AOZ1375DI
Rev. 1.1 May 2017 www.aosmd.com Page 12 of 17
There is no current limiting function when configured as
sink switch (current flow from VBUS to VINT), but VINT
startup rise time is still controlled by the SS pin.
If current limit function is not used, both CSP and CSN
pins must be connected to VINT.
Short-Circuit Protection (SCP)
AOZ1375DI implemented short circuit protection to
quickly turn off the power switch when output is severely
overloaded. A comparator monitors the voltage drop
between VINT and VBUS when switch is closed. Short-
circuit protection is activated when the above voltage
difference reaches approximately 75% of VIN. Short-
circuit protection functions in either sourcing or sinking
configuration.
Short-circuit protection is not active until soft start is com-
pleted.
Thermal Shutdown Protection
During current limit or short circuit, the power switch
resistance is increased to limit the load current. This
increases device power dissipation dramatically and
causes the die temperature to rise. When the die temper-
ature reaches 140°C the power switch is turned off.
There is a 35°C hysteresis. Over-temperature fault is
removed when die temperature drops below approxi-
mately 105°C.
Soft Start
When EN pin is asserted high, the soft start control
circuitry applies voltage on the gate of the power switch
in a manner such that the output voltage is ramped up
linearly until it reaches input voltage level. The soft start
can be adjusted by an external capacitor CSS connected
between SS pin and ground. The soft start can be
approximately set by the following equation:
Soft start of Output=VIN*2µA/Css -100µs
The actual soft start also depends on the output capaci-
tance and current limit setting if in-rush current reaches
current-limit level.
System Startup
The device is enabled when EN1.4V and either VBUS
or VINT is higher than UVLO threshold. The OVP thresh-
old is first selected by sensing POVP voltage set by
ROVP
. The device will then check if fault condition exist.
When no fault exists, the power switch is turned on and
the output is then ramped up, controlled by the soft start
and current limiting circuitry till it reaches input voltage.
Fault Protection
AOZ1375DI protects its load from the following fault
conditions: over-voltage, over-current, short circuit, and
over-temperature.
When device is first enabled, the power switch is off and
fault conditions are checked. If voltage at VBUS or VINT
is higher than the OVP threshold, or die temperature is
higher than thermal shutdown threshold. FLTB pin is
pulled low to report to host controller.
After the power switch turned on, device continuously
monitors all fault conditions. The switch is turned off
when OVP, short-circuit, or over-temperature is detected.
FLTB pin is pulled low.
In case of over-current, the device will limit the current
pass-through the switch to the value set by RLIM. The
switch is turned off if over-current last approximately
1ms. FLTB pin is pulled low about 500µs after OCP is
detected.
Auto-restart or Latch-off
AOZ1375DI-01 (auto-restart version): The device will
try to restart 64ms after the power switch is turned off
due to fault protection.
AOZ1375DI-02 ( latch-o ff versio n): The device keeps off
after fault occurs. It can only be re-enabled by either
toggle EN pin or recycle the power.
Input Capacitor Selection
The input capacitor prevents large voltage transients
from appearing at the input, and provides the instanta-
neous current needed each time the switch turns on to
charge output capacitors and to limit input voltage drop.
It is also to prevent high-frequency noise on the power
line from passing through to the output. The input
capacitor should be located as close to the pin as possi-
ble. A 10F ceramic capacitor is recommended. How-
ever, higher capacitor values further reduce the
transient voltage drop at the input.
Output Capacitor Selection
The output capacitor has to supply enough current for a
large load that it may encounter during system
transient. This bulk capacitor must be large enough to
supply fast transient load in order to prevent the output
from dropping.
There is an upper limit for output capacitor to ensure it
can be charged fully during start-up. This upper limit is
set by the current limit level and soft start time.
Output Capacitor (Max) = Current Limit * (t
ON
/ Input Voltage)