AP2401A/AP2411A
Document number: DS37744 Rev. 1 - 2
10 of 18
www.diodes.com
March 2015
© Diodes Incorporated
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.
Overcurrent and Short Circuit Protection
An internal sensing FET is employed to check for overcurrent 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 (7ms typical) is reached and the device is turned off. The device will remain latched off even the 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 AP2401A/AP2411A senses the short circuit and immediately clamps output current to a certain safe level namely
I
LIMIT
, and turns off after deglitch time (7ms typical).
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
overcurrent trip threshold), the device switches into current limiting mode and the current is clamped at I
LIMIT
current for deglitch time period
(7ms 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 threshold (I
TRIG
) is reached or until the thermal limit of the device is exceeded. The AP2401A/AP2411A 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 mode and output current is clamped at I
LIMIT
for deglitch time period (7ms typical), and then turned off.
FLG Response
When an overcurrent or over-temperature shutdown condition is encountered, the FLG open-drain output goes active low after a nominal 7ms
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 overcurrent condition, which does not trigger the FLG due to the 7ms deglitch timeout.
The AP2401A/AP2411A is designed to eliminate false, overcurrent 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) and R
DS(ON),
the power dissipation can be calculated by:
P
D
= R
DS(ON)×
I
2
Finally, calculate the junction temperature:
T
J
= P
D
x R
θ
JA
+ T
A
Where:
T
A
= Ambient Temperature°C
R
θ
JA
= Thermal Resistance
P
D
= Total Power Dissipation
Thermal Protection
Thermal protection prevents the IC from damage when heavy-overload or short-circuit faults are present for extended periods of time. The
AP2401A/AP2411A 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 overcurrent 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 7ms 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.
