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MIC5191 Datasheet(PDF) 10 Page - Micrel Semiconductor |
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MIC5191 Datasheet(HTML) 10 Page - Micrel Semiconductor |
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10 / 15 page  Micrel, Inc. MIC5191 December 2006 10 M9999-122206 compensation resistor, creating a higher mid-band gain. -20 0 20 40 60 80 100 0.01 0.1 1 10 100 1000 10000 100000 Frequency (KHz) -45 0 45 90 135 180 225 Increasing Cout reduces the load resistance and output capacitor pole allowing for an increase in mid-band gain Figure 7. Increasing Output Capacitance This will have the effect of both decreasing the voltage drop as well as returning closer and faster to the regulated voltage during the recovery time. MOSFET Selection The typical pass element for the MIC5191 is an N- Channel MOSFET. There are multiple considerations when choosing a MOSFET. These include: • VIN to VOUT differential • Output Current • Case Size/Thermal Characteristics • Gate Capacitance (CISS<10nF) • Gate to Source threshold The VIN(min) to VOUT ratio and current will determine the maximum RDSON required. For example, for a 1.8V (±5%) to 1.5V conversion at 5A of load current, dropout voltage can be calculated as follows (using VIN(min): () OUT OUT IN DSON I V V R − = () 5A V 5 . 1 1.71V RDSON − = RDSON = 42mΩ For performance reasons, we do not want to run the N- Channel in dropout. This will seriously affect transient response and PSRR (power supply ripple rejection). For this reason, we want to select a MOSFET that has lower than 42mΩ for our example application. Size is another important consideration. Most import- antly, the design must be able to handle the amount of power being dissipated. The amount of power dissipated can be calculated as follows (using VIN(max)): PD = (VIN – VOUT) × IOUT PD = (1.89V – 1.5V) × 5A PD = 1.95W Now that we know the amount of power we will be dissipating, we will need to know the maximum ambient air temperature. For our case we’re going to assume a maximum of 65°C ambient temperature, though different MOSFETs have different maximum operating junction temperatures. Most MOSFETs are rated to 150°C, while others are rated as high as 175°C. In this case, we’re going to limit our maximum junction temperature to 125°C. The MIC5191 has no internal thermal protection for the MOSFET so it is important that the design provides margin for the maximum junction temperature. Our design will maintain better than 125°C junction temperature with 1.95W of power dissipation at an ambient temperature of 65°C. Our thermal resistance calculates as follows: D J J JA P (ambient) T (max) T θ − = 1.95W C 65 C 125 θJA ° − ° = θJA = 31°C /W So our package must have a thermal resistance less than 31°C /W. Table 1 shows a good approximation of power dissipation and package recommendation. Package Power Dissipation TSOP-6 <850mW TSSOP-8 <950mW TSSOP-8 <1W PowerPAK™ 1212-8 <1.1W SO-8 <1.125W PowerPAK™ SO-8 D-Pack <1.4W TO-220/TO-263 (D 2pack) >1.4W Table 1. Power Dissipation and Package Recommendation In our example, our power dissipation is greater than 1.4W, so we’ll choose a TO-263 (D 2Pack) N-Channel MOSFET. θJA is calculated as follows: θJA = θJC + θCS + θSA Where θJC is the junction to case resistance, θCS is the case-to-sink resistance and the θSA is the sink-to-ambi- ent air resistance. In the D 2 package we’ve selected, the θ JC is 2°C/W. The θCS, assuming we are using the PCB as the heat sink, can be approximated to 0.2°C/W. This allows us to calculate the minimum θSA: θSA = θJA– θCS – θJC θSA = 31°C/W – 0.2°C/W – 2°C/W θSA = 28.8°C/W |
Similar Part No. - MIC5191_11 |
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Similar Description - MIC5191_11 |
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