AP2145/AP2155

0.5A SINGLE CHANNEL CURRENT-LIMITED POWER

SWITCH

AP2145/AP2155 Rev. 1

11 of 14

DECEMBER 2009

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Application Note

(Continued)

Power Dissipation and Junction Temperature

The low on-resistance of the internal MOSFET allows the small surface-mount packages to pass large

calculated by:

2

Finally, calculate the junction temperature:

Where:

°C

Thermal Protection

Thermal protection prevents the IC from damage when heavy-overload or short-circuit faults are present for

extended periods of time. The AP2145/AP2155 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 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 or over-current occurs with 7-ms

deglitch.

Under-voltage Lockout (UVLO)

Under-voltage lockout function (UVLO) keeps the internal power switch from being turned on until the power

supply has reached at least 1.9V, even if the switch is enabled. Whenever the input voltage falls below

approximately 1.9V, the power switch is quickly 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.

Generic Hot-Plug Applications

In many applications it may be necessary to remove modules or pc boards while the main unit is still

operating. These are considered hot-plug applications. Such implementations require the control of current

surges seen by the main power supply and the card being inserted. The most effective way to control these

surges is to limit and slowly ramp the current and voltage being applied to the card, similar to the way in

which a power supply normally turns on. Due to the controlled rise times and fall times of the

AP2145/AP2155, these devices can be used to provide a softer start-up to devices being hot-plugged into a

powered system. The UVLO feature of the AP2145/AP2155 also ensures that the switch is off after the card

has been removed, and that the switch is off during the next insertion.

these devices first after insertion. The typical rise time of the switch is approximately 1ms, providing a slow

voltage ramp at the output of the device. This implementation controls system surge current and provides a

hot-plugging mechanism for any device.