CS5201-1 Datasheet, PDF (5/9 Pages) Cherry Semiconductor Corporation – 1A Adjustable Linear Regulator

CS5201−1
R1 is chosen so that the minimum load current is at least
2.0 mA. R1 and R2 should be the same type, e.g. metal film
for best tracking overtemperature. While not required, a
bypass capacitor from the adjust pin to ground will improve
ripple rejection and transient response. A 0.1 mF tantalum
capacitor is recommended for “first cut” design. Type and
value may be varied to obtain optimum performance vs.
price.
V_IN
V_IN
V_OUT
V_OUT
CS5201−1
C₁
V_REF
C₂
Adj
R₁
I_Adj
R₂
C_Adj
fail−safe operation is required. One possible clamp circuit is
illustrated in Figure 12; 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.
EXTERNAL SUPPLY
V_IN
V_OUT
V_Adj
Figure 11. Resistor Divider Scheme
Short Circuit Protection
The CS5201−1 linear regulator has an absolute maximum
specification of 7.0 V for the voltage difference between V_IN
and V_OUT. However, the IC may be used to regulate voltages
in excess of 7.0 V. The main considerations in such a design
are power−up and short circuit capability.
In most applications, ramp−up of the power supply to V_IN
is fairly slow, typically on the order of several tens of
milliseconds, while the regulator responds in less than one
microsecond. In this case, the linear regulator begins
charging the load as soon as the V_IN to V_OUT differential is
large enough that the pass transistor conducts current. The
load at this point is essentially at ground, and the supply
voltage is on the order of several hundred millivolts, with the
result that the pass transistor is in dropout. As the supply to
V_IN increases, the pass transistor will remain in dropout, and
current is passed to the load until V_OUT reaches the point at
which the IC is in regulation. Further increase in the supply
voltage brings the pass transistor 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
V_IN to V_OUT differential of 7.0 V is not exceeded.
However, the possibility of destroying the IC in a short
circuit condition is very real for this type of design. 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. Additional circuitry may be required to
clamp the V_IN to V_OUT differential to less than 7.0 V if
V_OUT
Figure 12. Short Circuit Protection Circuit for
High Voltage Application.
Stability Considerations
The output compensation capacitor helps determine three
main characteristics of a linear regulator: startup 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 best, since a film or ceramic
capacitor with almost zero ESR can cause instability. The
aluminum 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 considerably. The capacitor manufacturer’s data sheet
provides this information.
A 22 mF tantalum capacitor will work for most
applications, but with high current regulators such as the
CS5201−1 the transient response and stability improve with
higher values of capacitance. 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:
DV + DI 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.
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