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B4251 Datasheet(PDF) 3 Page - List of Unclassifed Manufacturers |
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B4251 Datasheet(HTML) 3 Page - List of Unclassifed Manufacturers |
3 / 5 page Bay Linear, Inc 2478 Armstrong Street, Livermore, CA 94550 Tel: (925) 606-5950, Fax: (925) 940-9556 www.baylinear.com B4251 PRODUCT DESCRIPTION The B4251 is precision fixed output voltage regulator. Unlike bipolar regulators, the B4251 supply current does not increase with load current. In addition, Vout remains stable and within regulator at very low load currents (an important consideration in RTC and CMOS RAM battery back-up application). Figure 1 shows a typical application circuit. The regulator is enabled any time the shutdown input is at or above VIH. And shutdown (disabled) when SHDN is at or below VIL. SHDN maybe controlled by a CMOS logic gate, or I/O port of a micro controller. If the SHDN input is not. Required, it should be connected directly to the supply. While in shutdown, supply current decreases to 0.05 µA (typical) and Vout falls to zero volts. Bypass Input A 470pF capacitor connected from the bypass input to ground reduces noise present on the internal reference, which in turn significantly reduces output noise. If output noise is not a concern, this input maybe left unconnected. Larger capacitor values maybe used, but it results in a longer time period to rated output voltage power is initially applied. Output Capacitor A 1 µF (min) capacitor from Vout to ground is required. Then output capacitor should have an effective series resistance of 5 Ω or less. A 1µA capacitor should be connected from Vin to GND if there is more than 10 inches of wire between the regulator and the AC filter capacitor, or if a battery is used as the power source. Aluminum electrolytic or tantalum capacitor types can be used. (since many aluminum electrolytic capacitors freeze at approximately- 30 °C, solid tantalums are recommended for applications operating below –25 °C.) When operating from sources other than batteries, supply-noise rejection and transient response can be improved by increasing the value of the input and output capacitors and employing passive filtering techniques. Thermal Considerations Thermal Shutdown Integrated thermal protection circuitry shuts the regulator off when die temperature exceeds 160 °C. The regulator remaining off until the die temperature drops to approximately 150 °C. Power Dissipation The amount of power the regulator dissipates is primarily a function of input and output voltage, and output current. The following equation is used to calculate worst case power dissipation: PD=(VINMAX-VOUTMIN)ILOADMAX Where: PD= worst case actual power dissipation VINMAX= Maximum voltage on VIN VOUTMIN=Minimum regulator output voltage ILOADMAX= maximum output (LOAD) Current The maximum allowable power dissipation is function of the maximum ambient temperature (TAMAX). The maximum allowable die temperature (125 °C) and the thermal resistance from junction-to-air ( θJA). The 5- pin SOT-23A package has a θJA of approximately 220 °C/Watt when mounted on a single layer FR4 dielectric copper clad PC board. PDMAX=(TJMAX-TAMAX)/ θJA Where all terms are previously defined This can be caused in conjunction with other equation to ensure regulator thermal operation is within limit. For example: Given: VINMAX=3.0V+10% VOUTMIN=2.7V-2.5% ILOADMAX=40 mA TJMAX=125°C TAMAX=55°C Find: 1. Actual power dissipation 2. Maximum allowable dissipation Actual power dissipation: PD=(VINMAX-VOUTMIN)ILOADMAX =[(3.0 x 1.1)-(2.7 x 0.975)]40 x 10-3 =26.7mW Maximum allowable power dissipation: PDMAX=(TJMAX-TAMAX) =(125-55)/220 =318Mw These equations can be used to calculate maximum current and/or input voltage limits. |
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