Datasheet

LM2679
SNVS026N –MARCH 2000–REVISED APRIL 2013 www.ti.com
The peak switch current is equal to a factor of 37,125 divided by R
ADJ
. A resistance of 5.6KΩ sets the current
limit to typically 6.3A and an R
ADJ
of 8.25KΩ reduces the maximum current to approximately 4.4A. For
predictable control of the current limit it is recommended to keep the peak switch current greater than 3A. For
lower current applications a 3A 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 will turn back ON. If a load fault continues,
the switch will again exceed the threshold and switch back OFF. This will result in a low duty cycle pulsing of the
power switch to minimize the overall fault condition power dissipation.
C
SS
SOFTSTART CAPACITOR
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
voltage. The softstart turn-on time is programmable by the selection of Css.
The formula for selecting a softstart capacitor is:
(1)
Where:
I
SST
= Softstart Current, 3.7μA typical
t
SS
= Softstart time, from design requirements
V
SST
= Softstart Threshold Voltage, 0.63V typical
V
OUT
= Output Voltage, from design requirements
V
SCHOTTKY
= Schottky Diode Voltage Drop, typically 0.5V
V
IN
= Maximum Input Voltage, from design requirements
If this feature is not desired, leave the Softstart pin (pin 7) open circuited
With certain softstart capacitor values and operating conditions, the LM2679 can exhibit an overshoot on the
output voltage during turn on. Especially when starting up into no load or low load, the softstart function may not
be effective in preventing a larger voltage overshoot on the output. With larger loads or lower input voltages
during startup this effect is minimized. In particular, avoid using softstart capacitors between 0.033µF and 1µF.
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
the output capacitor varies as the square of the output voltage (½CV
2
), thus requiring an increased charging
current.
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