Datasheet

RUN
LM25011
SS
C
SS
STOP
LM25011, LM25011-Q1
www.ti.com
SNVS617G APRIL 2009REVISED FEBRUARY 2013
N-CHANNEL BUCK SWITCH AND DRIVER
The LM25011 integrates an N-Channel buck switch and associated floating high voltage gate driver. The gate
driver circuit works in conjunction with an external bootstrap capacitor (C
BST
) and an internal high voltage diode.
A 0.1 µF capacitor connected between BST and SW provides the supply voltage for the driver during the on-
time. During each off-time, the SW pin is at approximately -1V, and C
BST
is recharged from the internal 5V
regulator for the next on-time. The minimum off-time ensures a sufficient time each cycle to recharge the
bootstrap capacitor.
SOFT-START
The soft-start feature allows the converter to gradually reach a steady state operating point, thereby reducing
startup stresses and current surges. Upon turn-on, when V
IN
reaches its under-voltage lock-out threshold an
internal 10 µA current source charges the external capacitor at the SS pin to 2.51V (t1 in Figure 17). 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 1000 pF.
The LM25011 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 POWER GOOD OUTPUT (PGD)). 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 1 mA.
SHUTDOWN FUNCTION
The SS pin can be used to shutdown the LM25011 by grounding the SS pin as shown in Figure 20. Releasing
the pin allows normal operation to resume.
Figure 20. Shutdown Implementation
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 LM25011’s
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”.
Upon powering up the LM25011, 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 reference 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 approximately 92% of the internal reference,
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 LM25011 is used as a tracking regulator (see
SOFT-START), the PGD output is high as long as the voltage at the FB pin is above the thresholds mentioned
above.
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