RT9073 datasheet(12/14 Pages) RICHTEK

12 / 14 page

RT9073

DS9073-02 March 2015

www.richtek.com

is a registered trademark of Richtek Technology Corporation.

Figure 6. Derating Curve of Maximum Power Dissipation

0.0

0.2

0.4

0.6

0.8

100

125

Ambient Temperature (°C)

Four-Layer PCB

SOT-23-5

SC-70-5

where ISNS is the input Current at SNS pin (typical 550nA

at room temperature) and VSNS is determined by the

output voltage selections in the ordering information of

RT9073.

Current Limit

The RT9073 contains an independent current limiter, which

monitors and controls the pass transistor's gate voltage,

limiting the output current to 0.35A (typ.). The current

limiting level is reduced to around 250mA named fold-back

current limit when the output voltage is further decreased.

The output can be shorted to ground indefinitely without

damaging the part.

Thermal Considerations

For continuous operation, do not exceed absolute

maximum junction temperature. The maximum power

dissipation depends on the thermal resistance of the IC

package, PCB layout, rate of surrounding airflow, and

difference between junction and ambient temperature. The

maximum power dissipation can be calculated by the

following formula :

PD(MAX) = (TJ(MAX)

− TA) / θJA

where TJ(MAX) is the maximum junction temperature, TA is

the ambient temperature, and

θJAis the junction to ambient

thermal resistance.

For recommended operating condition specifications the

maximum junction temperature is 125

°C and TA is the

ambient temperature. The junction to ambient thermal

resistance,

θJA, is layout dependent. For SOT-23-5

package, the thermal resistance,

θJA, is 218.1°C/W on a

standard JEDEC 51-7 four-layer thermal test board. For

SC-70-5 package, the thermal resistance,

θJA, is 342.3°C/

W on a standard JEDEC 51-7 four-layer thermal test board.

The maximum power dissipation at TA = 25

°C can be

calculated by the following formula :

PD(MAX) = (125

°C − 25°C) / (218.1°CW) = 0.45W for

SOT-23-5 package

PD(MAX) = (125

°C − 25°C) / (342.3°CW) = 0.29W for

SC-70-5 package

The maximum power dissipation depends on the operating

ambient temperature for fixed TJ(MAX) and thermal

resistance,

θJA. The derating curve in Figure 6 allows the

designer to see the effect of rising ambient temperature

on the maximum power dissipation.

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