RT5047BF datasheet(14/17 Pages) RICHTEK

Il numero della parte	RT5047BF

Spiegazioni elettronici	Single Output LNB Supply and Control Voltage Regulator

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Produttore elettronici	RICHTEK [Richtek Technology Corporation]

Homepage	http://www.richtek.com

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14 / 17 page

RT5047BF

is a registered trademark of Richtek Technology Corporation.

www.richtek.com

DS5047BF-00

December

2017

14

Inrush Current

At start-up or during a LNB reconfiguration event, a

transient surge current above the normal DC operating

level can be provided by the IC. This current increase

can be as high as 550mA, typical, for as long as

required, up to a maximum of 6ms.

DC Current

The RT5047BF can handle up to 500mA during

continuous operation.

Thermal Considerations

The junction temperature should never exceed the

absolute maximum junction temperature TJ(MAX), listed

under Absolute Maximum Ratings, to avoid permanent

damage to the device. The maximum allowable power

dissipation depends on the thermal resistance of the IC

package, the PCB layout, the rate of surrounding

airflow, and the difference between the junction and

ambient

temperatures.

The

maximum

power

dissipation can be calculated using the following

formula :

PD(MAX) = (TJ(MAX) - TA) / JA

where TJ(MAX) is the maximum junction temperature,

TA is the ambient temperature, and JA is the

junction-to-ambient thermal resistance.

For continuous operation, the maximum operating

junction temperature indicated under Recommended

Operating

Conditions

is

125°C.

The

junction-to-ambient thermal resistance,

JA, is highly

package dependent. For a SOP-8 (Exposed Pad)

package, the thermal resistance,

JA, is 29°C/W on a

standard

JEDEC

51-7

high

effective-thermal-conductivity four-layer test board. The

maximum power dissipation at TA = 25°C can be

calculated as below :

PD(MAX) = (125°C - 25°C) / (29°C/W) = 3.44W for a

SOP-8 (Exposed Pad) package.

The maximum power dissipation depends on the

operating ambient temperature for the fixed TJ(MAX) and

the thermal resistance,

JA. The derating curves in

Figure 3 allows the designer to see the effect of rising

ambient

temperature

on

the

maximum

power

dissipation.

Figure 3. Derating Curve of Maximum Power

Dissipation

Layout Consideration

For high frequency switching power supplies, the PCB

layout is important to get good regulation, high

efficiency and stability. The following descriptions are

the guidelines for better PCB layout.



For good regulation, place the power components as

close as possible. The traces should be wide and

short enough especially for the high-current loop.



Minimize the size of the LX node and keep it wide

and shorter.

The exposed pad of the chip should be connected to a

strong

ground

plane

for

maximum

thermal

consideration.

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

0

25

50

75

100

125

Ambient Temperature (°C)

Four-Layer PCB

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