CS5257A-1GDP5 查看數據表（PDF） - Cherry semiconductor

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CS5257A-1GDP5

7A LDO 5-Pin Adjustable Linear Regulator

Cherry semiconductor 

Application Notes: continued

While not required, a bypass capacitor connected between

the adjust pin and ground will improve transient response

and ripple rejection. A 0.1µF tantalum capacitor is recom-

mended for Òfirst cutÓ design. Value and type may be var-

ied to optimize performance vs. price.

Other Adjustable Operation Considerations

The CS5257A-1 linear regulator has an absolute maximum

specification of 6V for the voltage difference between VIN

and VOUT. However, the IC may be used to regulate volt-

ages in excess of 6V. The two main considerations in such a

design are the sequencing of power supplies and short cir-

cuit capability.

Power supply sequencing should be such that the VCON-

TROL supply is brought up coincidentally with or before the

VPOWER supply. This allows the IC to begin charging the

output capacitor as soon as the VPOWER to VOUT differential

is large enough that the pass transistor conducts. As VPOW-

ER increases, the pass transistor will remain in dropout, and

current is passed to the load until VOUT is in regulation.

Further increase in the supply voltage brings the pass tran-

sistor out of dropout. In this manner, any output voltage

less than 13V may be regulated, provided the VPOWER to

VOUT differential is less than 6V. In the case where VCON-

TROL and VPOWER are shorted, there is no theoretical limit

to the regulated voltage as long as the VPOWER to VOUT dif-

ferential of 6V is not exceeded.

There is a possibility of damaging the IC when VPOWER-VIN

is greater than 6V if a short circuit occurs. Short circuit con-

ditions will result in the immediate operation of the pass

transistor outside of its safe operating area. Over-voltage

stresses will then cause destruction of the pass transistor

before overcurrent or thermal shutdown circuitry can

become active. Additional circuitry may be required to

clamp the VPOWER to VOUT differential to less than 6V if fail

safe operation is required. One possible clamp circuit is

illustrated in Figure 2; however, the design of clamp cir-

cuitry must be done on an application by application basis.

Care must be taken to ensure the clamp actually protects

the design. Components used in the clamp design must be

able to withstand the short circuit condition indefinitely

while protecting the IC.

External Supply

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 solution.

However, when the circuit operates at low temperatures,

both the value and ESR of the capacitor will vary consider-

ably. The capacitor manufacturer's data sheet provides this

information.

A 300µF tantalum capacitor will work for most applica-

tions, but with high current regulators such as the

CS5257A-1 the transient response and stability improve

with higher values of capacitor. The majority of applica-

tions for this regulator involve large changes in load cur-

rent 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 transient load conditions. The output capacitor net-

work should be as close to the load as possible for the best

results.

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 CS5257A-1 regulator, 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 Figure 3 is recommended.

Use of the diode has the added benefit of bleeding VOUT to

ground if VCONTROL is shorted. This prevents an unregulat-

ed output from causing system damage.

VControl

VSENSE

VPower

VAdjust

VOUT

VCONTROL VOUT

CS5257A-1

VPOWER VSENSE

Adjust

Figure 2: Example clamp circuitry for VPOWER - VOUT > 6V.

Stability Considerations

The output compensation capacitor helps determine three

main characteristics of a linear regulator: start-up delay,

load transient response, and loop stability.

Figure 3: Diode protection against VCONTROL short circuit conditions.

7

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