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Application information
Under-Voltage Lockout (UVLO)
Under-voltage lockout function (UVLO) guarantees that the internal power switch is initially off during start-up. The UVLO functions only when the
power supply has reached at least 2.5V (TYP). Whenever the input voltage falls below approximately 2.5V, the power switch is 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.
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.
The different overload conditions and the corresponding response of the AP2331 are outlined below:
1
-circuit condition at start-up
Output is shorted before input
voltage is applied or before the
part is powered up.
The IC senses the short circuit and immediately clamps output
current to a certain safe level namely I
LIMIT
2
-circuit or Over current
Short-Circuit or Overload
condition that occurs when the
part is powered up and above
UVLO.
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 limiting
mode and the current is clamped at I
.
3
from nominal
LIMIT
Load increases gradually until
the current-limit threshold.
The current rises until I
LIMIT
. Once the threshold has been
reached, the device switches into its current limiting mode and is
Reverse-Current Protection
The USB specification does not allow an output device to source current back into the USB port. In a normal MOSFET switch, current will flow in
reverse direction (from the output side to the input side) when the output side voltage is higher than the input side. A reverse current limit feature is
implemented in the AP2331 to limit such back currents. Reverse current limit is always active in AP2331. Reverse current is limited at IROCP level
and when the fault exists for more than 700µs, output device is disabled and shut down. This is called the "Deglitch time from reverse current trigger
to MOSFET turn off.” Recovery from IROCP occurs when the output voltage falls to 101% of input voltage.
Over-Voltage Protection
The device has an output over-voltage protection that triggers when the output voltage reaches 5.3V (MIN). When this fault condition stays on
for longer than 15µs, (This is called the “Debounce time from output over voltage to MOSFET turn off”) output device is disabled and shut down.
Recovery from ROVP occurs when the output voltage falls to 101% of input voltage.
Thermal Protection
Thermal protection prevents the IC from damage when the die temperature exceeds safe margins. This mainly occurs when heavy-overload or short-
circuit faults are present for extended periods of time. The AP2331 implements a thermal sensing to monitor the operating junction temperature of
the power distribution switch. Once the die temperature rises to approximately +150°C, the Thermal protection feature gets activated as follows: The
internal thermal sense circuitry turns the power switch off thus preventing the power switch from damage. Hysteresis in the thermal sense circuit
allows the device to cool down to approximately +20°C before the output is turned back on. This built-in thermal hysteresis feature is an excellent
feature, as it avoids undesirable oscillations of the thermal protection circuit. The switch continues to cycle in this manner until the load fault is
removed, resulting in a pulsed output.
Discharge Function
When input voltage falls below UVLO, the discharge function is active. The output capacitor is discharged through an internal NMOS that has a
discharge resistance of 800Ω. Hence, the output voltage drops down to zero. The time taken for discharge is dependent on the RC time constant of
the resistance and the output capacitor. Discharge time is calculated when UVLO falling threshold is reached to output voltage reaching 300mV.
AP2331
Document Number: DS35529 Rev. 5 - 2
7 of 12
www.diodes.com
September 2014
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