CS5258-1
Application Notes: continued
While not required, a bypass capacitor connected between
the adjust pin and ground will improve transient response
and ripple rejection. A 0.1µF tantalum capacitor is recom-
mended for Òfirst cutÓ design. Value and type may be var-
ied to optimize performance vs. price.
Other Adjustable Operation Considerations
The CS5258-1 linear regulator has an absolute maximum
specification of 6V for the voltage difference between V
IN
and V
OUT
. However, the IC may be used to regulate volt-
ages in excess of 6V. The two main considerations in such a
design are the sequencing of power supplies and short cir-
cuit capability.
Power supply sequencing should be such that the V
CON-
TROL
supply is brought up coincidentally with or before the
V
POWER
supply. This allows the IC to begin charging the
output capacitor as soon as the V
POWER
to V
OUT
differential
is large enough that the pass transistor conducts. As V
POW-
ER
increases, the pass transistor will remain in dropout, and
current is passed to the load until V
OUT
is in regulation.
Further increase in the supply voltage brings the pass tran-
sistor out of dropout. In this manner, any output voltage
less than 13V may be regulated, provided the V
POWER
to
V
OUT
differential is less than 6V. In the case where V
CON-
TROL
and V
POWER
are shorted, there is no theoretical limit to
the regulated voltage as long as the V
POWER
to V
OUT
differ-
ential of 6V is not exceeded.
There is a possibility of damaging the IC when V
POWER
-V
IN
is greater than 6V if a short circuit occurs. Short circuit con-
ditions will result in the immediate operation of the pass
transistor outside of its safe operating area. Over-voltage
stresses will then cause destruction of the pass transistor
before overcurrent or thermal shutdown circuitry can
become active. Additional circuitry may be required to
clamp the V
POWER
to V
OUT
differential to less than 6V if fail
safe operation is required. One possible clamp circuit is
illustrated in Figure 2; however, the design of clamp cir-
cuitry must be done on an application by application basis.
Care must be taken to ensure the clamp actually protects
the design. Components used in the clamp design must be
able to withstand the short circuit condition indefinitely
while protecting the IC.
External Supply
The capacitor value and type is based on cost, availability,
size and temperature constraints. A tantalum or aluminum
electrolytic capacitor is best, since a film or ceramic capaci-
tor with almost zero ESR can cause instability. The
aluminum electrolytic capacitor is the least expensive solu-
tion. However, when the circuit operates at low tempera-
tures, both the value and ESR of the capacitor will vary
considerably. The capacitor manufacturerÕs data sheet pro-
vides this information.
A 300µF tantalum capacitor will work for most applica-
tions, but with high current regulators such as the
CS5258-1 the transient response and stability improve with
higher values of capacitor. The majority of applications for
this regulator involve large changes in load current so the
output capacitor must supply the instantaneous load cur-
rent. The ESR of the output capacitor causes an immediate
drop in output voltage given by:
ÆV = ÆI
´
ESR.
For microprocessor applications it is customary to use an
output capacitor network consisting of several tantalum
and ceramic capacitors in parallel. This reduces the overall
ESR and reduces the instantaneous output voltage drop
under transient load conditions. The output capacitor net-
work should be as close to the load as possible for the best
results.
Protection Diodes
When large external capacitors are used with a linear regu-
lator it is sometimes necessary to add protection diodes. If
the input voltage of the regulator gets shorted, the output
capacitor will discharge into the output of the regulator.
The discharge current depends on the value of the capaci-
tor, the output voltage, and the rate at which V
CONTROL
drops. In the CS5258-1 regulator, the discharge path is
through a large junction and protection diodes are not usu-
ally needed. If the regulator is used with large values of
output capacitance and the input voltage is instantaneously
shorted to ground, damage can occur. In this case, a diode
connected as shown in Figure 3 is recommended. Use of
the diode has the added benefit of bleeding V
OUT
to
ground if V
CONTROL
is shorted. This prevents an unregulat-
ed output from causing system damage.
V
CONTROL
V
Control
V
Power
V
Adjust
V
SENSE
V
OUT
V
OUT
CS5258-1
V
POWER
V
SENSE
Adjust
Figure 2: Example clamp circuitry for V
POWER
-
V
OUT
> 6V.
Stability Considerations
The output compensation capacitor helps determine three
main characteristics of a linear regulator: start-up delay,
load transient response, and loop stability.
7
Figure 3: Diode protection against V
CONTROL
short circuit conditions.
CHERRY [ CHERRY SEMICONDUCTOR CORPORATION ]
