Datasheet LT3012B (Analog Devices) - 9

LT3012B
U U W U APPLICATIO S I FOR ATIO
can exhibit an effective value as low as 1µF to 2µF for the ing region. The protection is designed to provide some DC bias voltage applied and over the operating tempera- output current at all values of input voltage up to the device ture range. The X5R and X7R dielectrics result in more breakdown (see curve of Current Limit vs Input Voltage in stable characteristics and are more suitable for use as the the Typical Performance Characteristics). output capacitor. The X7R type has better stability across The LT3012B is limited for operating conditions by maxi- temperature, while the X5R is less expensive and is mum junction temperature. While operating at maximum available in higher values. Care still must be exercised input voltage, the output current range must be limited; when using X5R and X7R capacitors; the X5R and X7R when operating at maximum output current, the input codes only specify operating temperature range and maxi- voltage range must be limited. Device specifications will mum capacitance change over temperature. Capacitance not apply for all possible combinations of input voltage change due to DC bias with X5R and X7R capacitors is and output current. Operating the LT3012B beyond the better than Y5V and Z5U capacitors, but can still be maximum junction temperature rating may impair the life significant enough to drop capacitor values below appro- of the device. priate levels. Capacitor DC bias characteristics tend to improve as component case size increases, but expected
Thermal Considerations
capacitance at operating voltage should be verified. The power handling capability of the device will be limited Voltage and temperature coefficients are not the only by the maximum rated junction temperature (125°C). The sources of problems. Some ceramic capacitors have a power dissipated by the device will be made up of two piezoelectric response. A piezoelectric device generates components: voltage across its terminals due to mechanical stress, similar to the way a piezoelectric accelerometer or micro- 1. Output current multiplied by the input/output voltage phone works. For a ceramic capacitor the stress can be differential: IOUT • (VIN – VOUT) and, induced by vibrations in the system or thermal transients. 2. GND pin current multiplied by the input voltage: IGND • VIN.
Current Limit and Safe Operating Area Protection
The GND pin current can be found by examining the GND Like many IC power regulators, the LT3012B has safe Pin Current curves in the Typical Performance Character- operating area protection. The safe operating area protec- istics. Power dissipation will be equal to the sum of the two tion decreases the current limit as the input voltage components listed above. increases and keeps the power transistor in a safe operat- 20 40 BOTH CAPACITORS ARE 16V, 1210 CASE SIZE, 10µF 20 0 X5R 0 –20 X5R –20 –40 –40 Y5V –60 CHANGE IN VALUE (%) –60 Y5V CHANGE IN VALUE (%) –80 –80 BOTH CAPACITORS ARE 16V, 1210 CASE SIZE, 10µF –100 –100 0 2 4 6 8 10 12 14 16 –50 –25 0 25 50 75 100 125 DC BIAS VOLTAGE (V) TEMPERATURE (°C) 3012B F02 3012B F03
Figure 2. Ceramic Capacitor DC Bias Characterics Figure 3. Ceramic Capacitor Temperature Characterics
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