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AP1187S7A Datasheet(PDF) 4 Page - Anachip Corp |
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AP1187S7A Datasheet(HTML) 4 Page - Anachip Corp |
4 / 6 page AP1187 1A Ultra Low Dropout Positive Adjustable Regulator Anachip Corp. www.anachip.com.tw Rev.1.1 Aug 23, 2005 4/6 Function Description Introduction The AP1187 regulator is a 7-terminal device designed specifically to provide extremely low dropout voltages comparable to the PNP type without the disadvantage of the extra power dissipation due to the base current associated with PNP regulators. This is done by bringing out the control pin of the regulator that provides the base current to the power NPN and connecting it to a voltage that is greater than the voltage present at the VIN pin. This flexibility makes the AP1187 ideal for applications where dual inputs are available such as a computer motherboard with an ATX style power supply that provides 5V and 3.3V to the board. One such application is the new graphic chip sets that require anywhere from 2.4V to 2.7V supply. The AP1187-ADJ can easily be programmed with the addition of two external resistors to any voltages within the range of 1.25V to 15.5V. Another major requirement of these graphic chips is the need to switch the load current from zero to several amps in tens of nanoseconds at the processor pins, which translates to an approximately 300 to 500ns of current step at the regulator. In addition, the output voltage tolerances are also extremely tight and they include the transient response as part of the specification. The AP1187 is specifically designed to meet the fast current transient needs as well as providing an accurate initial voltage, reducing the overall system cost with the need for fewer number of output capacitors. Another feature of the device is its true remote sensing capability that allows accurate voltage setting at the load rather than at the device. Output Voltage Setting The AP1187 can be programmed to any voltages in the range of 1.25V to 15.5V with the addition of R1 and R2 external resistors according to the following formula: V OUT=VREF(1+R2/R1)+IADJ*R2 where: VREF=1.25V & IADJ=50uA Typically AP1187-ADJ V IN V CTRL ADJ V OUT V SENSE R1+ R2+ V REF I ADJ=50uA The AP1187 keeps a constant 1.25V between the VSENSE pin and the ADJ pin. By placing a resistor R1 across these two pins and connecting the VSENSE and VOUT pin together, a constant current flows through R1, adding to the IADJ current and into the R2 resistor producing a voltage equal to the (1.25/R1)*R2 + IADJ*R2. This voltage is then added to the 1.25V to set the output voltage. This is summarized in the above equation. Since the minimum load current requirement of the AP1187 is 10mA, R1 is typically selected to be a 121Ω resistor so that it automatically satisfies this condition. Notice that since the IADJ is typically in the range of 50uA it only adds a small error to the output voltage and should be considered when very precise output voltage setting is required. Load Regulation Since the AP1187 has separate pins for the output (VOUT) and the sense (VSENSE), it is ideal for providing true remote sensing of the output voltage at the load. This means that the voltage drops due to parasitic resistance such as PCB traces between the regulator and the load are compensated for using remote sensing. Figure following shows a typical application of the AP1187 with remote sensing. AP1187-ADJ V IN V CTRL V OUT V SENSE ADJ R1 R2 R L V CTRL V IN Stability The AP1187 requires the use of an output capacitor as part of the frequency compensation in order to make the regulator stable. Typical designs for the microprocessor applications use standard electrolytic capacitors with typical ESR in the range of 50 to 100mΩ and an output capacitance of 100uF to 1000uF. Fortunately as the capacitance increases, the ESR decreases resulting in a fixed RC time constant. The AP1187 takes advantage of the phenomena in making the overall regulator loop stable. For most applications a minimum of 100uF aluminum electrolytic capacitor insures both stability and good transient response. |
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