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,

voltage drop of the output diode.

electrical characteristics, that excludes MOSFET driver

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

which is typically much larger than the contoller’s DC

MOSFET, inductor and current sense resistor. The

average output current squared can be summed with

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:

losses, and inductor core losses, generally account for

less than 2% total additional loss.