Applications Information
The CS5207-1 linear regulator provides adjustable volt-
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
ages at currents up to 7A. 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 considerably. The capacitor manufacturers data sheet pro-
as the input-output differential voltage increases.
vides this information.
The CS5207-1 has a composite PNP-NPN output transistor
and requires an output capacitor for stability. A detailed
procedure for selecting this capacitor is included in the
Stability Considerations section.
A 22µF tantalum capacitor will work for most applications,
but with high current regulators such as the CS5207-1 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:
Adjustable Operation
The adjustable regulator has an output voltage 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.
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.
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 VOUT
is necessary.
Protection Diodes
The output voltage is set according to the formula:
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 CS5207-1 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 2 is recommended.
R1 + R2
VOUT = VREF
´
+ IAdj ´ R2
(
)
R1
The term IAdj ´ 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.
VIN
VOUT
VOUT
VIN
IN4002 (optional)
C1
CS5207-1
VREF
VIN
VOUT
VOUT
VIN
R
C2
1
C1
CS5207-1
Adj
R
C2
1
IAdj
Adj
R
2
CAdj
R
2
CAdj
Figure 1. Resistor divider scheme for the adjustable version.
Stability Considerations
Figure 2. Protection diode scheme for adjustable output regulator.
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.
Since the CS5207-1 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.
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
CHERRY [ CHERRY SEMICONDUCTOR CORPORATION ]
