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| 型号： | LTC1703IG#TRPBF |
| 厂商： | Linear Technology |
| 文件页数： | 27/36页 |
| 文件大小： | 0K |
| 描述： | IC REG SW DUAL SYNC VID 28SSOP |
| 标准包装： | 2,000 |
| 应用： | 控制器，移动式 Intel Pentium? III |
| 输入电压： | 3 V ~ 7 V |
| 输出数： | 2 |
| 输出电压： | 0.9 V ~ 2 V |
| 工作温度： | -40°C ~ 85°C |
| 安装类型： | 表面贴装 |
| 封装/外壳： | 28-SSOP（0.209"，5.30mm 宽） |
| 供应商设备封装： | 28-SSOP |
| 包装： | 带卷 (TR) |
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LTC1703
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:
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
Efficiency =
TotalOutputPower
TotalOutputPower + TotalPowerLost
( 100% )
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-
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
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.
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
100
90
V IN = 5V
V OUT = 3.3V
V OUT = 2.5V
V OUT = 1.6V
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
80
3.3V: 16.5W + 3.3V (4.4A + 0.5A) = 32.67W load
70
0
5 10
15
(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, 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.
LOAD CURRENT (A)
1703 G01
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
1703fa
27
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