AAT4685 Datasheet, PDF (10/17 Pages) Skyworks Solutions Inc. – thermal shutdown protection

DATA SHEET
AAT4685
Over-Voltage Protection Switch with OCP and BOVP
Under-Voltage Lockout (UVLO)
The AAT4685 has a fixed 2.7V under-voltage lockout
level (UVLO) trip level for IN rising. When the input volt-
age is less than 2.7V - V_{UVLO_HYS}, the OVP switch is turned
off. 100mV of hysteresis is included to ensure circuit
stability.
Over-Voltage Protection (OVP)
The AAT4685 adjustable version has a 1.0V ± 3% over-
voltage trip threshold on the OVP pin. With a resistor
divider on OVP pin from IN to GND, the over-voltage
protection point of IN can be adjusted between 4V and
7V. If the over-voltage trip level is triggered, the OVP
switch will start to be turned off in 1μs. The FLT pin is
driven low. When the input voltage pin returns below
V_OVPT – V_{OVP_HYS} (but is still above V_UVLO), the OVP switch
is turned on again after a T_{OVP_REC} recovery time to ensure
that the input supply has stabilized.
Fixed OVP products are also available where the resistive
divider is internally integrated. Pin 12 is designed to be
not connected.
Over-Current Protection (OCP)
The AAT4685 integrates adjustable over-current protec-
tion. Once the load current exceeds the pre-set limit of
I_OCP current (I_LOAD>I_OCP), the load current is limited after
the OCP blanking time (typical 180μs). During the OCP
blanking time, the OVP switch stays ON. Once the load
current drops below I_OCP, current restriction will be
removed. If the over-current state continues after the
blanking period, the OVP switch turns off and FLT is
pulled down. In order to prevent the input voltage from
spiking up due to the inductance of the input cable, the
OVP switch is turned off slowly, resulting in a "soft-stop."
The OVP switch will turn back on again after T_{OCP_REC}
(typical 64ms) and the current is monitored all over
again. Each time an OCP fault occurs, an internal counter
is incremented. If 15 OCP faults occur in one charge
cycle, the OVP switch is turned off permanently. The
counter is cleared either by removing and re-applying
input power, or by disabling and re-enabling the device
via the EN pin.
The OCP threshold is set by an external resistor (R_OCP)
connected between OCP and ground. I_OCP is approxi-
mated by the following formula:
30
I_OCP = _ROCP (current in A, resistance in kΩ)
Battery Over-Voltage Protection (BOVP)
The AAT4685 monitors the positive battery terminal and
turns the OVP switch off when the battery voltage
exceeds the BOVP threshold (typical 4.35V). The monitor
has blanking time T_{BOVP_BL} (typical 180μs) to prevent any
transient voltage from false triggering the BOVP. A resis-
tor is required between the BOVP pin and the battery, to
limit the current in case of a short circuit condition. If the
battery voltage exceeds the BOVP trip threshold V_BOVPT,
the OVP switch is turned off, and the FLT pin is driven
low. In order to prevent the input voltage from spiking
up due to the inductance of the input cable, the OVP
switch is turned off slowly , resulting in a "soft-stop." The
OVP switch is turned back on once the battery voltage
drops to V_BOVPT – V_{BOVP_HYS}. Each time a battery over-
voltage fault occurs, an internal counter is incremented.
If 15 such faults occur in one charge cycle, the OVP
switch is turned off permanently. The counter is cleared
either by removing and re-applying input power, or by
disabling and re-enabling the device via the EN pin.
Over-Temperature Protection
(OT Protection)
If the junction temperature of the device exceeds T_SHDN,
the OVP switch is turned off, and the FLT pin is driven
low. The OVP switch is turned back on when the junction
temperature falls below T_SHDN - 15°C.
Fault Indicator (FLT)
The FLT output is an active-low open-drain fault (OV/OC/
BOVP/OT) reporting output. A pull-up resistor should be
connected from FLT to the logic I/O voltage of the host
system. When EN is low, FLT is asserted by pulling down
in the case of an over-voltage, over-current, or battery
over-voltage or over-temperature fault occuring; FLT is
de-asserted into high impedance state with the fault
removed. When EN is high, FLT is in high impedance
status.
Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • [e-mail] • www.skyworksinc.com
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201960A • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • May 16, 2012