AP2301/AP2311

Document number: DS32241 Rev. 4 - 2

10 of 18

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March 2013

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AP2301/AP2311

Application Information

Power Supply Considerations

A 0.1μF 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 2.2μF 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 0.1μF 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 over-current condition is detected, the device maintains a constant output current and reduces the output

voltage accordingly. Complete shutdown occurs only if the fault stays long enough to activate thermal limiting.

In the second condition, an output short or an overload occurs while the device is enabled. At the instance the overload occurs, higher inrush

current may flow for a very short period of time before the current limit function can react. The input capacitor(s) rapidly discharge through the

device, activating current limit circuitry. Protection is achieved by momentarily opening the P-MOS high-side power switch and then gradually

turning it on. After the current limit function has tripped (reached the over-current trip threshold), the device switches into current limiting mode and

In the third condition, the load has been gradually increased beyond the recommended operating current. The current is permitted to rise until the

to the current-limit threshold without damaging the device. Once the threshold has been reached, the device switches into its current limiting mode

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. The FLG output remains low until both over-current and over-temperature conditions are 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 AP2301/AP2311 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

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

AP2301/AP2311 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 or short-circuit 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 20°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 or over-current occurs with 7-ms deglitch.

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.