Datasheet LT3002 (Analog Devices) - 7

link to page 7 LT3002
APPLICATIONS INFORMATION Step 1: Select the transformer turns ratio.
65V – V V IN(MAX) – VLEAKAGE IN NPS < VOUT + VF R1 EN/UVLO VLEAKAGE = Margin for transformer leakage spike = 15V RUN/STOP V LT3002 R2 CONTROL F = Output diode forward voltage = ~0.3V (OPTIONAL) Example: GND 3002 F01 65V – 32V – 15V NPS < = 3.4
Figure 1. Undervoltage Lockout (UVLO)
5V + 0.3V
Minimum Load Requirement
The choice of transformer turns ratio is critical in deter- mining output current capability of the converter. Table 1 The LT3002 samples the isolated output voltage from the shows the switch voltage stress and output current capa- primary-side flyback pulse waveform. The flyback pulse bility at different transformer turns ratio. occurs once the primary switch turns off and the sec- ondary winding conducts current. In order to sample the
Table 1. Switch Voltage Stress and Output Current Capability vs Turns Ratio
output voltage, the LT3002 has to turn on and off for a minimum amount of time and with a minimum frequency.
VSW(MAX) at IOUT(MAX) at NPS VIN(MAX) (V) VIN(MIN) (A) DUTY CYCLE (%)
The LT3002 delivers a minimum amount of energy even 1:1 37.3 0.92 14-40 during light load conditions to ensure accurate output 2:1 42.6 1.31 25-57 voltage information. The minimum energy delivery cre- 3:1 47.9 1.53 33-67 ates a minimum load requirement, which can be approxi- mately estimated as: Clearly, only N L PS = 3 can meet the 1.5A output current P 2 RI •I • fMIN requirement, so N I SW(MIN) PS = 3 is chosen as the turns ratio in LOAD(MIN) = this example. 2• VOUT L
Step 2: Determine the primary inductance.
PRI = Transformer primary inductance I Primary inductance for the transformer must be set above SW(MIN) = Minimum switch current limit = 1.04A (MAX) a minimum value to satisfy the minimum switch-off and fMIN = Minimum switching frequency = 12.7kHz (MAX) switch-on time requirements: The LT3002 typically needs less than 0.5% of its full out- t ( ) put power as minimum load. Alternatively, a Zener diode L OFF(MIN) • NPS • VOUT + VF PRI ≥ with its breakdown of 10% higher than the output voltage ISW(MIN) can serve as a minimum load if pre-loading is not accept- t able. For a 5V output, use a 5.6V Zener with cathode con- L ON(MIN) • VIN(MAX) PRI ≥ nected to the output. ISW(MIN)
Design Example
tOFF(MIN) = 350ns Use the following design example as a guide to designing tON(MIN) = 160ns applications for the LT3002. The design example involves ISW(MIN) = 0.87A designing a 5V output with a 1.5A load current and an input range from 8V to 32V. VIN(MIN) = 8V, VIN(NOM) = 12V, VIN(MAX) = 32V, VOUT = 5V, IOUT = 1.5A 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