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ADP667 датащи(PDF) 7 Page - Analog Devices |
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ADP667 датащи(HTML) 7 Page - Analog Devices |
7 / 8 page ADP667 REV. 0 –7– POWER DISSIPATION The ADP667 can supply currents up to 250 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) This may be expressed in terms of power dissipation as follows: PD = (TJ – TA)/( θ JA) where: TJ = Die Junction Temperature ( °C) TA = Ambient Temperature ( °C) PD = Power Dissipation (W) θ JA = Junction to Ambient Thermal Resistance ( °C/W) If the device is being operated at the maximum permitted ambi- ent temperature of 85 °C, the maximum power dissipation per- mitted is: PD (max) = (TJ (max) – TA)/(θJA) PD (max) = (125 – 85)/(θJA) = 40/ θ JA where: θ JA = 120°C/W for the 8-pin DIP (N-8) package θ JA = 170°C/W for the 8-pin SOIC (SO-8) package Therefore, for a maximum ambient temperature of 85 °C: PD (max) = 333 mW for N-8 PD (max) = 235 mW for SO-8 At lower ambient temperatures the maximum permitted power dissipation increases accordingly up to the maximum limits specified in the absolute maximum specifications. The thermal impedance ( θ JA) figures given are measured in still air conditions and are reduced considerably where fan assisted cooling is employed. Other techniques for reducing the thermal impedance include large contact pads on the printed circuit board and wide traces. The copper will act as a heat exchanger thereby reducing the effective thermal impedance. High Power Dissipation Recommendations Where excessive power dissipation due to high input-output differential voltages and/or high current conditions exists, the simplest method of reducing the power requirements on the regulator is to use a series dropper resistor. In this way the excess power can be dissipated in the external resistor. As an example, consider an input voltage of +12 V and an output voltage requirement of +5 V @ 100 mA with an ambient tem- perature of +85 °C. The package power dissipation under these conditions is 700 mW which exceeds the maximum ratings. By using a dropper resistor to drop 4 V, the power dissipation requirement for the regulator is reduced to 300 mW which is within the maximum specifications for the N-8 package at 85 °C. The resistor value is calculated as R = 4/0.1 = 40 Ω. A resistor power rating of 400 mW or greater may be used. IN OUT GND SET SHDN ADP667 +5V OUTPUT C2 10µF + 40 Ω 0.5W C1 1µF VIN 12V + Figure 14. Reducing Regulator Power Dissipation Transient Response The ADP667 exhibits excellent transient performance as illus- trated in the “Typical Performance Characteristics.” Figure 12 shows that an input step from 10 V to 6 V results in a very small output disturbance (50 mV). Adding an input capacitor would improve this even more. Figure 13 shows how quickly the regulator recovers from an output load change from 10 mA to 100 mA. The offset due to the load current change is less than 1 mV. Monitored µP Power Supply Figure 15 shows the ADP667 being used in a monitored µP supply application. The ADP667 supplies +5 V for the micro- processor. Monitoring the supply, the ADM705 will generate a reset if the supply voltage falls below 4.65 V. Early warning of an impending power fail is generated by a power fail comparator on the ADM705. A resistive divider network samples the pre- regulator input voltage so that failing power is detected while the regulator is still operating normally. An interrupt is gener- ated so that a power-down sequence can be completed before power is completely lost. The low dropout voltage on the ADP667 maximizes the available time to carry out the power- down sequence. The resistor divider network R1 and R2 should be selected so that the voltage on PFI is 1.25 V at the desired warning voltage. IN OUT GND SET SHDN ADP667 10µF + +5V ADM705 RESET PFO GND PFI UNREGULATED DC R1 R2 VCC µP RESET INTERRUPT VCC Figure 15. µP Regulator with Supply Monitoring and Early Power-Fail Warning |
Аналогичный номер детали - ADP667 |
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Аналогичное описание - ADP667 |
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