LM2679T-5.0 National Semiconductor, LM2679T-5.0 Datasheet - Page 12

• Manufacturer Part Number: LM2679T-5.0
• Description: IC, STEP-DOWN REGULATOR, TO-220-7
• Manufacturer: National Semiconductor

Specifications of LM2679T-5.0

• Primary Input Voltage: 12V
• No. Of Outputs: 1
• Output Voltage: 5V
• Output Current: 5A
• No. Of Pins: 7
• Operating Temperature Range: -40°C To +125°C
• Supply Voltage Range: 8V To 40V
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant

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The peak switch current is equal to a factor of 37,125 divided
by R
cally 6.3A and an R
current to approximately 4.4A. For predictable control of the
current limit it is recommended to keep the peak switch cur-
rent 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 fre-
quency 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.
Css 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 increas-
es 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:
Where:
I
t
V
V
V
V
If this feature is not desired, leave the Softstart pin (pin 7)
open circuited
With certain softstart capacitor values and operating condi-
tions, 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 out-
put 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.
SST
SS
SST
OUT
SCHOTTKY
IN
= Softstart time, from design requirements
= Maximum Input Voltage, from design requirements
= Softstart Current, 3.7μA typical
At the moment when the inductor current reaches the
current limit threshold, the ON-pulse is immediately
terminated. This happens for any application condition.
ADJ
= Softstart Threshold Voltage, 0.63V typical
= Output Voltage, from design requirements
. A resistance of 5.6KΩ sets the current limit to typi-
= Schottky Diode Voltage Drop, typically 0.5V
ADJ
of 8.25KΩ reduces the maximum
12
2.
3.
If the output capacitance is sufficiently ‘large’, it may be pos-
sible 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
quiring 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 condi-
tion. In an application with properly selected external compo-
nents, the output will recover smoothly.
Practical values of external components that have been ex-
perimentally found to work well under these specific operating
conditions are C
even with these components, for a device’s current limit of
I
the large current limit hysteresis can be minimized is I
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 confirmed to be at least
3A.
SIMPLE DESIGN PROCEDURE
Using the nomographs and tables in this data sheet (or use
the available design software at http://www.national.com) a
complete step-down regulator can be designed in a few sim-
ple steps.
Step 1: Define the power supply operating conditions:
Required output voltage
Maximum DC input voltage
Maximum output load current
Step 2: Set the output voltage by selecting a fixed output
LM2679 (3.3V, 5V or 12V applications) or determine the re-
quired feedback resistors for use with the adjustable LM2679
−ADJ
Step 3: Determine the inductor required by using one of the
four nomographs, Figure 3 through Figure 6. Table 1 provides
a specific manufacturer and part number for the inductor.
Step 4: Using Table 3 (fixed output voltage) or Table 6 (ad-
justable output voltage), determine the output capacitance
required for stable operation. Table 2 provides the specific
capacitor type from the manufacturer of choice.
Step 5: Determine an input capacitor from Table 4 for fixed
output voltage applications. Use Table 2 to find the specific
capacitor type. For adjustable output circuits select a capac-
itor from Table 2 with a sufficient working voltage (WV) rating
greater than Vin max, and an rms current rating greater than
one-half the maximum load current (2 or more capacitors in
parallel may be required).
Step 6: Select a diode from Table 5. The current rating of the
diode must be greater than I load max and the Reverse Volt-
age rating must be greater than Vin max.
Step 7: Include a 0.01μF/50V capacitor for Cboost in the de-
sign and then determine the value of a softstart capacitor if
desired.
CLIM
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.
, the maximum load current under which the possibility of
OUT
= 47µF, L = 22µH. It should be noted that
2
), thus re-
CLIM
/2.