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RS1117A-ADSJG Datasheet(PDF) 6 Page - Orister Corporation |
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RS1117A-ADSJG Datasheet(HTML) 6 Page - Orister Corporation |
6 / 13 page Page No. : 6/13 DS‐RS1117A‐16 September, 2009 www.Orister.com Detail Description Output Voltage Adjustment Like most regulators, RS1117A series regulate the output by comparing the output voltage to an internally generated reference voltage. On the adjustable version, the VREF is available externally as 1.25V between VOUT and ADJ. The voltage ratio formed by R1 and R2 should be set to conduct 10mA (minimum output load). The output voltage is given by the following equation: VOUT =VREF (1+R2/R1) + IADJ R2 On fixed versions of RS1117A series, the voltage divider is provided internally. Thermal Protection RS1117A series have thermal protection which limits junction temperature to 150°C. However, device functionality is only guaranteed to a maximum junction temperature of +125°C. The power dissipation and junction temperature for RS1117A in all packages given by PD=(VIN‐VOUT)IOUT, TJUNCTION=TAMBIENT+(PDxθJA), Note: TJUNCTION must not exceed 125°C Current Limit Protection RS1117A series are protected against overload conditions. Current protection is triggered at typically 1.5A. Stability and Load Regulation RS1117A series require a capacitor from VOUT to GND to provide compensation feedback to the internal gain stage. This is to ensure stability at the output terminal. Typically, a 47uF tantalum or 100uF aluminum electrolytic is sufficient. Note : It is important that the ESR for this capacitor does not exceed 0.5Ω. The output capacitor does not have a theoretical upper limit and increasing its value will increase stability. COUT=100uF or more is typical for high current regulator design. RS1117A series load regulation are limited by the resistance of the wire connecting it to the load(RP). For the adjustable version, the best load regulation is accomplished when the top of the resistor divider(R1) is connected directly to the output pin of the RS1117A series. When so connected, RP is not multiplied by the divider ratio. For fixed output versions, the top of R1 is internally connected to the output and ground pin can be connected to low side of the load as a negative side sense if, so desired. Thermal Consideration The RS1117A series contain thermal limiting circuitry designed to protect itself for over‐temperature conditions. Even for normal load conditions, maximum junction temperature ratings must not be exceeded. As mention in thermal protection section, we need to consider all sources of thermal resistance between junction and ambient. It contains junction‐to‐case, case‐to‐heat‐sink interface and heat sink resistance itself. An additional heat sink is applied externally sometimes. It can increase the maximum power dissipation. For example, the equivalent junction temperature of 300mA output current is 115°C without external heat sink. Under the same junction temperature IC can operates 500mA with an adequate heat sink. Therefore, to attach an extra heat sink is recommended. RL R1 R2 Connect R1 to Case Connect R2 to Load Out In Adj R P Parasitic Line Resistance V IN VOUT =VREF (1+R2/R1) + IADJ R2 VIN R1 R2 V REF In Out ADJ IADJ 10uA VOUT |
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