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PDF LTC3447 Data sheet ( Hoja de datos )
| Número de pieza | LTC3447 | |
| Descripción | I2C Controllable Buck Regulator in 3mm x 3mm DFN | |
| Fabricantes | Linear Technology | |
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LTC3447www.DataSheet4U.com
I2C Controllable Buck
Regulator in 3mm × 3mm DFN
FEATURES
■ I2C Programmable Output with 21.6mV Resolution
■ Overtemperature Protected
■ High Efficiency: Up to 93%
■ Very Low Quiescent Current: Only 33µA
■ 600mA Output Current at VIN = 3V
■ 2.5V to 5.5V Input Voltage Range
■ 1MHz Constant Frequency Operation
■ No Schottky Diode Required
■ Low Dropout Operation: 100% Duty Cycle
■ Stable with Ceramic Capacitors
■ Shutdown Mode Draws <1µA Supply Current
■ ±2% Output Voltage Accuracy
■ Standard (100kHz) or Fast Mode (400kHz) I2C
■ 6-Bit Voltage DAC (0.69V to 2.05V)
■ Disable Burst Mode Operation
■ Enable Power Good Blanking
■ Optional External Start-Up Resistors
■ Soft-Start
■ 10 Lead, 3mm × 3mm DFN Package
U
APPLICATIO S
■ Distributed Power Supplies
■ Notebook Computers
■ PDAs and Other Handheld Devices
, LTC and LT are registered trademarks of Linear Technology Corporation.
Burst Mode is a registered trademark of Linear Technology Corporation.
All other trademarks are the property of their respective owners.
Protected by U.S. Patents, including 5481178, 6580258, 6304066, 6127815,
6498466, 6611131.
DESCRIPTIO
The LTC®3447 is a high efficiency monolithic synchronous
current mode buck regulator. Using an I2C interface, the
output voltage can be set between 0.69V and 2.05V using
an internal 6-bit DAC.
The buck regulator has optional external feedback resis-
tors that can be used for setting the initial start up voltage.
The feedback voltage reference for this start-up option
is 0.6V. Once the voltage DAC is updated via the I2C, the
buck regulator switches from external to internal feedback
resistors. When there are no external resistors, the default
start-up voltage is 1.38V.
The switching frequency is internally set at 1MHz, al-
lowing the use of small surface mount inductors and
capacitors.
In Burst Mode® operation, supply current is only 33µA,
dropping to <1µA in shutdown. The 2.5V to 5.5V input
voltage range makes the LTC3447 ideally suited for single
cell Li-Ion battery-powered applications. 100% duty cycle
capability provides low dropout operation, extending
battery life in portable systems. Automatic Burst Mode
operation increases efficiency at light loads, further ex-
tending battery life.
The internal synchronous switch increases efficiency and
eliminates the need for an external Schottky diode.
TYPICAL APPLICATIO
VIN
2.5V TO 5.5V
C2
4.7µF
CERAMIC
I2C VCCD
10k
SDA
SCL
10k
PWREN
C3
4.7µF
* 600mA AT VIN = 3V
VIN
RUN PGOOD
LTC3447
VCCD
SW
SDA VOUT
SCL FB
GND
EXPOSED PADDLE
TO GROUND
VIN
RPU1
20k
L1
3.3µH
C1
10µF
VOUT
0.69V
TO 2.05V
AT 600mA*
R1
100k
R2
49.9k
3447 TA01
Efficiency and Power Loss vs
Load Current (VIN = 3.6V)
100 1000
90
80
70 100
60
50
40
10
30
20 PULSE SKIP EFFICIENCY
10 Burst Mode EFFICIENCY
LOSS
01
1 10 100 1000
LOAD CURRENT (mA)
3447 TA01b
3447f
1
1 page  
TYPICAL PERFOR A CE CHARACTERISTICS
LTC3447www.DataSheet4U.com
RDS(ON) vs Supply Voltage
0.60
0.55
RDS(ON) vs Temperature
0.60 VIN = 3.6V
0.55
0.50 0.50
0.45
PFET
0.40
0.45 PFET
0.40
0.35
NFET
0.30
0.35 NFET
0.30
0.25 0.25
0.20
2.5
3.5 4.5
SUPPLY VOLTAGE (V)
5.5
3447 G10
0.20
–40 –20 0 20 40 60 80 100 120
TEMPERATURE (°C)
3447 G11
Feedback Reference vs Supply
Voltage
610
Feedback Voltage vs
Temperature
618
Output Voltage vs Supply Voltage
2.060
0.720
2.055
2.050
DAC = MAX
0.715
0.710
2.045
0.705
2.040
0.700
2.035
2.030
DAC = MIN
0.695
0.690
2.025
0.685
2.020
2.5
3.5 4.5
SUPPLY VOLTAGE (V)
0.680
5.5
3447 G12
Load Step (200mA to 400mA)
612 VOUT
605 50mV/DIV
606
600 600
LOAD CURRENT
594
595 200mA/DIV
588
590
2.5
3.5 4.5
SUPPLY VOLTAGE (V)
5.5
3447 G13
582
–40 –20 0 20 40 60 80 100 120
TEMPERATURE (°C)
3447 G15
40µs/DIV
3447 G15
Soft-Start with No Load
500mV/DIV
5V/DIV
200mA/DIV
500mV/DIV
VOUT
PGOOD
INDUCTOR CURRENT
200µs/DIV
VOUT(MIN) to VOUT(MAX) Transition
PULSE SKIP MODE
500mV/DIV
VOUT
VOUT(MAX) to VOUT(MIN) Transition
BURST MODE OPERATION
500mV/DIV
VOUT
100mA/DIV
3447 G16
5V/DIV
INDUCTOR CURRENT
PGOOD
100µs/DIV
200mA/DIV
3447 G17
5V/DIV
INDUCTOR CURRENT
PGOOD
100µs/DIV
3447 G18
3447f
5
5 Page 
U
OPERATIO
Table 1. I2C Fast-Mode Timing Specifications (for Reference)
fI2C
tBUF
tHD,RSTA
tSU,RSTA
tSU,STOP
tHD,DAT
tSU,DAT
tLOW
tHIGH
tSP
tf
tr
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
CB = Capacitance of one bus line.
LTC3447www.DataSheet4U.com
0
1.3
0.6
0.6
0.6
0
100
1.3
0.6
0
20 + 0.1 • CB
20 + 0.1 • CB
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 CIN and COUT.
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 VIN or VOUT also increases
the ripple current as shown in Equation 1. A reasonable
starting point for setting ripple current is ΔIL = 240mA
(40% of 600mA).
∆IL
=
1
(f)(L)
VOUT
⎛
⎝⎜1–
VOUT
VIN(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 ΔIL) 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
D52LC-A914BYW-2R2M
59
1.63 5.0 x 5.0 x 2.0
3447f
11
11 Page | ||
| Páginas | Total 16 Páginas | |
| PDF Descargar | [ Datasheet LTC3447.PDF ] | |
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