Diodes AP2501/AP2511 User Manual
Page 10
AP2501/AP2511
Document number: DS35577 Rev. 6 - 2
10 of 18
March 2013
© Diodes Incorporated
AP2501/AP2511
Application Note
P
ower 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.
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 AP2501/AP2511 senses the short circuit and immediately clamps output current to a certain safe level namely I
LIMIT
.
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 the current is clamped at I
LIMIT
.
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 AP2501/AP2511 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 is set at I
LIMIT
.
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
ambient temperature (T
A
) 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
JA
+ T
A
Where:
T
A
= Ambient temperature
C
R
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
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.