Datasheet LT3002 (Analog Devices) - 8

LT3002
APPLICATIONS INFORMATION
Example: needs to conduct much higher current. Therefore, a con- 350ns • 3 • (5V + 0.3V) servative metric is 60% of the maximum switch current LPRI ≥ = 6.4µH limit multiplied by the turns ratio: 0.87A 160ns • 32V IDIODE(MAX) = 0.6 • ISW(MAX) • NPS L = 5.9µH PRI ≥ 0.87A Example: Most transformers specify primary inductance with a tol- IDIODE(MAX) = 8.1A erance of ±20%. With other component tolerance consid- Next calculate reverse voltage requirement using maxi- ered, choose a transformer with its primary inductance mum VIN: 40% to 60% larger than the minimum values calculated above. L V PRI = 9µH is then chosen in this example. V IN(MAX) REVERSE = VOUT + N Once the primary inductance has been determined, the PS maximum load switching frequency can be calculated as: Example: 1 1 fSW = = 32V t L L ON + tOFF PRI •ISW + PRI •ISW VREVERSE = 5V + = 15.7V V 3 IN NPS •(VOUT + VF) V The PDS835L (8A, 35V diode) from Diodes Inc. is chosen. I OUT •IOUT • 2 SW = η • V
Step 4: Choose the output capacitor.
IN • D The output capacitor should be chosen to minimize the Example: output voltage ripple while considering the increase in size (5V + 0.3V)•3 and cost of a larger capacitor. Use the following equation D = ( = 0.57 5V + 0.3V) • 3+ 12V to calculate the output capacitance: 5V •1.5A • 2 L I C PRI •ISW2 SW = OUT = 0.8 • 12V • 0.57 2 • VOUT • ΔVOUT f SW = 277kHz Example: The transformer also needs to be rated for the correct Design for output voltage ripple less than ±1% of VOUT, saturation current level across line and load conditions. i.e., 100mV. A saturation current rating larger than 7A is necessary to work with the LT3002. 9µH • (4.5A)2 COUT = = 182µF
Step 3: Choose the output diode.
2 • 5V • 0.1V Two main criteria for choosing the output diode include Remember ceramic capacitors lose capacitance with forward current rating and reverse-voltage rating. The applied voltage. The capacitance can drop to 40% of quoted maximum load requirement is a good first-order guess capacitance at the maximum voltage rating. So a 220µF, at the average current requirement for the output diode. 6.3V rating X5R or X7R ceramic capacitor is chosen. Under output short-circuit condition, the output diode 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