LM34922 Datasheet, PDF (13/17 Pages) Texas Instruments – 28V, 2A Constant On-Time Switching Regulator with Adjustable Current Limit

Shutdown Function
The SS pin can be used to shutdown the LM34922 by ground-
ing the SS pin as shown in Figure 4. Releasing the pin allows
normal operation to resume.
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FIGURE 4. Shutdown Implemetation
Power Good Output (PGD)
The Power Good output (PGD) indicates when the voltage at
the FB pin is close to the internal 2.51V reference voltage.
The rising threshold at the FB pin for the PGD output to switch
high is 95% of the internal reference. The falling threshold for
the PGD output to switch low is approximately 3.3% below the
rising threshold.
The PGD pin is internally connected to the drain of an N-
channel MOSFET switch. An external pull-up resistor
(R_PGD), connected to an appropriate voltage not exceeding
7V, is required at PGD to indicate the LM34922’s status to
other circuitry. When PGD is low, the pin’s voltage is deter-
mined by the current into the pin. See the graph “PGD Low
Voltage vs. Sink Current”.
Upon powering up the LM34922, the PGD pin is high until the
voltage at V_IN reaches 2V, at which time PGD switches low.
As V_IN is increased PGD stays low until the output voltage
takes the voltage at the FB pin above 95% of the internal ref-
erence voltage, at which time PGD switches high. As V_IN is
decreased (during shutdown) PGD remains high until either
the voltage at the FB pin falls below ≊92% of the internal ref-
erence, or when V_IN falls below its lower UVLO threshold,
whichever occurs first. PGD then switches low, and remains
low until V_IN falls below 2V, at which time PGD switches high.
If the LM34922 is used as a tracking regulator (see the Soft-
start section), the PGD output is high as long as the voltage
at the FB pin is above the thresholds mentioned above.
Thermal Shutdown
The LM34922 should be operated so the junction temperature
does not exceed 125°C. If the junction temperature increases
above that, an internal Thermal Shutdown circuit activates
(typically) at 155°C, taking the controller to a low power reset
state by disabling the buck switch and taking the SS pin to
ground. This feature helps prevent catastrophic failures from
accidental device overheating. When the junction tempera-
ture reduces below 135°C (typical hysteresis = 20°C) normal
operation resumes.
FIGURE 5. Example Circuit
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