LM34923 Datasheet, PDF (3/12 Pages) National Semiconductor (TI) – EVAL evaluation board provides the design engineer with a fully

Option B) Reduced Ripple Configuration: This configura-
tion generates less ripple at V_OUT than option A above by the
addition of one capacitor (C8) across R5, as shown in Figure
3.
FIGURE 3. Reduced Ripple Configuration
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Since the output ripple is passed by C8 to the FB pin with little
or no attenuation, R7 can be reduced so the minimum ripple
at V_OUT is ≊25 mVp-p. The minimum value for Cff is calculated
from:
where t_ON(max) is the maximum on-time (at minimum V_IN), and
R5//R6 is the parallel equivalent of the feedback resistors.
The ripple at V_OUT ranges from 28 mVp-p to 159 mVp-p over
the input voltage range. See Figure 8.
Option C) Minimum Ripple Configuration: To obtain mini-
mum ripple at V_OUT, R7 is set to 0Ω, and R8, C6, and C7 are
added to generate the required ripple for the FB pin. In this
configuration, the output ripple is determined primarily by the
characteristics of the output capacitance and the inductor’s
ripple current. See Figure 8.
The ripple voltage required by the FB pin is generated by R8,
and C6 since the SW pin switches from –0.1V to V_IN, and the
right end of C6 is a virtual ground. The values for R8 and C6
are chosen to generate a 30-100 mVp-p triangle waveform at
their junction. That triangle wave is then coupled to the FB pin
through C7. The following procedure is used to calculate val-
ues for R8, C6 and C7:
1) Calculate the voltage V_A:
V_A = V_OUT – (V_SW x (1 – (V_OUT/V_IN)))
where V_SW is the absolute value of the voltage at the SW pin
during the off-time, and V_IN is the minimum input voltage. For
this circuit, V_A calculates to 4.98V. This is the approximate DC
voltage at the R8/C6 junction, and is used in the next equa-
tion.
2) Calculate the R8 x C6 product:
where t_ON is the maximum on-time, V_IN is the minimum input
voltage, and ΔV is the desired ripple amplitude at the R8/C6
junction, 40 mVp-p for this example.
R8 and C6 are then chosen from standard value components
to satisfy the above product. Typically C6 is 3000 to 10000
pF, and R8 is 10 kΩ to 300 kΩ. C7 is chosen large compared
to C6, typically 0.1 µF. The ripple at V_OUT is typically less than
10 mVp-p. See Figure 4 and Figure 8.
3
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