Texas Instruments AN-1650 LM34919, User Manual, Page: 4 / 11

FB
SW
R4
BST
VCC
ISEN
SGND
VIN
R1
43.2k
SS
RTN
Gnd
D1
A1
A2
On
Timer
Minimum
Off
Timer
Logic
Regulation
Comparator
B3
A3 Current Limit
Detect
D3
C3
0
:
C1
IN
Gnd
C2
R5
1
P
F
8 V to 40V
RON/SD
C6
0.022 P F
0.1 P F
C3 1 P F
0.1 P F
C4
B1
V
IN
V
OUT
3300
pF
C10
C5
D1
D2
C1
2.49k
R3
2.49k
R2
C9
0.1
P F
P
F 10
P
F 10
5V
LM34919
15
P
H L1
R6 0 :
C8 C7
2.5V
0.022 P F
8.87 k
:
R5 x C10 =
(8V - 4.63V) x 875 ns
0.1V
= 29.5 x 10
-6
R5 x C10 =
(V
IN
±
V
A
) x t
ON
'
V
Monitor The Inductor Current
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1) Calculate the voltage V
A
:
V
A
= V
OUT
– (V
SW
x (1 – (V
OUT
/V
IN
))) (3)
where V
SW
is the absolute value of the voltage at the SW pin during the off-time (typically 1V), and V
IN
is
the minimum input voltage. For this circuit, V
A
calculates to 4.63V. This is the approximate DC voltage at
the R5/C10 junction, and is used in the next equation.
2) Calculate the R5 x C10 product:
(4)
where t
ON
is the maximum on-time ( ≊ 875 ns), V
IN
is the minimum input voltage, and Δ V is the desired
ripple amplitude at the R5/C10 junction, 100 mVp-p for this example.
(5)
R5 and C10 are then chosen from standard value components to satisfy the above product. Typically C10
is 3000 to 5000 pF, and R5 is 10k
Ω to 300 k Ω . C9 is chosen large compared to C10, typically 0.1 µF. See
Figure 4 and Figure 8 .
Figure 4. Minimum Output Ripple 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 LM34919 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.
4 AN-1650 LM34919 Evaluation Board SNVA250A – June 2007 – Revised April 2013
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