Datasheet LT3001 (Analog Devices) - 8

LT3001
APPLICATIONS INFORMATION
Most transformers specify primary inductance with a tol- Next calculate reverse voltage requirement using maxi- erance of ±20%. With other component tolerance consid- mum VIN: ered, choose a transformer with its primary inductance V 30% larger than the minimum values calculated above. V IN(MAX) REVERSE = VOUT + L N PRI = 40µH is then chosen in this example. PS Once the primary inductance has been determined, the Example: maximum load switching frequency can be calculated as: 32V 1 1 V = 15.6V f REVERSE = 5V + SW = = 3 t L L ON + tOFF PRI •ISW + PRI •ISW V The CMSH5-20 (5A, 20V diode) from Central IN NPS •(VOUT + VF) Semiconductor is chosen. V
Step 4: Choose the Output Capacitor.
I OUT •IOUT • 2 SW = η • V IN • D The output capacitor should be chosen to minimize the output voltage ripple while considering the increase in size Example: and cost of a larger capacitor. Use the equation below to (5V calculate the output capacitance: D + 0.3V) • 3 = = 0.57 (5V + 0.3V)• 3+12V L 2 C PRI •ISW 5V • 0.5A • 2 OUT = 2• V I OUT • ΔVOUT SW = = 0.86A 0.85 •12V • 0.57 Example: fSW = 199kHz Design for output voltage ripple less than 1% of V OUT, i.e., 50mV. The transformer also needs to be rated for the correct saturation current level across line and load conditions. 40µH •(0.86A)2 C = 60µF A saturation current rating larger than 2A is necessary OUT = 2 • 5V • 0.05V to work with the LT3001. The 750313974 from Würth is chosen as the flyback transformer. Remember ceramic capacitors lose capacitance with applied voltage. The capacitance can drop to 40% of
Step 3: Choose the Output Diode.
quoted capacitance at the maximum voltage rating. So a Two main criteria for choosing the output diode include 100µF, 10V rating ceramic capacitor is chosen. forward current rating and reverse voltage rating. The
Step 5: Design Snubber Circuit.
maximum load requirement is a good first-order guess as the average current requirement for the output diode. The snubber circuit protects the power switch from leak- A conservative metric is the maximum switch current limit age inductance voltage spike. A DZ snubber is recom- multiplied by the turns ratio, mended for this application because of lower leakage inductance and larger voltage margin. The Zener and the IDIODE(MAX) = ISW(MAX) • NPS diode need to be selected. Example: The maximum Zener breakdown voltage is set according I to the maximum V DIODE(MAX) = 4.125A IN: VZENER(MAX) ≤ 65V – VIN(MAX) Rev. 0 8 For more information www.analog.com 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