Datasheet LT3001 (Analog Devices) - 8
| Manufacturer | Analog Devices |
| Description | 36V 4W No-Opto Isolated Flyback Converter |
| Pages / Page | 10 / 8 — APPLICATIONS INFORMATION. Step 4: Choose the Output Capacitor. Step 3: … |
| File Format / Size | PDF / 626 Kb |
| Document Language | English |
APPLICATIONS INFORMATION. Step 4: Choose the Output Capacitor. Step 3: Choose the Output Diode
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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
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
