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AMS1505CTV-3.0 Datasheet(PDF) 5 Page - Advanced Monolithic Systems |
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AMS1505CTV-3.0 Datasheet(HTML) 5 Page - Advanced Monolithic Systems |
5 / 8 page AMS1505V APPLICATION HINTS CONTROL POWER SENSE AMS1505V OUTPUT ADJ LOAD + - V OUT R P R P R1 R2 5V 3.3V Figure 2. Remote Load Sensing ( ∆I OUT)(RP) TIME V OUT FIGURE 1 V OUT FIGURE 2 I OUT Figure 3. Remote Sensing Improves Load Regulation Voltage drops due to RP are not eliminated; they will add to the dropout voltage of the regulator regardless of whether they are inside or outside the regulation loop. The AMS1505V can control the voltage at the load as long as the input-output voltage is greater than the total of the dropout voltage of the device plus the voltage drop across RP. Stability The circuit design used in the AMS1505V series requires the use of an output capacitor as part of the device frequency compensation. The addition of 150 µF aluminum electrolytic or a 22 µF solid tantalum on the output will ensure stability for all operating conditions. For best frequency response use capacitors with an ESR of less than 1 Ω. In order to meet the transient requirements of the processor larger value capacitors are needed. Tight voltage tolerances are required in the power supply. To limit the high frequency noise generated by the processor high quality bypass capacitors must be used. In order to limit parasitic inductance (ESL) and resistance (ESR) in the capacitors to acceptable limits, multiple small ceramic capacitors in addition to high quality solid tantalum capacitors are required. When the adjustment terminal is bypassed to improve the ripple rejection, the requirement for an output capacitor increases. The Adjust pin is brought out on the fixed voltage device specifically to allow this capability. To ensure good transient response with heavy load current changes capacitor values on the order of 100 µF are used in the output of many regulators. To further improve stability and transient response of these devices larger values of output capacitor can be used. The modern processors generate large high frequency current transients. The load current step contains higher order frequency components than the output coupling network must handle until the regulator throttles to the load current level. Because they contain parasitic resistance and inductance, capacitors are not ideal elements. These parasitic elements dominate the change in output voltage at the beginning of a transient load step change. The ESR of the output capacitors produces an instantaneous step in output voltage ( ∆V=∆I)(ESR). The ESL of the output capacitors produces a droop proportional to the rate of change of the output current (V= L)( ∆I/∆t). The output capacitance produces a change in output voltage proportional to the time until the regulator can respond ( ∆V=∆t) (∆I/C). Figure 4 illustrates these transient effects. CAPACITANCE EFFECTS ESR EFFECTS ESL EFFECTS POINT AT WHICH REGULATOR TAKES CONTROL SLOPE, V/t = ∆I/C Figure 4. Output Voltage The AMS1505V series develops a 1.25V reference voltage between the Sense pin and the Adjust pin (Figure5). Placing a resistor between these two terminals causes a constant current to flow through R1 and down through R2 to set the overall output voltage. In general R1 is chosen so that this current is the specified minimum load current of 10mA.The current out of the Adjust pin is small, typically 50 µA and it adds to the current from R1. Because IADJ is very small it needs to be considered only when very precise output voltage setting is required. For best regulation the top of the resistor divider should be connected directly to the Sense pin. CONTROL POWER OUTPUT AMS1505V SENSE ADJ R1 R2 V OUT V REF V POWER I ADJ 50 µA + + + V CONTROL VOUT = VREF (1+ R2/R1)+IADJR2 Figure 5. Setting Output Voltage Advanced Monolithic Systems, Inc. www.advanced-monolithic.com Phone (925) 443-0722 Fax (925) 443-0723 |
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