LM26001BMH NSC [National Semiconductor], LM26001BMH Datasheet - Page 13

• Manufacturer Part Number: LM26001BMH
• Description: 1.5A Switching Regulator with High Efficiency Sleep Mode
• Manufacturer: NSC [National Semiconductor]

For example, at a maximum load of 1.5A and a ripple content
of 33%, peak inductor current is equal to 1.75A which is safely
below the minimum current limit of 1.80A. By increasing the
inductor size, ripple content and peak inductor current are
lowered, which increases the current limit margin.
The size of the output inductor can also be determined using
the desired output ripple voltage, Vrip. The equation to deter-
mine the minimum inductance value based on Vrip is as
follows:
Where Re is the ESR of the output capacitors, and Vrip is a
peak-to-peak value. This equation assumes that the output
capacitors have some amount of ESR. It does not apply to
ceramic output capacitors.
If this method is used, ripple content should still be verified to
be less than 40%.
OUTPUT CAPACITOR
The primary criterion for selecting an output capacitor is
equivalent series resistance, or ESR.
ESR (Re) can be selected based on the requirements for out-
put ripple voltage and transient response. Once an inductor
value has been selected, ripple voltage can be calculated for
a given Re using the equation above for Lmin. Lower ESR
values result in lower output ripple.
Re can also be calculated from the following equation:
Where ΔVt is the allowed voltage excursion during a load
transient, and ΔIt is the maximum expected load transient.
If the total ESR is too high, the load transient requirement
cannot be met, no matter how large the output capacitance.
If the ESR criteria for ripple voltage and transient excursion
cannot be met, more capacitors should be used in parallel.
For non-ceramic capacitors, the minimum output capacitance
is of secondary importance, and is determined only by the
load transient requirement.
If there is not enough capacitance, the output voltage excur-
sion will exceed the maximum allowed value even if the
maximum ESR requirement is met. The minimum capaci-
tance is calculated as follows:
It is assumed the total ESR, Re, is no greater than Re
Also, it is assumed that L has already been selected.
Generally speaking, the output capacitance requirement de-
creases with Re, ΔIt, and L. A typical value greater than 100
µF works well for most applications.
MAX
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INPUT CAPACITOR
In a switching converter, very fast switching pulse currents
are drawn from the input rail. Therefore, input capacitors are
required to reduce noise, EMI, and ripple at the input to the
LM26001B. Capacitors must be selected that can handle both
the maximum ripple RMS current at highest ambient temper-
ature as well as the maximum input voltage. The equation for
calculating the RMS input ripple current is shown below:
For noise suppression, a ceramic capacitor in the range of 1.0
µF to 10 µF should be placed as close as possible to the VIN
pin.
A larger, high ESR input capacitor should also be used. This
capacitor is recommended for damping input voltage spikes
during power on and for holding up the input voltage during
transients. In low input voltage applications, line transients
may fall below the UVLO threshold if there is not enough input
capacitance. Both tantalum and electrolytic type capacitors
are suitable for the bulk capacitor. However, large tantalums
may not be available for high input voltages and their working
voltage must be derated by at least 2X.
BOOTSTRAP
The drive voltage for the internal switch is supplied via the
BOOT pin. This pin must be connected to a ceramic capacitor,
Cboot, from the switch node, shown as C4 in the typical ap-
plication. The LM26001B provides the VDD voltage internally,
so no external diode is needed. A minimum value of 0.1 uF is
recommended for Cboot. Smaller values may result in insuf-
ficient hold up time for the drive voltage and increased power
dissipation.
During low Vin operation, when the on-time is extended, the
bootstrap capacitor is at risk of discharging. If the Cboot ca-
pacitor is discharged below approximately 2.5V, the
LM26001B enters a high frequency re-charge mode. The
Cboot cap is re-charged via the LG synchronous FET shown
in the block diagram. Switching returns to normal when the
Cboot cap has been recharged.
CATCH DIODE
When the internal switch is off, output current flows through
the catch diode. Alternately, when the switch is on, the diode
sees a reverse voltage equal to Vin. Therefore, the important
parameters for selecting the catch diode are peak current and
peak inverse voltage. The average current through the diode
is given by:
Where D is the duty cycle, defined as Vout/Vin. The catch
diode conducts the largest currents during the lowest duty
cycle. Therefore ID
mum input voltage. The diode should be rated to handle this
current continuously. For over-current or short circuit condi-
tions, the catch diode should be rated to handle peak currents
equal to the peak current limit.
The peak inverse voltage rating of the diode must be greater
than maximum input voltage.
A Schottky diode must be used. It's low forward voltage max-
imizes efficiency and BOOT voltage, while also protecting the
SW pin against large negative voltage spikes
AVE
ID
AVE
should be calculated assuming maxi-
= Iload x (1-D)
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