RT9722 datasheet(10/14 Pages) RICHTEK

10 / 14 page

RT9722

DS9722-00 August 2011

www.richtek.com

pin requires a pull-up resistor, this resistor should be large

in value to reduce energy drain. A 100k 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 a highly large capacitive load

is connected and causes a high transient inrush current

that exceeds the current limit threshold. The FLG response

delay time tD is typically 12ms at VIN = 5V.

Under-Voltage Lockout

Under-voltage lockout (UVLO) prevents the MOSFET

switch from turning on until input voltage exceeds

approximately 2V. Under-voltage detection functions only

when the chip enable input is enabled.

Thermal Shutdown

Thermal shutdown is employed to protect the device from

damage if the die temperature exceeds approximately

150

°C. If enabled, the switch automatically restarts when

the die temperature falls 20

°C (typ.). The output will

continue to cycle on and off until the device is disabled or

the fault is removed.

Short Circuit Protection

The current limit circuitry prevents damage to the MOSFET

switch and external load. 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.

Once this current limit threshold is exceeded, the device

enters constant current mode until the thermal shutdown

occurred or the fault is removed.

Supply Filter/Bypass Capacitor

A 1

μF low-ESR ceramic capacitor from VIN to GND (the

amount of the capacitance may be increased without limit),

located at the device is strongly recommended to prevent

the input voltage drooping during hot-plug events. However,

higher capacitor values will further reduce the voltage droop

on the input. Furthermore, without the bypass capacitor,

an output short may cause sufficient ringing on the input

(from source lead inductance) to destroy the internal

control circuitry. An important note to be award of is the

parasitic inductance of PCB traces can cause over-voltage

transients if the PCB trace has even a few tens of nH of

inductance. The input transient must not exceed 6.0V of

the absolute maximum supply voltage even for a short

duration.

Power Dissipation

The device's junction temperature depends on several

factors such as the load, PCB layout, ambient temperature

and package type. The maximum output current must be

derated at higher ambient temperature to ensure the

junction temperature does not exceed operating junction

temperature 125

°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

switch as below.

PD = RDS(ON) x IOUT2

The application may limit the amount of output current

based on the total power dissipation and the ambient

temperature.

Thermal Considerations

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, TA is the ambient temperature and the

θJAis

the junction to ambient thermal resistance.

For recommended operating conditions specification of

RT9722, the maximum operating junction temperature is

125

°C. The junction to ambient thermal resistance θJA for

SOT-23-5/SOT-23-6 package is 250

°C/W and WDFN-6L

2x2 package is 165

°C/W on the standard JEDEC 51-3

single-layer thermal test board. The maximum power

dissipation at TA = 25

°C can be calculated by following

formula :

PD(MAX) = (125

°C − 25°C) / (250°C/W) = 0.4W for

SOT-23-5/SOT-23-6 package

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