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PDF LTC3541-1 Data sheet ( Hoja de datos )
| Número de pieza | LTC3541-1 | |
| Descripción | High Efficiency Buck + VLDO Regulator | |
| Fabricantes | Linear Integrated | |
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LTC3541-1
High Efficiency
Buck + VLDO Regulator
FEATURES
DESCRIPTIO
■ High Efficiency, 500mA Buck Plus 300mA VLDOTM
Regulator
Auto Start-Up Powers VLDO/Linear Regulator
Output Prior to Buck Regulator Output
■ Independent High Efficiency, 500mA Buck
(VIN: 2.7V to 5.5V)
■ 300mA VLDO Regulator with 30mA Standalone Mode
■ No External Schottky Diodes Required
■ Buck Output Voltage Range: 0.8V to 5V
■ VLDO Input Voltage Range (LVIN): 0.9V to 5.5V
■ VLDO Output Voltage Range VLDO: 0.4V to 4.1V
■ Selectable Fixed Frequency, Pulse-Skip Operation or
Burst Mode® Operation
■ Short-Circuit Protected
■ Current Mode Operation for Excellent Line and Load
Transient Response
■ Constant Frequency Operation: 2.25MHz
■ Low Dropout Buck Operation: 100% Duty Cycle
■ Small, Thermally Enhanced, 10-Lead (3mm × 3mm)
DFN Package U
APPLICATIO S
■ PDAs/Palmtop PCs
■ Digital Cameras
■ Cellular Phones
■ PC Cards
■ Wireless and DSL Modems
■ Other Portable Power Systems
The LTC®3541-1 combines a synchronous buck DC/
DC converter with a very low dropout linear regulator
(VLDO) to provide up to two output voltages from a single
input voltage with minimal external components. When
configured for dual output operation, the LTC3541-1’s auto
start-up feature will bring the VLDO/linear regulator output
into regulation in a controlled manner prior to enabling the
Buck regulator output without the need for external pin
control. Buck output prior to VLDO/linear regulator output
sequencing may also be obtained via external pin control.
The input voltage range is ideally suited for Li-Ion battery-
powered applications, as well as powering sub-3.3V logic
from 5V or 3.3V rails.
The synchronous buck converter provides a high efficiency
output, typically 90%, capable of providing up to 500mA
of continuous output current while switching at 2.25MHz,
allowing the use of small surface mount inductors and ca-
pacitors. A mode-select pin allows Burst Mode operation
to be enabled for higher efficiency at light load currents, or
disabled for lower noise, constant frequency operation.
The VLDO regulator provides a low noise, low voltage
output capable of providing up to 300mA of continuous
output current using only a 2.2µF ceramic capacitor. The
input supply voltage of the VLDO regulator (LVIN) may
come from the buck regulator or a separate supply.
, LT, LTC and LTM are registered trademarks of Linear Technology Corporation.
VLDO is a trademark of Linear Technology Corporation.
All other trademarks are the property of their respective owners.
Protected by U.S. Patents, including 5481178, 6611131, 6304066, 6498466, 6580258
TYPICAL APPLICATIO
LTC3541-1 Typical Application
VIN
2.9V TO 5.5V
VIN ENVLDO
VOUT1
2.5V
500mA
2.2µH
ENBUCK MODE
LTC3541-1
SW GND
22pF 243k
10µF
115k
LVIN LFB
BUCKFB LVOUT
PGND
150k
412k
VOUT2
1.5V
300mA
2.2µF
35411 TA01a
Buck (Burst) Efficiency and Power Loss vs Load Current
100 1
90
EFFICIENCY
80
70
0.1
60
POWER LOSS
50 0.01
40
30
20
10 VIN = 3.3V
0 VOUT = 2.5V
1 10
100
LOAD CURRENT (mA)
0.001
0.0001
1000
35411 TA01b
35411f
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TYPICAL PERFOR A CE CHARACTERISTICS
LTC3541-1
VLDO Dropout Voltage vs
Load Current
100
VOUT = 1.5V
80
VIN = 3V
VIN = 3.6V
60
VIN = 4.2V
40
Buck (Burst) Plus VLDO Bias
Current vs VLDO Load Current
250
VIN = 3.6V
ILOAD(BUCK) = 0
200 IBIAS = IVIN + ILVIN – ILOAD
150
100
Output (Auto Start-Up Sequence,
Buck in Pulse Skip) vs Time
VOUT
2V/DIV
LVOUT
2V/DIV
VIN
2V/DIV
20
0
0 50 100 150 200 250 300
LOAD CURRENT (mA)
35411 G07
Oscillator Frequency
vs Temperature
2.50
VIN = 3.6V
2.45
2.40
2.35
2.30
2.25
2.20
2.15
2.10
2.05
2.00
–50 –25
0 25 50 75
TEMPERATURE (°C)
100 125
35411 G10
Buck Reference vs Temperature
0.820
VIN = 3.6V
0.816
0.812
0.808
0.804
0.800
0.796
0.792
0.788
0.784
0.780
–50 –25
0 25 50 75 100 125
TEMPERATURE (°C)
53411 G13
50
0
0.1
1 10 100
LOAD CURRENT (mA)
1000
35411 G08
IVOUT = 400mA 4ms/DIV
ILVOUT = 30mA
35411 G09
Oscillator Frequency
vs Supply Voltage
2.5
VIN = 3.6V
2.4
2.3
2.2
2.1
2.0
3
45
SUPPLY VOLTAGE (V)
6
35411 G11
VLDO/Linear Regulator Reference
vs Temperature
0.410
VIN = 3.6V
0.408
0.406
0.404
0.402
0.400
0.398
0.396
0.394
0.392
0.390
–50 –25
0 25 50 75 100 125
TEMPERATURE (°C)
53411 G12
RDS(ON) vs Temperature
0.700
Buck (Burst) and VLDO Output
0.600
0.500
0.400
SYNCH SWITCH
0.300
LVOUT
10mV/DIV
AC COUPLED
VOUT
10mV/DIV
AC COUPLED
0.200
0.100 MAIN SWITCH
0
–50 –25 0 25 50
VIN = 2.5V
VIN = 3.6V
VIN = 5.5V
75 100 125
TEMPERATURE (°C)
35411 G14
VIN = 3.6V
2µs/DIV
LVOUT = 1.5V
VOUT = 1.8V
ILOAD = 50mA
Burst Mode OPERATION
35411 G15
35411f
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LTC3541-1
APPLICATIO S I FOR ATIO
The basic LTC3541-1 application circuit is shown on the
first page of this data sheet. External component selection
is driven by the load requirement and requires the selection
of L, followed by CIN, COUT, and feedback resistor values
for the buck and the selection of the output capacitor and
feedback values for the VLDO and linear regulator.
BUCK REGULATOR
Inductor Selection
For most applications, the appropriate inductor value will
be in the range of 1.5µH to 3.3µH with 2.2µH the most
commonly used. The exact inductor value is chosen
largely based on the desired ripple current and burst
ripple performance. Generally, large value inductors re-
duce ripple current, and conversely, small value inductors
produce higher ripple current. Higher VIN or VOUT may
also increase the ripple current as shown in Equation 1.
A reasonable starting point for setting ripple current is
ΔIL = 200mA (40% of 500mA).
∆IL
=
(
1
f)(L
)
VOUT
⎛
⎝⎜
1−
VOUT
VIN
⎞
⎠⎟
(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 600mA rated
inductor should be enough for most applications (500mA
+ 100mA). For better efficiency, choose a low DC resis-
tance inductor.
Inductor Core Selection
Different core materials and shapes will change the
size/current and price/current relationship of an induc-
tor. Toroid or shielded pot cores in ferrite or permalloy
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 inductor to use often depends more on the price vs
size requirement and any radiated field/EMI requirements
rather than what the LTC3541-1 requires to operate. Table 2
shows some typical surface mount inductors that work
well in LTC3541-1 applications.
Table 2. Representative Surface Mount Inductors
PART
NUMBER
VALUE DCR
MAX DC
SIZE
(µH) (Ω MAX) CURRENT (A) W × L × H (mm3)
Sumida
1.0 0.025 2.0 3.9 × 3.9 × 2.4
CDRH3D23
1.5 0.029
1.5
2.2 0.038
1.3
3.3 0.048
1.1
Sumida
CMD4D06
2.2 0.116
3.3 0.174
0.950
0.770
3.5 × 4.3 × 0.8
Coilcraft
ME3220
1.0 0.058 2.7 2.5 × 3.2 × 2.0
1.5 0.068
2.2
2.2 0.104
1.0
3.3 0.138
1.3
Murata
LQH3C
1.0 0.060 1.00 2.5 × 3.2 × 2.0
2.2 0.097
0.79
Sumida
1.5 0.06 1.00 3.2 × 3.2 × 1.2
CDRH2D11/HP 2.2
0.10
0.72
CIN and COUT Selection
In continuous mode, the source current of the top MOSFET
is a square wave of duty cycle VOUT/VIN. 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 ⎡⎣VOUT
VIN − VOUT
VIN
⎤⎦1/2
This formula has a maximum at VIN = 2VOUT, where
IRMS = IOUT/2. This simple worst-case condition is common-
ly used for design. 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 capaci-
tor or choose a capacitor rated at a higher temperature
than required. Always consult the manufacturer with any
question regarding proper capacitor choice.
The selection of COUT for the buck regulator is driven by
the desired buck loop transient response, required effective
series resistance (ESR) and burst ripple performance.
The LTC3541-1 minimizes the required number of external
components by providing internal loop compensation
for the buck regulator loop. Loop stability, transient re-
sponse and burst performance can be tailored by choice
of output capacitance. For many applications, desirable
stability, trnasient response and ripple performance can
35411f
11
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| PDF Descargar | [ Datasheet LTC3541-1.PDF ] | |
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