LM25574EVAL National Semiconductor, LM25574EVAL Datasheet - Page 2

• Manufacturer Part Number: LM25574EVAL
• Description: BOARD EVALUATION FOR LM25574
• Manufacturer: National Semiconductor
• Series: PowerWise®, SIMPLE SWITCHER®r

Specifications of LM25574EVAL

• Main Purpose: DC/DC, Step Down
• Outputs And Type: 1, Non-Isolated
• Voltage - Output: 5V
• Current - Output: 500mA
• Voltage - Input: 7 ~ 42V
• Regulator Topology: Buck
• Frequency - Switching: 300kHz
• Board Type: Fully Populated
• Utilized Ic / Part: LM25574
• Lead Free Status / RoHS Status: Not applicable / Not applicable
• Power - Output: -
• Other names: *LM25574EVAL

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The selected operating frequency must be less than Fsw
(max) calculated above, if not reduce the operating frequen-
cy.
Step 4. Calculate the value of Rt for the selected operating
frequency.
Step 5. Calculate the value of L1.
Select the nearest standard inductor value. During an over-
load condition the peak inductor current is limited to 0.7A
nominal (0.8A maximum). The selected inductor must be rat-
ed for peak current of at least 0.8 Amps.
Step 6. Calculate the value of C
Step 7. Set Rfb2 to 5kOhms if Vout[1] is less than or equal to
5 Volts. If Vout[1] is greater than 5V set Rfb2 to 10K Ohms.
Calculate the value of Rfb1:
Step 8. Select the re-circulating diode, D1. A Schottky type
diode is required for all applications. Ultra-fast diodes are not
recommended and may result in damage to the IC due to re-
verse recovery current transients. The reverse breakdown
rating should be greater than the Input Voltage Max Spec[3],
plus some safety margin. For worst case design, assume a
short circuit load condition. In this case the diode will carry the
output current almost continuously. This current can be as
high as 0.8A. Assuming a 0.6V drop across the diode, the
maximum diode power dissipation can be as high as 0.5W.
An SMA case or similar package is recommended.
Selected operating frequency Fsw:
Selected value for R1:
Selected value of L1:
Selected value of C
Selected value of Rfb2:
Selected value of Rfb1:
C
RAMP
RAMP
:
= L1[7] x 5 x 10
RAMP
:
-6
Ohms [6]
Henrys [7]
Ohms [9]
Farads [8]
Ohms [10]
Hz [5]
2
Step 9. A good quality input capacitor(s) is necessary to limit
the ripple voltage at the VIN pin while supplying most of the
switch current during the on-time. The minimum RMS ripple
current rating for the input capacitor(s) is 0.25 Amp. A quality
ceramic capacitor with a low ESR is recommended. The input
capacitor voltage rating should be greater than the Input Volt-
age Max Spec [3], plus some safety margin. A guide to select
the input capacitor(s) value in proportion to the operating fre-
quency is:
Step 10. The output capacitor smoothes the inductor ripple
current and provide a source of charge for transient loading
conditions. A good choice for the output capacitor is to use a
ceramic capacitor (2.2 µF to 47 µF). Ceramic capacitors pro-
vides ultra low ESR necessary to reduce the output ripple
voltage and noise spikes. The output capacitor voltage rating
should be greater than the Output Voltage Spec [1], plus
some safety margin. An approximation for the output ripple
voltage is:
Step 11. Ccomp and Rcomp configure the error amplifier gain
characteristics to accomplish a stable overall loop gain. One
advantage of current mode control is the ability to close the
loop with only two feedback components. Calculate the value
of Rcomp:
Calculate the value of Ccomp
Step 12. Shown in the following table is the Bill of Materials
for your design. Transcribe each value [#] from worksheet
above into the following table. Congratulations, you’re done.
Selected diode part number:
Selected value for Cin:
Selected value for Cout:
Selected value of Rcomp:
Selected value of Ccomp:
Farads [12]
Farads [13]
Ohms [14]
Farads [15]
[11]