CS5253-1GDPR5 datasheet(8/9 Pages) CHERRY

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Application Notes: continued

A rule of thumb useful in determining if a protection diode

is required is to solve for current:

I =

where

is the current flow out of the load capacitance

when VCONTROL is shorted,

is the value of load capacitance

is the output voltage, and

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 typi-

cal short circuit current value provided in the specifica-

tions, serious thought should be given to the use of a pro-

tection 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 500mA 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 circuit-

ry has 25¡C of typical hysteresis, thereby allowing the reg-

ulator to recover from a thermal fault automatically.

Calculating Power Dissipation and Heat Sink

Requirements

High power regulators such as the CS5253-1 usually oper-

ate at high junction temperatures. Therefore, it is impor-

tant to calculate the power dissipation and junction tem-

peratures accurately to ensure that an adequate heat sink is

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

CS5253-1, electrical isolation may be required for some

applications. Also, as with all high power packages, ther-

mal compound in necessary to ensure proper heat flow.

For added safety, this high current LDO includes an inter-

nal thermal shutdown circuit

The thermal characteristics of an IC depend on the follow-

ing four factors: junction temperature, ambient tempera-

ture, die power dissipation, and the thermal resistance

from the die junction to ambient air. The maximum junc-

tion temperature can be determined by:

TJ(max) = TA(max) + PD(max) ´ RQJA

The maximum ambient temperature and the power dissi-

pation 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:

PD(max) = (VIN(max) -VOUT(min))IOUT(max) + VIN(max) ´ IIN(max)

A heat sink effectively increases the surface area of the

package to improve the flow of heat away from the IC and

into the surrounding air. Each material in the heat flow

path between the IC and the outside environment has a

thermal resistance which is measured in degrees per watt.

Like series electrical resistances, these thermal resistances

are summed to determine the total thermal resistance

between the die junction and the surrounding air, RQJA.

This total thermal resistance is comprised of three compo-

nents. These resistive terms are measured from junction to

case (RQJC), case to heat sink (RQCS), and heat sink to ambi-

ent air (RQSA). The equation is:

RQJA = RQJC + RQCS + RQSA

The value for RQJC is 2.5ûC/watt for the CS5253-1 in the D

PAK package. For a high current regulator such as the

CS5253-1 the majority of heat is generated in the power

transistor section. The value for RQSA depends on the heat

sink type, while the RQCS depends on factors such as pack-

age type, heat sink interface (is an insulator and thermal

grease used?), and the contact area between the heat sink

and the package. Once these calculations are complete, the

maximum permissible value of RQJA can be calculated and

the proper heat sink selected. For further discussion on

heat sink selection, see our Cherry application note

ÒThermal Management for Linear Regulators.Ó

´ V

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