Datasheet LT3001 (Analog Devices) - 9

LT3001
APPLICATIONS INFORMATION
Example:
Step 7: Select the EN/UVLO Resistors.
VZENER(MAX) ≤ 65V – 32V = 33V Determine the amount of hysteresis required and calcu- A 20V Zener with a maximum of 21V will provide optimal late R1 resistor value: protection and minimize power loss. So a 20V, 0.25W VIN(HYS) = 2.5µA • R1 Zener from Central Semiconductor (CMDZ5250B) Example: is chosen. Choose 2V of hysteresis, Choose a diode that is fast and has sufficient reverse volt- age breakdown: R1 = 806k VREVERSE > VSW(MAX) Determine the UVLO thresholds and calculate R2 resistor value: VSW(MAX) = VIN(MAX) + VZENER(MAX) 1.242V •(R1 Example: V +R2) +2.5µA •R1 IN(UVLO+) = R2 VREVERSE > 53V Example: A 100V, 0.25A diode from Central Semiconductor (CMHD4448) is chosen. Set VIN UVLO rising threshold to 7.5V,
Step 6: Select the R
R2 = 232k
FB Resistor.
Use the following equation to calculate the starting value VIN(UVLO+) = 7.5V for RFB: VIN(UVLO–) = 5.5V N R PS • (VOUT + VF ) FB =
Step 8: Ensure minimum load.
100µA The theoretical minimum load can be approximately esti- Example: mated as: 3 •(5V 40µH •(360mA)2 •10.6kHz R + 0.3V) FB = = 159k ILOAD(MIN) = = 5.5mA 100µA 2 • 5V Depending on the tolerance of standard resistor values, Remember to check the minimum load requirement in the precise resistor value may not exist. For 1% stan- real application. The minimum load occurs at the point dard values, a 158k resistor should be close enough. where the output voltage begins to climb up as the con- The final RFB value should be adjusted on the measured verter delivers more energy than what is consumed at output voltage. the output. The real minimum load for this application is about 6mA. In this example, a 820Ω resistor is selected as the minimum load. Rev. 0 Information furnished by Analog Devices is believed to be accurate and reliable. However, no responsibility is assumed by Analog Devices for its use, nor for any infrin Fo gem r m ents or of e in pat for entsm a or tion oth w er rww igh .a ts noaf lo th g. irdc om parties that may result from its use. Specifications subject to change without notice. No license is granted by implication or otherwise under any patent or patent rights of Analog Devices. 9 Document Outline Features Applications Typical Application Description Absolute Maximum Ratings Order Information Pin Configuration Electrical Characteristics Typical Performance Characteristics Pin Functions Operation Applications Information Package Description