AP2331

0.2A SINGLE CHANNEL CURRENT-LIMITED LOAD SWITCH

AP2331

Document Number: DS35529 Rev. 4 - 2

7 of 11

www.diodes.com

May 2012

© Diodes Incorporated

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:

S.NO

Conditions

Explanation

Behavior of the AP2331

1

Short-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

2

Short-circuit or Over current

condition

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

3

Gradual increase from nominal

Load increases gradually until

the current-limit threshold.

reached, the device switches into its current limiting mode

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

shutdown. 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 shutdown. 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 800

Ω. 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.

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

power dissipation can be calculated by:

2

Finally, calculate the junction temperature:

Where: