LTC3447_15 Datasheet, PDF (11/16 Pages) Linear Technology – I2C Controllable Buck Regulator

U
OPERATIO
Table 1. I²C Fast-Mode Timing Specifications (for Reference)
f_I2C
t_BUF
t_HD,RSTA
t_SU,RSTA
t_SU,STOP
t_HD,DAT
t_SU,DAT
t_LOW
t_HIGH
t_SP
t_f
t_r
I²C Operating Frequency
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
C_B = Capacitance of one bus line.
LTC3447
0
1.3
0.6
0.6
0.6
0
100
1.3
0.6
0
20 + 0.1 • C_B
20 + 0.1 • C_B
400
kHz
µs
µs
µs
µs
0.9
ns
ns
µs
µs
50
ns
300
ns
300
ns
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_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_IN or V_OUT also increases
the ripple current as shown in Equation 1. A reasonable
starting point for setting ripple current is ΔI_L = 240mA
(40% of 600mA).
∆I_L
=
1
(f)(L)
V_OUT
⎛
⎝⎜^1–
V_OUT
V_IN(MAX)
⎞
⎠⎟
(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 920mA rated
inductor should be enough for most applications (800mA
+ 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
50mA. Lower inductor values (higher ΔI_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/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 field/EMI requirements
than on what the LTC3447 requires to operate. Table 2
shows some typical surface mount inductors that work
well in LTC3447 applications.
Table 2.
Manufacturer
Part Number
Value DCR Max DC
Size
(µH) (mΩ max) Current (A) W x L x H (mm³)
Sumida
CDRH3D16/HP3R3
3.3 85
1.4 4.0 x 4.0 x 1.8
Sumida
CR434R7
4.7 109
1.15 4.0 x 4.5 x 3.5
Murata
LQH55DN2R2MO3
2.2 29
3.2 5.0 x 5.0 x 4.7
Toko
2.2 59
D52LC-A914BYW-2R2M
1.63 5.0 x 5.0 x 2.0
3447f
11