RT6908 datasheet(20/22 Pages) RICHTEK

20 / 22 page

RT6908

DS6908-01

March 2013

www.richtek.com

is a registered trademark of Richtek Technology Corporation.

VGL Negative Regulator

The VGL negative regulator controller provides low level

voltage for gate driver. The linear regulator can provide a

programmable output voltage. The output voltage can be

adjusted by setting the I2C register 02h [5:0]. The VGL

negative regulator controller has a fault protection. This

function can disable the VGL negative regulator controller

when the VGL voltage is detected to be above VGL + 2V.

If the voltage remains above VGL + 2V after 50ms, the IC

will shut down. Moreover, If VGL voltage above

−0.5V, the

IDRVN source current will be limited to 300

μA for short

circuit protection.

VGH Positive Regulator

The VGH positive regulator controller provides high level

voltage for gate driver. The linear regulator can provide a

programmable output voltage. The output voltage can be

adjusted by setting the I2C register 01h [4:0]. The VGH

positive regulator controller has a fault protection. This

function can disable the VGH regulator controller when

the VGH voltage is detected to be below 75%. If the VGH

voltage remains below 75% after 50ms, the IC will shut

down. Moreover, If VGH voltage remains below 20%, the

IDRVP sink current will be limited to 80

μA for short circuit

protection.

Voltage Detector

The voltage detector monitors the VDET voltage to generate

the RSTB pin signal. The RSTB pin will be floating and

pulled high by VI/O if VDET is higher than the detecting

level. Moreover, the detector power on delay can be

determined by connect capacitor to the CRST pin and

ground. The detecting level and delay time can be

determined as the following equations :

DET

R25

Detect voltage, falling = V

1 +

R26

where V

=0.6V (typ.)

Detector delay time = 0.1563 C17 10

(ms)

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Under Voltage Lockout Protection

The UVLO circuit compares the input voltage at the AVIN

pin with the UVLO threshold (8.6V rising, typ.) to ensure

that the input voltage is high enough for reliable operation.

The 1V (typ.) hysteresis prevents supply transients from

causing a shutdown. Once the input voltage exceeds the

UVLO rising threshold, start-up begins. When the input

voltage falls below the UVLO falling threshold, all switch

will be turned off and latched. Otherwise, input voltage

need below 5V (typ.), the IC can be reset.

Over Temperature Protection

The RT6908 equips an Over Temperature Protection (OTP)

circuitry to prevent overheating due to excessive power

dissipation. The OTP will shut down switching operation

when junction temperature exceeds 150

°C. Once the

junction temperature cools down by approximately 50

°C,

the RT6908 will resume operation. To maintain continuous

operation maximum, the junction temperature should be

prevented from rising above 125

°C.

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 of

the RT6908, the maximum junction temperature is 125

°C

and TA is the ambient temperature. The junction to ambient

thermal resistance,

θJA, is layout dependent. For

WQFN-40L 5x5 package, the thermal resistance,

θJA, is

°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) / (36°C/W) = 2.778W for

WQFN-40L 5x5 package

The maximum power dissipation depends on the operating

ambient temperature for fixed TJ(MAX) and thermal

resistance,

θJA. For the RT6908 package, the derating

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