LTC1703CG Linear Technology, LTC1703CG Datasheet - Page 27

• Manufacturer Part Number: LTC1703CG
• Description: IC REG SW DUAL SYNC VID 28SSOP
• Manufacturer: Linear Technology

Specifications of LTC1703CG

• Applications: Controller, Mobile Intel Pentium® III
• Voltage - Input: 3 ~ 7 V
• Number Of Outputs: 2
• Voltage - Output: 0.9 ~ 2 V
• Operating Temperature: 0°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 28-SSOP
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant

APPLICATIO S I FOR ATIO
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:
In our example 2-step system, the total output power is:
Assuming the LTC1703 provides 90% efficiency at each
output, the additional load on the 5V and 3.3V supplies is:
If the 5V and 3.3V supplies are each 94% efficient, the
power lost in each supply is:
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, L
to MOSFET gate charge. Maximizing efficiency in this
region of operation is as simple as minimizing these
terms.
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.
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
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
5V:
Total loss = 5.36W
Total system efficiency =
Efficiency
TotalOutputPower TotalPowerLost
48.75W/(48.75W + 5.36W) = 90.1%
(32.67W/94%) – 32.67W = 2.09W lost
15W + 5V (0.67A) = 18.4W load
(18.4W/94%) – 18.4W = 1.17W lost
EXT
TotalOutputPower
) and power lost in the gate drive circuits due
=
U
+
U
W
(
100%
U
)
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.
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
Figure 15. Typical LTC1703 Efficiency Curves
100
90
80
70
0
V
IN
= 5V
LOAD CURRENT (A)
5
V
V
V
OUT
OUT
OUT
= 3.3V
= 2.5V
= 1.6V
10
1703 G01
LTC1703
15
27
1703fa