Diodes AP2181D/AP2191D User Manual
Page 9
AP2181D/AP2191D
Document number: DS32251 Rev. 3 - 2
9 of 18
March 2013
© Diodes Incorporated
AP2181D/AP2191D
Application Note
The AP2181D and AP2191D are integrated high-side power switches optimized for Universal Serial Bus (USB) that require protection functions.
The power switches are equipped with a driver that controls the gate voltage and incorporates slew-rate limitation. This, along with the various
protection features and special functions, makes these power switches ideal for hot-swap or hot-plug applications.
Protection Features:
Under-Voltage Lockout (UVLO)
Under-voltage lockout function (UVLO) guarantees that the internal power switch is initially off during start-up. The UVLO functions only when the
switch is enabled. Even if the switch is enabled, the switch is not turned ON until the power supply has reached at least 1.9V. Whenever the input
voltage falls below approximately 1.9V, 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 AP2181D/AP2191D are outlined below:
S. NO
Conditions
Explanation
Behavior of the AP2181D/AP2191D
1
Short circuit condition at start-up
Output is shorted before
input voltage is applied or
before the part is enabled.
The IC senses the short circuit and immediately clamps
output current to a certain safe level namely I
LIMIT.
2
Short-circuit or over current
condition
Short-circuit or overload
condition that occurs when
the part is enabled.
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
LIMIT
.
3
Gradual increase from nominal
operating current to I
LIMIT
Load increases gradually
until the current-limit
threshold.(I
TRIG
)
The current rises until I
TRIG
or thermal limit. Once the
threshold has been reached, the device switches into its
current limiting mode and is set at I
LIMIT
.
Note that when the output has been shorted to GND at extremely low temperature (< -20°C), a minimum 120-
μF electrolytic capacitor on the
output pin is recommended. A correct capacitor type with capacitor voltage rating and temperature characteristics must be properly chosen so
that capacitance value does not drop too low at the extremely low temperature operation. A recommended capacitor should have temperature
characteristics of less than 10% variation of capacitance change when operated at extremely low temp. Our recommended aluminum electrolytic
capacitor type is Panasonic FC series.
Thermal Protection
Thermal protection prevents the IC from damage when the die temperature exceeds safe margins. This mainly occurs when heavy-overload or
short-circuit faults are present for extended periods of time. The AP2181D/AP2191D implements thermal sensing to monitor the operating junction
temperature of the power distribution switch. Once the die temperature rises to approximately +140°C, the Thermal protection feature gets
activated as follows: The internal thermal sense circuitry turns the power switch off and the FLG output is asserted thus preventing the power
switch from damage. Hysteresis in the thermal sense circuit allows the device to cool down to approximately +25°C before the output is turned
back on. The built-in thermal hysteresis feature avoids undesirable oscillations of the thermal protection circuit. The switch continues to cycle in
this manner until the load fault is removed, resulting in a pulsed output. The FLG open-drain output is asserted when an over-current occurs with
7-ms deglitch.
Reverse Current Protection
In a normal MOSFET switch, current can flow in reverse direction (from the output side to the input side) when the output side voltage is higher
than the input side, even when the switch is turned off. A reverse-current blocking feature is implemented in the AP21x1 series to prevent such
back currents. This circuit is activated by the difference between the output voltage and the input voltage. When the switch is disabled, this feature
blocks reverse current flow from the output back to the input.