Diodes AP2331 - Manual (Page 7)

AP2331
0.2A SINGLE CHANNEL CURRENT-LIMITED LOAD SWITCH
AP2331
Document Number: DS35529 Rev. 4 - 2
7 of 11
www.diodes.com
May 2012
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Application information
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 power supply has reached at least 2.5V
(TYP). Whenever the input voltage falls below approximately
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 over
current 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 AP2331 are outlined below:
S.NO Conditions Explanation Behavior of the AP2331
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
LIMIT
2
Short-circuit or Over current
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
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 AP2331
to limit such back currents. Reverse current limit is always active
in AP2331. Reverse current is limited at IROCP level and when
the fault exists for more than 700µs, 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 5.3V(MIN). When this fault
condition stays on for longer than 15µs, (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.
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 AP2331 implements a thermal sensing to
monitor the operating junction temperature of the power
distribution switch. Once the die temperature rises to
approximately +150°C, the Thermal protection feature gets
activated as follows: The internal thermal sense circuitry turns the
power switch off thus preventing the power switch from damage.
Hysteresis in the thermal sense circuit allows the device to cool
down to approximately +20°C before the output is turned back on.
This built-in thermal hysteresis feature is an excellent feature, as it
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.
Discharge Function
When input voltage falls below UVLO, the discharge function is
active. The output capacitor is discharged through an internal
NMOS that has a discharge resistance of 800
Ω. Hence, the output
voltage drops down to zero. The time taken for discharge is
dependent on the RC time constant of the resistance and the
output capacitor. Discharge time is calculated when UVLO falling
threshold is reached to output voltage reaching 300mV.
Power Dissipation and Junction Temperature
The low on-resistance of the internal MOSFET allows the small
surface-mount packages to pass large current. Using the
maximum operating ambient temperature (T
A ) and R DS(ON) , the
power dissipation can be calculated by:
P
D
= R
DS(ON)
× I 2
Finally, calculate the junction temperature:
T J = P D x R θJA + T A
Where:
T A = Ambient temperature °C
R θJA = Thermal resistance
P
D
= Total power dissipation