Power Management
A A
MICROELECTRONICS
GM1117
1.0A LOW DROPOUT PRECISION LINEAR REGULATORS
GM1117
APPLICATION INFORMATION
V
V
V
V
IN
OUT
OUT
IN
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.
V
Adj
C
C
REF
R1
1
2
I
Adj
R2
C
Adj
Figure 1. Resistor Divider Scheme
Stability Considerations
Adjustable Operation
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
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:
R1 + R2
V
= V
X (
) + I
X R2
OUT
REF
Adj
R1
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
given by:
DV = DI 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 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.
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.
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
GAMMA [ GAMMA MICROELECTRONICS INC. ]
