BAIDU MICRO ELECTRONS CO., LTD.
1.2A, 24V, 1.4MHz Step-down Converter
Setting the Output Voltage (Continued)
peak current is calculated from:
AE2359
Choose the inductor ripple current to be 30% of the maximum load current. The maximum inductor
Peak current determines the required saturation current rating, which influences the size of the
inductor. Saturating the inductor decreases the converter efficiency while increasing the
temperatures of the inductor, the MOSFET and the diode. Hence choosing an inductor with
appropriate saturation current rating is important.
A 1μH to 10μH inductor with a DC current rating of at least
25% percent higher than the maximum
load current is recommended for most applications.
For highest efficiency, the inductor’s DC resistance should be less than 200mΩ. Use a larger
inductance for improved efficiency under light load conditions.
Input Capacitor
The input capacitor reduces the surge current drawn from the input supply and the switching noise
from the device.
The input capacitor has to sustain the ripple current produced during the on time on the upper
MOSFET. It must hence have a low ESR to minimize the losses.
Due to large dI/dt through the input capacitors, electrolytic or ceramics should be used. If a tantalum
must be used, it must be surge protected. Otherwise, capacitor failure could occur. For most
applications, a 4.7μF ceramic
capacitor is sufficient.
Output Capacitor
The output capacitor keeps the output voltage ripple small, ensures feedback loop stability and
reduces the overshoot of the output voltage. The output capacitor is a basic component for the fast
response of the power supply. In fact, during load transient, for the first few microseconds it supplies
the current to the load. The converter recognizes the load transient and sets the duty cycle to
maximum, but the current slope is limited by the inductor value.
Maximum capacitance required can be calculated from the following equation:
Where
ΔV is the maximum output voltage overshoot.
ESR of the output capacitor dominates the output voltage ripple. The amount of ripple can be
calculated from the equation below:
Voutcapacitor = ΔIinductor
×ESR
An output capacitor with ample capacitance and low ESR is the best option. For most applications, a
22μF ceramic
capacitor will be sufficient.
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BAIDUMICRO [ BaiDu Micro electrons Co., Ltd ]
