APPLICATION INFORMATION
GM6605 series linear regulators provide fixed and ad-
justable output voltages at currents up to 5.0A. These
regulators are protected against over-current condi-
tions and include thermal shutdown protection. The
GM6605 has a composite PNP-NPN output transistor
and require an output capacitor for stability.
A detailed procedure for selecting this capacitor is as
followed.
Protection Diodes
When large external capacitors are used with most
linear regulator, it is wise to add protection diodes. If
the input voltage of the regulator is shorted, the out-
put capacitor will discharge into the output of the reg-
ulator. The discharge current depends on the value
of capacitor, output voltage, and rate at which V
IN
drops.
Figure 1
(a),(b)
Stability Considerations
The output compensation capacitor helps to determine
three main characteristics of a linear regulator's perfor-
mance: start-up delay, load transient response, and
loop stability. The capacitor value and type is based on
cost, availability, size, and temperature constraints. A
tantalum or aluminum electrolytic capacitor is pre-
ferred, as a film or ceramic capacitor with almost zero
ESR can cause instability. An aluminum electrolytic ca-
pacitor is the least expensive type. But when the circuit
operates at low temperatures, both the value and ESR
of the capacitor will vary widely. For optimum perfor-
mance over the full operating temperature range, a tan-
talum capacitor is the best. A 22µF tantalum capacitor
will work fine in most applications. But with high current
regulators, such as GM6605 higher capacitance values
will improve the transient response and stability. Most
applications for Gm6605 involve large changes in load
current, so the output capacitor must supply instanta-
neous load current. The ESR of the output capacitor
causes an immediate drop in output voltage given by:
C1
V
IN
C1
Protection Diode Scheme for
Large Output Capacitors
(a) Fixed Version
IN4002
V
IN
V
IN
V
OUT
V
OUT
GM6605-3.3
GND
C2
(b) Adjustable Version
V
IN
IN4002
V
OUT
V
OUT
GM6605-Adj
ADJ
R1
C
ADJ
C2
R2
DV
=
D
I x ESD
In microprocessor applications an output capacitor net-
work of several tantalum and ceramic capacitors in par-
allel 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 for
the best results.
In GM6605 linear regulators, the discharge path is
through a large junction, and protection diodes are
normally not needed. However, if the regulator is
used with large output capacitance values and the in-
put voltage is instantaneously shorted to ground,
as shown above in Figure 1.
damage can occur. In this case, a diode connected
GM6605
5
GAMMA [ GAMMA MICROELECTRONICS INC. ]
