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LTC1702A

1702afa

2-Step Efficiency Calculation

Calculating the efficiency of a 2-step converter system

involves some subtleties. Simply multiplying the effi-

ciency of the primary 5V or 3.3V supply by the efficiency

of the 1.8V or 1.5V supply underestimates the actual

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

TotalPowerLost

()

100%

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

Total output power =

15W + 16.5W + 1.25W + 3.6W + 15W = 51.35W

corresponding to 5V, 3.3V, 2.5V, 1.8V and 1.5V output

voltages.

Assuming the LTC1702A provides 90% efficiency at each

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

1.5V: 15W/90% = 16.6W/3.3V = 5A from 3.3V

1.8V: 3.6W/90% = 4W/5V = 0.8A 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:

1.5V: 17.55W – 15W = 2.55W

1.8V: 4.26W – 3.6W = 0.66W

2.5V: 1.77W – 1.25W = 0.52W

3.3V: 17.55W – 16.5W = 1.05W

5V:

15.96W – 15W = 0.96W

Total loss = 5.74W

Total system efficiency =

51.35W/(51.35W + 5.74W) = 90%

APPLICATIONS INFORMATION

Maximizing High Load Current Efficiency

Efficiency at high load currents (when the LTC1702A 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.

Figure 15. Typical LTC1702A Efficiency Curves

LOAD CURRENT (A)

100

510

1702A G01

VIN = 5V

VOUT = 3.3V

VOUT = 2.5V

VOUT = 1.6V

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