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LT1964 Datasheet(PDF) 11 Page - Linear Technology |
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LT1964 Datasheet(HTML) 11 Page - Linear Technology |
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11 / 16 page  11 LT3020/LT3020-1.2/ LT3020-1.5/LT3020-1.8 3020fc response to light tapping from a pencil. Similar vibration induced behavior can masquerade as increased output voltage noise. No-Load/Light-Load Recovery A possible transient load step that occurs is where the output current changes from its maximum level to zero current or a very small load current. The output voltage responds by overshooting until the regulator lowers the amount of current it delivers to the new level. The regulator loop response time and the amount of output capacitance control the amount of overshoot. Once the regulator has decreased its output current, the current provided by the resistor divider (which sets VOUT) is the only current remaining to discharge the output capacitor from the level to which it overshot. The amount of time it takes for the output voltage to recover easily extends to milliseconds with microamperes of divider current and a few microfar- ads of output capacitance. To eliminate this problem, the LT3020 incorporates a no-load or light-load recovery circuit. This circuit is a voltage-controlled current sink that significantly improves the light load transient response time by discharging the output capacitor quickly and then turning off. The current sink turns on when the output voltage exceeds 6% of the nominal output voltage. The current sink level is then proportional to the overdrive above the threshold up to a maximum of approximately 15mA. Consult the curve in the Typical Performance Characteristics for the No-Load Recovery Threshold. If external circuitry forces the output above the no load recovery circuit’s threshold, the current sink turns on in an attempt to restore the output voltage to nominal. The current sink remains on until the external circuitry releases the output. However, if the external circuitry pulls the output voltage above the input voltage, or the input falls below the output, the LT3020 turns the current sink off and shuts down the bias current/reference generator circuitry. Thermal Considerations The LT3020’s power handling capability is limited by its maximum rated junction temperature of 125 °C.Thepower dissipated by the device is comprised of two components: 1. Output current multiplied by the input-to-output volt- age differential: (IOUT)(VIN – VOUT) and 2. GND pin current multiplied by the input voltage: (IGND)(VIN). GND pin current is found by examining the GND pin current curves in the Typical Performance Characteristics. Power dissipation is equal to the sum of the two compo- nents listed above. The LT3020 regulator has internal thermal limiting (with hysteresis) designed to protect the device during overload conditions. For normal continuous conditions, do not exceed the maximum junction temperature rating of 125 °C. Carefully consider all sources of thermal resistance from junction to ambient including other heat sources mounted in proximity to the LT3020. The underside of the LT3020 DD package has exposed metal (4mm2) from the lead frame to where the die is attached. This allows heat to directly transfer from the die junction to the printed circuit board metal to control maximum operating junction temperature. The dual-in-line pin ar- rangement allows metal to extend beyond the ends of the package on the topside (component side) of a PCB. Con- nect this metal to GND on the PCB. The multiple IN and OUT pins of the LT3020 also assist in spreading heat to the PCB. The LT3020 MS8 package has pin 4 fused with the lead frame. This also allows heat to transfer from the die to the printed circuit board metal, therefore reducing the thermal resistance. Copper board stiffeners and plated through- holes can also be used to spread the heat generated by power devices. The following tables list thermal resistance for several different board sizes and copper areas for two different packages. Measurements were taken in still air on 3/32" FR-4 board with one ounce copper. Table 1. Measured Thermal Resistance for DD Package COPPER AREA THERMAL RESISTANCE TOPSIDE* BACKSIDE BOARD AREA (JUNCTION-TO-AMBIENT) 2500mm 2 2500mm 2 2500mm 2 35 °C/W 900mm 2 2500mm 2 2500mm 2 40 °C/W 225mm 2 2500mm 2 2500mm 2 55 °C/W 100mm 2 2500mm 2 2500mm 2 60 °C/W 50mm 2 2500mm 2 2500mm 2 70 °C/W APPLICATIO S I FOR ATIO |
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