LM5008AEVAL/NOPB National Semiconductor, LM5008AEVAL/NOPB Datasheet - Page 4

• 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: -

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Current Limit Off-Time
When current limit is detected the on-time period is immedi-
ately terminated, and the off-time forced by the LM5008A
must be greater than the maximum normal off-time, which
occurs at maximum input voltage. The longer-than-normal
off-time is necessary to allow the inductor current to decrease
at least as much, if not more, than the current increase which
occurred during the on-time leading to the current limit detec-
tion. The forced off-time is determined by the resistor at the
RCL pin (R2), and is calculated from the following:
where V
limit detection. In this evaluation board, the maximum normal
off-time is approximately 7.2 µs (at 75V). Due to the 25% tol-
erance of the on-time, the off-time tolerance is also 25%,
yielding a maximum possible off-time of 9 µs. Allowing for the
response time of the current limit detection circuit (350 ns) the
maximum off-time, for the purpose of this calculation, is in-
creased to 9.35 µs. This is increased an additional 25% to
FB
T
OFF
is the voltage at the FB pin at the time of the current
= 10
-5
/(0.285 + (V
FIGURE 5. Complete Evaluation Board Schematic (As Supplied)
FB
/6.35 x 10
FIGURE 4. Minimum Output Ripple Configuration
-6
x R2))
4
11.7 µs to allow for the tolerances of the above equation. Us-
ing the above equation, R2 calculates to 691 kΩ at V
A standard value 715 kΩ resistor is used.
Monitor The Inductor Current
The inductor’s current can be monitored or viewed on a scope
with a current probe. Remove R6, and install an appropriate
current loop across the two large pads where R6 was located.
In this way the inductor’s ripple current and peak current can
be accurately determined.
Scope Probe Adapters
Scope probe adapters are provided on this evaluation board
for monitoring the waveform at the SW pin, and at the circuit’s
output (V
can pick up noise from the switching waveforms. The probe
adapters are suitable for Tektronix P6137 or similar probes,
with a 0.135” diameter.
OUT
), without using the probe’s ground lead which
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30086309
FB
= 2.5V.