LM22673EVAL National Semiconductor, LM22673EVAL Datasheet - Page 2

• Manufacturer Part Number: LM22673EVAL
• Description: BOARD EVALUATION FOR LM22673
• Manufacturer: National Semiconductor
• Series: SIMPLE SWITCHER®r

Specifications of LM22673EVAL

• Main Purpose: DC/DC, Step Down
• Outputs And Type: 1, Non-Isolated
• Voltage - Output: 3.3V
• Current - Output: 3A
• Voltage - Input: 4.5 ~ 42V
• Regulator Topology: Buck
• Frequency - Switching: 500kHz
• Board Type: Fully Populated
• Utilized Ic / Part: LM22673
• Lead Free Status / RoHS Status: Not applicable / Not applicable
• Power - Output: -
• Other names: *LM22673EVAL

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Evaluation Board Setup
Before applying power to the LM22673 evaluation board, all
external connections should be verified. The external power
supply must be turned off and connected with proper polarity
to the VIN and GND posts. A load resistor or electronic load
should be connected between the VOUT and GND posts as
desired. Both the VIN and VOUT connections should use the
closest GND posts respective to VIN or VOUT. The output
voltage can be monitored with a multi-meter or oscilloscope
at the VOUT post.
Once all connections to the evaluation board have been ver-
ified, input power can be applied. A load resistor or electronic
load does not require connection during startup. The output
voltage will ramp up when an input voltage is applied. Make
sure that the external power supply (input voltage power
source) is capable of providing enough current so that the
adjusted output voltage can be obtained. Keep in mind that
the startup current will be greater than the steady state cur-
rent.
Soft-Start
The capacitor C6 sets the amount of soft-start. A 1 µF soft-
start capacitor is used for slow start-up. See the LM22673
datasheet for details about the soft-start function.
Current Limit Adjust
Current limit can be reduced from the nominal LM22673 peak
current limit by using a resistor R3. See the LM22673
datasheet for information about the current limit adjust func-
tion. R3 is not populated on the evaluation board. This sets
the current limit to the nominal current limit as specified in the
LM22673 datasheet.
Ref #
C1
C2
C3
C4
C5
C6
D1
R1
R2
R3
U1
L1
TABLE 1. LM22673EVAL Bill of Materials for VOUT = 3.3V, Designed for 3A Output Current
8.2 µH 6.25A WE-PD L
22 µF 63V electrolytic
120 µF 6.3V 24mohm
6.8 µF 50V ceramic
10 nF 50V ceramic
60V 5A CMSH5-60
1 µF 10V ceramic
Not populated
Not populated
1.54 kΩ 1%
976Ω 1%
Value
2
Component Selection
Before changing the default components refer to the
LM22673 datasheet for information regarding component se-
lection. The WEBENCH
tool is also available at www.national.com.
The output voltage is adjustable with resistors R1 and R2
shown in the schematic. Any changes to these evaluation
board feedback resistors may require changes to the inductor
and output capacitor values. It is especially important to
change the output capacitor, C4, if the output voltages are
adjusted higher than 5V. The Schottky diode, D1, has a volt-
age rating of 60V to allow for a 42V maximum input voltage.
If the input voltage is below 38V, a 40V Schottky diode may
be used with a lower forward voltage to improve efficiency.
For improved heat dissipation, a Schottky diode in a D2PAK
package may be selected for higher efficiency. The input ca-
pacitor, C2, is not always required. This capacitor is placed
on the evaluation board to make the application robust and
minimize input voltage ringing if power is suddenly applied.
Capacitor C2 also helps stabilize the transfer function of the
converter loop. The input capacitor C1 provides the main
contribution to the switch current. Both capacitors are select-
ed with appropriate values intended only for the evaluation of
the LM22673. For production designs, the impedance of the
power source as well as the ripple current rating of the se-
lected input capacitors need to be taken into consideration
and modified accordingly. See the LM22673 datasheet for
more information.
Output capacitor C5 is not populated but space is provided to
add a second output capacitor. This second output capacitor
may be used to further reduce output voltage ripple.
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