CS5205-1,-3,-5
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.
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.
A 22µF tantalum capacitor will work for most applications,
but with high current regulators such as the CS5205-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:
ÆV = ÆI
´
ESR
V
IN
IN4002
V
IN
C
1
V
IN
(optional)
V
OUT
V
OUT
CS5205-X
C
2
Gnd
Figure 2b. Protection diode scheme for fixed output regulators.
Output Voltage Sensing
Since the CS5205-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.
conductor parasitic
resistance
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 network
should be as close as possible to the load for the best results.
Protection Diodes
V
IN
V
OUT
R
C
CS5205-X
Gnd
R
LOAD
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 CS5205-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 val-
ues of output capacitance and the input voltage is instanta-
neously shorted to ground, damage can occur. In this case,
a diode connected as shown in Figures 2a and 2b is recom-
mended.
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, R
C
is multiplied by the divider ratio and the effective
resistance between the regulator and the load becomes
R
C
´
R1 + R2
R1
(
)
R
C
conductor parasitic
resistance
R
C
= conductor parasitic resistance
IN4002
V
IN
C
1
V
IN
(optional)
V
OUT
V
OUT
V
IN
V
IN
V
OUT
CS5205-1
R
1
Adj
R
LOAD
CS5205-1
R
1
Adj
C
2
R
2
C
Adj
R
2
Figure 2a. Protection diode scheme for adjustable output regulator.
Figure 4. Grounding scheme for the adjustable output regulator to min-
imize parasitics.
5
CHERRY [ CHERRY SEMICONDUCTOR CORPORATION ]
