RT9702 Просмотр технического описания (PDF) - Richtek Technology

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RT9702

80mΩ, 500mA/1.1A High-Side Power Switches with Flag

Richtek Technology 

Applications Information

The RT9702 and RT9702A are single N-Channel MOSFET

high-side power switches with active-high enable input,

optimized for self-powered and bus-powered Universal

Serial Bus (USB) applications. The RT9702/A equipped

with a charge pump circuitry to drive the internal NMOS

switch; the switch's low RDS(ON), 80mΩ, meets USB

voltage drop requirements; and a flag output is available

to indicate fault conditions to the local USB controller.

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.

Unlike a normal MOSFET, there is no a parasitic body

diode between drain and source of the MOSFET, the

RT9702/A prevents reverse current flow if VOUT being

externally forced to a higher voltage than VIN when the

output disabled (VEN < 0.8V).

D

SD

S

G

Normal MOSFET

G

RT9702/A

Chip Enable Input

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

low condition. During this condition, the internal circuitry

and MOSFET are turned off, reducing the supply current

to 0.1μA 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 RT9702/A back

on. Floating the input may cause unpredictable operation.

EN should not be allowed to go negative with respect to

GND. The EN pin may be directly tied to VIN to keep the

part on.

RT9702/A

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 RT9702/A provides a FLG signal pin which is an N-

Channel open drain MOSFET output. This open drain

output goes low when VOUT < VIN – 1V, current limit or

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 100kΩ 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,

tD, has elapsed. This ensures that FLG is asserted only

upon valid over-current conditions and that erroneous error

reporting is eliminated.

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. The FLG

response delay time tD is typically 10ms.

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 RT9702 and 1.5A for RT9702A

respectively. When a heavy load or short circuit is applied

to an enabled switch, a large transient current may flow

DS9702/A-11 June 2007

www.richtek.com

11

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