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LM3460M5X-1.5 数据表(PDF) 7 Page - National Semiconductor (TI) |
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LM3460M5X-1.5 数据表(HTML) 7 Page - National Semiconductor (TI) |
7 / 8 page App Circuit Technical Information (Continued) A given thermal resistance can be obtained by using differ- ent combinations of heatsink and airflow (refer to heatsink manufacturers datasheets). The design tradeoff here is that heatsinks which are smaller, lighter, and cheaper require more airflow to get the desired value of thermal resistance. TRANSIENT RESPONSE: If the regulator is to respond quickly to changes in load current demand, the input and output capacitors must be selected carefully. The output capacitor C4 is most critical, as it must supply current to the load in the time it takes the regulator loop to sense the output voltage change and turn on the pass tran- sistor. A Sanyo Oscon type (or equivalent) will give the best performance here. The input capacitor C3 is also important, as it provides an energy reservoir from which the regulator sources current to force the output back up to the nominal value. A good, low ESR electrolytic such as a Panasonic HFQ type is a good choice for C3. LAYOUT TIPS: In order to optimize performance, parasitic inductance due to connecting traces must be minimized. All paths shown as heavy lines on the schematic must be made by traces which are wide and short as possible (component placement should be optimized for minimum lead length). POWER TRANSISTOR AND DRIVER: The power transistor used at Q4 must have very good current gain at 7A, and wide bandwidth (high f T) for this circuit to work as specified. The D44H8 is an excellent choice for cost and performance. The current gain of Q4 dictates the power dissipation in its driver (Q3) which must supply the base current to Q4. If the gain of Q4 is lowered, Q3 must source more current into its base (and the power dissipation in Q3 goes up proportion- ately). The D44H8 has a guaranteed minimum gain of 40 @ 4A, with typical gain much higher. Assuming the gain of Q4 is about 30% lower at 7A, it will still be > 28. Therefore, to support 7A of load current, Q3 must supply 250 mA to the base of Q4 (worst case). The power dissipation in Q3 (assuming 3.3V input) will never exceed approximately 250 mW, which is easily handled by 2N3906 in a TO-92 case (which has a thermal resistance of about 180˚C/W), but could be a problem for a very small sur- face mount device. If substitutions are made for Q3 or Q4, careful attention must be paid to the current gain as well as the f T. TRANSISTOR BANDWIDTH: Fast transient response that the regulator be able to respond quickly to any change in output voltage (which will occur if the current drawn by the load suddenly changes). All of the transistors specified in the schematic are very wide-band devices (have high f T values) which is necessary for fast response. If substitutions are made for any of the transistors, this specification must be considered. 1.2V/7A TYPICAL APPLICATION The 1.2V @ 7A design in Figure 2 is very similar in function to the design shown in Figure 1. Most of the circuit descrip- tions previously detailed for that circuit apply unchanged to Figure 2, will not be repeated. Detailed information will be presented in the areas which dif- fer between the two circuits. HEATSINKING The 1.2V design needs a little more heatsinking because the lower output voltage means more power dissipation in Q4 at any value of load current. Figure 7 shows the maximum allowable values of thermal resistance (from heatsink-to-ambient) that must be provided for various values of the load current. Q1 DRIVE CIRCUITRY In the circuit shown in Figure 1, the output of U1 drives the base of Q1 with current when the voltage at V OUT reaches the regulation point. As Q1 turns ON, it steals drive from Q2 which holds the loop in regulation. The circuit of Figure 2 uses a different drive configuration for Q1, required because of the lower voltage across U1. With only 1.2V across U1, the OUT pin of the LM3460 can- not swing up high enough in voltage to turn on the V BE of Q1. In the circuit of Figure 2, drive for Q1 is provided by R7, but only when U1 sources current: The operation of the drive scheme is as follows: If the voltage at V OUT is below 1.2V, no current flows from the OUT pin of U1. Q1 is held OFF as the current flowing down through R7 goes through D1 and R5 to ground. IMPORTANT: Diode D1 is a 1N4001 because its V F must be much less than the V BE of Q1 (a signal diode like 1N4148 will not work here). When U1 is not sourcing current, the voltage at the OUT pin (and the cathode of D1) will be held at about 50 mV by the R7/D1/R5 divider. The current flowing to ground through these components is about 110 µA. Because D1 is a 1A power diode, the V F across D1 at this small value of current will be much less than the V BE needed to turn ON Q1 (so Q1 is held off by D1). When U1 begins to source current (to cut off the pass tran- sistor and regulate V OUT) it forces the voltage at the cathode of D1 to rise. This action causes the current that was flowing through D1 to flow into the base of Q1, turning it ON and taking drive away from the base of Q2. This action provides the negative feedback required to regu- late V OUT and allows the LM3460 to operate with only 1.2V of total supply voltage across the device. DS012603-13 FIGURE 7. Q4 Heatsink Requirements for Circuit shown in Figure 2 www.national.com 7 |
类似零件编号 - LM3460M5X-1.5 |
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类似说明 - LM3460M5X-1.5 |
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