CS5208-1 Datasheet, PDF (6/8 Pages) Cherry Semiconductor Corporation – 8A LDO 3-Pin Adjustable Linear Regulator

CS5208−1
STABILITY CONSIDERATIONS
The output compensation capacitor helps determine three
main characteristics of a linear regulator: 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 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 300 μF tantalum capacitor will work for most
applications, but with high current regulators such as the
CS5208−1 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.
Protection Diodes
When large external capacitors are used with a linear
regulator 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 capacitor, the
output voltage and the rate at which V_IN drops. In the
CS5208−1 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,
damage can occur. In this case, a diode connected as shown
in Figure 13 is recommended.
A rule of thumb useful in determining if a protection diode
is required is to solve for current
where:
I+^C _T ^V
I is the current flow out of the load capacitance when V_IN
is shorted,
C is the value of the load capacitance,
V is the output voltage, and
T is the time duaration required for V_IN to transition from
high to being shorted.
If the calculated current is greater than or equal to the
typical short circuit current value provided in the
specifications, serious thought should be given to including
a protection diode.
V_IN
V_OUT
CS5208−1
Adj
Figure 13.
Current Limit
The internal current limit circuit limits the output current
under excessive load conditions and protects the regulator.
Short Circuit Protection
The device includes foldback short circuit current limit
that clamps the output current at approximately two amperes
less than its current limit value.
Thermal Shutdown
The thermal shutdown circuitry is guaranteed by design to
become activated above a die junction temperature of 150°C
and to shut down the regulator output. This circuitry
includes a thermal hysteresis circuit with 25°C of typical
hysteresis, thereby allowing the regulator to recover from a
thermal fault automatically.
Calculating Power Dissipation and Heat Sink
Requirements
High power regulators such as the CS5208−1 usually
operate at high junction temperatures. Therefore, it is
important to calculate the power dissipation and junction
temperatures accurately to ensure that an adequate heat sink
is used. Since the package tab is connected to V_OUT on the
CS5208−1, electrical isolation may be required for some
applications. Also, as with all high power packages, thermal
compound is necessary to ensure proper heat flow. For
added safety, this high current LDO includes an internal
thermal shutdown circuit
The thermal characteristics of an IC depend on the
following four factors. Junction temperature, ambient
temperature, die power dissipation, and the thermal
resistance from the die junction to ambient air. The
maximum junction temperature can be determined by:
TJ(max) + TA(max) ) PD(max) R_QJA
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