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LTC3407 Datasheet(PDF) 11 Page - Linear Technology

No. de Pieza. LTC3407
Descripción  Dual Synchronous, 600mA, 1.5MHz Step-Down DC/DC Regulator
Descarga  16 Pages
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Fabricante  LINER [Linear Technology]
Página de inicio  http://www.linear.com
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LTC3407 Datasheet(HTML) 11 Page - Linear Technology

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LTC3407
11
sn3407 3407fs
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 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 flowing
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 does not
dissipate much heat due to its high efficiency. However, in
applications where the LTC3407 is running at high ambi-
ent 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 be turned off and the SW node will
become high impedance.
To prevent the LTC3407 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 is in
dropout on both channels at an input voltage of 2.7V with
a load current of 600mA 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) = 153mW
The MS package junction-to-ambient thermal resistance,
θJA, is 45°C/W. Therefore, the junction temperature of the
APPLICATIO S I FOR ATIO


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