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GM11172.5TB3R Datasheet(PDF) 6 Page - Gamma Microelectronics Inc. |
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GM11172.5TB3R Datasheet(HTML) 6 Page - Gamma Microelectronics Inc. |
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6 / 13 page  APPLICATION INFORMATION GM1117 linear regulators provide fixed and adjustable output voltages at currents up to 1.0A. These regula- tors are protected against overcurrent conditions and in- clude thermal shutdown protection. The GM1117's have a composite PNP-NPN output transistor and re- quire an output capacitor for stability. A detailed proce- dure for selecting this capacitor follows. Adjustable Operation The GM1117 has an output voltage range of 1.25 V to 5.5 V. An external resistor divider sets the output volt- age as shown in Figure 1. The regulator maintains a fixed 1.25V (typical) reference between the output pin and the adjust pin. A resistor divider network R1 and R2 causes a fixed current to flow to ground. This current creates a voltage across R2 that adds to the 1.25 V across R1 and sets the overall output voltage. The adjust pin current (typically 50 µA) also flows through R2 and adds a small error that should be taken into account if precise adjustment of V is OUT necessary. The output voltage is set according to the formula: The term IAdj × R2 represents the error added by the adjust pin current. R1 is chosen so that the minimum load current is at least 2.0 mA. R1 and R2 should be the same type, e.g. metal film for best tracking over temperature. While not required, a bypass capacitor from the adjust pin to ground will improve ripple rejection and transient response. A 0.1 µF tantalum capacitor is recommended for “first cut” design. Type and value may be varied to obtain optimum performance vs. price. Stability Considerations The output compensation capacitor helps to deter- mine three main characteristics of a linear regula- tor's performance: start-up delay, load transient re- sponse, and loop stability. The capacitor value and type is based on cost, availability, size and tempera- ture constraints. A tantalum or aluminum electrolytic capacitor is preferred, as a film or ceramic capacitor with almost zero ESR can cause instability. An alu- minum electrolytic capacitor is the least expensive type, but when the circuit operates at low tempera- tures, both the value and ESR of the capacitor will vary widely. For optimum performance over the full operating temperature range, a tantalum capacitor is best. A 22µF tantalum capacitor will work fine in most applications, but with high current regulators such as the GM1117 higher capacitance values will improve the transient response and stability. Most ap- plications for the GM1117's involve large changes in load current, so the output capacitor must supply in- stantaneous load current. The ESR of the output ca- pacitor causes an immediate drop in output voltage given by: In microprocessor applications an output capacitor network of several tantalum and ceramic capacitors in parallel is commonly used. This reduces overall ESR and minimizes the instantaneous output volt- age drop under transient load conditions. The output capacitor network should be placed as close to the load as possible for the best results. Used with large output capacitance values and the input voltage is instantaneously shorted to ground, damage can occur. In this case, a diode connected as shown above in Figure 1. 6 V = V X ( ) + I X R2 OUT REF Adj R1 + R2 R1 DV = I x ESR D VREF IAdj CAdj R1 R2 C2 C1 VIN VOUT VIN VOUT Adj GM1117 Figure 1. Resistor Divider Scheme A A MICROELECTRONICS GM1117 1.0A LOW DROPOUT PRECISION LINEAR REGULATORS Power Management |
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