Datasheet

LM2679
www.ti.com SNVS026N –MARCH 2000–REVISED APRIL 2013
The RMS current and working voltage (WV) ratings of the output capacitor are also important considerations. In a
typical step-down switching regulator, the inductor ripple current (set to be no more than 30% of the maximum
load current by the inductor selection) is the current that flows through the output capacitor. The capacitor RMS
current rating must be greater than this ripple current. The voltage rating of the output capacitor should be
greater than 1.3 times the maximum output voltage of the power supply. If operation of the system at elevated
temperatures is required, the capacitor voltage rating may be de-rated to less than the nominal room temperature
rating. Careful inspection of the manufacturer's specification for de-rating of working voltage with temperature is
important.
INPUT CAPACITOR
Fast changing currents in high current switching regulators place a significant dynamic load on the unregulated
power source. An input capacitor helps to provide additional current to the power supply as well as smooth out
input voltage variations.
Like the output capacitor, the key specifications for the input capacitor are RMS current rating and working
voltage. The RMS current flowing through the input capacitor is equal to one-half of the maximum dc load current
so the capacitor should be rated to handle this. Paralleling multiple capacitors proportionally increases the
current rating of the total capacitance. The voltage rating should also be selected to be 1.3 times the maximum
input voltage. Depending on the unregulated input power source, under light load conditions the maximum input
voltage could be significantly higher than normal operation and should be considered when selecting an input
capacitor.
The input capacitor should be placed very close to the input pin of the LM2679. Due to relative high current
operation with fast transient changes, the series inductance of input connecting wires or PCB traces can create
ringing signals at the input terminal which could possibly propagate to the output or other parts of the circuitry. It
may be necessary in some designs to add a small valued (0.1μF to 0.47μF) ceramic type capacitor in parallel
with the input capacitor to prevent or minimize any ringing.
CATCH DIODE
When the power switch in the LM2679 turns OFF, the current through the inductor continues to flow. The path for
this current is through the diode connected between the switch output and ground. This forward biased diode
clamps the switch output to a voltage less than ground. This negative voltage must be greater than −1V so a low
voltage drop (particularly at high current levels) Schottky diode is recommended. Total efficiency of the entire
power supply is significantly impacted by the power lost in the output catch diode. The average current through
the catch diode is dependent on the switch duty cycle (D) and is equal to the load current times (1-D). Use of a
diode rated for much higher current than is required by the actual application helps to minimize the voltage drop
and power loss in the diode.
During the switch ON time the diode will be reversed biased by the input voltage. The reverse voltage rating of
the diode should be at least 1.3 times greater than the maximum input voltage.
BOOST CAPACITOR
The boost capacitor creates a voltage used to overdrive the gate of the internal power MOSFET. This improves
efficiency by minimizing the on resistance of the switch and associated power loss. For all applications it is
recommended to use a 0.01μF/50V ceramic capacitor.
R
ADJ
, ADJUSTABLE CURRENT LIMIT
A key feature of the LM2679 is the ability to control the peak switch current. Without this feature the peak switch
current would be internally set to 7A or higher to accommodate 5A load current designs. This requires that both
the inductor (which could saturate with excessively high currents) and the catch diode be able to safely handle
up to 7A which would be conducted under load fault conditions.
If an application only requires a load current of 3A or 4A the peak switch current can be set to a limit just over the
maximum load current with the addition of a single programming resistor. This allows the use of less powerful
and more cost effective inductors and diodes.
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