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 varied with almost zero ESR can cause instability. The aluminum
to optimize performance vs. price.
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 capacitor
electrolytic capacitor is the least expensive solution.
However, when the circuit operates at low temperatures,
both the value and ESR of the capacitor will vary consider-
ably. The capacitor manufacturers data sheet provides this
information.
Other Adjustable Operation Considerations
The CS52510-1 linear regulator has an absolute maximum
specification of 6V for the voltage difference between VIN
and VOUT. However, the IC may be used to regulate voltages
in excess of 6V. The two main considerations in such a
design are the sequencing of power supplies and short cir-
cuit capability.
A 300µF tantalum capacitor will work for most applications,
but with high current regulators such as the CS52510-1 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 current. The ESR of the
output capacitor causes an immediate drop in output voltage
given by:
Power supply sequencing should be such that the VCONTROL
supply is brought up coincidentally with or before the VPOW-
ER supply. This allows the IC to begin charging output capac-
itor as soon as the VPOWER to VOUT differential is large
enough that the pass transistor conducts. As VPOWER increas-
es, the pass transistor will remain in dropout, and current is
passed to the load until VOUT is in regulation. Further
increase in the supply voltage brings the pass transistor out
of dropout. In this manner, any output voltage less than 13V
may be regulated, provided the VPOWER to VOUT differential
is less than 6V. In the case where VCONTROL and VPOWER are
shorted, there is no theoretical limit to the regulated voltage
as long as the VPOWER to VOUT differential of 6V is not
exceeded.
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 network
should be as close to the load as possible for the best results.
Protection Diodes
There is a possibility of damaging the IC when VPOWER-VIN
is greater than 6V if a short circuit occurs. Short circuit condi-
tions will result in the immediate operation of the pass tran-
sistor outside of its safe operating area. Over-voltage stresses
will then cause destruction of the pass transistor before over-
current or thermal shutdown circuitry can become active.
Additional circuitry may be required to clamp the VPOWER to
VOUT 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 circuitry 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.
When large external capacitors are used with a linear regula-
tor it is sometimes necessary to add protection diodes. If the
input voltage of the regulator gets shorted, the output capac-
itor will discharge into the output of the regulator. The dis-
charge current depends on the value of the capacitor, the
output voltage, and the rate at which VCONTROL drops. In the
CS52510-1 regulator, the discharge path is through a large
junction and protection diodes are not usually needed. If the
regulator is used with large values of output capacitance and
the input voltage is instantaneously shorted to ground, dam-
age can occur. In this case, a diode connected as shown in
Figure 3 is recommended. Use of this diode has the added
benefit of bleeding VOUT to ground if VCONTROL is shorted.
This prevents an unregulated output from causing system
damage.
External Supply
VCONTROL VOUT
VControl
VPower
CS52510-1
VSENSE
VOUT
VPOWER
Adjust
VSENSE
VAdjust
Figure 2: Example clamp circuitry for VPOWER - VOUT > 6V.
Figure 3: Diode protection against VCONTROL short circuit conditions.
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
CHERRY [ CHERRY SEMICONDUCTOR CORPORATION ]
