CS52015-1 データシートの表示（PDF） - ON Semiconductor

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CS52015-1

1.5 A Adjustable Linear Regulator

ON Semiconductor 

CS52015−1

R1 is chosen so that the minimum load current is at least

2.0 mA. R1 and R2 should be the same type, e.g. metal film

for best tracking over temperature. While not required, a

bypass capacitor from the adjust pin to ground will improve

ripple rejection and transient response. A 0.1 mF tantalum

capacitor is recommended for “first cut” design. Type and

value may be varied to obtain optimum performance vs.

price.

VIN

VIN

VOUT

VOUT

CS52015−1

C1

VREF

C2

Adj

R1

IAdj

R2

CAdj

Figure 11. Resistor Divider Scheme

Short Circuit Protection

The CS52015−1 linear regulator has an absolute

maximum specification of 7.0 V for the voltage difference

between VIN and VOUT. However, the IC may be used to

regulate voltages in excess of 7.0 V. The main considerations

in such a design are power−up and short circuit capability.

In most applications, ramp−up of the power supply to VIN

is fairly slow, typically on the order of several tens of

milliseconds, while the regulator responds in less than one

microsecond. In this case, the linear regulator begins

charging the load as soon as the VIN to VOUT differential is

large enough that the pass transistor conducts current. The

load at this point is essentially at ground, and the supply

voltage is on the order of several hundred millivolts, with the

result that the pass transistor is in dropout. As the supply to

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

current is passed to the load until VOUT reaches the point at

which the IC is in regulation. Further increase in the supply

voltage brings the pass transistor out of dropout. The result

is that the output voltage follows the power supply ramp−up,

staying in dropout until the regulation point is reached. In

this manner, any output voltage may be regulated. There is

no theoretical limit to the regulated voltage as long as the

VIN to VOUT differential of 7.0 V is not exceeded.

However, the possibility of destroying the IC in a short

circuit condition is very real for this type of design. Short

circuit conditions 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 VIN to VOUT differential to less than 7.0 V if

fail−safe operation is required. One possible clamp circuit is

illustrated in Figure 12; however, the design of clamp

circuitry 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

VIN

VOUT

VAdj

VOUT

Figure 12. Short Circuit Protection Circuit for

High Voltage Application.

Stability Considerations

The output 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

capacitor with almost zero ESR can cause instability. The

aluminum 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 considerably. The capacitor manufacturer’s data sheet

provides this information.

A 22 mF tantalum capacitor will work for most

applications, but with high current regulators such as the

CS52015−1 the transient response and stability improve

with higher values of capacitance. The majority of

applications for this regulator 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:

DV + DI 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 network

should be as close as possible to the load for the best results.

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