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PDF LTC3407-2 Data sheet ( Hoja de datos )
| Número de pieza | LTC3407-2 | |
| Descripción | 2.25MHz Step-Down DC/DC Regulator | |
| Fabricantes | Linear Technology | |
| Logotipo |  |
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FEATURES
■ High Efficiency: Up to 95%
■ Very Low Quiescent Current: Only 40µA
■ 2.25MHz Constant Frequency Operation
■ High Switch Current: 1.2A on Each Channel
■ No Schottky Diodes Required
■ Low RDS(ON) Internal Switches: 0.35Ω
■ Current Mode Operation for Excellent Line
and Load Transient Response
■ Short-Circuit Protected
■ Low Dropout Operation: 100% Duty Cycle
■ Ultralow Shutdown Current: IQ < 1µA
■ Output Voltages from 5V down to 0.6V
■ Power-On Reset Output
■ Externally Synchronizable Oscillator
■ Small Thermally Enhanced MSOP and 3mm × 3mm
DFN Packages
U
APPLICATIO S
■ PDAs/Palmtop PCs
■ Digital Cameras
■ Cellular Phones
■ Portable Media Players
■ PC Cards
■ Wireless and DSL Modems
LTC3407-2www.DataSheet4U.com
Dual Synchronous, 800mA,
2.25MHz Step-Down
DC/DC Regulator
DESCRIPTIO
The LTC®3407-2 is a dual, constant frequency, synchro-
nous step down DC/DC converter. Intended for low power
applications, it operates from 2.5V to 5.5V input voltage
range and has a constant 2.25MHz switching frequency,
allowing the use of tiny, low cost capacitors and inductors
with a profile ≤1.2mm. Each output voltage is adjustable
from 0.6V to 5V. Internal synchronous 0.35Ω, 1.2A power
switches provide high efficiency without the need for
external Schottky diodes.
A user selectable mode input is provided to allow the user
to trade-off noise ripple for low power efficiency. Burst
Mode® operation provides high efficiency at light loads,
while Pulse Skip Mode provides low noise ripple at light
loads.
To further maximize battery life, the P-channel MOSFETs
are turned on continuously in dropout (100% duty cycle),
and both channels draw a total quiescent current of only
40µA. In shutdown, the device draws <1µA.
, LTC and LT are registered trademarks of Linear Technology Corporation.
Burst Mode is a registered trademark of Linear Technology Corporation.
TYPICAL APPLICATIO
VIN = 2.5V*
TO 5.5V
C1
10µF
RUN2 VIN RUN1
MODE/SYNC
POR
VOUT2 = 2.5V
AT 800mA
L2
2.2µH
C5, 22pF
LTC3407-2
SW2 SW1
R5
100k
RESET
L1
2.2µH
C4, 22pF
C3
10µF
R4
887k R3
280k
VFB2
GND
VFB1
R2
R1 604k
301k
VOUT1 = 1.8V
AT 800mA
C2
10µF
C1, C2, C3: TAIYO YUDEN JMK316BJ106ML
L1, L2: MURATA LQH32CN2R2M33
*VOUT CONNECTED TO VIN FOR VIN ≤ 2.8V
Figure 1. 2.5V/1.8V at 800mA Step-Down Regulators
3407 TA01
LTC3407-2 Efficiency Curve
100
95 2.5V
90
1.8V
85
80
75
70
VIN = 3.3V
65 Burst Mode OPERATION
NO LOAD ON OTHER CHANNEL
60
1
10 100
LOAD CURRENT (mA)
1000
3407 TA02
sn34072 34072fs
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1 page  
LTC3407-2www.DataSheet4U.com
PI FU CTIO S
VFB1 (Pin 1): Output Feedback. Receives the feedback
voltage from the external resistive divider across the
output. Nominal voltage for this pin is 0.6V.
RUN1 (Pin 2): Regulator 1 Enable. Forcing this pin to VIN
enables regulator 1, while forcing it to GND causes regu-
lator 1 to shut down.
VIN (Pin 3): Main Power Supply. Must be closely decoupled
to GND.
SW1 (Pin 4): Regulator 1 Switch Node Connection to the
Inductor. This pin swings from VIN to GND.
GND (Pin 5): Main Ground. Connect to the (–) terminal of
COUT, and (–) terminal of CIN.
MODE/SYNC (Pin 6): Combination Mode Selection and
Oscillator Synchronization. This pin controls the operation
of the device. When tied to VIN or GND, Burst Mode
operation or pulse skipping mode is selected, respec-
tively. Do not float this pin. The oscillation frequency can
be syncronized to an external oscillator applied to this pin
and pulse skipping mode is automatically selected.
SW2 (Pin 7): Regulator 2 Switch Node Connection to the
Inductor. This pin swings from VIN to GND.
POR (Pin 8): Power-On Reset . This common-drain logic
output is pulled to GND when the output voltage is not
within ±8.5% of regulation and goes high after 117ms
when both channels are within regulation.
RUN2 (Pin 9): Output Feedback. Forcing this pin to VIN
enables regulator 2, while forcing it to GND causes regu-
lator 2 to shut down.
VFB2 (Pin 10): Output Feedback. Receives the feedback
voltage from the external resistive divider across the
output. Nominal voltage for this pin is 0.6V.
Exposed Pad (GND) (Pin 11): Power Ground. Connect to
the (–) terminal of COUT, and (–) terminal of CIN. Must be
connected to electrical ground on PCB.
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LTC3407-2www.DataSheet4U.com
APPLICATIO S I FOR ATIO
produce the most improvement. Percent efficiency can be
expressed as:
%Efficiency = 100% - (L1 + L2 + L3 + ...)
where L1, L2, etc. are the individual losses as a percentage
of input power.
Although all dissipative elements in the circuit produce
losses, 4 main sources usually account for most of the
losses in LTC3407-2 circuits: 1)VIN quiescent current, 2)
switching losses, 3) I2R losses, 4) other losses.
1) The VIN current is the DC supply current given in the
Electrical Characteristics which excludes MOSFET driver
and control currents. VIN current results in a small (<0.1%)
loss that increases with VIN, even at no load.
2) The switching current is the sum of the MOSFET driver
and control currents. The MOSFET driver current results
from switching the gate capacitance of the power MOSFETs.
Each time a MOSFET gate is switched from low to high to
low again, a packet of charge dQ moves from VIN to
ground. The resulting dQ/dt is a current out of VIN that is
typically much larger than the DC bias current. In continu-
ous mode, IGATECHG = fO(QT + QB), where QT and QB are the
gate charges of the internal top and bottom MOSFET
switches. The gate charge losses are proportional to VIN
and thus their effects will be more pronounced at higher
supply voltages.
3) I2R losses are calculated from the DC resistances of the
internal switches, RSW, and external inductor, RL. In
continuous mode, the average output current flows through
inductor L, but is “chopped” between the internal top and
bottom switches. Thus, the series resistance looking into
the SW pin is a function of both 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 Characteristics
curves. Thus, to obtain I2R losses:
I2R losses = IOUT2(RSW + RL)
4) Other ‘hidden’ losses such as copper trace and internal
battery resistances can account for additional efficiency
degradations in portable systems. It is very important to
include these “system” level losses in the design of a
system. The internal battery and fuse resistance losses
can be minimized by making sure that CIN has adequate
charge storage and very low ESR at the switching fre-
quency. Other losses including diode conduction losses
during dead-time and inductor core losses generally ac-
count for less than 2% total additional loss.
Thermal Considerations
In a majority of applications, the LTC3407-2 does not
dissipate much heat due to its high efficiency. However, in
applications where the LTC3407-2 is running at high
ambient temperature with low supply voltage and high
duty cycles, such as in dropout, the heat dissipated may
exceed the maximum junction temperature of the part. If
the junction temperature reaches approximately 150°C,
both power switches will turn off and the SW node will
become high impedance.
To prevent the LTC3407-2 from exceeding the maximum
junction temperature, the user will need to do some
thermal analysis. The goal of the thermal analysis is to
determine whether the power dissipated exceeds the
maximum junction temperature of the part. The tempera-
ture rise is given by:
TRISE = 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 = TRISE + TAMBIENT
As an example, consider the case when the LTC3407-2 is
in dropout on both channels at an input voltage of 2.7V
with a load current of 800mA and an ambient temperature
of 70°C. From the Typical Performance Characteristics
graph of Switch Resistance, the RDS(ON) resistance of the
main switch is 0.425Ω. Therefore, power dissipated by
each channel is:
PD = I2 • RDS(ON) = 272mW
The MS package junction-to-ambient thermal resistance,
θJA, is 45°C/W. Therefore, the junction temperature of the
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11 Page | ||
| Páginas | Total 16 Páginas | |
| PDF Descargar | [ Datasheet LTC3407-2.PDF ] | |
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