RT9728

9

DS9728-05

October 2014

www.richtek.com

©

Copyright

2014 Richtek Technology Corporation. All rights reserved.

is a registered trademark of Richtek Technology Corporation.

capacitor itself should have a low dissipation factor to allow

decoupling at higher frequencies.

Chip Enable Input

The RT9728 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 1

μA typical. The maximum guaranteed voltage for a

logic-low at the EN pin is 0.66V. A minimum guaranteed

voltage of 1.1V at the EN pin will turn off the RT9728.

Floating the input may cause unpredictable operation. EN

should not be allowed to go negative with respect to GND.

Under Voltage Lockout

Under Voltage Lockout (UVLO) prevents the MOSFET

switch from turning on until input voltage exceeds

approximately 2.3V. If input voltage drops below

approximately 2.1V, UVLO turns off the MOSFET switch

and FAULT will be asserted accordingly. The under voltage

lockout detection functions only when the switch is

enabled.

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 :

the ambient temperature, and

thermal resistance.

For recommended operating condition specifications, the

maximum junction temperature is 125

°C. The junction to

ambient thermal resistance,

WDFN-6L 2x2 packages, the thermal resistance,

165

°C/W on a standard JEDEC 51-3 single layer thermal

°C

can be calculated by the following formula :

°C − 25°C) / (165°C/W) = 0.606W for

WDFN-6L 2x2 package

Fault Flag

The RT9728 provides a FAULT signal pin which is an N-

Channel open drain MOSFET output. This open drain

output goes low when current exceeds current limit

threshold. The FAULT output is capable of sinking a 1mA

load to typically 180mV above ground. The FAULT 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 case of an over current

condition, FAULT will be asserted only after the flag

FAULT is asserted 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, which induces a high transient inrush current

that exceeds the current limit threshold. The FAULT

Supply Filter/Bypass Capacitor

A 10

GND and located close to the device is strongly

recommended to prevent input voltage drooping during hot-

plug events. However, higher capacitor values may be used

to further reduce the voltage droop on the input. Without

this bypass capacitor, an output short may cause sufficient

ringing on the input (from source lead inductance) to

destroy the internal control circuitry. Note that the input

transient voltage must never exceed 6V as stated in the

Absolute Maximum Ratings.

Output Filter Capacitor

A low-ESR 150

μF aluminum electrolytic capacitor

recommended to meet the USB standard maximum droop

should be used to minimize inductance and resistance

between the bypass capacitor and the downstream

connector to reduce EMI and decouple voltage droop

caused by hot-insertion transients in downstream cables.

0.1

μF bypass capacitors at the power connector pins are

recommended for EMI and ESD protection. The bypass