Datasheet
LM26480
SNVS543I –JANUARY 2008–REVISED MAY 2013
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Shutdown Mode
During shutdown the PFET switch, reference, control and bias circuitry of the converters are turned off. The
NFET switch will be on in shutdown to discharge the output. When the converter is enabled, soft start is
activated. It is recommended to disable the converter during the system power up and under voltage conditions
when the supply is less than 2.8V.
Soft Start
The soft-start feature allows the power converter to gradually reach the initial steady state operating point, thus
reducing startup stresses and surges. The two LM26480 buck converters have a soft-start circuit that limits in-
rush current during startup. During startup the switch current limit is increased in steps. Soft start is activated
only if EN goes from logic low to logic high after V
IN
reaches 2.8V. Soft start is implemented by increasing switch
current limit in steps of 250 mA, 500 mA, 950 mA and 2A for both bucks (typ. switch current limit). The startup
time thereby depends on the output capacitor and load current demanded at start-up.
Low Dropout Operation
The LM26480 can operate at 100% duty cycle (no switching; PMOS switch completely on) for low dropout
support of the output voltage. In this way the output voltage will be controlled down to the lowest possible input
voltage. When the device operates near 100% duty cycle, output voltage ripple is approximately 25 mV. The
minimum input voltage needed to support the output voltage is
V
IN,
MIN = I
LOAD
* (R
DSON, PFET
+ R
INDUCTOR
) + V
OUT
I
LOAD
Load current
R
DSON, PFET
Drain to source resistance of
PFET switch in the triode region
R
INDUCTOR
Inductor resistance
Flexible Power-On Reset (i.e., Power Good with Delay)
The LM26480 is equipped with an internal Power-On-Reset (“POR”) circuit which monitors the output voltage
levels on bucks 1 and 2. The nPOR is an open drain logic output which is logic LOW when either of the buck
outputs are below 92% of the rising value, or when one or both outputs fall below 82% of the desired value. The
time delay between output voltage level and nPOR is enabled is (130 µs, 60 ms, 100 ms, 200 ms), 60 ms by
default. For any other delay option, other than the default, please consult a Texas Instruments Sales
Representative. The system designer can choose the external pull-up resistor (i.e. 100 kΩ ) for the nPOR pin.
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