Datasheet LT3001 (Analog Devices) - 7

link to page 7 LT3001
APPLICATIONS INFORMATION Minimum Load Requirement
Example: The LT3001 samples the isolated output voltage from 65V − 32V −15V the primary-side flyback pulse waveform. The flyback N = 3.4 PS < 5V + 0.3V pulse occurs once the primary switch turns off and the secondary winding conducts current. In order to sample The choice of transformer turns ratio is critical in deter- the output voltage, the LT3001 has to turn on and off at mining output current capability of the converter. Table 1 least for a minimum amount of time and with a minimum shows the switch voltage stress and output current capa- frequency. The LT3001 delivers a minimum amount of bility at different transformer turns ratio. energy even during light load conditions to ensure accu-
Table 1. Switch Voltage Stress and Output Current Capability
rate output voltage information. The minimum energy
vs Turns Ratio
delivery creates a minimum load requirement, which can
VSW(MAX) at IOUT(MAX) at
be approximately estimated as:
NPS VIN(MAX) (V) VIN(MIN) (mA) DUTY CYCLE (%)
1:1 37.3 330 14-40 L 2 • f I PRI •ISW(MIN) MIN 2:1 42.6 470 25-57 LOAD(MIN) = 2 • VOUT 3:1 47.9 540 33-67 LPRI = Transformer primary inductance Since only NPS = 3 can meet the 500mA output current ISW(MIN) = Minimum switch current limit = 360mA (max) requirement, NPS = 3 is chosen in this example. fMIN = Minimum switching frequency = 10.6kHz (max)
Step 2: Determine the Primary Inductance.
The LT3001 typically needs less than 0.5% of its full out- Primary inductance for the transformer must be set above put power as minimum load. Alternatively, a Zener diode a minimum value to satisfy the minimum switch-off and with its breakdown of 20% higher than the output volt- switch-on time requirements: age can serve as a minimum load if pre-loading is not acceptable. For a 5V output, use a 6V Zener with cathode t ( ) L OFF(MIN) • N PS • VOUT + VF connected to the output. PRI ≥ ISW(MIN)
Design Example
t L ON(MIN) • VIN(MAX) Use the following design example as a guide to design PRI ≥ ISW(MIN) applications for the LT3001. The design example involves designing a 5V output with a 500mA load current and an t input range from 8V to 32V. OFF(MIN) = 450ns t V ON(MIN) = 170ns IN(MIN) = 8V, VIN(NOM) = 12V, VIN(MAX) = 32V, V I OUT = 5V, IOUT = 500mA SW(MIN) = 290mA (typ)
Step 1: Select the Transformer Turns Ratio.
Example: 65V − V 450ns • 3 •(5V + 0.3V) N IN(MAX) − VLEAKAGE LPRI ≥ = 25µH PS < V 290mA OUT + VF 170ns • 32V V L = 19µH LEAKAGE = Margin for transformer leakage spike = 15V PRI ≥ 290mA V F = Output diode forward voltage = ~0.3V Rev. 0 For more information www.analog.com 7 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