5

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

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.

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-

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 2a. Protection diode scheme for adjustable output regulator.

Figure 2b. Protection diode scheme for fixed output regulators.

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.

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

resistance between the regulator and the load becomes

()

Figure 4. Grounding scheme for the adjustable output regulator to min-

imize parasitics.

conductor parasitic

resistance

CS5205-1

Adj

R1 + R2

R1

conductor parasitic

resistance

CS5205-X

Gnd

Output Voltage Sensing

CS5205-X

Gnd

IN4002

(optional)

CS5205-1

Adj

IN4002

(optional)

Protection Diodes

Stability Considerations

Applications Information: continued