LM5008AEVAL/NOPB National Semiconductor, LM5008AEVAL/NOPB Datasheet - Page 3

• Manufacturer Part Number: LM5008AEVAL/NOPB
• Description: BOARD EVAL LM5008
• Manufacturer: National Semiconductor

Specifications of LM5008AEVAL/NOPB

• Main Purpose: DC/DC, Step Down
• Outputs And Type: 1, Non-Isolated
• Voltage - Output: 5V
• Current - Output: 300mA
• Voltage - Input: 8 ~ 75 V
• Regulator Topology: Buck
• Frequency - Switching: 130kHz
• Board Type: Fully Populated
• Utilized Ic / Part: LM5008A
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Power - Output: -

Option B) Intermediate Ripple Configuration: This config-
uration generates less ripple at V
Since the output ripple is passed by Cff to the FB pin with little
or no attenuation, R5 can be reduced so the minimum ripple
at V
from:
where t
R3//R4 is the parallel equivalent of the feedback resistors.
The ripple at V
the input voltage range. See
Option C) Minimum Ripple Configuration: To obtain mini-
mum ripple at V
added to generate the required ripple for the FB pin. In this
configuration, the output ripple is determined primarily by the
characteristics of the output capacitance and the inductor’s
ripple current. See
The ripple voltage required by the FB pin is generated by RA,
and CA since the SW pin switches from -1V to V
right end of CA is a virtual ground. The values for RA and CA
are chosen to generate a 50-100 mVp-p triangle waveform at
their junction. That triangle wave is then coupled to the FB pin
through CB. The following procedure is used to calculate val-
ues for RA, CA and CB:
OUT
ON(max)
is
≊
25 mVp-p. The minimum value for Cff is calculated
is the maximum on-time (at minimum V
OUT
OUT
ranges from 26 mVp-p to 64 mVp-p over
, R5 is set to 0Ω, and RA, CA, and CB are
Figure
4.
Figure
OUT
8.
than option A above by
FIGURE 3. Intermediate Ripple Configuration
IN
, and the
IN
), and
3
the addition of one capacitor (Cff) across R3, as shown in
Figure
1) Calculate the voltage V
where V
during the off-time (typically 0.6V), and V
input voltage. For this circuit, V
the approximate DC voltage at the RA/CA junction, and is
used in the next equation.
2) Calculate the RA x CA product:
where t
imum input voltage, and ΔV is the desired ripple amplitude at
the RA/CA junction, 50 mVp-p for this example.
RA and CA are then chosen from standard value components
to satisfy the above product. Typically CA is 3000 to 10000
pF, and RA is 10 kΩ to 300 kΩ. CB is chosen large compared
to CA, typically 0.1 µF. The ripple at V
10 mVp-p. See
3.
ON
SW
is the maximum on-time (
is the absolute value of the voltage at the SW pin
V
A
= V
Figure 4
OUT
– (V
and
A
SW
:
Figure
x (1 – (V
A
calculates to 4.78V. This is
≊
8.
OUT
4.85 µs), V
OUT
is typically less than
/V
IN
IN
is the minimum
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30086304
IN
www.national.com
is the min-