LTC1872
9
1872fa
For more information
www.linear.com/LTC1872
applicaTions inForMaTion
For most applications, an 80k resistor is suggested for
R1. To prevent stray pickup, locate resistors R1 and R2
close to LTC1872.
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% – (
η
1 +
η
2 +
η
3 + ...)
where
η
1,
η
2, etc. are the individual losses as a percent-
age of input power.
Although all dissipative elements in the circuit produce
losses, four main sources usually account for most of the
losses in LTC1872 circuits: 1) LTC1872 DC bias current, 2)
MOSFET gate charge current, 3) I
2
R losses and 4) voltage
drop of the output diode.
1. The V
IN
current is the DC supply current, given in the
electrical characteristics, that excludes MOSFET driver
and control currents. V
IN
current results in a small loss
which increases with V
IN
.
2. MOSFET gate charge current results from switching
the gate capacitance of the power MOSFET. Each
time a MOSFET 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 a current out of V
IN
which is typically much larger than the contoller’s DC
supply current. In continuous mode, I
GATECHG
= f(Qp).
3. I
2
R losses are predicted from the DC resistances of
the MOSFET, inductor and current sense resistor.
The MOSFET R
DS(ON)
multiplied by duty cycle times
the average output current squared can be summed
with I
2
R losses in the inductor ESR in series with the
current sense resistor.
4. The output diode is a major source of power loss at high
currents. The diode loss is calculated by multiplying
the forward voltage by the load current.
5. Transition losses apply to the external MOSFET and
increase at higher operating frequencies and input
voltages. Transition losses can be estimated from:
Transition Loss = 2(V
IN
)
2
I
IN(MAX)
C
RSS
(f)
Other losses, including C
IN
and C
OUT
ESR dissipative
losses, and inductor core losses, generally account for
less than 2% total additional loss.
