Application Notes: continued
EXTERNAL SUPPLY
VIN
CS5207A-1
Adj
VOUT
R1
R2
VIN
VOUT
VAdj
Figure 1.
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.
Figure 2.
Stability Considerations
The output compensation capacitor helps determine three
main characteristics of a linear regulator: start-up delay,
load transient response, and loop stability.
Other Adjustable Operation Considerations
The CS5207A-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 volt-
ages in excess of 6V. The main considerations in such a
design are power-up and short circuit capability.
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 alu-
minum 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 manufacturerÕs data sheet provides this
information.
In most applications, ramp-up of the power supply to VIN
is fairly slow, typically on the order of several tens of mil-
liseconds, while the regulator responds in less than one
microsecond. In this case, the linear regulator begins charg-
ing the output capacitor as soon as the VIN to VOUT differ-
ential is large enough that the pass transistor conducts cur-
rent. VOUT is essentially at ground, and VIN is on the order
of several hundred millivolts, so the pass transistor is in
dropout. As VIN increases, the pass transistor will remain
in dropout, and current is passed to the load until VOUT is
in regulation. Further increase in VIN brings the pass tran-
sistor out of dropout. The result is that the output voltage
follows the power supply ramp-up, staying in dropout
until the regulation point is reached. In this manner, any
output voltage may be regulated. There is no theoretical
limit to the regulated voltage as long as the VIN to VOUT
differential of 6V is not exceeded.
A 300µF tantalum capacitor will work for most applica-
tions, but with high current regulators such as the
CS5207A-1 the transient response and stability improve
with higher values of capacitance. The majority of applica-
tions for this regulator involve large changes in load cur-
rent 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:
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.
However, maximum ratings of the IC will be exceeded in a
short circuit condition. Short circuit conditions 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.
Protection Diodes
Additional circuitry may be required to clamp VIN to VOUT
differential to less than 6V if failsafe operation is required.
One possible clamp circuit is illustrated below; however,
the design of clamp circuitry must be done on an applica-
tion 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 cir-
cuit conditions indefinitely while protecting the IC.
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 VIN drops. In
the CS5207A-1 regulator, the discharge path is through a
large junction and protection diodes are not usually need-
ed. If the regulator is used with large values of output
capacitance and the input voltage is instantaneously short-
ed to ground, damage can occur. In this case, a diode con-
nected as shown in Figure 3 is recommended.
5
CHERRY [ CHERRY SEMICONDUCTOR CORPORATION ]
