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ADP667 датащи(PDF) 5 Page - Analog Devices |
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ADP667 датащи(HTML) 5 Page - Analog Devices |
5 / 12 page ADP667 REV. 0 –5– Dropout Detector The ADP667 features an extremely low dropout voltage making it suitable for low voltage systems where headroom is limited. A dropout detector is also provided. The dropout detector output, DD, changes as the dropout voltage approaches its limit. This is useful for warning that regulation can no longer be maintained. The dropout detector output is an open collector output from a PNP transistor. Under normal operating conditions with the in- put voltage more than 300 mV above the output, the PNP tran- sistor is off and no current flows out the DD pin. As the voltage differential reduces to less than 300 mV, the transistor switches on and current is sourced. This condition indicates that regulation can no longer be maintained. Please refer to Figure 10 in the “Typical Performance Characteristics.” The current output can be translated into a voltage output by connecting a resistor from DD to GND. A resistor value of 100 k Ω is suitable. A digital status signal can be obtained using a comparator. The on-chip comparator LBI may be used if it is not being used to monitor a battery voltage. This is illustrated in Figure 5. + C1 10µF IN OUT LBO ADP667 +5V OUTPUT R2 10k Ω LBI GND SET SHDN DROPOUT STATUS OUTPUT DD VIN R1 100k Ω + Figure 5. Dropout Status Output Output Capacitor Selection An output capacitor is required on the ADP667 to maintain stability and also to improve the load transient response. Ca- pacitor values from 10 µF upwards are suitable. All specifica- tions are tested and guaranteed with 10 µF. Capacitors larger than 10 µF will further improve the dynamic transient response characteristics of the regulator. Tantalum or aluminum electro- lytics are suitable for most applications. For temperatures below about –25 °C, solid tantalums should be used as many alumi- num electrolytes freeze at this temperature. Quiescent Current Considerations The ADP667 uses a PNP output stage to achieve low dropout voltages combined with high output current capability. Under normal regulating conditions the quiescent current is extremely low. However if the input voltage drops so that it is below the desired output voltage, the quiescent current increases consider- ably. This happens because regulation can no longer be main- tained and large base current flows in the PNP output transistor in an attempt to hold it fully on. For minimum quiescent cur- rent, it is therefore important that the input voltage is main- tained higher than the desired output level. If the device is being powered using a battery that can discharge down below the rec- ommended level, there are a couple of techniques that can be applied to reduce the quiescent current, but at the expense of dropout voltage. The first of these is illustrated in Figure 6. By connecting DD to SHDN the regulator is partially disabled with input voltages below the desired output voltage and therefore the quiescent current is reduced considerably. + C1 10µF IN OUT ADP667 +5V OUTPUT GND SET SHDN DD VIN R1 47k Ω + C2 0.1µF Figure 6. IQ Reduction 1 Another technique for reducing the quiescent current near drop- out is illustrated in Figure 7. The DD output is used to modify the output voltage so that as VIN drops, the desired output volt- age setpoint also drops. This technique only works when exter- nal resistors are used to set the output voltage. With VIN greater than VOUT, DD has no effect. As VIN reduces and dropout is reached, the DD output starts sourcing current into the SET input through R3. This increases the SET voltage so that the regulator feedback loop does not drive the internal PNP transis- tor as hard as it otherwise would. As the input voltage continues to decrease, more current is sourced, thereby reducing the PNP drive even further. The advantage of this scheme is that it main- tains a low quiescent current down to very low values of VIN at which point the batteries are well outside their useful operating range. The output voltage tracks the input voltage minus the dropout. The SHDN function is also unaffected and may be used normally if desired. + C1 10µF IN OUT ADP667 +5V OUTPUT GND SET SHDN DD R1 332k Ω R2 1M Ω R3 1M Ω + VIN Figure 7. IQ Reduction 2 A |
Аналогичный номер детали - ADP667_15 |
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Аналогичное описание - ADP667_15 |
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