Datasheet

Irms =
Vin
Vout x (Vin ± Vout)
Iload x
LM26003
SNVS576D –AUGUST 2008–REVISED MARCH 2013 www.ti.com
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 temperature as well as the
maximum input voltage. The equation for calculating the RMS input ripple current is shown below:
(15)
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 capacitor 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 filtering can be implemented using a small resistor between PVIN and
AVIN. In Figure 19 the resistor value of R7 is selected to be 0Ω but can be increased to filter with different time
constants 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 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 C6 in the typical application. 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 increased power
dissipation.
During low Vin operation, when the on-time is extended, the bootstrap capacitor is at risk of discharging. If the
Cboot capacitor 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 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:
I
DAVE
= Iload x (1-D) (16)
Where D is the duty-cycle, defined as Vout/Vin. The catch diode conducts the largest currents during the lowest
duty-cycle. Therefore ID
AVE
should be calculated assuming maximum input voltage. The diode should be rated to
handle this current continuously. For over-current or short circuit conditions, 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 maximizes 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 leakage current. Also keep in mind that the reverse leakage current of a Schottky diode increases with
temperature and with reverse voltage. Reverse voltage equals roughly the input voltage in a buck converter. At
hot, the diode reverse leakage current may be larger than the current consumption of the LM26003.
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