CS5257A-1GDP5 Просмотр технического описания (PDF) - ON Semiconductor

Номер в каталоге

Компоненты Описание

производитель

CS5257A-1GDP5

7.0 A LDO 5−Pin Adjustable Linear Regulator

ON Semiconductor 

CS5257A−1

DESIGN GUIDELINES

Adjustable Operation

This LDO adjustable regulator has an output voltage

range of 1.25 V to 5.0 V. An external resistor divider sets the

output voltage as shown in Figure 16. The regulator’s

voltage sensing error amplifier maintains a fixed 1.253 V

reference between the output pin and the adjust pin.

VCONTROL

VOUT

CS5257A−1

VPOWER

VSENSE

Adjust

R1

R2

Figure 16. An External Resistor Divider Sets the

Value of VOUT. The 1.253 V Reference Voltage

Drops Across R1.

A resistor divider network R1 and R2 causes a fixed

current to flow to ground. This current creates a voltage

across R2 that adds to the 1.253 V across R1 and sets the

overall output voltage. The adjust pin current (typically

50 mA) also flows through R2 and adds a small error that

should be taken into account if precise adjustment of VOUT

is necessary. The output voltage is set according to the

formula:

VOUT + 1.253 V

R1

)

R1

R2

)

R2

IADJ

The term IADJ × R2 represents the error added by the

adjust pin current. R1 is chosen so that the minimum load

current is a least 10 mA. R1 and R2 should be of the same

composition for best tracking over temperature. The divider

resistors should be placed physically as close to the load as

possible.

While not required, a bypass capacitor connected between

the adjust pin and ground will improve transient response

and ripple rejection. A 0.1 mF tantalum capacitor is

recommended for “first cut” design. Value and type may be

varied to optimize performance vs. price.

Other Adjustable Operation Considerations

The CS5257A−1 linear regulator has an absolute

maximum specification of 6.0 V for the voltage difference

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

regulate voltages in excess of 6.0 V. The two main

considerations in such a design are the sequencing of power

supplies and short circuit capability.

Power supply sequencing should be such that the

VCONTROL 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 VPOWER 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 transistor out of dropout. In this manner, any

output voltage less than 13 V may be regulated, provided the

VPOWER to VOUT differential is less than 6.0 V. In the case

where VCONTROL and VPOWER are shorted, there is no

theoretical limit to the regulated voltage as long as the

VPOWER to VOUT differential of 6.0 V is not exceeded.

There is a possibility of damaging the IC when VPOWER

− VIN is greater than 6.0 V if a short circuit occurs. Short

circuit conditions will result in the immediate operation of

the pass transistor outside of its safe operating area.

Overvoltage 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 6.0 V

if fail safe operation is required. One possible clamp circuit

is illustrated in Figure 17; 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

VCONTROL VSENSE

VPOWER

VADJ

VOUT

Figure 17. Example Clamp Circuitry for

VPOWER − VOUT > 6.0 V

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

http://onsemi.com

7

Share Link: