Datasheet LT3002 (Analog Devices) - 9
| Manufacturer | Analog Devices |
| Description | 36V 10W No-Opto Isolated Flyback Converter |
| Pages / Page | 12 / 9 — APPLICATIONS INFORMATION Step 5: Design snubber circuit. Step 7: Adjust … |
| File Format / Size | PDF / 811 Kb |
| Document Language | English |
APPLICATIONS INFORMATION Step 5: Design snubber circuit. Step 7: Adjust RFB resistor based on output voltage
Model Line for this Datasheet
Text Version of Document
LT3002
APPLICATIONS INFORMATION Step 5: Design snubber circuit. Step 7: Adjust RFB resistor based on output voltage.
The snubber circuit protects the power switch from leak- Build and power up the application with application com- age inductance voltage spike. A (RC + DZ) snubber is ponents and measure the regulated output voltage. Adjust recommended for this application. A 470pF capacitor in RFB resistor based on the measured output voltage: series with a 39Ω resistor is chosen as the RC snubber. VOUT The maximum Zener breakdown voltage is set according RFB(NEW) = •R V FB OUT(MEASURED) to the maximum V IN: V Example: ZENNER(MAX) ≤ 60V – VIN(MAX) Example: 5V RFB = • 158k = 154k 5.14V V ZENNER(MAX) ≤ 60V – 32V = 28V A 24V Zener with a maximum of 26V will provide optimal
Step 8: Select RTC resistor based on output voltage tem-
protection and minimize power loss. So a 24V, 1.5W Zener
perature variation.
from Central Semiconductor (CMZ5934B) is chosen. Measure output voltage in a controlled temperature envi- Choose a diode that is fast and has sufficient reverse volt- ronment like an oven to determine the output temperature age breakdown: coefficient. Measure output voltage at a consistent load current and input voltage, across the operating tempera- VREVERSE > VSW(MAX) ture range. VSW(MAX) = VIN(MAX) + VZENNER(MAX) Calculate the temperature coefficient of VF: Example: V ( )– V ( ) –(δV ) = OUT T1 OUT T2 V F/δT REVERSE > 60V T1– T2 A 100V, 1A diode from Diodes Inc. (DFLS1100) is chosen. 3.35mV/°C ⎛ R ⎞ R FB TC = ( ) •
Step 6: Select the R
– δVF/δT ⎝⎜ NPS ⎠⎟
REF and RFB resistors.
Use the fol owing equation to calculate the starting values Example: for RREF and RFB: 5.189V – 5.041V –(δV ) = R ( ) F/δT 100°C – (0°C) = 1.48mV / °C R REF • NPS • VOUT + VF T0 FB = VREF 3.35mV/°C ⎛ 154 R ⎞ TC = • 1.48mV/°C ⎝⎜ 3 ⎠⎟ = 115k RREF = 10k Example:
Step 9: Select the EN/UVLO resistors.
10k • 3 • (5V + 0.3V) Determine the amount of hysteresis required and calcu- RFB = = 159k late R1 resistor value: 1.00V VIN(HYS) = 2.5µA • R1 For 1% standard values, a 158k resistor is chosen. Example: Choose 2V of hysteresis, R1 = 806k Rev. 0 For more information www.analog.com 9 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 Step 5: Design snubber circuit. Step 7: Adjust RFB resistor based on output voltage.
The snubber circuit protects the power switch from leak- Build and power up the application with application com- age inductance voltage spike. A (RC + DZ) snubber is ponents and measure the regulated output voltage. Adjust recommended for this application. A 470pF capacitor in RFB resistor based on the measured output voltage: series with a 39Ω resistor is chosen as the RC snubber. VOUT The maximum Zener breakdown voltage is set according RFB(NEW) = •R V FB OUT(MEASURED) to the maximum V IN: V Example: ZENNER(MAX) ≤ 60V – VIN(MAX) Example: 5V RFB = • 158k = 154k 5.14V V ZENNER(MAX) ≤ 60V – 32V = 28V A 24V Zener with a maximum of 26V will provide optimal
Step 8: Select RTC resistor based on output voltage tem-
protection and minimize power loss. So a 24V, 1.5W Zener
perature variation.
from Central Semiconductor (CMZ5934B) is chosen. Measure output voltage in a controlled temperature envi- Choose a diode that is fast and has sufficient reverse volt- ronment like an oven to determine the output temperature age breakdown: coefficient. Measure output voltage at a consistent load current and input voltage, across the operating tempera- VREVERSE > VSW(MAX) ture range. VSW(MAX) = VIN(MAX) + VZENNER(MAX) Calculate the temperature coefficient of VF: Example: V ( )– V ( ) –(δV ) = OUT T1 OUT T2 V F/δT REVERSE > 60V T1– T2 A 100V, 1A diode from Diodes Inc. (DFLS1100) is chosen. 3.35mV/°C ⎛ R ⎞ R FB TC = ( ) •
Step 6: Select the R
– δVF/δT ⎝⎜ NPS ⎠⎟
REF and RFB resistors.
Use the fol owing equation to calculate the starting values Example: for RREF and RFB: 5.189V – 5.041V –(δV ) = R ( ) F/δT 100°C – (0°C) = 1.48mV / °C R REF • NPS • VOUT + VF T0 FB = VREF 3.35mV/°C ⎛ 154 R ⎞ TC = • 1.48mV/°C ⎝⎜ 3 ⎠⎟ = 115k RREF = 10k Example:
Step 9: Select the EN/UVLO resistors.
10k • 3 • (5V + 0.3V) Determine the amount of hysteresis required and calcu- RFB = = 159k late R1 resistor value: 1.00V VIN(HYS) = 2.5µA • R1 For 1% standard values, a 158k resistor is chosen. Example: Choose 2V of hysteresis, R1 = 806k Rev. 0 For more information www.analog.com 9 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
