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LTC3409A Datasheet(PDF) 12 Page - Linear Technology

Part No. LTC3409A
Description  600mA Low VIN Buck Regulator in 3mm 3mm DFN
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Maker  LINER [Linear Technology]
Homepage  http://www.linear.com
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LTC3409A Datasheet(HTML) 12 Page - Linear Technology

 
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LTC3409A
12
3409af
1. The VIN quiescent current is due to two components:
the DC bias current as given in the Electrical Charac-
teristics and the internal main switch and synchronous
switch gate charge currents. The gate charge current
results from switching the gate capacitance of the
internal power MOSFET switches. Each time the gate
is switched from high to low to high again, a packet
of charge, dQ, moves from VIN to ground. The result-
ing dQ/dt is the current out of VIN that is typically
larger than the DC bias current. In continuous mode,
IGATECHG = f(QT + QB) where QT and QB are the gate
charges of the internal top and bottom switches. Both
the DC bias and gate charge losses are proportional to
VIN and thus their effects will be more pronounced at
higher supply voltages.
2. I2R losses are calculated from the resistances of the
internal switches, RSW, and external inductor RL. In
continuous mode, the average output current flowing
through inductor L is “chopped” between the main
switch and the synchronous switch. 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)forboththetopandbottomMOSFETscanbe
obtained from the Typical Performance Characteristics.
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 LTC3409A does not dissipate
much heat due to its high efficiency. But, in applications
where the LTC3409A is running at high ambient tem-
perature 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 avoid the LTC3409A 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 LTC3409A in dropout at an
input voltage of 1.6V, a load current of 600mA and an
ambient temperature of 75°C. From the typical perfor-
mance graph of switch resistance, the RDS(ON) of the
P-channel switch at 75°C is approximately 0.48Ω. There-
fore, power dissipated by the part is:
PD = ILOAD2 • RDS(ON) = 172.8mW
For the DD8 package, the
θJA is 43°C/W. Thus, the junction
temperature of the regulator is:
TJ = 75°C + (0.1728)(43) = 82.4°C
which is well below the maximum junction temperature
of 125°C.
Figure 2. Power Loss
APPLICATIONS INFORMATION
LOAD CURRENT (mA)
0.1
1.0000
0.0100
0.1000
0.0010
0.0001
10
3409A F02
VOUT = 1.8V
1000
100
1
BURST
PULSE SKIP
2.5VIN
2.5VIN
4.2VIN
4.2VIN
3.6VIN
3.6VIN


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