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AMS2954 Datasheet(PDF) 9 Page - Advanced Monolithic Systems |
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AMS2954 Datasheet(HTML) 9 Page - Advanced Monolithic Systems |
9 / 15 page Advanced Monolithic Systems, Inc. www.advanced-monolithic.com Phone (925) 443-0722 Fax (925) 443-0723 AMS2954 APPLICATION HINTS External Capacitors A 1.0 µF or greater capacitor is required between output and ground for stability at output voltages of 5V or more. At lower output voltages, more capacitance is required (2.2 µ or more is recommended for 2.5V, 3.0V and 3.3V versions). Without this capacitor the part will oscillate. Most types of tantalum or aluminum electrolytic works fine here; even film types work but are not recommended for reasons of cost. Many aluminum types have electrolytes that freeze at about -30 °C, so solid tantalums are recommended for operation below -25 °C. The important parameters of the capacitor are an ESR of about 5 Ω or less and resonant frequency above 500 kHz parameters in the value of the capacitor. The value of this capacitor may be increased without limit. At lower values of output current, less output capacitance is required for stability. The capacitor can be reduced to 0.33 µF for currents below 10 mA or 0.1 µF for currents below 1 mA. Using the adjustable versions at voltages below 5V runs the error amplifier at lower gains so that more output capacitance is needed. For the worst-case situation of a 300mA load at 1.23V output (Output shorted to Feedback) a 3.3 µF (or greater) capacitor should be used. Unlike many other regulators, the AMS2954, will remain stable and in regulation with no load in addition to the internal voltage divider. This is especially important in CMOS RAM keep-alive applications. When setting the output voltage of the AMS2954 version with external resistors, a minimum load of 1 µA is recommended. A 1 µF tantalum or aluminum electrolytic capacitor should be placed from the AMS2954/AMS2954 input to the ground if there is more than 10 inches of wire between the input and the AC filter capacitor or if a battery is used as the input. Stray capacitance to the AMS2954 Feedback terminal can cause instability. This may especially be a problem when using a higher value of external resistors to set the output voltage. Adding a 100 pF capacitor between Output and Feedback and increasing the output capacitor to at least 3.3 µF will fix this problem. Error Detection Comparator Output The comparator produces a logic low output whenever the AMS2954 output falls out of regulation by more than approximately 5%. This figure is the comparator’s built-in offset of about 60 mV divided by the 1.235 reference voltage (Refer to the block diagram). This trip level remains “5% below normal” regardless of the programmed output voltage of the 2951. For example, the error flag trip level is typically 4.75V for a 5V output or 11.4V for a 12V output. The out of regulation condition may be due either to low input voltage, current limiting, or thermal limiting. Figure 1 gives a timing diagram depicting the ERROR signal and the regulator output voltage as the AMS2954 input is ramped up and down. For 5V versions the ERROR signal becomes valid (low) at about 1.3V input. It goes high at about 5V input (the input voltage at which Vout = 4.75 ). Since the AMS2954’s dropout voltage is load dependent (see curve in typical performance characteristics), the input voltage trip point (about 5V) will vary with the load current. The output voltage trip point (approx. 4.75V) does not vary with load. The error comparator has an open-collector output which requires an external pull-up resistor. This resistor may be returned to the output or some other supply voltage depending on system requirements. In determining a value for this resistor, note that the output is rated to sink 400 µA, this sink current adds to battery drain in a low battery condition. Suggested values range from 100K to 1M Ω. The resistor is not required if this output is unused. OUTPUT VOLTAGE ERROR* INPUT VOLTAGE 4.75V 5V 1.3V FIGURE 1. ERROR Output Timing *When VIN ≤1.3V the error flag pin becomes a high impedance, and the error flag voltage rises to its pull-up voltage. Using Vout as the pull-up voltage (see Figure 2), rather than an external 5V source, will keep the error flag voltage under 1.2V (typ.) in this condition. The user may wish to drive down the error flag voltage using equal value resistors (10 k Ω suggested), to ensure a low- level logic signal during any fault condition, while still allowing a valid high logic level during normal operation. Programming the Output Voltage The AMS2954 may be pin-strapped for the nominal fixed output voltage using its internal voltage divider by tying the output and sense pins together, and also tying the feedback and VTAP pins together. Alternatively, it may be programmed for any output voltage between its 1.235V reference and its 30V maximum rating. As seen in Figure 2, an external pair of resistors is required. The complete equation for the output voltage is: Vout = VREF × (1 + R1/ R2)+ IFBR1 where VREF is the nominal 1.235 reference voltage and IFB is the feedback pin bias current, nominally -20 nA. The minimum recommended load current of 1 µA forces an upper limit of 1.2 M Ω on value of R 2, if the regulator must work with no load (a condition often found in CMOS in standby) IFB will produce a 2% typical error in VOUT which may be eliminated at room temperature by trimming R1. For better accuracy, choosing R2 = 100k reduces this error to 0.17% while increasing the resistor program current by 12 µA. Since the AMS2954 typically draws 60 µA at no load with Pin 2 open-circuited, this is a small price to pay. |
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