Datasheet
R1
280k
0.1 PF
C5
LM5008A
C4
D1
L1
220 PH
V
OUT
GND
VIN
RT/SD
RCL
VCC
BST
SW
FB
RTN
V
IN
GND
SHUTDOWN
(TP1SD)
R6
8V to 75V
Input
0:
1 PF
C1
R2
715k
0.1 PF
C3
0.47 PF
R3
3.01k
R4
3.01k
R5
0.82:
C2
22 PF
5V
V
OUT
SW
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Current Limit Off-Time
6 Current Limit Off-Time
When current limit is detected the on-time period is immediately 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
detection. The forced off-time is determined by the resistor at the RCL pin (R2), and is calculated from the
following:
T
OFF
= 10
-5
/(0.285 + (V
FB
/6.35 x 10
-6
x R2)) (6)
where V
FB
is the voltage at the FB pin at the time of the current limit detection. In this evaluation board, the
maximum normal off-time is approximately 7.2 µs (at 75V). Due to the 25% tolerance 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 increased to 9.35 µs. This is increased an additional 25% to 11.7 µs to allow for the tolerances of the
above equation. Using the above equation, R2 calculates to 691 kΩ at V
FB
= 2.5V. A standard value 715
kΩ resistor is used.
7 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.
8 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
OUT
), without using the probe’s ground lead which can pick up noise from the
switching waveforms. The probe adapters are suitable for Tektronix P6137 or similar probes, with a 0.135”
diameter.
Figure 5. Complete Evaluation Board Schematic (As Supplied)
5
SNVA380C–March 2009–Revised April 2013 AN-1925 LM5008A Evaluation Board
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