LM26003MHX/NOPB National Semiconductor, LM26003MHX/NOPB Datasheet - Page 14

• Manufacturer Part Number: LM26003MHX/NOPB
• Description: IC REG BUCK SW 3A 20-TSSOP
• Manufacturer: National Semiconductor
• Series: PowerWise®r
• Type: Step-Down (Buck)r

Specifications of LM26003MHX/NOPB

• Internal Switch(s): Yes
• Synchronous Rectifier: No
• Number Of Outputs: 1
• Voltage - Output: 1.25 ~ 35 V
• Current - Output: 3A
• Frequency - Switching: 150kHz ~ 500kHz
• Voltage - Input: 4 ~ 38 V
• Operating Temperature: -40°C ~ 125°C
• Mounting Type: Surface Mount
• Package / Case: 20-TSSOP Exposed Pad, 20-eTSSOP, 20-HTSSOP
• Power - Output: 3.1W
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Other names: LM26003MHX

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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% and that the peak currents do not exceed
the minimum current threshold.
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 120
µF works well for most applications.
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
LM26003. 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:
MAX
.
14
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 PVIN
pin. For the AVIN pin also some decoupling is necessary. It
is very important that the pin is decoupled with such a capac-
itor close to the AGND pin and the GND pin of the IC to avoid
switching noise to couple into the IC. Also some RC input fil-
tering can be implemented using a small resistor between
PVIN and AVIN. In figure 7 the resistor value of R7 is selected
to be 0Ω but can be increased to filter with different time con-
stants depending on the capacitor value used. When using a
R7 resistor, keep in mind that the resistance will increase the
minimum input voltage threshold due to the voltage drop
across the resistor.
The PVIN decoupling should be implemented in a way to
minimize the trace length between the Cin capacitor gnd and
the Schottky diode gnd. 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 thresh-
old if there is not enough input capacitance. Both tantalum
and electrolytic type capacitors are suitable for the bulk ca-
pacitor. 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 C6 in the typical ap-
plication. The LM26003 provides the VDD voltage internally,
so no external diode is needed. A maximum value of 0.1 µF
is recommended for Cboot. Values smaller than 0.022 µF may
result in insufficient hold up time for the drive voltage and in-
creased 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 LM26003
enters a high frequency re-charge mode. The Cboot cap is
re-charged via the synchronous FET shown in the block dia-
gram. 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.
When selecting the catch diode for high efficiency low output
load applications, select a Schottky diode with low reverse
AVE
I
DAVE
should be calculated assuming maxi-
= Iload x (1-D)