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参数资料
| 型号： | LT3412AEFE |
| 厂商： | LINEAR TECHNOLOGY CORP |
| 元件分类： | 稳压器 |
| 英文描述： | 3 A SWITCHING REGULATOR, 4 kHz SWITCHING FREQ-MAX, PDSO16 |
| 封装： | 4.4 MM, PLASTIC, TSSOP-16 |
| 文件页数： | 5/20页 |
| 文件大小： | 267K |
| 代理商： | LT3412AEFE |
LTC3412A
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Efficiency Considerations
The efficiency of a switching regulator is equal to the
output power divided by the input power times 100%. It is
often useful to analyze individual losses to determine what
is limiting the efficiency and which change would produce
the most improvement. 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, two main sources usually account for most of the
losses: VIN quiescent current and I2R losses.
The VIN quiescent current loss dominates the efficiency
loss at very low load currents whereas the I2R loss
dominates the efficiency loss at medium to high load
currents. In a typical efficiency plot, the efficiency curve at
very low load currents can be misleading since the actual
power lost is of no consequence.
1. The VIN quiescent current is due to two components:
the DC bias current as given in the electrical characteristics
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 resulting 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; thus, their effects will be more pro-
nounced at higher supply voltages.
2. I2R losses are calculated from the resistances of the
internal switches, RSW, and external inductor RL. In con-
tinuous 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) for both the top and bottom MOSFETs can be
obtained from the Typical Performance Characteristics
curves. 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% of the total loss.
Thermal Considerations
In most applications, the LTC3412A does not dissipate
much heat due to its high efficiency.
However, in applications where the LTC3412A 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 be turned off and the SW
node will become high impedance.
To avoid the LTC3412A 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:
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. For the 16-lead exposed TSSOP
package, the
θJA is 38°C/W. For the 16-lead QFN package
the
θJA is 34°C/W.
The junction temperature, TJ, is given by:
TJ = TA + tr
where TA is the ambient temperature.
Note that at higher supply voltages, the junction tempera-
ture is lower due to reduced switch resistance (RDS(ON)).
To maximize the thermal performance of the LTC3412A,
the exposed pad should be soldered to a ground plane.
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.
APPLICATIO S I FOR ATIO
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