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ADP3367AR Datasheet(PDF) 6 Page - Analog Devices |
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ADP3367AR Datasheet(HTML) 6 Page - Analog Devices |
6 / 8 page ADP3367 –6– REV. 0 + C1 10µF IN OUT LBO ADP3367 +5V OUTPUT R2 10k Ω LBI GND SET SHDN DROPOUT STATUS OUTPUT DD VIN R1 100k Ω + Figure 11. Dropout Status Output Output Capacitor An output capacitor is required on the ADP3367 to maintain sta- bility and also to improve the load transient response. Capacitor values from 10 µF upwards are recommended. Capacitors larger than 10 µF will further improve the transient response. Tantalum or aluminum electrolytics are suitable for most applications. For temperatures below about –25 °C, solid tantalums should be used as many aluminum electrolytes freeze at this temperature. Quiescent Current Considerations The ADP3367 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 12. 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 ADP3367 +5V OUTPUT GND SET SHDN DD VIN R1 47k Ω + C2 0.1µF Figure 12. IQ Reduction 1 Another technique for reducing the quiescent current near drop- out is illustrated in Figure 13. 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 ADP3367 +5V OUTPUT GND SET SHDN DD R1 610k Ω R2 2M Ω R3 1M Ω + VIN QUIESCENT CURRENT BELOW DROPOUT 0 16 23 5 400 200 900 1mA 800 700 600 500µA 300 100 4 1.2mA 900µA VIN – V Figure 13. IQ Reduction 2 POWER DISSIPATION The ADP3367 can supply currents up to 300 mA and can oper- ate with input voltages as high as 16.5 V, but not simultaneously. It is important that the power dissipation and hence the internal die temperature be maintained below the maximum limits. Power Dissipation is the product of the voltage differential across the regulator times the current being supplied to the load. The maximum package power dissipation is given in the Absolute Maximum Ratings. In order to avoid excessive die temperatures, these ratings must be strictly observed. PD = (VIN – VOUT) (IL ) The die temperature is dependent on both the ambient tempera- ture and on the power being dissipated by the device. The inter- nal die temperature must not exceed 125 °C. Therefore, care must be taken to ensure that, under normal operating condi- tions, the die temperature is kept below the thermal limit. TJ = TA + PD ( θ JA) |
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