LTC1702 Datasheet, PDF (27/36 Pages) Linear Technology – Dual 550kHz Synchronous 2-Phase Switching Regulator Controller

LTC1702
APPLICATIONS INFORMATION
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
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 LTC1702 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: 16.6W – 15W = 1.6W
1.8V: 4W – 3.6W = 0.4W
2.5V: 1.66W – 1.25W = 0.4W
3.3V: 17.55W – 16.5W = 1W
5V: 16W – 15W = 1W
Total loss = 4.4W
Total system efficiency =
51.35W/(51.35W + 4.4W) = 92.1%
Maximizing High Load Current Efficiency
Efficiency at high load currents (when the LTC1702 is
operating in continuous mode) is primarily controlled by
the resistance of the components in the power path
(QT, QB, L_EXT) 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 usually
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.
100
V_IN = 5V
V_OUT = 3.3V
V_OUT = 2.5V
90
V_OUT = 1.6V
80
70
0
5
10
LOAD CURRENT (A)
15
1702 G01
Figure 15. Typical LTC1702 Efficiency Curves
27