Datasheet
LM26400Y
SNVS457C –FEBRUARY 2007–REVISED APRIL 2013
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In an extreme case where the load goes to less than 100mA during a large load step, output voltage may exhibit
extra undershoot. This usually happens when the load toggles high at the time V
OUT
just ramps down to its
regulation level from an overshoot. Figure 33 shows such a case where the load toggles between 1.7A and only
50mA.
Figure 33. Extreme Load Step
In the example, the load first goes down to 50mA quickly (0.9A/µs), causing a 90µs no-switching period, and
then quickly goes up to 1.7A when V
OUT1
just hits its regulation level (1.2V), resulting in a large dip of 440mV in
the output voltage.
If it is known in a system design that the load can go down to less than 100mA during a load step, and that the
load can toggle high any time after it toggles low, take the following measures to minimize the potential extra
undershoot. First is to add the Cff mentioned above. Second is to increase the output capacitance.
For example, to meet a ±10% V
OUT
excursion requirement for a 100mA to 2A load step, approximately 200µF
output capacitance is needed for a 1.2V output, and about 44µF is needed for a 5V output.
LOW INPUT VOLTAGE CONSIDERATIONS
When V
IN
is between 3V and 5V, it is recommended that an external bootstrap bias voltage and a Schottky diode
be used to handle load currents up to 2A. See Figure 34 for an illustration.
Figure 34. External Bootstrap for Low V
IN
The recommended voltage for the external bias is 5V. Due to the absolute maximum rating of V
BST
- V
SW
, the
external 5V bias should not be higher than 6V.
THERMAL SHUTDOWN
Whenever the junction temperature of the LM26400Y exceeds 165°C, the MOSFET switch will be kept off until
the temperature drops below 150°C, at which point the regulator will go through a hard-start to quickly raise the
output voltage back to normal. Since it is a hard-start, there will be an overshoot at the output. See Thermal
Shutdown in the Typical Performance Characteristics section.
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