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LM2671

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SNVS008L – SEPTEMBER 1998 – REVISED JUNE 2016

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Copyright © 1998–2016, Texas Instruments Incorporated

9 Application and Implementation

NOTE

Information in the following applications sections is not part of the TI component

specification, and TI does not warrant its accuracy or completeness. TI’s customers are

responsible for determining suitability of components for their purposes. Customers should

validate and test their design implementation to confirm system functionality.

9.1 Application Information

The LM2671 is a step-down DC-DC regulator. The device is typically used to convert a higher DC voltage to a

lower DC voltage with a maximum output current of 0.5 A. The following design procedure can be used to select

When generating a design, the WEBENCH software uses iterative design procedure and accesses

comprehensive databases of components. See ti.com for more details.

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

greater than approximately 50%, the designer must 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. Therefore, 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 might 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

the output capacitor varies as the square of the output voltage (½ CV2), 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 recovers smoothly. Practical values of external components that have

NOTE

Even with these components, for a device’s current limit of ICLIM, the maximum load

current under which the possibility of the large current limit hysteresis can be minimized is

ICLIM/2.

For example, if the input is 24 V and the set output voltage is 18 V, then for a desired maximum current of 1.5 A,

the current limit of the chosen switcher must be confirmed to be at least 3 A. Under extreme overcurrent or short-

circuit conditions, the LM267X employs frequency foldback in addition to the current limit. If the cycle-by-cycle

inductor current increases above the current limit threshold (due to short circuit or inductor saturation for

example) the switching frequency is automatically reduced to protect the IC. Frequency below 100 kHz is typical

for an extreme short-circuit condition.