RT9301 datasheet(7/9 Pages) RICHTEK

7 / 9 page

RT9301

DS9301-02

March 2007

www.richtek.com

GPIO Control

Figure 4 shows an application circuit for backlight with

GPIO. The three setting resistors are connected to GPIO.

The LED current can be controlled by GPIO directly. The

RT9301 provides low dropout voltage and 5% maximum

current matching. It also allows dimming control frequency

up to 10kHz.

The LED current can be set at different value with proper

setting resistor. For typical application of GPIO 1.8V/2.8V

and LED current 15mA/20mA, the recommended current

setting resistors are showed as below table.

Figure 5 shows another application circuit for keypad

backlight with GPIO. There are 9 LEDs operation in parallel.

A battery or a regulated power source drives the LEDs.

Each channel supports three LEDs. The LED brightness

adjustment can be set with proper setting resistor for each

channel and be controlled from GPIO.

Thermal Considerations

The Rt9301 can support LED current up to 50mA over the

operation junction temperature range. However, the

maximum current must be derated at higher ambient

temperature to ensure the junction temperature does not

exceed 125

°C. With all possible conditions, the junction

temperature must be within the range specified under

recommended operating conditions. The power dissipation

can be calculated based on the LED current and the voltage

drop across the regulator.

PD =

∑ (VIN -VFLED) x ILED + VIN x IQ

where VIN is input voltage, VFLED is the forward voltage of

LED, ILED is the current of LED, and IQ is the quiescent

current of this chip.

For continuous operation, do not exceed the maximum

operation junction temperature 125

°C. The maximum power

dissipation depends on the thermal resistance of IC

package, PCB layout, the rate of surroundings airflow and

temperature difference between junction and 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, TAis the ambient temperature and the

θJA is the junction to ambient thermal resistance.

For recommended operating conditions specification of

RT9301, 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 TSOT-23-8

packages, the thermal resistance

θJA is 262°C/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) / 262°C/W = 0.382 W for TSOT-

23-8 packages

Table 1. RSET Value Selection

GPIO

(V)

ILED

(mA)

RSET

(kΩ)

Nearest Standard Values for

RSET (kΩ)

47.5

1.8

101

100

2.8

BAT

1uF

LED3

LED2

LED1

ISET1

VIN

ISET3

ISET2

RT9301

SET3

SET2

SET1

GPIO

GND

Figure 5. Application Circuit for Keypad Backlight

LED3

LED2

LED1

GND

ISET1

BAT

VIN

ISET3

ISET2

SET3

SET2

GPIO

RT9301

SET1

1uF

Figure 4. Application Circuit for Backlight with GPIO

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