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CS5253-1GDP5

3.0 A LDO 5−Pin Adjustable Linear Regulator

ON Semiconductor 

CS5253−1

External Supply

External

Supply

VCONTROL VSENSE

CS5253−1

VPOWER

VOUT

VADJ

Figure 20. This Circuit Is an Example of How the

CS5253−1 Can Be Short−Circuit Protected When

Operating With VOUT > 6.0 V

Stability Considerations

The output compensation capacitor helps determine three

main characteristics of a linear regulator: loop stability, startup

delay, and load transient response. Different capacitor types

vary widely in tolerance, Equivalent Series Resistance (ESR),

Equivalent Series Inductance (ESI), and variation over

temperature. Tantalum and aluminum electrolytic capacitors

work best, with electrolytic capacitors being less expensive in

general, but varying more in capacitor value and ESR over

temperature.

The CS5253−1 requires an output capacitor to guarantee

loop stability. The Stability vs ESR graph in the typical

performance section shows the minimum ESR needed to

guarantee stability, but under ideal conditions. These include:

having VOUT connected to VSENSE directly at the IC pins; the

compensation capacitor located right at the pins with a

minimum lead length; the adjust feedback resistor divider

ground, (bottom of R2 in Figure 19), connected right at the

capacitor ground; and with power supply decoupling

capacitors located close to the IC pins. The actual

performance will vary greatly with board layout for each

application. In particular, the use of the remote sensing feature

will require a larger capacitor with less ESR. For most

applications, a minimum of 33 mF tantalum or 150 mF

aluminum electrolytic, with an ESR less than 1.0 W over

temperature, is recommended. Larger capacitors and lower

ESR will improve stability.

The load transient response, during the time it takes the

regulator to respond, is also determined by the output

capacitor. For large changes in load current, the ESR of the

output capacitor causes an immediate drop in output voltage

given by:

DV + DI ESR

There is then an additional drop in output voltage given

by:

DV + DI TńC

where T is the time for the regulation loop to begin to respond.

The very fast transient response time of the CS5253−1 allows

the ESR effect to dominate. 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 transient load conditions. The output

capacitor network should be as close to the load as possible

for the best transient response.

Protection Diodes

When large external capacitors are used with a linear

regulator, 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 capacitor, the

output voltage, and the rate at which VCONTROL drops. In the

CS5253−1 regulator, 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 Figure 21 is recommended.

VCONTROL

VOUT

CS5253−1

VPOWER

VSENSE

Adjust

Figure 21. Diode Protection Circuit

A rule of thumb useful in determining if a protection diode

is required is to solve for current:

I+C T V

where:

I is the current flow out of the load capacitance when

VCONTROL is shorted,

C is the value of load capacitance

V is the output voltage, and

T is the time duration required for VCONTROL to transition

from high to being shorted.

If the calculated current is greater than or equal to the

typical short circuit current value provided in the

specifications, serious thought should be given to the use of

a protection diode.

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