Datasheet

Operation STBP120
10/36 Doc ID 15492 Rev 5
3 Operation
The STBP120 provides overvoltage protection for positive input voltage up to 28 V using
a built-in low R
DS(on)
N-channel MOSFET switch.
3.1 Power-up
At power-up, with EN = low, the MOSFET switch is turned on after a 50 ms delay, t
on
, after
the input voltage exceeds the UVLO threshold to ensure the input voltage is stabilized. After
an additional 50 ms delay, t
start
, the FLT indication output is deactivated (see Figure 6).
The FLT
output state is valid for V
IN
input voltage 1.2 V or higher.
3.2 Normal operation
The device continuously monitors the input voltage and its own internal temperature so the
output voltage is kept within the specified range. Internal MOSFET switch is turned on and
the FLT
output is not active.
The STBP120 enters normal operation state if the input voltage returns to the interval
between V
UVLO
and V
OVLO
- V
HYS(OVLO)
and the junction temperature falls below T
OFF
-
T
HYS(OFF)
. Internal MOSFET is turned on after the 50 ms delay t
on
to ensure that the
conditions have stabilized. Then, after an additional 50 ms delay, t
start
, the FLT output is
deactivated (i.e. driven high). This behavior is equivalent to the startup shown on Figure 6.
Note: The STBP120 MOSFET switch topology allows the current to also flow in the reverse
direction, i.e. from OUT to IN, which can be useful e.g. for powering external peripherals
from the system connector (see the supply current in Figure 4). At first, the current flows
through the MOSFET body diode. If the voltage that appears on the IN terminal is above the
UVLO threshold, the MOSFET is (after the 50 ms startup delay) turned on so the voltage
drop across STBP120 is significantly reduced. The charger IC should not contain the
reverse diode to prevent the battery pack voltage from appearing on the system connector.
If the reverse current is undesirable, it may be prevented by connecting a properly rated low
drop Schottky diode in series with the OUT pin. The voltage drop between IN and charger is
increased by the voltage drop across the diode.
Due to the MOSFET body diode, thermal shutdown protection is not functional for the
supply current.
3.3 Undervoltage lockout (UVLO)
To ensure proper operation under any conditions, the STBP120 has an undervoltage lockout
(UVLO) threshold. For rising input voltage, the output remains disconnected from input until
V
IN
voltage exceeds the V
UVLO
threshold (3.25 V typ). The FLT output is driven low as long
as V
IN
is below the UVLO threshold (assuming the input voltage is above 1.2 V). For falling
input voltage, the UVLO circuit has a 50 mV hysteresis, V
HYS(UVLO)
, to improve noise
immunity under transient conditions.