LTC3614_15 Datasheet, PDF (23/30 Pages) Linear Technology – 4A, 4MHz Monolithic Synchronous Step-Down DC/DC Converter

LTC3614
Applications Information
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 percent-
age of input power.
Although all dissipative elements in the circuit produce
losses, two main sources usually account for most of
the losses: V_IN quiescent current and I²R losses. The V_IN
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
usually of no consequence.
1. The V_IN 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 V_IN to ground. The resulting dQ/dt is the current
out of V_IN 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 V_IN; thus, their effects
will be more pronounced at higher supply voltages.
2. I²R losses are calculated from the resistances of the
internal switches, R_SW , and external inductor, R_L. 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 R_DS(ON) and the duty cycle
(DC) as follows:
R_SW = (R_DS(ON)TOP)(DC) + (R_DS(ON)BOT)(1 – DC)
The R_DS(ON) for both the top and bottom MOSFETs can
be obtained from the Typical Performance Character-
istics curves. To obtain I²R losses, simply add R_SW to
R_L and multiply the result by the square of the average
output current.
Other losses including C_IN and C_OUT ESR dissipative
losses and inductor core losses generally account for
less than 2% of the total loss.
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