CS5205A-1
Applications Information
The CS5205A-1 linear regulator provides an adjustable
voltage at currents up to 5A. The regulator is protected
against short circuit, and include thermal shutdown and
safe area protection (SOA) circuitry. The SOA protection
circuitry decreases the maximum available output current
as the input-output differential voltage increases.
The CS5205A-1 has a composite PNP-NPN output transis-
tor and requires an output capacitor for stability. A
detailed procedure for selecting this capacitor is included
in the Stability Considerations section.
Adjustable Operation
The adjustable regulator (CS5205A-1) has an output volt-
age range of 1.25V to 13V. An external resistor divider sets
the output voltage as shown in Figure 1. The regulator
maintains a fixed 1.25V (typical) reference between the
output pin and the adjust pin.
A resistor divider network R1 and R2 causes a fixed cur-
rent to flow to ground. This current creates a voltage
across R2 that adds to the 1.25V across R1 and sets the
overall output voltage. The adjust pin current (typically
50µA) also flows through R2 and adds a small error that
should be taken into account if precise adjustment of V
OUT
is necessary.
The output voltage is set according to the formula:
V
OUT
= V
REF
´
R1 + R2 + I
Adj
´
R2
R1
The term I
Adj
´
R2 represents the error added by the adjust
pin current.
R1 is chosen so that the minimum load current is at least
10mA. R1 and R2 should be the same type, e.g. metal film
for best tracking over temperature. The adjust pin is
bypassed to improve the transient response and ripple
rejection of the regulator.
tor with almost zero ESR, can cause instability. The alu-
minum electrolytic capacitor is the least expensive solu-
tion. However, when the circuit operates at low tempera-
tures, both the value and ESR of the capacitor will vary
considerably. The capacitor manufacturers data sheet pro-
vides this information.
A 22µF tantalum capacitor will work for most applications,
but with high current regulators such as the CS5205A-1 the
transient response and stability improve with higher val-
ues of capacitor. The majority of applications for this regu-
lator 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:
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 load transient conditions. The output capacitor net-
work should be as close as possible to the load for the best
results.
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 V
IN
drops. In
the CS5205A-1 linear regulator, the discharge path is
through a large junction and protection diodes are not usu-
ally needed. If the regulator is used with large values of
output capacitance and the input voltage is instantaneous-
ly shorted to ground, damage can occur. In this case, a
diode connected as shown in Figure2 is recommended.
V
IN
C
1
V
IN
V
OUT
V
OUT
V
IN
V
REF
R
1
C
2
C
1
V
IN
IN4002
(optional)
V
OUT
V
OUT
CS5205A-1
Adj
CS5205A-1
R
1
Adj
C
2
I
Adj
C
Adj
R
2
C
Adj
R
2
Figure 1. Resistor divider scheme for the adjustable version.
Figure 2. Protection diode scheme for adjustable output regulator.
Stability Considerations
Output Voltage Sensing
The output or 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-
4
Since the CS5205A-1 is a three terminal regulator, it is not
possible to provide true remote load sensing. Load regula-
tion is limited by the resistance of the conductors connect-
ing the regulator to the load.
CHERRY [ CHERRY SEMICONDUCTOR CORPORATION ]
