Datasheet LT3014B (Analog Devices) - 10

LT3014B
APPLICATIONS INFORMATION
Continuous operation at large input/output voltage dif- Example 2: Given an output voltage of 5V, an input voltage ferentials and maximum load current is not practical due of 48V that rises to 72V for 5ms(max) out of every 100ms, to thermal limitations. Transient operation at high input/ and a 5mA load that steps to 20mA for 50ms out of every output differentials is possible. The approximate thermal 250ms, what is the junction temperature rise above ambi- time constant for a 2500sq mm 3/32" FR-4 board with ent? Using a 500ms period (well under the time constant maximum topside and backside area for one ounce cop- of the board), power dissipation is as follows: per is 3 seconds. This time constant will increase as more P1(48V in, 5mA load) = 5mA • (48V – 5V) thermal mass is added (i.e. vias, larger board, and other + (100µA • 48V) = 0.22W components). P2(48V in, 20mA load) = 20mA • (48V – 5V) For an application with transient high power peaks, average + (0.55mA • 48V) = 0.89W power dissipation can be used for junction temperature calculations as long as the pulse period is significantly less P3(72V in, 5mA load) = 5mA • (72V – 5V) than the thermal time constant of the device and board. + (100µA • 72V) = 0.34W P4(72V in, 20mA load) = 20mA • (72V – 5V)
Calculating Junction Temperature
+ (0.55mA • 72V) = 1.38W Example 1: Given an output voltage of 5V, an input volt- Operation at the different power levels is as follows: age range of 24V to 30V, an output current range of 0mA to 20mA, and a maximum ambient temperature of 50°C, 76% operation at P1, 19% for P2, 4% for P3, and what will the maximum junction temperature be? 1% for P4. The power dissipated by the device will be equal to: P EFF = 76%(0.22W) + 19%(0.89W) + 4%(0.34W) + 1%(1.38W) = 0.36W IOUT(MAX) • (VIN(MAX) – VOUT) + (IGND • VIN(MAX)) With a thermal resistance in the range of 40°C/W to where: 62°C/W, this translates to a junction temperature rise IOUT(MAX) = 20mA above ambient of 20°C. VIN(MAX) = 30V
Protection Features
IGND at (IOUT = 20mA, VIN = 30V) = 0.55mA The LT3014B incorporates several protection features So: which make it ideal for use in battery-powered circuits. P = 20mA In addition to the normal protection features associated • (30V – 5V) + (0.55mA • 30V) = 0.52W with monolithic regulators, such as current limiting and The thermal resistance for the DFN package will be in the thermal limiting, the device is protected against reverse- range of 40°C/W to 62°C/W depending on the copper input voltages, and reverse voltages from output to input. area. So the junction temperature rise above ambient will be approximately equal to: Current limit protection and thermal overload protection are intended to protect the device against current overload 0.52W • 50°C/W = 26°C conditions at the output of the device. For normal opera- The maximum junction temperature will then be equal to tion, the junction temperature should not exceed 125°C. the maximum junction temperature rise above ambient The input of the device will withstand reverse voltages of plus the maximum ambient temperature or: 80V. Current flow into the device will be limited to less T than 6mA (typically less than 100µA) and no negative JMAX = 50°C + 26°C = 76°C 3014bfb 10 For more information www.linear.com/LT3014B Document Outline Features Description Applications Typical Application Absolute Maximum Ratings Pin Configuration Electrical Characteristics Typical Performance Characteristics Pin Functions Applications Information