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LTM8005 Arkusz danych(PDF) 16 Page - Linear Technology |
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LTM8005 Arkusz danych(HTML) 16 Page - Linear Technology |
16 / 26 page LTM8005 16 8005f For more information www.linear.com/LTM8005 APPLICATIONS INFORMATION to do so with a 28µA source. Once above 1.7V, the pull- up source is disabled and a discharge current of about 2.8µA is activated. While the SS pin is discharging, the integrated switching MOSFET is turned off. When the SS pin is discharged below about 0.2V, a new cycle is initiated. This is hiccup mode operation. If the fault still exists when SS crosses below about 0.2V, then a full SS charge/discharge cycle has to complete before switching is enabled. If a resistor, typically 402k , is placed between the VREF pin and SS pin to hold SS pin higher than 0.2V during a fault, then the LTM8005 will enter latchoff mode with switching stopped and the load disconnected from VOUT. To exit latchoff mode, the EN/UVLO pin must be toggled low to high. Capacitor Selection Considerations 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 applied voltage and give dependable service. Other types, includ- ing Y5V and Z5U have very large temperature and voltage coefficients of capacitance. In an application circuit they may have only a small fraction of their nominal capaci- tance resulting in much higher output voltage ripple than expected. Another precaution regarding ceramic capacitors con- cerns the maximum input voltage rating of the LTM8005. A ceramic input capacitor combined with trace or cable inductance forms a high Q (under damped) tank circuit. If the LTM8005 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; see the Hot-Plugging Safely section. Input Capacitor Selection The input capacitor supplies the transient input current for the power inductor of the converter and must be placed and sized according to the transient current require- ments. The switching frequency, output current and tol- erable input voltage ripple are key inputs to estimating the capacitor value. An X7R type ceramic capacitor is a good choice because it has the least variation with tem- perature and DC bias. Typically, boost and SEPIC con- verters require a lower value capacitor than a buck mode converter. In the buck mode configuration, the input capacitor has large pulsed currents due to the current returned through the Schottky diode when the switch is off. It is important to place the capacitor as close as possible to the Schottky diode and to the GND return of the switch. It is also impor- tant to consider the ripple current rating of the capacitor. For best reliability, this capacitor should have low ESR and ESL and have an adequate ripple current rating. Output Capacitor Selection The selection of the output capacitor depends on the load and converter configuration, i.e., step-up or step-down and the operating frequency. For LED applications, the equivalent resistance of the LED is typically low and the output filter capacitor should be sized to attenuate the cur- rent ripple. Use of an X5R or X7R type ceramic capacitor is recommended. To achieve the same LED ripple current, the required fil- ter capacitor is larger in the boost and buck-boost mode applications than that in the buck mode applications. Lower operating frequencies will require proportionately higher capacitor values. The component values shown in the data sheet applications are appropriate to drive the specified LED string. The product of the output capacitor and LED string impedance decides the second dominant pole in the LED current regulation loop. It is prudent to validate the power supply with the actual load (or loads). Inductor Selection The inductor used with the LTM8005 should have a satu- ration current rating appropriate to the peak inductor cur- rent under all expected operating conditions. Choose an inductor value based on operating frequency to provide a peak-to-peak inductor ripple current appropriate to the 12A (typical) switch current limit and duty cycle. |
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