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GM1117S

1A LOW DROPOUT PRECISION VOLTAGE REGULATOR

Gamma Microelectronics Inc. 

GM1117S

1A LOW DROPOUT

PRECISION VOLTAGE REGULATOR

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.

GM1117S

VIN

VIN VOUT

VOUT

C1

Adj

VREF R1

C2

VOUT

=1.25V ( 1+

R2

R1

)

IADJ

CADJ

R2

Figure 1. Resistor Divider Scheme

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.

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