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LTC1703

efficiency, since a significant fraction of the total power is

drawn from the 3.3V and 5V rails in a typical system. The

correct way to calculate system efficiency is to calculate

the power lost in each stage of the converter, and divide

the total output power from all outputs by the sum of the

output power plus the power lost:

Efficiency

TotalOutputPower

TotalOutputPower TotalPowerLost

()

100%

In our example 2-step system, the total output power is:

Total output power =

15W + 16.5W + 1.25W + 3W + 13W = 48.75W

corresponding to 5V, 3.3V, 2.5V, 1.5V and 1.3V output

voltages.

Assuming the LTC1703 provides 90% efficiency at each

output, the additional load on the 5V and 3.3V supplies is:

1.3V: 13W/90% = 14.4W/3.3V = 4.4A from 3.3V

1.5V: 3W/90% = 3.3W/5V = 0.67A from 5V

2.5V: 1.25W/75% = 1.66W/3.3V = 0.5A from 3.3V

If the 5V and 3.3V supplies are each 94% efficient, the

power lost in each supply is:

1.3V: 14.4W – 13W = 1.4W

1.5V: 3.3W – 3W = 0.3W

2.5V: 1.66W – 1.25W = 0.4W

3.3V: 16.5W + 3.3V (4.4A + 0.5A) = 32.67W load

(32.67W/94%) – 32.67W = 2.09W lost

5V:

15W + 5V (0.67A) = 18.4W load

(18.4W/94%) – 18.4W = 1.17W lost

Total loss = 5.36W

Total system efficiency =

48.75W/(48.75W + 5.36W) = 90.1%

Maximizing High Load Current Efficiency

Efficiency at high load currents (when the LTC1703 is

operating in continuous mode) is primarily controlled by

the resistance of the components in the power path

(QT, QB, LEXT) and power lost in the gate drive circuits due

to MOSFET gate charge. Maximizing efficiency in this

region of operation is as simple as minimizing these

terms.

The behavior of the load over time affects the efficiency

strategy. Parasitic resistances in the MOSFETs and the

inductor set the maximum output current the circuit can

supply without burning up. A typical efficiency curve

(Figure 15) shows that peak efficiency occurs near 30% of

this maximum current. If the load current will vary around

the efficiency peak and will spend relatively little time at the

maximum load, choosing components so that the average

load is at the efficiency peak is a good idea. This puts the

maximum load well beyond the efficiency peak, but usu-

ally gives the greatest system efficiency over time, which

translates to the longest run time in a battery-powered

system. If the load is expected to be relatively constant at

the maximum level, the components should be chosen so

that this load lands at the peak efficiency point, well below

the maximum possible output of the converter.

LOAD CURRENT (A)

100

510

1703 G01

VIN = 5V

VOUT = 3.3V

VOUT = 2.5V

VOUT = 1.6V

Figure 15. Typical LTC1703 Efficiency Curves

Maximizing Low Load Current Efficiency

Low load current efficiency depends strongly on proper

operation in discontinuous and Burst Mode operations. In

an ideally optimized system, discontinuous mode reduces

conduction losses but not switching losses, since each

power MOSFET still switches on and off once per cycle. In

a typical system, there is additional loss in discontinuous

mode due to a small amount of residual current left in the

inductor when QB turns off. This current gets dissipated

across the body diode of either QT or QB. Some LTC1703

systems lose as much to body diode conduction as they

save in MOSFET conduction. The real efficiency benefit of

APPLICATIO S I FOR ATIO

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