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MSK5046-1.7RHG Datasheet(PDF) 3 Page - M.S. Kennedy Corporation |
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MSK5046-1.7RHG Datasheet(HTML) 3 Page - M.S. Kennedy Corporation |
3 / 10 page APPLICATION NOTES PRELIMINARY Rev. G 2/06 3 INPUT CAPACITOR SELECTION: The MSK 5046RH should have an external high fre- quency ceramic capacitor (0.1uF) between VIN and power ground. Connect a low-ESR bulk capacitor directly to the input pin of the MSK 5046RH. Select the bulk input filter capacitor according to input ripple-current require- ments and voltage rating, rather than capacitor value. Electrolytic capacitors that have low enough ESR to meet the ripple-current requirement invariably have more than adequate capacitance values. Aluminum-electrolytic ca- pacitors are preferred over tantalum types, which could cause power-up surge-current failure when connecting to robust AC adapters or low-impedance batteries. INPUT BIAS AND UVLO: The VBIAS pin of the MSK 5046RH provides bias to the control circuitry. The VBIAS pin can be connected directly to the input bus for 12V to 18V operation or it can be biased separately with a 12V to 18V source to extend the input range of the device. Refer to the para- graph titled "INPUT VOLTAGE RANGE". For bias volt- ages greater than 16V series diodes or resistors are rec- ommended to bring VBIAS below 16V. The MSK 5046RH's built-in under voltage lockout feature prevents damage to downstream devices in the event of a drop in bias voltage. Under voltage lockout occurs at bias volt- ages of approximately 10.3V rising and 9.5V falling. The internal bias draws approximately 40mA under normal operation. INPUT VOLTAGE RANGE The MSK 5046RH's input range of 12V to 18V can be further extended down to 3.1V by using a separate bias supply. In this configuration very efficient low V to low V conversion can be achieved. OUTPUT CAPACITOR SELECTION: The MSK 5046RH has no internal output capacitance. Use between 200 and 400µF of low ESR bulk capaci- tance for optimum performance. For optimal performance use three AVX part number TAZH107M015L (MIL part number CWR29H-107M). For loads with large excur- sions use surge rated capacitors. Additional 0.1µF ce- ramic capacitors close to the load will help suppress high frequency switching noise. Custom compensation to maximize performance with a variety of capacitive loads is available; consult the factory. SOFT START: The internal 0.1µF soft start capacitor charges from a 12µA (nominal) current source. Connect additional capacitance to the soft start pin to slow the output rise further. The output rise time is dependant on the input to ouput voltage ratio as well as the soft start capaci- tance. The output rise time at 50% load may be ap- proximated with the following equation: TRISE= (VOUT + 0.6V) 2 Css (VIN + 0.3V) 14 Where VOUT = the output voltage in volts. VIN = the input bus voltage in volts. Css = 0.1 + external soft start capacitance in µF. POWER CIRCUIT LAYOUT CONSIDERATIONS: Layout of the power circuitry will affect the overall performance of the MSK 5046RH. Input power should be tied as close to VIN (pins 12-22) as possible. Create a single point ground as close to the MSK 5046RH power ground pins (23-33) as possible. The input power return should be tied between the MSK 5046RH power ground pins and the load. Keeping the input power return con- nection as close to the MSK 5046RH power ground pins as possible and keeping the distance between the MSK 5046RH power ground pins and the load to a minimum will maximize the performance. Tie the signal ground pins (4 and 7) to the single point ground with a short low impedance connection. See remaining notes for more layout and application performance. The soft start will begin as soon as the bias voltage exceeds the under voltage lockout threshold or the in- hibit pin is released; whichever comes last. If bias volt- age is applied and the inhibit pin is released before the input voltage is applied, the soft start will have risen early and the output will rise at approximately the same rate as VIN and may overshoot. Any circuit that holds the enable pin low until VIN rises will ensure soft start when VBIAS must rise before VIN. The circuit below may be adapted to suit a variety of applications. |
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