RT9285CPJ6 datasheet(9/13 Pages) RICHTEK

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RT9285C

Preliminary

DS9285C-00

August 2007

www.richtek.com

As GPIO = 1.8V, VOUT = RA x {(0.25/RB) + (0.25-1.8)/

RGPIO)} + 0.25

(4)

For Efficiency Consideration :

Set RA = 990k

Ω,

If 2 levels are 16V (GPIO = 0V) and 14V (GPIO = 1.8V)

Get RB = 16k

Ω, RGPIO = 890kΩ

Table 2. Suggested Resistance for Output Voltage

Control

Conditions

Ω)

RGPIO

Ω)

Case A :

Normal Voltage = 16V

(GPIO = 0V)

Dimming Voltage = 12V

(GPIO = 1.8V)

1100

495

Case B :

Normal Voltage = 16V

(GPIO = 0V)

Dimming Voltage = 12V

(GPIO = 2.8V)

1200

19.5

840

Considering the output voltage deviation from the GPIO

voltage tolerance, as GPIO voltage vibrated by 0

± 50mV

and 1.8(2.8)

±5% ,the output voltage could be kept within

±2.5%.

Thermal Considerations

For continuous operation, do not exceed absolute

maximum operation junction temperature. The maximum

power dissipation depends on the thermal resistance of

IC package, PCB layout, the rate of surroundings airflow

and temperature difference between junction to ambient.

The maximum power dissipation can be calculated by

following formula:

PD(MAX) = ( TJ(MAX) - TA ) /

θJA

Where TJ(MAX) is the maximum operation junction

temperature 125

°C, TA is the ambient temperature and

the

θJA is the junction to ambient thermal resistance.

For recommended operating conditions specification of

RT9285C, where TJ (MAX) is the maximum junction

temperature of the die (125

°C) and TA is the maximum

ambient temperature. The junction to ambient thermal

resistance

θJA is layout dependent. For XDFN/WDFN 2x2

packages, the thermal resistance

θJA is 165蚓 /W on the

standard JEDEC 51-3 single layer thermal test board. The

maximum power dissipation at TA= 25

°C can be calculated

by following formula:

PD(MAX) = (125

°C − 25°C) / (165°C/W) = 0.606 W forWDFN/

XDFN 2x2 packages

PD(MAX) = (125

°C − 25°C) / (255°C/W) = 0.392 W for TSOT-

23-6 packages

The maximum power dissipation depends on operating

ambient temperature for fixed TJ (MAX) and thermal

resistance

θJA. For RT9285C packages, the Figure 6 of

derating curves allows the designer to see the effect of

rising ambient temperature on the maximum power

allowed.

Figure 6. Derating Curves for RT9285C Packages

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

100

125

Ambient Temperature (

°C)

TSOT-23-6

XDFN/WDFN-8L 2x2

Layout guide

} A full GND plane without gap break.

} Traces in bold need to be routed first and should be

kept as short as possible.

} VDD to GND noise bypass : Short and wide connection

for the 1uF MLCC capacitor between Pin 6 and Pin 2.

} LX node copper area should be minimized for reducing

EMI. (*1)

} The input capacitor C1 should be placed as closed as

possible to Pin 6. (*2)

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