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GM1117S-AST3R Datasheet(PDF) 10 Page - Gamma Microelectronics Inc. |
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GM1117S-AST3R Datasheet(HTML) 10 Page - Gamma Microelectronics Inc. |
10 / 15 page 10 GM1117S 1A LOW DROPOUT PRECISION VOLTAGE REGULATOR Figure 1. Resistor Divider Scheme Application Notes GM1117S linear regulators provide fixed and adjustable output voltage at current up to 1.0A. These regulators are protected against over current conditions and include thermal shutdown protection. The GM1117S has a composite PNP-NPN output transistor and require an output capacitor for stability. A detailed procedure for selecting this capacitor follows. Adjustable Operation The GM1117S has an output voltage range for 1.25V to 5.5V. An external resistor divider sets the output voltage as show 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 R1 causes a fixed current to follow to ground. This current creates a voltage across R2 that adds to the 1.25V across R1 and sets the overall output voltage. The adjust pin current (typically 50µA) also flows through R2 and adds a small error that should be taken into account if precise adjustment of VOUT is necessary. The term IADJ X R2 represents the error added by the adjust pin current. R1 is chosen such that the minimum load current is at least 2mA. R1 and R2 should be the same type, e.g., metal film for best tracking over temperature. While not require, a bypass capacitor from the adjust pin to ground will improve ripple rejection and transient response. A 0.1µF tantalum capacitor is recommended. Stability Considerations The output compensation capacitor helps to determine three feature characteristics of a linear regulator's performance: start-up delay, load transient response, and loop stability. The capacitance and type is based on cost, availability, size and temperature constraints. A tantalum or aluminum electrolytic capacitor is preferred, as a film or ceramic capacitor with almost zero ESR can cause instability. An aluminum electrolytic capacitor is the least expensive type, but when the circuit operates at low temperatures, both the capacitance and ESR 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 GM1117S higher capacitance values will improve the transient response and stability. Most applications for the GM1117S involve large changes in load current, so the output capacitor must supply instantaneous load current. The ESR of the output capacitor causes an immediate drop in output voltage given by: △ V = △I X ESR 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 voltage drop under transient load conditions. The output capacitor network should be placed as close to the load as possible. R2 VOUT =1.25V ( 1+ R1 ) VIN VOUT GM1117S VIN VOUT Adj C1 C2 R2 R1 VREF CADJ IADJ |
Similar Part No. - GM1117S-AST3R |
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Similar Description - GM1117S-AST3R |
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