RUN

LM34922

SS

STOP

LM34922

SNVS813A – JUNE 2012 – REVISED MARCH 2013

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Soft-Start

The soft-start feature allows the converter to gradually reach a steady state operating point, thereby reducing

internal 10µA current source charges the external capacitor at the SS pin to 2.51V (t1 in Figure 15). The ramping

voltage at SS ramps the non-inverting input of the regulation comparator, and the output voltage, in a controlled

manner. For proper operation, the soft-start capacitor should be no smaller than 1000pF.

The LM34922 can be employed as a tracking regulator by applying the controlling voltage to the SS pin. The

regulator’s output voltage tracks the applied voltage, gained up by the ratio of the feedback resistors. The applied

voltage at the SS pin must be within the range of 0.5V to 2.6V. The absolute maximum rating for the SS pin is

3.0V. If the tracking function causes the voltage at the FB pin to go below the thresholds for the PGD pin, the

PGD pin will switch low (see the Power Good Output (PGD) section). An internal switch grounds the SS pin if the

input voltage at VIN is below its under-voltage lock-out threshold or if the Thermal Shutdown activates. If the

tracking function (described above) is used, the tracking voltage applied to the SS pin must be current limited to

a maximum of 1mA.

Shutdown Function

The SS pin can be used to shutdown the LM34922 by grounding the SS pin as shown in Figure 18. Releasing

the pin allows normal operation to resume.

Figure 18. 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

status to other circuitry. When PGD is low, the pin’s voltage is determined by the current into the pin. See the

graph “ PGD Low Voltage vs. Sink Current”.

PGD remains high until either the voltage at the FB pin falls below

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 temperature reduces

below 135°C (typical hysteresis = 20°C) normal operation resumes.

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