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LTC1705 Datasheet(PDF) 24 Page - Linear Technology

Part No. LTC1705
Description  Dual 550kHz Synchronous Switching Regulator Controller with 5-Bit VID and 150mA LDO
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Manufacturer  LINER [Linear Technology]
Direct Link  http://www.linear.com
Logo LINER - Linear Technology

LTC1705 Datasheet(HTML) 24 Page - Linear Technology

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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 1.5V or 1.3V supply under estimates 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:
Total Output Power
Total Output Power
Total Output Lost
In our example 2-step system, the total output power is:
Total Output Power =
15W + 16.5W + 0.375W + 3W + 19.5W = 54.375W
(corresponding to 5V, 3.3V, 2.5V, 1.5V and
1.3V output voltages)
Assuming the LTC1705 provides 90% efficiency at the
core and I/O channels, and 75% efficiency at the LDO, the
additional loads on the 5V and 3.3V supplies are:
1.3V: 19.5W/90% =21.67W
⇒ 6.6A from 3.3V
1.5V: 3W/90% =3.3W
⇒ 0.66A from 5V
2.5V: 0.375W/75% =0.5W
⇒ 0.152A from 3.3V
If the 5V and 3.3V supplies are each 94% efficient, the
power lost in each supply is:
1.3V: 21.67W – 19.5W
= 2.17W
1.5V: 3.3W – 3W
= 0.3W
2.5V: 0.5W – 0.375W
= 0.125W
3.3V: 16.5W + 3.3V(6.6A + 0.152A) = 38.78W Load
(38.78W/94%) – 38.78W
= 2.48W Lost
5V: 15W + 5V(0.66A)
= 18.3W Load
(18.3W/94%) – 18.3W
= 1.17W Lost
Total Loss
= 6.25W
Total System Efficiency =
54.375W/(54.375W + 6.25W) = 89.7%
Maximizing High Load Current Efficiency
Efficiency at high load currents 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
shows that peak efficiency occurs near 30% of this maxi-
mum 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.

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