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LM5069
SNVS452G –SEPTEMBER 2006–REVISED JAUNUARY 2020
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8.3 Feature Description
8.3.1 Current Limit
The current limit threshold is reached when the voltage across the sense resistor RS(VIN to SENSE) reaches
55 mV. In the current limiting condition, the GATE voltage is controlled to limit the current in MOSFET Q1. While
the current limit circuit is active, the fault timer is active as described in Fault Timer and Restart. If the load
current falls below the current limit threshold before the end of the fault timeout period, the LM5069 resumes
normal operation. For proper operation, the RSresistor value must be no larger than 100 mΩ.
8.3.2 Circuit Breaker
If the load current increases rapidly (for example, the load is short-circuited) the current in the sense resistor (RS)
may exceed the current limit threshold before the current limit control loop is able to respond. If the current
exceeds twice the current limit threshold (105 mV/RS), Q1 is quickly switched off by the 230-mA pulldown current
at the GATE pin, and a fault timeout period begins. When the voltage across RSfalls below 105 mV the 230-mA
pulldown current at the GATE pin is switched off, and the gate voltage of Q1 is then determined by the current
limit or the power limit functions. If the TIMER pin reaches 4 V before the current limiting or power limiting
condition ceases, Q1 is switched off by the 2-mA pulldown current at the GATE pin as described in Fault Timer
and Restart.
8.3.3 Power Limit
An important feature of the LM5069 is the MOSFET power limiting. The Power Limit function can be used to
maintain the maximum power dissipation of MOSFET Q1 within the device SOA rating. The LM5069 determines
the power dissipation in Q1 by monitoring its drain-source voltage (SENSE to OUT), and the drain current
through the sense resistor (VIN to SENSE). The product of the current and voltage is compared to the power
limit threshold programmed by the resistor at the PWR pin. If the power dissipation reaches the limiting threshold,
the GATE voltage is modulated to reduce the current in Q1. While the power limiting circuit is active, the fault
timer is active as described in Fault Timer and Restart.
8.3.4 Undervoltage Lockout (UVLO)
The series pass MOSFET (Q1) is enabled when the input supply voltage (VSYS) is within the operating range
defined by the programmable undervoltage lockout (UVLO) and overvoltage lockout (OVLO) levels. Typically the
UVLO level at VSYS is set with a resistor divider (R1-R3) as shown in Figure 30. When VSYS is below the UVLO
level, the internal 21-µA current source at UVLO is enabled, the current source at OVLO is off, and Q1 is held off
by the
2-mA pulldown current at the GATE pin. As VSYS is increased, raising the voltage at UVLO above 2.5 V, the
21-µA current source at UVLO is switched off, increasing the voltage at UVLO, providing hysteresis for this
threshold. With the UVLO pin above 2.5 V, Q1 is switched on by the 16-µA current source at the GATE pin if the
insertion time delay has expired (Figure 22). See Application and Implementation for a procedure to calculate the
values of the threshold setting resistors (R1-R3). The minimum possible UVLO level at VSYS can be set by
connecting the UVLO pin to VIN. In this case Q1 is enabled when the VIN voltage reaches the POREN threshold.
8.3.5 Overvoltage Lockout (OVLO)
The series pass MOSFET (Q1) is enabled when the input supply voltage (VSYS) is within the operating range
defined by the programmable undervoltage lockout (UVLO) and overvoltage lockout (OVLO) levels. If VSYS raises
the OVLO pin voltage above 2.5 V, Q1 is switched off by the 2-mA pulldown current at the GATE pin, denying
power to the load. When the OVLO pin is above 2.5 V, the internal 21-µA current source at OVLO is switched on,
raising the voltage at OVLO to provide threshold hysteresis. When VSYS is reduced below the OVLO level Q1 is
enabled. See Application and Implementation for a procedure to calculate the threshold setting resistor values.
8.3.6 Power Good Pin
During turnon, the Power Good pin (PGD) is high until the voltage at VIN increases above ≊5 V. PGD then
switches low, remaining low as the VIN voltage increases. When the voltage at OUT increases to within 1.25 V of
the SENSE pin (VDS <1.25 V), PGD switches high. PGD switches low if the VDS of Q1 increases above 2.5 V. A
pullup resistor is required at PGD as shown in Figure 20. The pullup voltage (VPGD) can be as high as 80 V, with
transient capability to 100 V, and can be higher or lower than the voltages at VIN and OUT.