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RT9718

Over Voltage Protection IC

Richtek Technology 

RT9718

Applications Information

Power Up

The RT9718 has a threshold of 2.7V power on reset (POR)

with a built-in hysteresis of 100mV. Before the input voltage

reaches the POR threshold, the RT9718 is off. When the

input voltage is over the POR threshold; the RT9718 will

delay for 8ms and the soft-start will be activated after the

8ms delay. The 8ms delay allows any transients at the

input during a hot insertion of the power supply to settle

down before the IC starts to operate.

During the soft-start transition, the RT9718 slowly turns

on the internal MOSFET to reduce the inrush current.

Enable Control

The RT9718 offers an enable (EN) input. When the EN pin

is pulled to logic high (>1.5V), the RT9718 will be shut

down. When the EN pin is pulled to logic low (<0.4V), the

RT9718 will be powered on. The EN pin has an internal

pull-down resistor. Leaving the EN pin floating can enable

the IC.

Warning Indication Output

The WRN pin is an open-drain output that indicates a LOW

signal when any protection event occurs (Input OVP, Output

OCP and Battery OVP). When the protection events are

released and then the WRN pin indicates a HIGH signal.

Over Temperature Protection (OTP)

The RT9718 monitors its own internal temperature to prevent

thermal failures. The chip turns off the MOSFET when the

internal temperature reaches 140°C. The IC will resume

after the internal temperature is cooled down 20°C.

Input Over Voltage Protection

The RT9718 monitors input voltage to prevent the input

voltage lead to output system failures. The RT9718 input

OVP threshold is set by the internal resistor. When the

input voltage exceeds the threshold, the RT9718 outputs

a logic signal to turn off the internal MOSFET within 1us

to prevent the high input voltage from damaging the

electronics in the handheld system. The hysteresis of the

input OVP threshold is 100mV. When the input voltage

returns to normal operation voltage rage, the RT9718 re-

enables the MOSFET.

Battery Over Voltage Protection

The battery OVP threshold voltage is set at 4.35V in typical

and the RT9718 has a built-in 180us blanking time to prevent

any transient voltage from triggering the battery OVP. If

the OVP situation still exists after 180us, the internal

MOSFET will be turned off and the WRN pin indicates a

LOW signal. The battery OVP threshold has a 30mV built-

in hysteresis. The control logic contains a 4-bit binary

counter. If the battery over voltage event occurs for

consecutive 16 times, the MOSFET will be turned off

permanently unless the input power or the enable pin is

reset.

Selecting RVB

The RT9718 monitors the battery voltage by the VB pin.

The RT9718 will be turned off when the battery voltage

exceeds the 4.35V battery OVP threshold. The VB pin is

connected to the battery pack positive terminal via an

isolation resistor (RVB) and the resistor is an important

component. The RVB determines some parameters such

as battery OVP threshold error and VB pin leakage current.

Generally, it is necessary to decrease the RVB for reducing

the battery OVP threshold error. However, this will increase

the VB pin leakage current. So, it is an important issue to

get a trade-off between the battery OVP threshold error

and the VB pin leakage current. The resistance of 200kΩ

to 1MΩ is allowed for RVB.

Over Current Protection (OCP)

The RT9718 monitors the output current to prevent the

output short or the charging of the battery with an excessive

current. The OCP (Over Current protection) threshold can

be set by the ILIM pin. The RT9718 has a built-in 180us

delay time to prevent any transient noise from triggering

the OCP. If the OCP situation exists for 180us, the internal

MOSFET will be turned off and the WRN pin indicates a

LOW signal. When the OCP happens for consecutive 16

times, the internal MOSFET will be turned off permanently

unless the input power is recycled or the enable pin is

toggled.

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DS9718-02 May 2011

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