ltc3445euf-pbf Linear Technology Corporation, ltc3445euf-pbf Datasheet - Page 17

• Manufacturer Part Number: ltc3445euf-pbf
• Description: I2c Controllable Buck Regulator With Two Ldos In A 4mm ? 4mm Qfn
• Manufacturer: Linear Technology Corporation

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
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
ripple current as shown in Equation 1. A reasonable
starting point for setting ripple current is ∆I
(40% of 600mA).
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
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
∆ =
I
L
( )( )
f L
1
V
OUT
CC1
U
⎛
⎜
⎝
1
IN
or lower V
–
and C
U
V
V
OUT
CC
1
OUT
⎞
⎟
⎠
OUT
.
W
also increases the
L
) will cause this
L
U
= 240mA
(1)
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
Sumida CDRH3D16/
HP2R2
Sumida CR434R7
TDK TDK7030T-
2R2M5R4
Coilcraft D03316P-222
C
In continuous mode, the source current of the top MOSFET
is a square wave of duty cycle V
voltage transients, a low ESR input capacitor sized for the
maximum RMS current must be used. The maximum
RMS capacitor current is given by:
This formula has a maximum at V
= I
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
series resistance (ESR). Typically, once the ESR require-
ment for C
generally far exceeds the I
output ripple ∆V
C
IN
IN
OUT
∆
and C
required I
V
OUT
/2. This simple worst-case condition is commonly
OUT
≅ ∆
OUT
Selection
RMS
I ESR
L
has been met, the RMS current rating
⎛
⎜
⎝
OUT
OUT
VALUE
≅
(µH) (mΩ MAX)
2.2
4.7
2.2
2.2
is determined by:
I
OMAX
+
is driven by the required effective
8
fC
DCR
109
[
RIPPLE(P-P)
1
72
12
12
OUT
V
OUT
OUT
⎞
⎟
⎠
CC1
(
/V
MAX DC
V
1.15
CC
CC1
= 2V
(A)
1.2
5.5
7
V
1
CC
requirement. The
. To prevent large
–
OUT
LTC3445
1
V
12.45 × 9.4 × 5.21
L × W × H (mm
OUT
4.0 × 4.0 × 1.8
4.0 × 4.5 × 3.5
7.3 × 6.8 × 3.2
, where I
)
SIZE
]
1 2
17
/
3445fa
RMS
(2)
(3)
3
)