APPLICATION NOTE | AN42 |
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where:
•VIN = Input Voltage to the Converter
•VSW = Voltage Across the MOSFET = ILOAD x RDS,ON
•VD = Forward Voltage of the Schottky Diode
•T = The Switching Period of the Converter = 1/fS, Where fS = Switching Frequency.
For an input voltage of 5V, an output voltage of 3.3V, an inductor value of 1.3∝H, and a switching frequency of 650KHz (using CEXT = 39pF), the inductor current can be calculated as follows:
(Ipk | – Imin ) | (5.0 – 14.5 ⋅ 0.037 – 3.3) | ⋅ | ||||
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| 2 | 1.3 ⋅ 10 |
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| (3.3 + 0.5) |
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650 ⋅ | 3 | = 1.048A | |||||
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| 10 |
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Therefore, for a continued load current of 14.5A, the peak current through the inductor, Ipk, is found to be:
Table 7. Comparison of Sense Resistors
ISC ≥ Iinductor = ILoad, max + | (IPK – Imin ) | = 14.5 + 1 = 15.5A |
For continuous operation at 14.5A, the short circuit detection threshold must be at least 15.5A.
The next step is to determine the value of the sense resistor. Including tolerance, the sense resistor value can be approxi- mated as follows:
RSENSE = | Vth,min | ⋅ (1 – TF) = | Vth,min | ⋅ (1 – TF) | |
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where TF = Tolerance Factor for the sense resistor.
Several different types of sense resistors exist. Table 7 describes tolerance, size, power capability, temperature coefficient and cost of various sense resistors.
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| Discrete Metal |
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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) |
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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 | $0.31 | $0.47 | $0.09 | $0.09 |
@10,000 piece | included in |
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| motherboard |
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Refer to Appendix A for Directory of component suppliers
Based on the Tolerance in the above table, for embedded PC trace resistor and for Iload,max = 14.5A:
RSENSE = | Vth,min | ⋅ (1 – TF) = |
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1.0A + ILoad, max |
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100mV | ⋅ (1 – 29%) = 4.6mΩ | |
For a discrete resistor and Iload, max = 14.5A:
RSENSE = | Vth,min | ⋅ (1 – TF) = |
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1.0A + ILoad, max |
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100mV | ⋅ (1 – 5%) = 6.1mΩ | |
For user convenience, Table 8 lists the recommended values for sense resistor at various load currents using an embedded PC trace resistor or discrete resistor.
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