ltc3447edd-trpbf Linear Technology Corporation, ltc3447edd-trpbf Datasheet - Page 11

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ltc3447edd-trpbf

Manufacturer Part Number
ltc3447edd-trpbf
Description
I2c Controllable Buck Regulator In 3mm ? 3mm Dfn
Manufacturer
Linear Technology Corporation
Datasheet
OPERATIO
Table 1. I
f
t
t
t
t
t
t
t
t
t
t
t
C
APPLICATIO S I FOR ATIO
The basic LTC3447 application circuit is shown on the
front page of the data sheet. External component selec-
tion is driven by the load requirement and begins with the
selection of L1 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
the ripple current as shown in Equation 1. A reasonable
starting point for setting ripple current is ΔI
(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 920mA rated
inductor should be enough for most applications (800mA
+ 120mA). For better effi ciency, 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
50mA. Lower inductor values (higher ΔI
I2C
BUF
HD,RSTA
SU,RSTA
SU,STOP
HD,DAT
SU,DAT
LOW
HIGH
SP
f
r
B
= Capacitance of one bus line.
∆I
L
=
2
C Fast-Mode Timing Specifi cations (for Reference)
( )( )
I
Bus free time between Stop and Start Condition
Hold Time after (Repeated) Start Condition
Repeated Start Condition Setup Time
Stop Condition Setup Time
Data Hold Time
Data Setup Time
Clock Low Period
Clock High Period
Pulse Width of Spikes Suppressed by Input Filter
Clock, Data Fall Time
Clock, Data Rise Time
2
f L
C Operating Frequency
1
V
OUT
U
U
1
V
U
IN MAX
V
OUT
(
IN
and C
IN
)
or V
W
OUT
OUT
L
.
) will cause this
also increases
L
U
= 240mA
(1)
Table 2.
Manufacturer
Part Number
Sumida
CDRH3D16/HP3R3
Sumida
CR434R7
Murata
LQH55DN2R2MO3
Toko
D52LC-A914BYW-2R2M
to occur at lower load currents, which can cause a dip in
effi ciency 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/cur-
rent and price/current relationship of an inductor. Toroid
or shielded pot cores in ferrite or perm alloy 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 induc-
tor to use often depends more on the price versus size
requirements and any radiated fi eld/EMI requirements
than on what the LTC3447 requires to operate. Table 2
shows some typical surface mount inductors that work
well in LTC3447 applications.
Value
(µH) (mΩ max) Current (A) W x L x H (mm
3.3
4.7
2.2
2.2
20 + 0.1 • C
20 + 0.1 • C
100
1.3
0.6
0.6
0.6
1.3
0.6
0
0
0
DCR
109
85
29
59
B
B
Max DC
1.15
1.63
1.4
3.2
LTC3447
400
300
300
0.9
50
4.0 x 4.0 x 1.8
4.0 x 4.5 x 3.5
5.0 x 5.0 x 4.7
5.0 x 5.0 x 2.0
Size
11
kHz
µs
µs
µs
µs
ns
ns
µs
µs
ns
ns
ns
3447f
3
)

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