Application Notes: continued
If the calculated current is greater than or equal to the typi-
Stability Considerations
cal short circuit current valued provided in the specifica-
tions, serious thought should be given to include a protec-
tion diode.
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 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.
VIN
VOUT
CS5210-1
Adj
A 300µF tantalum capacitor will work for most applica-
tions, but with high current regulators such as the CS5210
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 out-
put capacitor must supply the instantaneous load current.
The ESR of the output capacitor causes an immediate drop
in output voltage given by:
Figure 3:
ÆV = ÆI ´ ESR.
Current Limit
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.
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.
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 VIN drops. In
the CS5210-1 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.
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 circuit-
ry 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
A rule of thumb useful in determining if a protection diode
is required is to solve for current
High power regulators such as the CS5210-1 usually oper-
ate at high junction temperatures. Therefore, it is important
to calculate the power dissipation and junction tempera-
tures accurately to ensure that an adequate heat sink is
used. Since the package tab is connected to Vout on the
CS5210-1, electrical isolation may be required for some
applications. Also, as with all high power packages, ther-
mal compound in necessary to ensure proper heat flow.
For added safety, this high current LDO includes an inter-
nal thermal shutdown circuit.
C ´V
T
I=
, where
I
is the current flow out of the load capacitance
when VIN is shorted,
C
V
T
is the value of load capacitance
is the output voltage, and
The thermal characteristics of an IC depend on the follow-
ing four factors. Junction temperature, ambient tempera-
ture, die power dissipation, and the thermal resistance
is the time duration required for VIN to transition
from high to being shorted.
6
CHERRY [ CHERRY SEMICONDUCTOR CORPORATION ]
