ap2171 Diodes, Inc., ap2171 Datasheet - Page 12

• Manufacturer Part Number: ap2171
• Description: 1a Single Channel Current-limited Power Switch
• Manufacturer: Diodes, Inc.

AP2161/AP2171
1A SINGLE CHANNEL CURRENT-LIMITED POWER
SWITCH
Application Note
(Continued)
Thermal Protection
Thermal protection prevents the IC from damage when heavy-overload or short-circuit faults are present for
extended periods of time. The AP2161/AP2171 implements a thermal sensing to monitor the operating
junction temperature of the power distribution switch. Once the die temperature rises to approximately 145°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 And Bus-Powered HUBs
Hosts and self-powered hubs 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 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
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
AP2161/AP2171, 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 AP2161/AP2171 also ensures that the switch is off after the card
has been removed, and that the switch is off during the next insertion.
By placing the AP2161/AP2171 between the V
input and the rest of the circuitry, the input power reaches
CC
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.
AP2161/AP2171 Rev. 5
12 of 17
FEBRUARY 2009
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