LM34923 Datasheet, PDF (14/26 Pages) National Semiconductor (TI) – EVAL evaluation board provides the design engineer with a fully

LM34923
SNVS695A – MARCH 2011 – REVISED FEBRUARY 2013
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During each off-time, the SW pin is at approximately 0V, and the bootstrap capacitor charges from Vcc through
the internal diode. The minimum OFF timer, set to 260 ns, ensures a minimum time each cycle to recharge the
bootstrap capacitor.
The internal pre-charge switch at the SW pin is turned on for ≊150 ns during the minimum off-time period,
ensuring sufficient voltage exists across the bootstrap capacitor for the on-time. This feature helps prevent
operating problems which can occur during very light load conditions, involving a long off-time, during which the
voltage across the bootstrap capacitor could otherwise reduce below the Gate Drive UVLO threshold. The pre-
charge switch also helps prevent startup problems which can occur if the output voltage is pre-charged prior to
turn-on. After current limit detection, the pre-charge switch is turned on for the entire duration of the forced off-
time .
Under Voltage Detector
The Under Voltage Detector can be used to monitor the input voltage, or any other system voltage as long as the
voltage at the UV pin does not exceed its maximum rating.
The Under Voltage Output indicator pin (UVO) is connected to the drain of an internal N-channel MOSFET
capable of sustaining 10V in the off-state. An external pull-up resistor is required at UVO to an appropriate
voltage to indicate the status to downstream circuitry. The off-state voltage at the UVO pin can be higher or lower
than the voltage at VIN, but must not exceed 10V.
The UVO pin switches low when the voltage at the UV input pin is above its threshold. Typically the monitored
voltage threshold is set with a resistor divider (R_UV1, R_UV2) as shown in the Block Diagram. When the voltage at
the UV pin is below its threshold, the internal 5 µA current source at UV is enabled. As the input voltage
increases, taking UV above its threshold, the current source is disabled, raising the voltage at UV to provide
threshold hysteresis.
The UVO output is high when the VCC voltage is below its UVLO threshold, or when the LM34923 is shutdown
using the RT/SD pin (see Figure 32), regardless of the voltage at the UV pin.
Thermal Protection
The LM34923 should be operated so the junction temperature does not exceed 125°C during normal operation.
An internal Thermal Shutdown circuit is provided to shutdown the LM34923 in the event of a higher than normal
junction temperature. When activated, typically at 165°C, the controller is forced into a low power reset state by
disabling the buck switch. This feature prevents catastrophic failures from accidental device overheating. When
the junction temperature reduces below 145°C (typical hysteresis = 20°C) normal operation is resumed.
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