RT9703CS datasheet(12/14 Pages) RICHTEK

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RT9703

Preliminary

The normal current limit value, ILIMIT, is set with an external

resistor between IP (pin 8) and GND (pin 4). 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. It

is important to note that the maximum current limit value

shall be set properly in accordance with its supply voltage

otherwise it may result in the failure occurrence. The graph

below shows the maximum current limit and supply

voltage on the safe operation area.

Thermal Shutdown

Thermal shutdown is employed to protect the device from

damage if the die temperature exceeds approxi- mately

130

°C. If enabled, the switch automatically restarts when

the die temperature falls 10

°C. The output and FLG signal

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

or the fault is removed.

Power Dissipation

The device's junction temperature depends on several

factors such as the load, PCB layout, ambient temperature

and package type. The output pin of RT9703 can deliver

a current of up to 3A over the full operating junction

temperature range. However, the maximum output current

must be derated 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 switch as below.

PD = RDS(ON) x IOUT2

Although the devices are rated for 3A of output current,

but the application may limit the amount of output current

based on the total power dissipation and the ambient

temperature. The final operating junction temperature for

any set of conditions can be estimated by the following

thermal equation:

PD (MAX) = ( TJ (MAX) - TA ) / θθθθθJA

Where TJ (MAX) is the maximum junction temperature of

the die (100° C) and TA is the maximum ambient

temperature. The junction to ambient thermal resistance

(

θJA) for SOP-8 package at recommended minimum

footprint is 104°C/W (

θJA is layout dependent).

Supply Filter/Bypass Capacitor

A 10

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. It is recommended to use

aluminum electrolytic acted the supply capacitor to prevent

the device from being damaged. The input transient must

not exceed 6.5V of the absolute maximum supply voltage

even for a short duration.

Maximun Current Limit vs. Supply Voltage

0.5

1.5

2.5

3.5

4.5

2.5

3.5

4.5

5.5

Supply Voltage (V)

TA = 25

°C

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