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LTM8040EV-PBF Datasheet(PDF) 11 Page - Linear Technology |
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LTM8040EV-PBF Datasheet(HTML) 11 Page - Linear Technology |
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11 / 16 page  LTM8040 11 8040p Ceramic capacitors are also piezoelectric. While the LTM8040 is a fixed frequency device, the internal regulators may skip cycles at light loads and extend the switching cycle on time as the input voltage falls towards the to output. Under either of these conditions, the LTM8040 can excite a ceramic capacitor at audio frequencies, generating audible noise. If this audible noise is unacceptable, use a high performance electrolytic capacitor at the output. The input capacitor can be a parallel combination of a 4.7μF ceramic capacitor and a low cost electrolytic capacitor. A final precaution regarding ceramic capacitors concerns the maximum input voltage rating of the LTM8040. A ceramic input capacitor combined with trace or cable inductance forms a high Q (under damped) tank circuit. If the LTM8040 circuit is plugged into a live supply, the input voltage can ring to twice its nominal value, possi- bly exceeding the device’s rating. This situation is easily avoided by introducing a small series damping resistance into the circuit. This is most often taken care of by the presence of an electrolytic bulk capacitor in the board. High Temperature Considerations The internal operating temperature of the LTM8040 must be lower than 125°C rating, so care should be taken in the layout of the circuit to ensure good heat sinking of the LTM8040. To estimate the junction temperature, ap- proximate the power dissipation within the LTM8040 by applying the typical efficiency stated in this datasheet to the desired output power, or, if you have and actual module, by taking a power measurement. Then calculate the tem- perature rise of the LTM8040 junction above the surface of the printed circuit board by multiplying the module’s power dissipation by the thermal resistance. The actual thermal resistance of the LTM8040 to the printed circuit board depends on the layout of the circuit board, but the thermal resistance given on page 2, which is based upon a 40.3cm2 4 layer FR4 PC board, can be used a guide. The LTM8040 is equipped with thermal protection that reduces the output LED current if the internal operating temperature is too high. This thermal protection is active above the 125°C temperature rating of the LTM8040, so Total dimming ratio (DIMRATIO) is the product of the PWM dimming ratio and the current dimming ratio. Example: IMAX = 1A, IMIN = 0.1A, tMAX = 1.0ms, tMIN = 25μs I RATIO = 1A 0.1A =10:1 PWM RATIO = 10ms 25µs = 400:1 DIM RATIO = 10 • 400 = 4000 :1 Minimum Input Voltage The LTM8040 is a step down converter, so a minimum amount of headroom is required to keep the output in regulation. For most applications at full load, the input needs to be at least 1.5V above the desired output. In addition, it takes more input voltage to initially start than is required for continuous operation. This start voltage is also dependent on whether turn-on is controlled by the LTM8040’s SHDN pin or UVLO (that is, the SHDN pin is tied to VIN). See Typical Performance Characteristics for details. Capacitor Selection Considerations The CIN and capacitor values in tables 1 and 2 are the minimum recommended values for the associated oper- ating conditions. Applying capacitor values below those indicated in table 1 is not recommended, and may result in undesirable operation. Using larger values is generally acceptable, and can yield improved performance, if it is necessary. Again, it is incumbent upon the user to verify proper operation over the intended system’s line, load and environmental conditions. Ceramic capacitors are small, robust and have very low ESR. However, not all ceramic capacitors are suitable. X5R and X7R types are stable over temperature and ap- plied voltage and give dependable service. Other types, including Y5V and Z5U have very large temperature and voltage coefficients of capacitance. In an application cir- cuit they may have only a small fraction of their nominal capacitance resulting in much higher output voltage ripple than expected. APPLICATION INFORMATION |
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