FB
LM34917A
V
OUT
RON/SD
VIN
C1
SS
RTN
GND
3B
2A
On
Timer
Minimum
Off Timer
Logic
IN
Regulation
Comparator
Current Limit
Detect
8V to 33V
1.0
P
F
2C
C3
R1
22.1k
0.1
P
F
2B
1C
3A
C6
0.022
P
F
V
IN
10
P
F
SW
ISEN
SGND
5V
1D
3D
0.047
P
F
C4
0.1
P
F
L1
15
P
H
R6 0
:
2D
1B
D1
1A
R2
2.49k
R3
2.49k
R4
0.5
:
C7
C8
+
-
VCC
3C
BST
C5
2.5V
10
P
F
C2
GND
1.0
P
F
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Monitor the Inductor Current
C) Lowest Cost Configuration: This configuration is the same as option B, but without Cff. Since
≥
25
mVp-p are required at the FB pin, R4 is chosen to generate
≥
50 mV at V
OUT
, knowing that the minimum
ripple current in this circuit is 105 mAp-p at minimum V
IN
. Using 0.5
Ω
for R4, the ripple at V
OUT
ranges from
≊
80 mVp-p to
≊
150 mVp-p over the input voltage range. If the application can tolerate this ripple level, this
is the most economical solution. The circuit is shown in
Figure 4
.
Figure 4. Lowest Cost Configuration
6
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.
7
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.
8
Minimum Load Current
The LM34917A requires a minimum load current of
≊
1 mA 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 allowing the board’s minimum load current at V
OUT
to be specified at zero.
5
SNOA484D – June 2007 – Revised April 2013
AN-1601 LM34917A Evaluation Board
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