4.5V to 24V
Input
V
IN
GND
10
P
F
C1
FB
LM25085A
PGATE
ISEN
RT
VIN
GND
VCC
ADJ
SW
Q1
V
OUT
1V
Pads for
wire loop
L1 6.8
P
H
C10
3300 pF
0.01
P
F
R5
10 m
:
R6
0
:
C9
R3 2.05k
C7
R1
1.1k
C11
10
P
F
C8
0.1
P
F
R4
21k
ENABLE
TP1
0.47
P
F
C3
C4
1000 pF
JP1
A
B
C
D1
A
B
C
R7
48.7k
V
OUT
GND
68
P
F
C6
R2
10k
JP2
C5
C2
10
P
F
FB
L1
LM25085A
PGATE
D1
GND
Q1
V
OUT
GND
1V
Pads for
wire loop
6.8
P
H
R2
10k
R6
0.051
:
68
P
F
C6
R1
1.1k
Monitor The Inductor Current
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C) Lowest Cost Configuration: This configuration is the same as option B above, but with C5 removed.
The ripple at the FB pin is attenuated from that at V
OUT
by the feedback resistors (R1, R2). Since
≥
25
mVp-p are required at the FB pin, R6 is chosen to generate
≥
28 mV at V
OUT
. Since the minimum ripple
current in this circuit is 622 mAp-p the minimum value for R6 calculates to 45 m
Ω
. Using a standard value
51 m
Ω
resistor for R6, the ripple at V
OUT
ranges from
≊
32 mVp-p to
≊
43 mVp-p over the input voltage
range. See
Figure 10
. If the application can accept this ripple level, this is the most economical solution.
The circuit is shown in
Figure 5
.
Figure 5. Lowest Cost Configuration
7
Monitor The Inductor Current
The inductor’s current can be monitored or viewed on a scope with a current probe. Remove the jumper
from the WIRE LOOP pads, and install an appropriate current loop across the pads. 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 node,
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 6. Complete Evaluation Board Schematic
6
AN-1933 LM25085A Evaluation Board
SNVA384B – February 2009 – Revised April 2013
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