Diodes AP2156 - Manual (Page 13)

AP2146/AP2156
Document number: DS31813 Rev. 3 - 2
13 of 17
www.diodes.com
March 2013
© Diodes Incorporated
AP2146/AP2156
Application Information
(cont.)
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.
3.3V
IN
GND
EN2
Power Supply
USB
Controller FLG2
OUT1
68uF 0.1uF
4
3
7 8
6
5V
Downstream
USB Ports
D+
D-
V
BUS
GND
AP2146
0.1uF
EN1
FLG1
1
2
OUT2
68uF 0.1uF
5
D+
D-
V
BUS
GND
Figure 2 Typical Two-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 AP2146/AP2156, 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 AP2146/AP2156 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 AP2146/AP2156 between the V
CC
input and the rest of the circuitry, the input power reaches 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.