参数资料
型号: LTC3614EUDD#TRPBF
厂商: LINEAR TECHNOLOGY CORP
元件分类: 稳压器
英文描述: 10.5 A SWITCHING REGULATOR, 2700 kHz SWITCHING FREQ-MAX, PQCC24
封装: 3 X 5 MM, LEAD FREE, PLASTIC, QFN-24
文件页数: 16/30页
文件大小: 438K
代理商: LTC3614EUDD#TRPBF
LTC3614
23
3614fa
Efciency Considerations
The efciency 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 efciency and which change would produce
the most improvement. Efciency can be expressed as:
Efciency = 100% – (L1 + L2 + L3 + ...)
where L1, L2, etc. are the individual losses as a percent-
age 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 efciency loss at
very low load currents whereas the I2R loss dominates
the efciency loss at medium to high load currents. In a
typical efciency plot, the efciency curve at very low load
currents can be misleading since the actual power lost is
usually 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
low to high to low again, a packet of charge dQ moves
from VIN to ground. The resulting dQ/dt is the current
out of VIN due to gate charge, and it is typically larger
than the DC bias current. Both the DC bias and gate
charge losses are proportional to VIN; 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 owing
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 Character-
istics 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.
APPLICATIONS INFORMATION
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