AP2501/AP2511

Document number: DS35577 Rev. 5 - 2

10 of 18

www.diodes.com

August 2012

© Diodes Incorporated

AP2501/AP2511

Application Note

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 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.

Three possible overload conditions can occur. In the first condition, the output has been shorted to GND before the device is enabled or before

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

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

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

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 AP2501/AP2511 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

AP2501/AP2511 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.