LM25007EVAL National Semiconductor, LM25007EVAL Datasheet - Page 2

• Manufacturer Part Number: LM25007EVAL
• Description: BOARD EVALUATION LM25007
• Manufacturer: National Semiconductor

Specifications of LM25007EVAL

• Main Purpose: DC/DC, Step Down
• Outputs And Type: 1, Non-Isolated
• Voltage - Output: 5V
• Current - Output: 450mA
• Voltage - Input: 9 ~ 42V
• Regulator Topology: Buck
• Frequency - Switching: 306kHz
• Board Type: Fully Populated
• Utilized Ic / Part: LM25007
• Lead Free Status / RoHS Status: Not applicable / Not applicable
• Power - Output: -

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Board Connection/Start-up
the intended load current. Before start-up a voltmeter should
be connected to the input terminals, and to the output termi-
nals. The load current should be monitored with an ammeter
or a current probe. It is recommended that the input voltage
be increased gradually to 9V, at which time the output volt-
age should be 5V. If the output voltage is correct with 9V at
V
with evaluating the circuit.
Output Ripple Control
The LM25007 requires a minimum of 25 mVp-p ripple at the
FB pin, in phase with the switching waveform at the SW pin,
for proper operation. In the simplest configuration that ripple
is derived from the ripple at V
tor’s ripple current flowing through R4. That ripple voltage is
attenuated by the feedback resistors, requiring that the ripple
amplitude at V
by the gain factor. Options for reducing the output ripple are
discussed below, and the results are shown in the graph of
Figure 8.
A) Minimum Output Ripple: This evaluation board is sup-
plied configured for minimum ripple at V
zero ohms, and including components R6, C7 and C8. The
output ripple, which ranges from 2 mVp-p at V
mVp-p at V
output capacitor (C2), and the inductor’s ripple current,
which ranges from 75 mAp-p to 144 mAp-p over the input
voltage range. This performance applies only to continuous
conduction mode as the ripple amplitude is higher in discon-
tinuous conduction mode. The ripple voltage required by the
FB pin is generated by R6, C7 and C8 since the SW pin
IN
, then increase the input voltage as desired and proceed
IN
OUT1
= 42V, is determined primarily by the ESR of
be higher than the minimum of 25 mVp-p
OUT1
, generated by the induc-
OUT1
FIGURE 2. Minimum Ripple Using R6, C7, C8
by setting R4 to
IN
(Continued)
= 9V to 7
2
switches from -1V to V
ground. The values for R6 and C7 are chosen to generate a
30-40 mVp-p triangle waveform at their junction. That tri-
angle wave is then coupled to the FB pin through C8. The
following procedure is used to calculate values for R6, C7
and C8:
1) Calculate the voltage V
where V
during the off-time (typically 1V), and V
input voltage. For this circuit V
the DC voltage at the R6/C7 junction, and is used in the next
equation.
2) Calculate the R6 x C7 product:
where t
minimum input voltage, and ∆V is the desired ripple ampli-
tude at the R6/C7 junction, 30 mVp-p for this example.
R6 and C7 are then chosen from standard value compo-
nents to satisfy the above product. For example, C7 can be
2200 pF requiring R6 to be 121 kΩ. C8 is chosen to be 0.01
µF, large compared to C7. This portion of the circuit, as
supplied on this EVB, is shown in Figure 2.
ON
SW
is the maximum on-time ()1800 ns), V
V
is the absolute value of the voltage at the SW pin
A
= V
OUT
IN
- (V
20185006
, and the right end of C7 is a virtual
A
SW
:
x (1 - (V
A
calculates to 4.55V. This is
OUT
IN
/V
is the minimum
IN
)))
IN
is the