Datasheet

FB
L1
LM25085
PGATE
D1
GND
Q1
V
OUT
GND
5V
Pads for
wire loop
15 PH
R2
3.4k
47 PF
C6
R6
1:
47 PF
C7
R1
10k
www.ti.com Monitor The Inductor Current
8.3 Lowest Cost Configuration
This configuration is the same as Section 8.2, 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 ≥100 mV at V
OUT
. Since the minimum ripple current in this circuit is 116 mAp-p the
minimum value for R6 calculates to 0.86 Ωs. Using a standard value 1.0 Ωs resistor for R6, the ripple at
V
OUT
ranges from ≊116 mVp-p to ≊1080 mVp-p over the input voltage range, see Figure 11. If the
application can accept this ripple level, this is the most economical solution. The circuit is shown in
Figure 6.
Figure 6. Lowest Cost Configuration
9 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.
10 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.
7
SNVA375B–October 2008–Revised April 2013 AN-1905 LM25085 Evaluation Board
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