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PDF LTC3405A Data sheet ( Hoja de datos )
| Número de pieza | LTC3405A | |
| Descripción | Dual DC/DC Converter with USB Power Manager and Li-Ion Battery Charger | |
| Fabricantes | Linear Technology | |
| Logotipo |  |
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FEATURES
■ High Efficiency: Up to 96%
■ Very Low Quiescent Current: Only 20µA
During Operation
■ 300mA Output Current at VIN = 3V
■ 2.5V to 5.5V Input Voltage Range
■ 1.5MHz Constant Frequency Operation
■ No Schottky Diode Required
■ Low Dropout Operation: 100% Duty Cycle
■ Stable with Ceramic Capacitors
■ 0.8V Reference Allows Low Output Voltages
■ Shutdown Mode Draws < 1µA Supply Current
■ ±2% Output Voltage Accuracy
■ Current Mode Operation for Excellent Line and
Load Transient Response
■ Overtemperature Protected
■ Low Profile (1mm) ThinSOTTM Package
U
APPLICATIO S
■ Cellular Telephones
■ Personal Information Appliances
■ Wireless and DSL Modems
■ Digital Still Cameras
■ MP3 Players
■ Portable Instruments
LTC3405A
1.5MHz, 300mA
Synchronous Step-Down
Regulator in ThinSOT
DESCRIPTIO
The LTC®3405A is a high efficiency monolithic synchro-
nous buck regulator using a constant frequency, current
mode architecture. Supply current during operation is
only 20µA and drops to <1µA in shutdown. The 2.5V to
5.5V input voltage range makes the LTC3405A ideally
suited for single Li-Ion battery-powered applications. 100%
duty cycle provides low dropout operation, extending
battery life in portable systems.
Switching frequency is internally set at 1.5MHz, allowing
the use of small surface mount inductors and capacitors.
The LTC3405A is specifically designed to work well with
ceramic output capacitors, achieving very low output
voltage ripple and a small PCB footprint.
The internal synchronous switch increases efficiency and
eliminates the need for an external Schottky diode. Low
output voltages are easily supported with the 0.8V feed-
back reference voltage. The LTC3405A is available in a low
profile (1mm) ThinSOT package.
For fixed 1.5V and 1.8V output versions, refer to the
LTC3405A-1.5/LTC3405A-1.8 data sheet.
, LTC and LT are registered trademarks of Linear Technology Corporation.
ThinSOT is a trademark of Linear Technology Corporation.
Protected by U.S. Patents, including 6580258, 5481178.
TYPICAL APPLICATIO
VIN
2.7V
TO 5.5V
CIN†
2.2µF
CER
4
VIN
3
SW
LTC3405A
1
RUN
65
MODE VFB
GND
2
4.7µH**
22pF
887k
280k
3405A F01a
*VOUT CONNECTED TO VIN FOR 2.7V < VIN < 3.3V
**MURATA LQH3C4R7M34
†TAIYO YUDEN LMK212BJ225MG
††TAIYO YUDEN JMK212BJ475MG
VOUT*
3.3V
COUT††
4.7µF
CER
Figure 1a. High Efficiency Step-Down Converter
100
95 VIN = 3.6V
90
85
VIN = 4.2V
80
75
VIN = 5.5V
70
65
60
0.1
1 10 100
OUTPUT CURRENT (mA)
1000
3405A F01b
Figure 1b. Efficiency vs Load Current
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1
1 page  
TYPICAL PERFOR A CE CHARACTERISTICS
(From Figure 1a Except for the Resistive Divider Resistor Values)
LTC3405A
Load Step
VOUT
100mV/DIV
AC
COUPLED
IL
200mA/DIV
Load Step
VOUT
100mV/DIV
AC
COUPLED
IL
200mA/DIV
Load Step
VOUT
100mV/DIV
AC
COUPLED
IL
200mA/DIV
ILOAD
200mA/DIV
VIN = 3.6V
40µs/DIV
VOUT = 1.8V
ILOAD = 20mA TO 250mA
PULSE SKIPPING MODE
ILOAD
200mA/DIV
3405A G20
VIN = 3.6V
40µs/DIV
VOUT = 1.8V
ILOAD = 20mA TO 250mA
Burst Mode OPERATION
ILOAD
200mA/DIV
3405A G21
VIN = 3.6V
40µs/DIV
VOUT = 1.8V
ILOAD = 0mA TO 250mA
Burst Mode OPERATION
3405A G22
PI FU CTIO S
RUN (Pin 1): Run Control Input. Forcing this pin above
1.5V enables the part. Forcing this pin below 0.3V shuts
down the device. In shutdown, all functions are disabled
drawing <1µA supply current. Do not leave RUN floating.
GND (Pin 2): Ground Pin.
SW (Pin 3): Switch Node Connection to Inductor. This pin
connects to the drains of the internal main and synchro-
nous power MOSFET switches.
VIN (Pin 4): Main Supply Pin. Must be closely decoupled
to GND, Pin 2, with a 2.2µF or greater ceramic capacitor.
VFB (Pin 5): Feedback Pin. Receives the feedback voltage
from an external resistive divider across the output.
MODE (Pin 6): Mode Select Input. To select pulse skip-
ping mode, tie to VIN. Grounding this pin selects Burst
Mode operation. Do not leave this pin floating.
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5 Page 
LTC3405A
APPLICATIO S I FOR ATIO
The junction temperature, TJ, is given by:
TJ = TA + TR
where TA is the ambient temperature.
As an example, consider the LTC3405A in dropout at an
input voltage of 2.7V, a load current of 300mA and an
ambient temperature of 70°C. From the typical perfor-
mance graph of switch resistance, the RDS(ON) of the
P-channel switch at 70°C is approximately 0.94Ω. There-
fore, power dissipated by the part is:
PD = ILOAD2 • RDS(ON) = 84.6mW
For the SOT-23 package, the θJA is 250°C/ W. Thus, the
junction temperature of the regulator is:
TJ = 70°C + (0.0846)(250) = 91.15°C
which is well below the maximum junction temperature of
125°C.
Note that at higher supply voltages, the junction tempera-
ture is lower due to reduced switch resistance (RDS(ON)).
Checking Transient Response
The regulator loop response can be checked by looking at
the load transient response. Switching regulators take
several cycles to respond to a step in load current. When
a load step occurs, VOUT immediately shifts by an amount
equal to (∆ILOAD • ESR), where ESR is the effective series
resistance of COUT. ∆ILOAD also begins to charge or
discharge COUT, which generates a feedback error signal.
The regulator loop then acts to return VOUT to its steady-
state value. During this recovery time VOUT can be moni-
tored for overshoot or ringing that would indicate a stability
problem. For a detailed explanation of switching control
loop theory, see Application Note 76.
A second, more severe transient is caused by switching in
loads with large (>1µF) supply bypass capacitors. The
discharged bypass capacitors are effectively put in parallel
with COUT, causing a rapid drop in VOUT. No regulator can
deliver enough current to prevent this problem if the load
switch resistance is low and it is driven quickly. The only
solution is to limit the rise time of the switch drive so that
the load rise time is limited to approximately (25 • CLOAD).
Thus, a 10µF capacitor charging to 3.3V would require a
250µs rise time, limiting the charging current to about
130mA.
PC Board Layout Checklist
When laying out the printed circuit board, the following
checklist should be used to ensure proper operation of the
LTC3405A. These items are also illustrated graphically in
Figures 5 and 6. Check the following in your layout:
–
VOUT
+
COUT
L1
16
RUN MODE
LTC3405A
25
GND VFB
3 SW
4
VIN
CIN
R2 R1
CFWD
VIN
BOLD LINES INDICATE HIGH CURRENT PATHS
3405A F05
Figure 5. LTC3405A Layout Diagram
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11 Page | ||
| Páginas | Total 16 Páginas | |
| PDF Descargar | [ Datasheet LTC3405A.PDF ] | |
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