LM25007EVAL National Semiconductor, LM25007EVAL Datasheet - Page 4

• 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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Output Ripple Control
D) Alternate Low Ripple Configuration: A low ripple output
can be obtained by connecting the load to V
circuits of options B or C above. Since R4 degrades load
regulation, this alternative may be viable for applications
where the load current is relatively constant. If this method is
used, ensure R4’s power rating is appropriate for the load
current.
Current Limit
The LM25007 contains an intelligent current limit off-timer.
The current limit threshold is 725 mA,
the buck switch (the peak of the inductor’s current waveform)
reaches the threshold the present on-time cycle is immedi-
ately terminated, and a non-resetable off-time is initiated.
The length of the off-time is controlled by an external resistor
(R5) and the voltage at the FB pin. If FB = 0V (output is
shorted to ground) the off-time is the preset maximum of 17
µs. This off-time ensures safe short circuit operation to the
maximum input voltage of 42V. In cases of less severe
overload where the output voltage, and the voltage at FB, is
above ground the current limit off-time is less than 17 µs.
The shorter off-times reduces the amount of foldback, recov-
ery time, and also reduces the startup time. The current limit
off-time is calculated from the following equation:
FIGURE 5. Complete Evaluation Board Schematic
±
25%. If the current in
(Continued)
OUT2
in the
4
The current limit off-time ranges from 4.3 µs to 17 µs as V
varies from 2.5V to 0V, with R5 = 200 kΩ. The guideline for
selecting R5’s value is that the current limit off-time (at V
2.5V) should be slightly longer than the maximum off-time
encountered in normal operation. Setting a shorter off-time
could result in inadequate overload protection, and setting a
much longer off-time can affect the startup operation.
Minimum Load Current
The LM25007 requires a minimum load current of )500 µA
to ensure the boost capacitor (C5) is recharged sufficiently
during each off-time. In this evaluation board, the minimum
load current is provided by the feedback resistors (R2, R3),
allowing the board’s minimum load current to be specified at
zero.
20185011
FB
FB
=