5
GM6605
APPLICATION INFORMATION
The GM6605 series linear regulators provide fixed and
adjustable output voltages at currents up to 5.0A.
These regulators are protected against overcurrent con-
ditions and include thermal shutdown protection. The
GM6605's have a composite PNP-NPN output transis-
tor and require an output capacitor for stability.
A detailed procedure for selecting this capacitor fol-
lows.
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 best. A 22µF tantalum capacitor will
work fine in most applications, but with high current reg-
ulators such as the GM6605 higher capacitance values
will improve the transient response and stability. Most
applications for the GM6605'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
given by:
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.
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.
In the 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,
damage can occur. In this case, a diode connected
as shown above in Figure 1.
DV = I x ESD
D
(a),(b)
Figure 1
Protection Diode Scheme for
Large Output Capacitors
(a) Fixed Version
(b) Adjustable Version
V
IN
V
OUT
C
ADJ
GM6605-Adj
ADJ
V
IN
V
OUT
C1
C2
R1
R2
GM6605-3.3
GND
V
IN
V
OUT
C1
IN4002
C2
V
IN
V
OUT
IN4002
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