AN938 Datasheet, PDF (18/33 Pages) STMicroelectronics – The L4973 family is a 3.5 A monolithic DC-DC converter

Typical application
AN938
3.2
Input capacitor
The input capacitor has to be able to support the maximum input operating voltage of the
device and the maximum RMS input current. The input current is squared and the quality of
these capacitors has to be very high to minimize its power dissipation generated by the
internal ESR, improving the system reliability. Moreover, input capacitors also affect the
system efficiency.
The maximum I_rms current flowing through the input capacitors is:
Equation 8
I_rms = I_O ⋅
D – -²----^â‹---^D-----²- + -^D----²-
η _η²
where η is the expected system efficiency, D is the duty cycle and Io the output DC current.
This function reaches the maximum value at D = 0.5 and the equivalent RMS current is
equal to Io/2. The following diagram Figure 23 is the graphical representation of Equation 8
with an estimated efficiency of 85% at different output currents.
The maximum and minimum duty cycles are:
Equation 9
D_max
=
-----^V-----^O-----+-----^V----^f-----
V_ccmin + V_f
=
0.66
D_max
=
------^V----^O-----+-----^V-----^f-----
V_ccmax + V_f
=
0.1
where V_f is the freewheeling diode forward voltage. This formula does not take into account
the power MOSFET R_DS(on), and considers negligible the inherent voltage drop, with
respect to input and output voltages. At full load, 3.5 A, and D=0.5, the RMS capacitor
current to be sustained is 1.75 A.
The selected 470 µF/63 V KY, guaranteeing a lifetime of 10000 hours at an ambient
temperature of 105°C and switching frequency of 100 kHz, can support 2.3 A RMS current.
Figure 23. Input capacitance RMS current vs. duty cycle
)_2-3
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Doc ID 5410 Rev 10