LTC3541
3541fa
High Efficiency
Buck + VLDO Regulator
?/DIV>
PDAs/Palmtop PCs
?/DIV>
Digital Cameras
?/DIV>
Cellular Phones
?/DIV>
PC Cards
?/DIV>
Wireless and DSL Modems
?/DIV>
Other Portable Power Systems
?/DIV>
High Effciency, 500mA Buck Plus 300mA VLDO
TM
Regulator
Auto Start-Up Powers Buck Output Prior to
VLDO/Linear Regulator Output
?/DIV>
Independent High Effciency, 500mA Buck
(V
IN
: 2.7V to 5.5V)
?/DIV>
300mA VLDO Regulator with 30mA Standalone Mode
?/DIV>
No External Schottky Diodes Required
?/DIV>
Buck Output Voltage Range: 0.8V to 5V
?/DIV>
VLDO Input Voltage Range (LV
IN
): 0.9V to 5.5V
?/DIV>
VLDO Output Voltage Range VLDO: 0.4V to 4.1V
?/DIV>
Selectable Fixed Frequency, Pulse-Skip Operation or
Burst Mode
?/DIV>
Operation
?/DIV>
Short-Circuit Protected
?/DIV>
Current Mode Operation for Excellent Line and Load
Transient Response
?/DIV>
Constant Frequency Operation: 2.25MHz
?/DIV>
Low Dropout Buck Operation: 100% Duty Cycle
?/DIV>
Small, Thermally Enhanced, 10-Lead (3mm ?3mm)
DFN Package
PPLICATIO S
EATURES
ESCRIPTIO
YPICAL APPLICATIO
, LT, LTc and LTM are registered trademarks of Linear Technology corporation.
LDO is a trademark of Linear Technology corporation.
ll other trademarks are the property of their respective owners.
r t t . .P t nt in l in 4 117 111 1 4 4 4
2
The LTC
?/DIV>
3541 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 LTC3541s auto
start-up feature will bring the Buck output into regulation
in a controlled manner prior to enabling the VLDO regulator
output without the need for external pin control. VLDO/
linear regulator output prior to Buck 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 2.2礔 ceramic capacitor. The
input supply voltage of the VLDO regulator (LV
IN
) may
come from the buck regulator or a separate supply.
SW
V
IN
ENBUCK
BUCKFB
LV
IN
ENVLDO
MODE
GND
LFB
LV
OUT
PGND
LTC3541
150k
V
OUT2
1.5V
300mA
2.2礔
115k
2.2礖
10礔
V
IN
2.9V TO 5.5V
243k
22pF
412k
OUT1
2.5V
00mA
LTC3541 Typical Application
Buck (Burst) Efficiency and Power Loss vs Load Current
LOAD CURRENT (mA)
1
40
50
60
70
80
10
100
1000
3541 TA01b
30
20
10
0
90
100
0.001
0.01
0.1
0.0001
1
V
IN
= 3.3V
V
OUT
= 2.5V
EFFICIENCY
POWER LOSS
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