RT9706

10

DS9706-02 March 2007

www.richtek.com

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.

Fault Flag

The RT9706 provides a FLG signal pin which is an

N-Channel open drain MOSFET output. This open drain

the die temperature exceeds 130

°C approximately. The

FLG output is capable of sinking a 10mA load to typically

200mV above ground. The FLG pin requires a pull-up

resistor, this resistor should be large in value to reduce

energy drain. A 100k

Ω pull-up resistor works well for most

applications. In the case of an over-current condition, FLG

will be asserted only after the flag response delay time,

upon valid over-current conditions and that erroneous error

is typically 12ms.

For example, false over-current conditions may occur

during hot-plug events when extremely large capacitive

loads are connected and causes a high transient inrush

current that exceeds the current limit threshold.

Under-Voltage Lockout

Under-voltage lockout (UVLO) prevents the MOSFET

switch from turning on until input voltage exceeds

approximately 1.7V. If input voltage drops below

approximately 1.3V, UVLO turns off the MOSFET switch,

FLG will be asserted accordingly. Under-voltage detection

functions only when the switch is enabled.

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 800mA

through the switch of RT9706. When a heavy load or short

circuit is applied to an enabled switch, a large transient

current may flow until the current limit circuitry responds.

Applications Information

The RT9706 is a single N-Channel MOSFET high-side

power switch with active-low enable input, optimized for

self-powered and bus-powered Universal Serial Bus (USB)

applications. The RT9706 equipped with a charge pump

circuitry to drive the internal NMOS switch; the switch's

Ω, meets USB voltage drop requirements;

and a flag output is available to indicate fault conditions

to the local USB controller.

Input and Output

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

Unlike a normal MOSFET, there is no a parasitic body

diode between drain and source of the MOSFET, the

Chip Enable Input

The switch will be disabled when the EN pin is in a logic

high condition. During this condition, the internal circuitry

and MOSFET are turned off, reducing the supply current

to 0.1

μA typical. The maximum guaranteed voltage for a

logic low at the EN pin is 0.8V. A minimum guaranteed

voltage of 2V at the EN pin will turn the RT9706 off. Floating

the input may cause unpredictable operation. EN should

not be allowed to go negative with respect to GND.

D

G

S

D

G

S

Normal MOSFET

RT9706

Figure 1