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 VOUT
VIN
VOUT
C1
Adj
VREF
C2
R1
R2
IADJ
CADJ
R2
R1
VOUT =1.25V
(
1+
)
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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GAMMA [ GAMMA MICROELECTRONICS INC. ]
