RT9052 datasheet(7/9 Pages) RICHTEK

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RT9052

DS9052-01 April 2011

www.richtek.com

Application Information

The RT9052 is a low cost single channel LED current

source controller with a specific FAULT indicating scheme.

This device can drive an external NPN-BJT for various

applications. The RT9052 is operated with VCC power

ranging from 3.8V to 13.5V. With such a topology, it is

very flexible and cost effective.

Capacitors Selection

Careful selection of the external capacitors for the RT9052

is necessary to maintain high stability and performance.

A capacitor

≥ 1μF must be connected between VCC and

ground to improve supply voltage stability for proper

operation.

FAULT Function

The RT9052 has a FAULT function with delay. The FAULT

output is an open drain output. Connect a 100k

Ω pull up

resistor to external 5V source to obtain an output voltage.

When the ISET voltage reaches 90% of normal value,

FAULT will become active and be pulled high by external

circuits with a typical 3ms delay.

LED Current Setting

The RT9052 includes a 0.8V reference voltage for easy

setting of the LED current source. As shown in application

circuit, the LED current is easily set via an RISET resistor.

)

LED

ISET

0.8 (V)

(

( A)

PWM Dimming Operation

For controlling the LEDbrightness, the RT9052 can perform

dimming control by applying a PWM signal to the DIM

pin. The average LED current is proportional to the PWM

signal duty cycle. Note that the magnitude of the PWM

signal needs to be higher than the maximum dimming

voltage of the DIM pin, in order to have correct dimming

control.

NPN Transistor Selection

The RT9052 drives the NPN transistor via the DRI pin

(source Base current IB ). When making an NPN transistor

selection, the following criteria Should be considered :

DC current gain hFE , threshold voltage VBE, collector-

emitter voltage VCE, maximum collector current IC

package thermal resistance

θ(JA).

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 :

D(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 of

the RT9052, 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-23-6 packages, the thermal resistance,

θJA, is

250

°C/W on a standard JEDEC 51-3 single-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) / (250°C/W) = 0.400W for

SOT-23-6 package

The maximum power dissipation depends on the operating

ambient temperature for fixed TJ(MAX) and thermal

resistance,

θJA. For the RT9052 package, the derating

curve in Figure 1 allows the designer to see the effect of

rising ambient temperature on the maximum power

dissipation.

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