For most applications, the value of the inductor will fall in the range of 4.7μH to 47μH. Its value is chosen
based on the desired ripple current. Large value inductors lower ripple current and small value inductors
result in higher ripple currents. Higher VIN or VOUT also increases the ripple current as shown in equation.
A reasonable starting point for setting ripple current is △IL=840mA (40% of 2.1A).
The DC current rating of the inductor should be at least equal to the maximum load current plus half the
ripple current to prevent core saturation. Thus, a 2.94A rated inductor should be enough for most
applications (2.1A + 840mA). For better efficiency, choose a low DC-resistance inductor.
Different core materials and shapes will change the size/current and price/current relationship of an
inductor. Toroid or shielded pot cores in ferrite or perm alloy materials are small and don’t radiate much
energy, but generally cost more than powdered iron core inductors with similar electrical characteristics.
The choice of which style inductor to use often depends more on the price vs. size requirements and
any radiated field/EMI requirements than on what the HX1304G requires to operate.
Output and Input Capacitor Selection
In continuous mode, the source current of the top MOSFET is a square wave of duty cycle VOUT/VIN. To
prevent large voltage transients, a low ESR input capacitor sized for the maximum RMS current must be
used. The maximum RMS capacitor current is given by:
This formula has a maximum at VIN = 2VOUT, where IRMS = IOUT/2. This simple worst-case condition is
commonly used for design because even significant deviations do not offer much relief. Note that the
capacitor manufacturer’s ripple current ratings are often based on 2000 hours of life. This makes it
advisable to further derate the capacitor, or choose a capacitor rated at a higher temperature than
required. Always consult the manufacturer if there is any question.
The selection of COUT is driven by the required effective series resistance (ESR).Typically, once the
ESR requirement for COUT has been met, the RMS current rating generally far exceeds the IRIPPLE(P-P)
requirement. The output ripple ΔVOUT is determined by:
Where f = operating frequency, COUT = output capacitance and ΔIL = ripple current in the inductor. For a
fixed output voltage, the output ripple is highest at maximum input voltage since ΔIL increases with input
Aluminum electrolytic and dry tantalum capacitors are both available in surface mount configurations. In
the case of tantalum, it is critical that the capacitors are surge tested for use in switching power supplies.
An excellent choice is the AVX TPS series of surface mount tantalum. These are specially constructed
and tested for low ESR so they give the lowest ESR for a given volume.