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AVDD Block Function – continued
About the phase characteristic, please consider it based on enough evaluations with the actual model.
(1) Setting the Output L Constant (Boost Converter)
The coil to use for output is decided by the rating current ILR and input current maximum value IINMAX of the coil.
Adjust so that IINMAX
+∆IL does not reach the rating current value ILR. ∆IL can be obtained by the following equation.
AVDD - VIN
Set with sufficient margin because the coil value may have the dispersion of
30%. If the coil current exceeds the
rating current ILR of the coil, it may damage the IC internal element.
BM81810MUF-M uses the current mode DC/DC converter control and has the optimized design at the coil value. A coil
inductance (L) of 4.7
μH to 10 μH is recommended from viewpoints of electric power efficiency, response, and stability.
(2) Output Capacity Settings
For the capacitor to use for the output, select the capacitor which has the larger value in the ripple voltage VPP
allowance value and the drop voltage allowance value at the time of sudden load change. Output ripple voltage is
decided by the following equation.
Perform setting so that the voltage is within the allowable ripple voltage range.
For the drop voltage during sudden load change; VDR, please perform the rough calculation by the following equation.
μs is the rough calculation value of the DC/DC response speed. Please set the capacitance considering
the sufficient margin so that these two values are within the standard value range.
(3) Selecting the Input Capacitor
Since the peak current flows between the input and output at the DC/DC converter, a capacitor is required to install at
the input side. For the reason, the low ESR capacitor is recommended as an input capacitor which has the value more
μF and less than 100 mΩ. If a capacitor out of this range is selected, the excessive ripple voltage is
superposed on the input voltage, accordingly it may cause the malfunction of IC.
However these conditions may vary according to the load current, input voltage, output voltage, inductance and
switching frequency. Be sure to perform the margin check using the actual product.
Figure 81. Coil Current Waveform
Figure 82. Output Application Circuit Diagram
Here, f is the switching frequency.
Here, f is the switching frequency
and RESR is ESR of output capacitor.
IINMAX + 1/2 x ΔIL should not reach
the rating value level