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PDF LTC3405 Data sheet ( Hoja de datos )
| Número de pieza | LTC3405 | |
| Descripción | Synchronous Step-Down Regulator | |
| 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
■ 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
U■ Low Profile (1mm) ThinSOTTM Package
APPLICATIO S
■ Cellular Telephones
■ Personal Information Appliances
■ Wireless and DSL Modems
■ Digital Still Cameras
■ MP3 Players
■ Portable Instruments
LTC3405
1.5MHz, 300mA
Synchronous Step-Down
Regulator in ThinSOT
DESCRIPTIO
The LTC®3405 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 LTC3405 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 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 LTC3405 is available in a low
profile (1mm) ThinSOT package.
For new designs, refer to the LTC3405A data sheet. 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
LTC3405
1
RUN
65
MODE VFB
GND
2
4.7µH**
22pF
887k
280k
+
3405 F01a
*VOUT CONNECTED TO VIN FOR 2.7V < VIN < 3.3V
**MURATA LQH3C4R7M34
†TAIYO YUDEN LMK212BJ225MG
††AVX TPSB336K006R0600
VOUT*
3.3V
COUT††
33µF
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
3405 F01b
Figure 1b. Efficiency vs Load Current
3405fa
1
1 page  
TYPICAL PERFOR A CE CHARACTERISTICS
(From Figure 1a Except for the Resistive Divider Resistor Values)
LTC3405
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
Load Step
VOUT
100mV/DIV
AC
COUPLED
IL
200mA/DIV
ILOAD
200mA/DIV
3405 G20
VIN = 3.6V
40µs/DIV
VOUT = 1.8V
ILOAD = 20mA TO 250mA
Burst Mode OPERATION
Load Step
VOUT
100mV/DIV
AC
COUPLED
IL
200mA/DIV
ILOAD
200mA/DIV
3405 G21
VIN = 3.6V
40µs/DIV
VOUT = 1.8V
ILOAD = 0mA TO 250mA
Burst Mode OPERATION
3405 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 
LTC3405
APPLICATIO S I FOR ATIO
top and bottom MOSFET RDS(ON) and the duty cycle
(DC) as follows:
RSW = (RDS(ON)TOP)(DC) + (RDS(ON)BOT)(1 – DC)
The RDS(ON) for both the top and bottom MOSFETs can
be obtained from the Typical Performance Charateristics
curves. Thus, to obtain I2R losses, simply add RSW to
RL and multiply the result by the square of the average
output current.
Other losses including CIN and COUT ESR dissipative
losses and inductor core losses generally account for less
than 2% total additional loss.
Thermal Considerations
In most applications the LTC3405 does not dissipate
much heat due to its high efficiency. But, in applications
where the LTC3405 is running at high ambient tempera-
ture with low supply voltage and high duty cycles, such
as in dropout, the heat dissipated may exceed the maxi-
mum junction temperature of the part. If the junction
temperature reaches approximately 150°C, both power
switches will be turned off and the SW node will become
high impedance.
To avoid the LTC3405 from exceeding the maximum
junction temperature, the user will need to do a thermal
analysis. The goal of the thermal analysis is to determine
whether the operating conditions exceed the maximum
junction temperature of the part. The temperature rise is
given by:
TR = (PD)(θJA)
where PD is the power dissipated by the regulator and θJA
is the thermal resistance from the junction of the die to the
ambient temperature.
The junction temperature, TJ, is given by:
TJ = TA + TR
where TA is the ambient temperature.
As an example, consider the LTC3405 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.
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11 Page | ||
| Páginas | Total 16 Páginas | |
| PDF Descargar | [ Datasheet LTC3405.PDF ] | |
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