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| 型号： | SI8540-B-FSR |
| 厂商： | Silicon Laboratories Inc |
| 文件页数： | 17/18页 |
| 文件大小： | 0K |
| 描述： | IC CURRENT SENSOR AMP 8SOIC |
| 标准包装： | 3,000 |
| 放大器类型： | 电流检测 |
| 电路数： | 1 |
| 电流 - 电源： | 90µA |
| 电流 - 输出 / 通道： | 10mA |
| 电压 - 电源，单路/双路(±)： | 5 V ~ 36 V |
| 工作温度： | -40°C ~ 85°C |
| 安装类型： | 表面贴装 |
| 封装/外壳： | 8-SOIC（0.154"，3.90mm 宽） |
| 供应商设备封装： | 8-SOICN |
| 包装： | 带卷 (TR) |
Si8540
8
Rev. 1.2
3.1.1. Selecting RSENSE
Selecting RSENSE involves making the best trade-off between power efficiency and accuracy. Low RSENSE
values dissipate less power while higher values maximize accuracy. In general, it is best to choose a relatively high
value for RSENSE in applications where the measured current is small. For higher current applications, the sense
resistor should be able to dissipate the heat from its power loss; otherwise, its value may drift or it may fail open,
possibly causing a large differential voltage across RG1 and RG2 that may damage the device. In most
applications, RSENSE should have low inductance to reduce the impact of any high-frequency components in the
current being measured (low inductance metal film resistors are recommended). Also, note that the Si8540
requires at least 3.5 V of voltage headroom between the voltage at pin RG1 and pin OUT. This voltage headroom
decreases as RSENSE increases. A good guideline for determining the maximum value for RSENSE is shown in the
following equation:
RSENSEmax =(VSOURCE – VOUTmax – 3.5 V)/ILOAD
Where:
VSOURCE is the high-side voltage
VOUTmax is the full-scale output voltage at the OUT pin
ILOAD is the current passing through RSENSE measured by the Si8540
3.1.2. Selecting RG1 and RG2
The values of resistors RG1 and RG2 determine the sense amp current-gain. These two resistors must have the
same value, and resulting current gain is equal to RSENSE / RG (where RG =RG1 =RG2).
The minimum value of RG is determined by the maximum current at the OUT pin (1.5 mA) and by the resistance
between the internal current sense amp input and the sense resistor (approximately 0.2
). As the value of R
G is
reduced, the input resistance becomes a larger portion of the total gain-setting resistance. This gain error can be
compensated by trimming RG or ROUT. A good guideline for determining the maximum value for RG is shown in the
following equation:
RGmax =(VSENSEmax / 1.5 mA)
Where:
RGmax is the largest value for RG1 and RG2
VSENSEmax is the value of VSENSE at maximum ILOAD
Note that for a given value of VSENSE, a decrease of the RG resistor values causes a corresponding increase in
current at the OUT pin. This causes additional power to be dissipated in ROUT rather than in the load, which can
reduce efficiency. Note also that mismatches in the currents passing RG1 and RG2 (IOS) together with RG affect the
full scale error.
This error can be reduced by lowering the values of RG1, RG2 and/or lowering their tolerances. This error can also
be reduced by increasing the value of RSENSE.
3.1.3. Choosing ROUT
ROUT must be chosen to generate the required full-scale output voltage at the full scale IOUT, which, in turn, is
determined by RG1, RG2, and RSENSE. The upper limit of ROUT is determined by the input impedance of the device
that it drives. This input impedance should be much larger than ROUT; otherwise, measurement accuracy will be
degraded. A good guideline for choosing the value of ROUT is shown in the following equation:
(VOUTfullscale x RG) / (ILOAD x RSENSE)
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