technology – I2C Controllable Buck Regulator with Two LDOs

LTC3445
APPLICATIO S I FOR ATIO
BUCK REGULATOR
The basic LTC3445 application circuit is shown on the first
page of this data sheet. External component selection is
driven by the load requirement and begins with the selec-
tion of L followed by C_IN and C_OUT.
Inductor Selection
For most applications, the value of the inductor will fall in
the range of 1µH to 4.7µH. Its value is chosen based on the
desired ripple current. Large value inductors lower ripple
current and small value inductors result in higher ripple
currents. Higher V_CC1 or lower V_OUT also increases the
ripple current as shown in Equation 1. A reasonable
starting point for setting ripple current is ∆I_L = 240mA
(40% of 600mA).
∆I_L
=
1
(^f)(^L)
V_OUT
⎛
⎝⎜
1–
V_OUT
V_CC1
⎞
⎠⎟
(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 720mA rated
inductor should be enough for most applications (600mA
+ 120mA). For better efficiency, choose a low DC-resis-
tance 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
200mA. 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.
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 characteristics. The choice of which style
inductor to use often depends more on the price vs size
requirements and any radiated field/EMI requirements
than on what the LTC3445 requires to operate. Table 1
shows some typical surface mount inductors that work
well in LTC3445 applications.
Table 1
MANUFACTURER
PART NUMBER
VALUE DCR MAX DC
SIZE
(µH) (mΩ MAX) (A) L × W × H (mm³)
Sumida CDRH3D16/ 2.2
72
HP2R2
1.2 4.0 × 4.0 × 1.8
Sumida CR434R7
4.7
109
1.15 4.0 × 4.5 × 3.5
TDK TDK7030T-
2R2M5R4
2.2
12
5.5 7.3 × 6.8 × 3.2
Coilcraft D03316P-222 2.2
12
7 12.45 × 9.4 × 5.21
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_CC1. 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:
[ ^{( )}] _{CIN required IRMS ≅ IOMAX}
V_OUT
V_CC1 – V_OUT
V_CC1
1/2
(2)
This formula has a maximum at V_CC1 = 2V_OUT, where I_RMS
= I_OUT/2. This simple worst-case condition is commonly
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 manufacturer 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_OUT
≅
∆I_L ^{⎛⎝⎜}ESR
+
1
8fC_OUT
⎞
⎠⎟
(3)
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