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LM2672

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SNVS136L – SEPTEMBER 1998 – REVISED JUNE 2016

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Copyright © 1998–2016, Texas Instruments Incorporated

10 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.

10.1 Application Information

The LM2672 is a step-down DC-DC regulator. It is typically used to convert a higher DC voltage to a lower DC

voltage with a maximum output current of 1 A. The following design procedure can be used to select components

for the LM2672.

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. 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

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

been experimentally found to work well under these specific operating conditions are COUT = 47 µF, L = 22 µH.

Note that even with these components, for a device’s current limit of ICLIM, the maximum load current under

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.