LM26400YMH NSC [National Semiconductor], LM26400YMH Datasheet - Page 14

• Manufacturer Part Number: LM26400YMH
• Description: Dual 2A, 500kHz Wide Input Range Buck Regulator
• Manufacturer: NSC [National Semiconductor]

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dershoot. This usually happens when the load toggles high at
the time V
overshoot. Figure 4 shows such a case where the load tog-
gles between 1.7A and only 50mA.
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
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
a 100mA to 2A load step, approximately 200µF output ca-
pacitance is needed for a 1.2V output, and about 44µF is
needed for a 5V output.
LOW INPUT VOLTAGE CONSIDERATIONS
When V
external bootstrap bias voltage and a Schottky diode be used
to handle load currents up to 2A. See Figure 5 for an illustra-
tion.
FIGURE 5. External Bootstrap for Low V
IN
OUT
is between 3V and 5V, it is recommended that an
FIGURE 4. Extreme Load Step
just ramps down to its regulation level from an
OUT
OUT1
excursion requirement for
just hits its regulation
20200241
20200244
IN
14
The recommended voltage for the external bias is 5V. Due to
the absolute maximum rating of V
bias should not be higher than 6V.
THERMAL SHUTDOWN
Whenever the junction temperature of the LM26400Y ex-
ceeds 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 over-
shoot at the output. See Thermal Shutdown in the Typical
Performance Characteristics section.
POWER LOSS ESTIMATION
The total power loss in the LM26400Y comprises of three
parts - the power FET conduction loss, the power FET switch-
ing loss and the IC's housekeeping power loss. Use the
following equation to estimate the conduction loss.
where T
temperature if the former is unknown. R
tance of the internal FET at room temperature. Use 180mΩ
for R
Use the following equation to estimate the switching loss.
Another loss in the IC is the housekeeping loss. It is the power
dissipated by circuitry in the IC other than the power FETs.
The equation is:
The 15mW is gate drive loss. Do the calculation for both
channels and find out the total power loss in the IC.
The power loss calculation can help estimate the overall pow-
er supply efficiency.
Example:
V
Target junction temperature is 90°C.
So conduction loss in Channel 1 is:
Conduction loss in Channel 2 is:
Switching loss in either channel is:
IN
= 12V, V
DS
if the actual value is unknown.
J
is the junction temperature or the target junction
OUT1
= 1.2V, I
OUT1
= 2A, V
BST
OUT2
- V
SW
DS
= 2.5V, I
, the external 5V
is the ON resis-
OUT2
= 2A.