AP2181/AP2191

1.5A SINGLE CHANNEL CURRENT-LIMITED POWER

SWITCH

AP2181/AP2191 Rev. 6

11 of 18

JUNE 2009

DS31563

www.diodes.com

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 AP2181/AP2191 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.

Host/Self-Powered HUBs

Hosts and self-powered hubs (SPH) have a local power supply that powers the embedded functions and the

downstream ports (see Figure 2). This power supply must provide from 5.25V to 4.75V to the board side of

the downstream connection under both full-load and no-load conditions. Hosts and SPHs are required to

have current-limit protection and must report over-current conditions to the USB controller. Typical SPHs are

desktop PCs, monitors, printers, and stand-alone hubs.

Figure 2. Typical One-Port USB Host / Self-Powered Hub