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LM2679

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SNVS026O – MARCH 2000 – REVISED JUNE 2016

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Copyright © 2000–2016, Texas Instruments Incorporated

Application Information (continued)

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 7 A or higher to accommodate 5-A 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 7 A which would be conducted under load fault conditions.

If an application only requires a load current of 3 A or 4 A 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.

predictable control of the current limit, TI recommends keeping the peak switch current greater than 3 A. For

lower current applications a 3-A switching regulator with adjustable current limit, the LM2673, is available.

When the power switch reaches the current limit threshold it is immediately turned OFF and the internal switching

frequency is reduced. This extends the OFF time of the switch to prevent a steady-state high current condition.

As the switch current falls below the current limit threshold, the switch turns back ON. If a load fault continues,

the switch again exceeds the threshold and switch back OFF. This results in a low duty cycle pulsing of the

power switch to minimize the overall fault condition power dissipation.

This optional capacitor controls the rate at which the LM2679 starts up at power on. The capacitor is charged

linearly by an internal current source. This voltage ramp gradually increases the duty cycle of the power switch

until it reaches the normal operating duty cycle defined primarily by the ratio of the output voltage to the input

The formula for selecting a soft-start capacitor is Equation 1.

where

•

•

•

•

•

•

(1)

If this feature is not desired, leave the soft-start pin (pin 7) open circuited.

With certain soft-start capacitor values and operating conditions, the LM2679 can exhibit an overshoot on the

output voltage during turnon. Especially when starting up into no load or low load, the soft-start function may not

be effective in preventing a larger voltage overshoot on the output. With larger loads or lower input voltages

during start-up this effect is minimized. In particular, avoid using soft-start capacitors between 0.033 µF and 1 µF.

8.1.9 Additional Application Information

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.