APPLICATION NOTE | AN50 |
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Therefore, for load current of 14.5A, the peak current through the inductor, Ipk, is found to be approximately 15.5A:
ISC ≥ Iinductor = ILoad, max + | (IPK – Imin ) | = 14.5 + 2 = 16.5A |
Therefore, the short circuit detection threshold must be at least 16.5A.
The next step is to determine the value of the sense resistor. Including sense resistor tolerance, the sense resistor value can be approximated as follows
RSENSE = | Vth,min | × (1 – TF) = | Vth,min | × (1 – TF) | |
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Where TF = Tolerance Factor for the sense resistor.
Table 7 describes tolerance, size, power capability, tempera- ture coefficient and cost of various type of sense resistors.
Table 7. Comparison of Sense Resistors
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| Discrete Metal |
| Discrete |
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| Discrete Iron | Strip Surface | Discrete MnCu | CuNi Alloy |
| Motherboard | Alloy | Mount Resistor | Alloy Wire | Wire Resistor |
Description | Trace Resistor | Resistor (IRC) | (Dale) | Resistor | (Copel) |
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Tolerance | ±29% | ±5% | ±1% | ±10% | ±10% |
Factor (TF) |
| (±1% available) |
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Size | 2" x 0.2" x 0.001" | 0.45" x 0.065" x | 0.25" x 0.125" x | 0.200" x 0.04" x | 0.200" x 0.04" x |
(L x W x H) | (1 oz Cu trace) | 0.200" | 0.025" | 0.160" | 0.100" |
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Power capability | >50A/in | 1 watt | 1 watt | 1 watt | 1 watt |
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Temperature | +4,000 ppm | +30 ppm | ±75 ppm | ±30 ppm | ±20 ppm |
Coefficient |
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Cost | Low included in | $0.31 | $0.47 | $0.09 | $0.09 |
@10,000 piece | motherboard |
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Refer to Appendix A for Directory of component suppliers
Based on the Tolerance Factor in the above table, for an embedded PC trace resistor and for Iload,max = 14.5A:
RSENSE = | Vth,min | × (1 – TF) = |
2.0A + ILoad, max |
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100mV | × (1 – 29%) = 4.3mΩ | |
For a discrete resistor and for Iload, max = 14.5A:
RSENSE = | Vth,min | × (1 – TF) = |
2.0A + ILoad, max |
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100mV | × (1 – 5%) = 5.8mΩ | |
For user convenience, Table 8 lists the recommended values for sense resistors for various load currents using embedded PC trace resistors and discrete resistors.
Table 8. Rsense for Various Load Currents
| RSENSE | RSENSE |
ILoad,max | PC Trace | Discrete |
(A) | Resistor (mΩ) | Resistor (mΩ) |
10.0 | 5.9 | 7.9 |
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11.2 | 5.4 | 7.2 |
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12.4 | 4.9 | 6.6 |
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13.9 | 4.5 | 6.0 |
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14.0 | 4.4 | 5.9 |
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14.5 | 4.3 | 5.8 |
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Discrete Sense Resistor
Discrete Iron Alloy resistors come in variety of tolerances and power ratings, and are most ideal for precision imple- mentation. MnCu Alloy wire resistors or CuNi Alloy wire resistors are ideal for low cost implementations.
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