STBP112 Datasheet, PDF (10/38 Pages) STMicroelectronics – Overvoltage protection device

Operation
3
Operation
STBP112
The STBP112 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 the startup delay, t_on,
after the input voltage exceeds the UVLO threshold to ensure the input voltage is stabilized
(see Figure 5).
3.2
Note:
Normal operation
The device continuously monitors the input voltage and its own internal temperature so the
output voltage is kept within the specified range. The internal MOSFET switch is turned on
and the FLT output is deasserted.
The STBP112 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). The internal MOSFET is turned on after the t_rec delay to ensure that the conditions
have stabilized and the FLT output is deasserted.
The STBP112 MOSFET switch topology allows the current to flow also in a reverse
direction, i.e. from OUT to IN, which can be useful 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 startup delay) turned on so the voltage drop across
STBP112 is significantly reduced.
If the reverse current is undesirable, it may be prevented by connecting an external, 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.
3.3
Undervoltage lockout (UVLO)
To ensure proper operation under any condition, the STBP112 has an undervoltage lockout
(UVLO) threshold. When the input voltage is rising, the output remains disconnected from
input until the V_IN voltage exceeds the V_UVLO threshold. This circuit is equipped with
hysteresis, V_HYS(UVLO), to improve noise immunity under transient conditions.
3.4
Overvoltage lockout (OVLO)
If the input voltage V_IN rises above the threshold level V_OVLO, the MOSFET switch is
immediately turned off. At the same time, the fault indication output FLT is activated
(i.e. driven low), see Figure 6. This device is equipped with hysteresis, V_HYS(OVLO), to
improve noise immunity under transient conditions.
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Doc ID 023357 Rev 3