RT9741 datasheet(7/11 Pages) RICHTEK

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RT9741

DS9741-01 March 2015

www.richtek.com

is a registered trademark of Richtek Technology Corporation.

Application Information

The RT9741 is a single N-MOSFET high-side power

switches, optimized for self-powered and bus-powered

Universal Serial Bus (USB) applications. The RT9741 is

equipped with a charge pump circuitry to drive the internal

N-MOSFET switch; the switch's low RDS(ON) meets USB

voltage drop requirements.

Input and Output

VIN (input) is the power source connection to the internal

circuitry and the drain of the MOSFET. VOUT (output) is

the source of the MOSFET. In a typical application, current

flows through the switch from VIN to VOUT toward the load.

If VOUT is greater than VIN, current will flow from VOUT to

VIN since the MOSFET is bidirectional when on.

Soft-Start for Hot Plug-In Applications

In order to eliminate the upstream voltage droop caused

by the large inrush current during hot-plug events, the

“soft-start” feature effectively isolates the power source

from extremely large capacitive loads, satisfying the USB

voltage droop requirements.

Under-Voltage Lockout

Under-Voltage Lockout (UVLO) prevents the MOSFET

switch from turning on until input the voltage exceeds

approximately 1.7V. If input voltage drops below

approximately 1.3V, UVLO turns off the MOSFET switch.

Current Limiting and Short-Circuit Protection

The current limit circuitry prevents damage to the MOSFET

switch and the hub downstream port but can deliver load

current up to the current limit threshold of typically 2A

through the switch of the RT9741A and 1A for RT9741C

respectively. When a heavy load or short circuit is applied

to switch, a large transient current may flow until the

current limit circuitry responds. Once this current limit

threshold is exceeded, the device enters constant current

mode until the thermal shutdown occurs or the fault is

removed.

Thermal Shutdown

Thermal protection limits the power dissipation in RT9741.

When the operation junction temperature exceeds 120

°C,

the OTP circuit starts the thermal shutdown function and

turns the pass element off. The pass element turns on

again after the junction temperature cools to 80

°C. The

RT9741 lowers its OTP trip level from 120

°C to 100°C

when output short circuit occurs (VOUT < 1V) as shown in

Figure 1.

Figure 1. Short Circuit Thermal Folded Back Protection

when Output Short Circuit Occurs (Patent)

VOUT Short to GND

VOUT

IOUT

Thermal

Shutdown

OTP Trip Point

120 C



100 C



100 C



80 C



IC Temperature

Power Dissipation

The junction temperature of the RT9741 series depends

on several factors such as the load, PCB layout, ambient

temperature and package type. The output pin of the

RT9741 can deliver the current of up to 2A (RT9741A) and

1A (RT9741C) respectively over the full operating junction

temperature range. However, the maximum output current

must be decreased at higher ambient temperature to

ensure the junction temperature does not exceed 100

°C.

With all possible conditions, the junction temperature must

be within the range specified under operating conditions.

Power dissipation can be calculated based on the output

current and the RDS(ON) of the switch as below.

PD = RDS(ON) x IOUT2

RT9741 Datasheet(PDF) 7 Page - Richtek Technology Corporation

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