Diodes AP2337 - Manual (Page 7)

AP2337
1.0A SINGLE CHANNEL CURRENT-LIMITED LOAD SWITCH
AP2337
Document number: DS35060 Rev. 2 - 2
7 of 10
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October 2011
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Application Note
Protection Features:
Under-voltage Lockout (UVLO)
Whenever the input voltage falls below UVLO threshold (~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 overcurrent 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 AP2337 are outlined below:
NO Conditions Explanation Behavior of the AP2337
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 output current to a certain safe level
namely I
short
2
Short-circuit or
Overcurrent 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 current limiting mode
and the current is clamped at I
short
/I
LIMIT
.
3
Gradual increase from
nominal operating current
to I
LIMIT
Load increases gradually
until the current-limit
threshold. The current rises until I
LIMIT
. Once the threshold
has been reached, the device switches into its
current limiting mode and is clamped at I
LIMIT
.
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 AP2337 to limit such back currents. This circuit is
activated by the difference between the output voltage and the input voltage (100mV TYP). After the reverse current circuit
has tripped (reached the reverse current trip threshold), the current rises until I
ROCP
level. When the fault exists for more than
600us, 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 114% (TYP) of input
voltage. When this fault condition stays on for longer than 15us (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.