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LT3021-1.5 Datasheet(PDF) 10 Page - Linear Technology |
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LT3021-1.5 Datasheet(HTML) 10 Page - Linear Technology |
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10 / 16 page  10 LT3021/LT3021-1.2/ LT3021-1.5/LT3021-1.8 3021fa APPLICATIO S I FOR ATIO of noise. A ceramic capacitor produced Figure 4’s trace in response to light tapping from a pencil. Similar vibration induced behavior can masquerade as increased output voltage noise. No-Load/Light-Load Recovery A transient load step occurs when the output current changes from its maximum level to zero current or a very small load current. The output voltage responds by over- shooting 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 microfarads of output capacitance. To eliminate this problem, the LT3021 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 LT3021 turns the current sink off and shuts down the bias current/reference generator circuitry. Thermal Considerations The LT3021’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. DC BIAS VOLTAGE (V) 3021 F02 20 0 –20 –40 –60 –80 –100 0 4 8 10 26 12 14 X5R Y5V 16 BOTH CAPACITORS ARE 16V, 1210 CASE SIZE, 10 µF TEMPERATURE ( °C) –50 40 20 0 –20 –40 –60 –80 –100 25 75 3021 F03 –25 0 50 100 125 Y5V X5R BOTH CAPACITORS ARE 16V, 1210 CASE SIZE, 10 µF Figure 2. Ceramic Capacitor DC Bias Characteristics Figure 3. Ceramic Capacitor Temperature Characteristics 1ms/DIV 3021 F04 1mV/DIV VOUT = 1.3V COUT = 10µF ILOAD = 0 Figure 4. Noise Resulting from Tapping on a Ceramic Capacitor |
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