AP2401/AP2411

2.0A SINGLE CHANNEL CURRENT-LIMITED

POWER SWITCH WITH LATCH-OFF

AP2401/AP2411

Document number: DS35113 Rev. 3 - 2

10 of 15

www.diodes.com

June 2011

© Diodes Incorporated

Application Notes

Power Supply Considerations

A 0.1

μF to 2.2μF X7R or X5R ceramic bypass capacitor

placed between IN and GND, close to the device, is

recommended. When an external power supply is used,

or an additional ferrite bead is added to the input, high

inrush current may cause voltage spikes higher than the

device maximum input rating during short circuit condition.

In this case a 2.2

μF or bigger capacitor is recommended.

Placing a high-value electrolytic capacitor on the input

and output pin(s) is recommended when the output load

is

heavy.

This

precaution

reduces

power-supply

transients

that

may

cause

ringing

on

the

input.

Additionally, bypassing the output with a 0.1

μF to 1.0μF

ceramic capacitor improves the immunity of the device to

short circuit transients.

Over-current and Short Circuit Protection

An internal sensing FET is employed to check for over-

current conditions. Unlike current-sense resistors, sense

FETs do not increase the series resistance of the current

path. When an overcurrent condition is detected, the

devices will limit the current until the overload condition is

removed or the internal deglitch time (7-ms typical) is

reached and the device is turned off. The device will

remain latched off even overload condition is removed

until power is cycled or the device enable is toggled.

Three possible overload conditions can occur. In the first

condition, the output has been shorted to GND before the

device is enabled or before VIN has been applied. The

AP2401/AP2411 senses the short circuit and immediately

clamps output current to a certain safe level namely

In the second condition, an output short or an overload

occurs while the device is enabled. At the instance the

overload occurs, higher current may flow for a very short

period of time before the current limit function can react.

After the current limit function has tripped (reached the

over-current trip threshold), the device switches into

current for deglitch time period (7-ms typical), and then

turned off.

In the third condition, the load has been gradually

increased beyond the recommended operating current.

The current is permitted to rise until the current-limit

device is exceeded. The AP2401/AP2411 is capable of

delivering current up to the current-limit threshold without

damaging the device. Once the threshold has been

reached, the device switches into its current limiting

time period (7-ms typical), and then turned off.

FLG Response

When an over-current or over-temperature shutdown

condition is encountered, the FLG open-drain output

goes active low after a nominal 7-ms deglitch timeout.

When that happens, the FLG will remain low and the

switch will be latched off until the fault condition is

removed. Connecting a heavy capacitive load to the

output of the device can cause a momentary over-current

condition, which does not trigger the FLG due to the 7-ms

deglitch timeout. The AP2401/AP2411 is designed to

eliminate false over-current reporting without the need of

external components to remove unwanted pulses.

Power Dissipation and Junction Temperature

The low on-resistance of the internal MOSFET allows the

small surface-mount packages to pass large current.

Using the maximum operating ambient temperature (TA)

2

Finally, calculate the junction temperature:

Where:

Thermal Protection

Thermal protection prevents the IC from damage when

heavy-overload or short-circuit faults are present for

extended

periods

of

time.

The

AP2401/AP2411

implements a thermal sensing to monitor the operating

junction temperature of the power distribution switch.

Once the die temperature rises to approximately 140°C

due to excessive power dissipation in an over-current

condition, the internal thermal sense circuitry turns the

power switch off, thus preventing the power switch from

damage. Hysteresis is built into the thermal sense circuit

allowing the device to cool down approximately 25°C

before the switch turns back on. The switch continues to

cycle in this manner until the load fault or input power is

removed. The FLG open-drain output is asserted when

an over-temperature shutdown occurs with 7-ms deglitch.

When the FLG is asserted, the switch will be latched off

until the temperature drops to 20

°C below the thermal

shutdown threshold and the power or EN pin is cycled.

Under-voltage Lockout (UVLO)

Under-voltage lockout function (UVLO) keeps the internal

power switch from being turned on until the power supply

has reached at least 2V, even if the switch is enabled.

Whenever the input voltage falls below approximately 2V,

the power switch is quickly turned off. This facilitates the

design of hot-insertion systems where it is not possible to

turn off the power switch before input power is removed.