LM2673S-5.0 National Semiconductor, LM2673S-5.0 Datasheet - Page 11

• 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

Application Hints
In some cases multiple capacitors are required either to re-
duce the ESR of the output capacitor, to minimize output
ripple (a ripple voltage of 1% of Vout or less is the assumed
performance condition), or to increase the output capaci-
tance to reduce the closed loop unity gain bandwidth (to less
than 40KHz). When parallel combinations of capacitors are
required it has been assumed that each capacitor is the ex-
act same part type.
The RMS current and working voltage (WV) ratings of the
output capacitor are also important considerations. In a typi-
cal step-down switching regulator, the inductor ripple current
(set to be no more than 30% of the maximum load current by
the inductor selection) is the current that flows through the
output capacitor. The capacitor RMS current rating must be
greater than this ripple current. The voltage rating of the out-
put capacitor should be greater than 1.3 times the maximum
output voltage of the power supply. If operation of the system
at elevated temperatures is required, the capacitor voltage
rating may be de-rated to less than the nominal room tem-
perature rating. Careful inspection of the manufacturer’s
specification for de-rating of working voltage with tempera-
ture is important.
INPUT CAPACITOR
Fast changing currents in high current switching regulators
place a significant dynamic load on the unregulated power
source. An input capacitor helps to provide additional current
to the power supply as well as smooth out input voltage
variations.
Like the output capacitor, the key specifications for the input
capacitor are RMS current rating and working voltage. The
RMS current flowing through the input capacitor is equal to
one-half of the maximum dc load current so the capacitor
should be rated to handle this. Paralleling multiple capacitors
proportionally increases the current rating of the total capaci-
tance. The voltage rating should also be selected to be 1.3
times the maximum input voltage. Depending on the unregu-
lated input power source, under light load conditions the
maximum input voltage could be significantly higher than
normal operation and should be considered when selecting
an input capacitor.
The input capacitor should be placed very close to the input
pin of the LM2673. Due to relative high current operation
with fast transient changes, the series inductance of input
connecting wires or PCB traces can create ringing signals at
the input terminal which could possibly propagate to the out-
put or other parts of the circuitry. It may be necessary in
some designs to add a small valued (0.1µF to 0.47µF) ce-
ramic type capacitor in parallel with the input capacitor to
prevent or minimize any ringing.
CATCH DIODE
When the power switch in the LM2673 turns OFF, the current
through the inductor continues to flow. The path for this cur-
rent is through the diode connected between the switch out-
put and ground. This forward biased diode clamps the switch
output to a voltage less than ground. This negative voltage
must be greater than −1V so a low voltage drop (particularly
at high current levels) Schottky diode is recommended. Total
efficiency of the entire power supply is significantly impacted
by the power lost in the output catch diode. The average cur-
rent through the catch diode is dependent on the switch duty
cycle (D) and is equal to the load current times (1-D). Use of
a diode rated for much higher current than is required by the
actual application helps to minimize the voltage drop and
power loss in the diode.
(Continued)
11
During the switch ON time the diode will be reversed biased
by the input voltage. The reverse voltage rating of the diode
should be at least 1.3 times greater than the maximum input
voltage.
BOOST CAPACITOR
The boost capacitor creates a voltage used to overdrive the
gate of the internal power MOSFET. This improves efficiency
by minimizing the on resistance of the switch and associated
power loss. For all applications it is recommended to use a
0.01µF/50V ceramic capacitor.
R
A key feature of the LM2673 is the ability to control the peak
switch current. Without this feature the peak switch current
would be internally set to 5A or higher to accommodate 3A
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 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
cally 4.5A. For predictable control of the current limit it is rec-
ommended to keep the peak switch current greater than 1A.
For lower current applications 500mA and 1A switching regu-
lators, 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
SIMPLE DESIGN PROCEDURE
SST
SS
SST
OUT
SCHOTTKY
IN
ADJ
= Softstart time, from design requirements
= Maximum Input Voltage, from design requirements
ADJ
= Softstart Current, 3.7µA typical
, ADJUSTABLE CURRENT LIMIT
= Softstart Threshold Voltage, 0.63V typical
= Output Voltage, from design requirements
. A resistance of 8.2K
= Schottky Diode Voltage Drop, typically 0.5V
sets the current limit to typi-
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