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LM3460M5-1.5 Fiches technique(PDF) 6 Page - National Semiconductor (TI) |
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LM3460M5-1.5 Fiches technique(HTML) 6 Page - National Semiconductor (TI) |
6 / 8 page Setting the Output Voltage (Continued) The simplest way to calculate the resistor values is to as- sume a value for R A and then solve the equation shown for R B. To assure best output voltage accuracy, the value selected for R A should be < 500Ω, and 1% tolerance resistors should be used. As the ohmic value of R A is increased, the internal resistive divider inside the LM3460 will cause the output voltage to deviate from the value predicted by the formula shown. App Circuit Technical Information Figure 1 and Figure 2 highlight two applications of the LM3460. This section provides details of circuit function. 1.5V/7A TYPICAL APPLICATION Figure 1 shows the schematic of a wide-bandwidth linear regulator which provides a regulated 1.5V output at up to 7A of load current from a 3V-3.6V input. The pass element of the regulator (which supplies the load current) is made up of a three-transistor complimentary Dar- lington composed of Q2, Q3, and Q4. The bias current flow- ing through R1 will drive the pass element ON, until such time as Q1 pulls down and takes the drive away from the base of Q2. The circuit regulates the output to 1.5V using the LM3460 precision controller, which sources current from its output whenever the voltage at the IN pin reaches 1.5V. When the LM3460 sources current from its output, it turns on Q1 (stealing the base drive for Q2) which reduces the cur- rent from the 1.5V regulated output. In this way, a negative feedback loop is established which locks the output at 1.5V. C1 and C2 are used for compensation, and should be ce- ramic capacitors. C4 is required for regulator stability, and both C3 and C4 af- fect transient response. Circuit performance should be care- fully evaluated if substitutions are made for these two com- ponents. PERFORMANCE DATA All data taken at 20˚C ambient: LOAD/LINE REGULATION: The output voltage changed <0.1 mV as the load was increased from 0-7A, and the input voltage was varied from 3.0V-3.6V. DROPOUT VOLTAGE: The dropout voltage (which is de- fined as the minimum input-output voltage differential re- quired to maintain a regulated output) was measured at 7A and found to be 1.4V. This means that a minimum input volt- age of 2.9V is required to keep the 1.5V output in regulation. TRANSIENT RESPONSE: Transient response was tested using a 0.2 Ω power resistor connected to the output using a mechanical contact to provide a 0-7A load current step. When the load was applied, the change in output voltage was seen to be < 5 mV with a total recovery time of about 30 µs (see Figure 5). HEATSINKING/COMPONENT SELECTION HEATSINKING: As with any linear regulator, the power dissi- pated in the pass transistor (Q4) is approximately: P = (V IN−VOUT)XILOAD Q4 must be provided with adequate heatsinking so that the junction temperature never exceeds 150˚C. Figure 6 shows the maximum allowable values of thermal resistance (from heatsink-to-ambient) that must be provided for various values of the load current. These values are calculated assuming a maximum ambient of 50˚C, 3.3V input, and a TO-220 power transistor mounted using thermal grease and a mica insulator. DS012603-10 FOR BEST RESULTS: SELECT R A < 500Ω FIGURE 4. Setting the Output Voltage DS012603-11 FIGURE 5. Output Transient Response DS012603-12 FIGURE 6. Q4 Heatsink Requirements for Circuit Shown in Figure 1 www.national.com 6 |
Numéro de pièce similaire - LM3460M5-1.5 |
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Description similaire - LM3460M5-1.5 |
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