LM2673S-5.0 National Semiconductor, LM2673S-5.0 Datasheet - Page 12

• Manufacturer Part Number: LM2673S-5.0
• Description: DC/DC Converter IC
• Manufacturer: National Semiconductor

Specifications of LM2673S-5.0

• Input Voltage: 40V
• Output Current: 3A
• Output Voltage: 5V
• No. Of Pins: 7
• Termination Type: SMD
• Mounting Type: Through Hole
• Voltage Regulator Type: Switching
• Output Current Max: 3A
• Peak Reflow Compatible (260 C): No
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant

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Application Hints
(which could saturate with excessively high currents) and the
catch diode be able to safely handle up to 5A which would be
conducted under load fault conditions.
If an application only requires a load current of 2A or so the
peak switch current can be set to a limit just over the maxi-
mum load current with the addition of a single programming
resistor. This allows the use of less powerful and more cost
effective inductors and diodes.
The peak switch current is equal to a factor of 37,125 divided
by R
typically 4.5A. For predictable control of the current limit it is
recommended to keep the peak switch current greater than
1A. For lower current applications 500mA and 1A switching
regulators, the LM2674 and LM2672, are 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 LM2673
starts up at power on. The capacitor is charged linearly by an
internal current source. This voltage ramp gradually in-
creases 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
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 applica-
tion designed to these specific operating conditions is sub-
jected 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
SST
SS
SST
OUT
SCHOTTKY
IN
= Softstart time, from design requirements
= Maximum Input Voltage, from design requirements
current limit threshold, the ON-pulse is immediately ter-
minated. This happens for any application condition.
= Softstart Current, 3.7µA typical
ADJ
= Softstart Threshold Voltage, 0.63V typical
= Output Voltage, from design requirements
. A resistance of 8.2KΩ sets the current limit to
= Schottky Diode Voltage Drop, typically 0.5V
(Continued)
12
2. However, the current limit block is also designed to
3. Thereafter, once the inductor current falls below the
If the output capacitance is sufficiently ‘large’, it may be
possible that as the output tries to recover, the output ca-
pacitor 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 out-
put capacitor varies as the square of the output voltage
(
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 will recover smoothly.
Practical values of external components that have been
experimentally found to work well under these specific oper-
ating conditions are C
noted that even with these components, for a device’s cur-
rent limit of I
possibility of the large current limit hysteresis can be mini-
mized is I
output voltage is 18V, then for a desired maximum current of
1.5A, the current limit of the chosen switcher must be con-
firmed 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
simple 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
LM2673 (3.3V, 5V or 12V applications) or determine the
required feedback resistors for use with the adjustable
LM2673−ADJ
Step 3: Determine the inductor required by using one of the
four nomographs, Figure 3 through Figure 6. Table 1 pro-
vides a specific manufacturer and part number for the induc-
tor.
Step 4: Using Table 3 (fixed output voltage) or Table 6
(adjustable output voltage), determine the output capaci-
tance 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 capaci-
tor 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
Voltage rating must be greater than Vin max.
1
⁄
2
CV
momentarily reduce the duty cycle to below 50% to
avoid subharmonic oscillations, which could cause the
inductor to saturate.
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.
2
), thus requiring an increased charging current.
CLIM
CLIM
/2. For example, if the input is 24V and the set
, the maximum load current under which the
OUT
= 47µF, L = 22µH. It should be