LTC3602

13

3602fb

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

the efficiency loss at medium to high load currents.

The DC Supply Current as given in the electrical char-

acteristics, the internal MOSFET gate charge currents

and the internal topside MOSFET transition losses. The

MOSFET gate charge current results from switching the

gate capacitance of the internal power MOSFET switches.

supply. Each time the gate is switched from high to

low to high again, a packet of charge dQ moves from

current. In continuous mode, the gate charge current

of efficiency considerations.

Transition losses apply only to the internal topside

MOSFET and become more prominent at higher input

voltages. Transition losses can be estimated from:

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

(DC) as follows:

be obtained from the Typical Performance Characteristics

output current:

losses and inductor core losses, generally account for

less than 2% of the total power loss.

LTC3602

3602 F05a

TRACK/SS

TIME

(5b) Ratiometric Tracking

TIME

3602 F05b,c

(5c) Coincident Tracking