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MAX8862 Datasheet(PDF) 10 Page - Maxim Integrated Products |
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MAX8862 Datasheet(HTML) 10 Page - Maxim Integrated Products |
10 / 12 page __________Applications Information Power Dissipation and Operating Region The MAX8862’s maximum power dissipation depends on the thermal resistance of the case and circuit board, the temperature difference between the die junction and ambient air, and the rate of air flow. The GND pins of the MAX8862 SO package perform the dual function of providing an electrical connection to ground and channeling heat away. Connect all GND pins to ground using a large pad or ground plane. Where this is impossible, place a copper plane on an adjacent layer. For a given power dissipation, the pad should exceed the associated dimensions in Figure 3. This figure shows a typical thermal resistance for a 35µm-thick copper foil as a function of its area1. The power dissipation across the device is given by: P = IOUT1 (VIN1 - VOUT1) + IOUT2 (VIN2 - VOUT2). The resulting power dissipation is as follows: P = (TJ - TA) / ( θJB + θBA) where (TJ - TA) is the temperature difference between the MAX8862 die junction and the surrounding air, θJB (or θJC) is the thermal resistance of the package, and θBA is the thermal resistance through the printed circuit board, copper traces, and other materials to the sur- rounding air. The MAX8862’s narrow SO package has a thermal resistance of θJB = +50°C/W. The MAX8862 regulators deliver the rated output cur- rents and operate with input voltages up to 11.5V, but not simultaneously. High output currents can only be sustained when input-output differential voltages are small, as shown in Figure 4. Capacitor Selection and Regulator Stability Filter capacitors are required at the MAX8862’s inputs and outputs. 1µF ceramic capacitors are required at the inputs. The minimum output capacitance required for stability is 3.3µF for OUT1 and 2.2µF for OUT2. The capacitor values depend primarily on the desired power-up time and load-transient response. Load- transient response is improved by using larger capaci- tor values. Input and output filter capacitors should be soldered directly to pins to minimize lead inductance of PC board traces. The output capacitor’s equivalent series resistance (ESR) affects stability and output noise. Surface-mount ceramic capacitors have a very low ESR and are avail- able up to 10µF. Otherwise, other low-ESR (<0.5 Ω) capacitors should be used. If the selected capacitor’s ESR is higher than the recommended value, the capac- itor value should be increased proportionally to main- tain minimum output noise under all input voltage and output load conditions. Paralleling two or more capaci- tors also results in lower ESR. Low-Cost, Low-Dropout, Dual Linear Regulator 10 ______________________________________________________________________________________ SUPPLY VOLTAGE (V) 2 6 3 4 5 7 8 9 10 11 12 400 0 300 200 100 350 250 150 50 (R) (T) (L) OPERATING REGION AT TA = +25°C, TJ = +125°C MAXIMUM SUPPLY VOLTAGE MAXIMUM CURRENT (R) (T) (L) 0 0 PC-BOARD COPPER FOIL AREA (Cm2) 20 40 60 80 100 510 15 20 25 30 35 Figure 3. Typical Copper Thermal Resistance vs. Copper Ground Pad Area Figure 4. Safe Operating Regions: Main and Secondary Regulators Maximum Output Current vs. Supply Voltage 1This graph was generated by Mr. Kieran O’Malley of Cherry Semiconductor Corp. and was published in the October 26, 1995, issue of EDN magazine. |
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