RT9705B datasheet(10/12 Pages) RICHTEK

10 / 12 page

RT9705B

DS9705B-03 April 2011

www.richtek.com

Application Information

The RT9705B is dual input single output power multiplexer

specifically designed to provide seamless voltage transition

between two independent power suppliers. Equipped with

two low RDS(ON) N-Channel MOSFETs driven by internal

charge pump circuitry, the RT9705B is able to deliver 1A

output current with only 80mV voltage drop. The RT9705B

provides comprehensive protection functions, including

adjustable current limit, over temperature protection, soft

start function for minimum inrush current, cross-conduction

protection, and reverse conduction protection. These

features greatly simplify power multiplexer design.

Manual Switching Mode

The RT9705B provides two logic input D0 and D1 for switch

selection as shown in Table 1. The RT9705B selects the

manual-switching mode when the D0 is pulled low. In this

mode VOUT connects to VIN1 if D1 pulled high, otherwise

VOUT connects to VIN2.

Auto Switching Mode

RT9705B selects the auto-switching mode when the D0 is

pulled high andD1 is pulled low. In this mode VOUT connects

to the higher of VIN1 and VIN2.

Shutdown Mode

When both D0 and D1 are selected high, the RT9705B

enters shutdown mode and consumes minimum power.

An internal MOSFET with 2k

Ω RDS(ON) turns on and softly

discharges the output voltage in the shutdown mode. Since

no body diode exists between VINX and VOUT, output voltage

is allowed to be high than the input voltages in the shutdown

mode.

Switch Status Indication

A STAT pin with open drain output is provided to indicate

the switch status. STAT pin outputs high impedance if VIN2

is active, otherwise STAT pin outputs low.

Current Limiting

The current limit circuitry prevents damage to the MOSFET

switch and external load. A resistor RILIM from ILIM to GND

sets the current limit to 500/R ILIM and the adjustable

current limiting up to 1.25 A. A setting resistor RILIM equal

to zero is not recommended as that disables current

limiting.

Thermal Considerations

Thermal protection limits power dissipation in RT9705B.

When the operation junction temperature exceeds 135

°C,

the OTP circuit starts the thermal shutdown function and

turns the pass element off. The pass element turn on again

after the junction temperature cools by 10

°C.

For continuous operation, do not exceed absolute

maximum operation junction temperature 125

°C. The

power dissipation definition in device is :

PD = (VIN-VOUT) x IOUT + VIN x IQ

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

RT9705B, 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 TSSOP-8 package

is 230

°C/W on standard JEDEC 51-3 thermal test board.

The maximum power dissipation at TA = 25

°C can be

calculated by following formula :

PD(MAX) = (125

°C -25°C) / 230°C/W = 430 mW (TSSOP-8)

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