Datasheet

9
LTC1872
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% – (η1 + η2 + η3 + ...)
where η1, η2, 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
.
APPLICATIONS INFORMATION
WUU
U
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 multiply-
ing 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.