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

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 300 mF tantalum capacitor will work for most

applications, but with high current regulators such as the

CS5257A−1 the transient response and stability improve

with higher values of capacitor. 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 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 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 18 is recommended.

VCONTROL

VOUT

CS5257A−1

VPOWER

VSENSE

Adjust

Figure 18. Diode Protection Against VCONTROL

Short Circuit Conditions

Use of the diode has the added benefit of bleeding VOUT

to ground if VCONTROL is shorted. This prevents an

unregulated output from causing system damage.

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.

Current Limit

The internal current limit circuit limits the output current

under excessive load conditions.

Short Circuit Protection

The device includes short circuit protection circuitry that

clamps the output current at approximately two amperes less

than its current limit value. This provides for a current

foldback function, which reduces power dissipation under a

direct shorted load.

Thermal Shutdown

The thermal shutdown circuitry is guaranteed by design to

activate above a die junction temperature of approximately

150°C and to shut down the regulator output. This circuitry

has 25°C of typical hysteresis, thereby allowing the

regulator to recover from a thermal fault automatically.

Calculating Power Dissipation and

Heat Sink Requirements

High power regulators such as the CS5257A−1 usually

operate at high junction temperatures. Therefore, it is

important to calculate the power dissipation and junction

temperatures accurately to ensure that an adequate heat sink

is used. Since the package tab is connected to VOUT on the

CS5257A−1, electrical isolation may be required for some

applications. Also, as with all high power packages, thermal

compound in necessary to ensure proper heat flow. For

added safety, this high current LDO includes an internal

thermal shutdown circuit.

The thermal characteristics of an IC depend on the

following four factors: junction temperature, ambient

temperature, die power dissipation, and the thermal

resistance from the die junction to ambient air. The

maximum junction temperature can be determined by:

TJ(max) + TA(max) ) PD(max) RQJA

The maximum ambient temperature and the power

dissipation are determined by the design while the

maximum junction temperature and the thermal resistance

depend on the manufacturer and the package type. The

maximum power dissipation for a regulator is:

http://onsemi.com

8

Share Link: