LM2678 datasheet(12/27 Pages) NSC

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Application Hints (Continued)

frequency. With the fixed 260KHz switching frequency of the

LM2678, the output capacitor is selected to provide a unity

gain bandwidth of 40KHz maximum. Each recommended

capacitor value has been chosen to achieve this result.

In some cases multiple capacitors are required either to

reduce the ESR of the output capacitor, to minimize output

ripple (a ripple voltage of 1% of Vout or less is the assumed

performance condition), or to increase the output capaci-

tance to reduce the closed loop unity gain bandwidth (to less

than 40KHz). When parallel combinations of capacitors are

required it has been assumed that each capacitor is the

exact same part type.

The RMS current and working voltage (WV) ratings of the

output capacitor are also important considerations. In a typi-

cal 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 maxi-

mum 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 tempera-

ture 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 capaci-

tance. The voltage rating should also be selected to be 1.3

times the maximum input voltage. Depending on the unregu-

lated 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 LM2678. 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 LM2678 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 recom-

mended. Total efficiency of the entire power supply is signifi-

cantly 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.

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 applica-

tion designed to these specific operating conditions is sub-

jected 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:

At the moment when the inductor current reaches the

current limit threshold, the ON-pulse is immediately ter-

minated. This happens for any application condition.

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.

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 ca-

pacitor 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 the out-

put capacitor varies as the square of the output voltage

(1⁄2CV

2), thus requiring an increased charging current.

A simple test to determine if this condition might exist for a

suspect application is to apply a short circuit across the

output of the converter, and then remove the shorted output

condition. In an application with properly selected external

components, the output will recover smoothly.

Practical values of external components that have been

experimentally found to work well under these specific oper-

ating conditions are C

OUT = 47µF, L = 22µH. It should be

noted that even with these components, for a device’s cur-

rent limit of I

CLIM, the maximum load current under which the

possibility of the large current limit hysteresis can be mini-

mized is I

CLIM/2. For example, if the input is 24V and the set

output voltage is 18V, then for a desired maximum current of

1.5A, the current limit of the chosen switcher must be con-

firmed to be at least 3A.

SIMPLE DESIGN PROCEDURE

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