Applications Information: continued
Stability Considerations
The output or compensation capacitor helps determine
three main characteristics of a linear regulator: start-up
delay, load transient response and loop stability.
IN4002 (optional)
VOUT
VIN
VOUT
VIN
C1
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 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.
CS5206-X
C2
Gnd
Figure 2b. Protection diode scheme for fixed output regulators.
Output Voltage Sensing
A 22µF tantalum capacitor will work for most applications,
but with high current regulators such as the CS5206 -X 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:
Since the CS5206 -X 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. For best results the fixed reg-
ulators should be connected as shown in Figure 3.
ÆV = ÆI ´ ESR
conductor parasitic
resistance
R
C
VIN
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.
VOUT
VIN
CS5206-X
R
LOAD
Gnd
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 CS5206-X family of linear regulators, 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 Figures 2a and 2b
is recommended.
Figure 3. Conductor parasitic resistance can be minimized with the
above grounding scheme for fixed output regulators.
For the adjustable regulator, the best load regulation
occurs when R1 is connected directly to the output pin of
the regulator as shown in Figure 4. If R1 is connected to the
load, RC is multiplied by the divider ratio and the effective
resistance between the regulator and the load becomes
R1 + R2
RC ´
(
)
R1
RC = conductor parasitic resistance
conductor parasitic
resistance
R
C
VIN
VOUT
VIN
IN4002 (optional)
VIN
VOUT
VOUT
VIN
CS5206-1
R
LOAD
R
C1
1
CS5206-1
Adj
R
C2
1
Adj
R
2
R
2
CAdj
Figure 4. Grounding scheme for the adjustable output regulator to min-
imize parasitics.
Figure 2a. Protection diode scheme for adjustable output regulator.
5
CHERRY [ CHERRY SEMICONDUCTOR CORPORATION ]
