SLVS426 − MAY 2002
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11
OVER CURRENT PROTECTION
Over current protection (OCP) is achieved by comparing the drain-to-source voltage of the high-side and low-side MOSFET
to a set-point voltage, which is defined by both the internal current source, I(TRIP), and the external resistor connected
between the VIN_SENSEx and the TRIPx pins. I(TRIP) has a typical value of 13 µA at 25°C. When the drain-to-source
voltage exceeds the set-point voltage during low-side conduction, the high-side current comparator becomes active, and
the low-side pulse is extended until this voltage comes back below the threshold. If the set-point voltage is exceeded during
high-side conduction in the following cycle, the current limit circuit terminates the high-side driver pulse. Together this action
has the effect of decreasing the output voltage until the under voltage protection circuit is activated to latch both the
high-side and low-side drivers OFF. In the TPS5130, trip current I(TRIP) has a temperature coefficient of 3400 ppm/°C in
order to compensate for temperature drift of the MOSFET on-resistance.
OCP FOR THE LDO
To achieve the LDO current limit, a sense resistor must be placed in series with the N-channel MOSFET drain, connected
between the LDO_IN and LDO_CUR pins (see reference schematic). If the voltage drop across this sense resistor exceeds
50 mV, the output voltage is reduced to approximately 22% of the nominal value, thus it activates the UVP to start the FLT
latch timer. When the time is up, the LDO_GATE pin is pulled LOW to makes the LDO regulator shut down. Note that all
of the SBRCs are latched OFF at the same time since the LDO and the SBRCs share the same FLT capacitor.
OVER VOLTAGE PROTECTION
For overvoltage protection (OVP), the TPS5130 monitors the INVx and INV_LDO pin voltages. When the INVx or INV_LDO
pin voltage is higher than 0.95 V (0.85 V +12%), the OVP comparator output goes low and the FLT timer starts to charge
an external capacitor connected to FLT pin. After a set time, the FLT circuit latches the high-side MOSFET driver, the
low-side MOSFET drivers, and the LDO. The latched state of each block is summarized in Table 2. The timer source current
for the OVP latch is 125 µA(typ.), and the time-up voltage is 1.185 V (typ.). The OVP timer is designed to be 50 times faster
than the under voltage protection timer described in Table 2.
Table 2. OVP Logic
OVP OCCURRED AT HIGH-SIDE MOSFET DRIVER LOW -SIDE MOSFET DRIVER LDO
SBRC OFF ON OFF
LDO OFF OFF OFF
UNDER VOLTAGE PROTECTION
For under voltage protection (UVP), the TPS5130 monitors the INVx and INV_LDO pin voltages. When the INVx or
INV_LDO pin voltage is lower than 0.55 V (0.85 V - 35 %), the UVP comparator output goes low, and the FLT timer starts
to charge the external capacitor connected to FLT pin. Also, when the current comparator triggers the OCP, the UVP
comparator detects the under voltage output and starts the FLT capacitor charge, too. After a set time, the FLT circuit latches
all of the MOSFET drivers to the OFF state. The timer latch source current for UVP is 2.3 µA (typ.), and the time-up voltage
is also 1.185 V (typ.). The UVP function of the LDO controller is disabled when voltage across the pass transistor is less
than 0.23 V (typ.).
FLT
When a n OVP or UVP comparator output goes low, the FLT circuit starts to charge the FLT capacitor. I f the FLT pin voltage
goes beyond a constant level, the TPS5130 latches the MOSFET drivers. At this time, the state of MOSFET is different
depending o n the OVP alert and the UVP alert (see Table 2). The enable time used to latch the MOSFET drivers is decided
by the value of the FLT capacitor . The charging constant current value depends on whether it is an OVP alert or a UVP
alert as shown in the following equation:
FLT source current (OVP) = FLT source current (UVP) × 50