LM2676

SNVS031J – APRIL 2000 – REVISED APRIL 2013

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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 LM2676. 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 (0.1

μF 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 LM2676 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 0.01

μF/50V ceramic capacitor.

ADDITIONAL APPLICATION INFORMATION

When the output voltage is greater than approximately 6V, and the duty cycle at minimum input voltage is greater

than approximately 50%, the designer should exercise caution in selection of the output filter components. When

an application designed to these specific operating conditions is subjected to a current limit fault condition, it may

be possible to observe a large hysteresis in the current limit. This can affect the output voltage of the device until

the load current is reduced sufficiently to allow the current limit protection circuit to reset itself.

Under current limiting conditions, the LM267x is designed to respond in the following manner:

1. At the moment when the inductor current reaches the current limit threshold, the ON-pulse is immediately

terminated. This happens for any application condition.

2. However, the current limit block is also designed to momentarily reduce the duty cycle to below 50% to avoid

subharmonic oscillations, which could cause the inductor to saturate.

3. Thereafter, once the inductor current falls below the current limit threshold, there is a small relaxation time

during which the duty cycle progressively rises back above 50% to the value required to achieve regulation.

If the output capacitance is sufficiently ‘large’, it may be possible that as the output tries to recover, the output

capacitor charging current is large enough to repeatedly re-trigger the current limit circuit before the output has

fully settled. This condition is exacerbated with higher output voltage settings because the energy requirement of

current.

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