SM72485 Datasheet, PDF (13/16 Pages) National Semiconductor (TI) – SolarMagic 100V, 150 mA Constant On-Time Buck Switching Regulator

FIGURE 7. Reduced Ripple Configuration
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b) Minimum ripple configuration: If the application requires
a lower value of ripple (<10 mVp-p), the circuit of Figure 8 can
be used. R3 is removed, and the resulting output ripple volt-
age is determined by the inductor’s ripple current and C2’s
characteristics. RA and CA are chosen to generate a saw-
tooth waveform at their junction, and that voltage is AC-
coupled to the FB pin via CB. To determine the values for RA,
CA and CB, use the following procedure:
Calculate V_A = V_OUT - (V_SW x (1 - (V_OUT/V_IN(min))))
where V_SW is the absolute value of the voltage at the SW pin
during the off-time (typically 1V). VA is the DC voltage at the
RA/CA junction, and is used in the next equation.
- Calculate RA x CA = (V_IN(min) - V_A) x t_ON/ΔV
where t_ON is the maximum on-time (at minimum input volt-
age), and ΔV is the desired ripple amplitude at the RA/CA
junction (typically 40-50 mV). RA and CA are then chosen
from standard value components to satisfy the above product.
Typically CA is 1000 pF to 5000 pF, and RA is 10 kΩ to 300
kΩ. CB is then chosen large compared to CA, typically 0.1 µF.
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FIGURE 8. Minimum Output Ripple Using Ripple Injection
c) Alternate minimum ripple configuration: The circuit in
Figure 9 is the same as that in the Block Diagram, except the
output voltage is taken from the junction of R3 and C2. The
ripple at V_OUT is determined by the inductor’s ripple current
and C2’s characteristics. However, R3 slightly degrades the
load regulation. This circuit may be suitable if the load current
is fairly constant.
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